Eos,Vol. 84, No. 13, 1 April 2003

VOLUME 84 NUMBER 13 1 APRIL 2003

EOS,TRANSACTIONS, AMERICAN GEOPHYSICAL UNION PAGES 117–124

Glacier Monitoring using ASTER Imagery Rapid ASTER Imaging Facilitates High-resolution satellite instruments such as Landsat ETM+ (Enhanced Thematic Mapper Plus; Timely Assessment of Glacier 15-m resolution for panchromatic band),ASTER (15-m for Visible and Near-Infrared— VNIR– bands),and others of their class help in recog- Hazards and Disasters nizing important details of natural hazards; e.g., avalanche and debris-flow traces, glacier crevasses, and lakes and their changes over PAGES 117, 121 Ice avalanches are able to trigger far-reaching time.ASTER proved to be very suitable for hazards such as enormous snow avalanches, assessing glacier hazards [Kääb, 2002; Wessels Glacier- and permafrost-related hazards lake outbursts,and mudflows.The 1970 Huascaran et al., 2002].With topography being a crucial increasingly threaten human lives,settlements, (Peru) and 2002 Kolka-Karmadon rock/ice ava- parameter for the understanding of high- and infrastructure in high-mountain regions. lanches and mudflows were among the most mountain hazards, DEMs generated from the Present atmospheric warming particularly devastating of all glacier catastrophes. Given ASTER along-track stereo band are especially affects terrestrial systems where surface and the lack of clear statistical evidence,it is difficult helpful [Kääb, 2002]. Imaging opportunities sub-surface ice are involved.Changes in glacier to know whether some of these hazards are a by ASTER are governed by TERRA’s 16-day and permafrost equilibrium are shifting beyond normal part of glacier and permafrost behavior, nadir-track repeat period, and the fact that the historical knowledge. Human settlement and or whether they signal dramatic new threats ASTER VNIR sensor can be pointed cross-track activities are extending toward danger zones from a changing cryosphere.A number of recent, by up to ± 24°,which allows for repeat imaging in the cryospheric system.A number of recent high-magnitude rock fall events from glacierized as frequently as every second day in response glacier hazards and disasters underscore these and perennially frozen rock slopes [e.g.Bottino to urgent priorities. trends. Difficult site access and the need for et al.,2002] have also directed attention toward The rapid development of the 3-million-m3 fast data acquisition make satellite remote the possible influence of atmospheric warming glacier lake on the Belvedere glacier in the sensing of crucial importance in high-mountain on high-mountain rock faces. Italian Alps during late spring 2002, and the hazard management and disaster mapping. Here,rapid imagery from the ASTER (Advanced Spaceborne Thermal Emission and reflection Radiometer) instrument aboard the NASA TERRA satellite is used to support those authorities and scientists responsible for assessing the condition of the 20 September 2002 rock/ice avalanche at Kolka-Karmadon, North Ossetian Caucasus,Russia,and the spring 2002 glacier lake on the Belvedere glacier in the Italian Alps.

Glacier-related Hazards Glacier- and permafrost-related hazards include glacier lake outbursts,ice avalanches, and destabilization of rock faces due to glacier and permafrost retreat. Glacier lake outbursts are often accompanied by heavy floods and debris flows. Recent increases in retreat rates of glaciers in virtually all mountain ranges of the world have led to the development of numerous new glacier lakes. In spring 2002, for example, the United Nations Environment Programme (UNEP) launched a high-level warning in view of the dramatic growth of gigantic glacier lakes in the Himalayas.

Fig. 1.ASTER false color image of 6 October 2002 showing the trace (yellow dotted line) and BY ANDY KÄÄB,RICK WESSELS,WILFRIED HAEBERLI, deposits from the 20 September 2002 Kolka-Karmadon rock/ice avalanche. North is on the right- CHRISTIAN HUGGEL,JEFFREY S.KARGEL,AND SIRI JODHA hand side of the image.The red area in the inset indicates the location of the satellite image. SINGH KHALSA Image details for the zones in white-outlined rectangles are shown in Figure 2. Eos,Vol. 84, No. 13, 1 April 2003 devastating rock/ice avalanche from Kolka glacier,Caucasus, were given the highest acquisition priority by the ASTER science team at the NASA Jet Propulsion Laboratory (JPL) and the Japanese Earth Remote Sensing Data Analysis Center (ERSDAC). Both cases are very unusual in that they have no recorded precedent. A sound glaciological investigation of case details is,therefore,urgently required to update the knowledge base of scientists and authorities involved in glacier hazard mitigation.ASTER imagery has been rapidly available on site at the Kolka- Karmadon event, and has helped much with the disaster assessment.

The Rock/Ice Avalanche at Kolka-Karmadon During the late evening of 20 September 2002,a combined rock/ice avalanche of several million m3 started from a nearly 1.5-km-wide zone between around 3600 and 4200 m eleva- tion in the perennially frozen north face of the Dzimarai-khokh peak in the Kazbek Massif (Figure 1).The unusually large avalanche fell down onto the Kolka glacier tongue located at around 3200 m elevation.The Kolka glacier is known to have surged in the past (e.g.,1969), but no such activity was reported in 2002. Descriptions about two similar avalanche events in 1902 in the same area are available to the authors and are presently being carefully evaluated and compared to the 2002 event. The impact of the initial rock/ice avalanche sheared off a major part of the Kolka glacier tongue and started a sledge-like rock/ice ava- lanche of tens of millions m3 (Figure 2,bottom). Such a total detachment of a rather flat glacier tongue has no known precedent.The Kolka rock/ice avalanche crossed the tongue of the Maili glacier (Figure 2, bottom). It thereby removed and picked up lateral moraines from both glaciers. Even at this time, the speed of Fig. 2.ASTER false color imagery from before (left) and after (right) the 20 September 2002 Kol- the mass must have been around 100 km/h as ka-Karmadon disaster.A large rock/ice avalanche from the north face of the Kazbek massive (cf. suggested by the vertical rise of approximately Figure 1) sheared off the entire Kolka glacier tongue (bottom, yellow outline).Approximately 80 300 m as the avalanche rose up the valley million m3 of ice and debris were deposited at the village Karmadon, south of the Karmadon wall, turning from northeast to north to follow gorge (top).The rivers in the area (blue arrows) were dammed by the avalanche deposits and the main valley (Figure 2 bottom, eastward formed lakes (dark blue line: lake extent as of 27 September 2002; light blue line: lake extent as bulge in path).On its devastating journey,the of 22 October 2002). North is to the top. avalanche picked up a large amount of loose sediments in the valley bottom.An avalanche the Kolka-Karmadon avalanche was produced a certain extent, also have contributed to the track extracted in three-dimensional from the largely from ice melt due to friction heating formation of the lakes.These lakes, estimated ASTER stereo data will be used for a thorough during the avalanche event, or whether the from the 22 October 2002 ASTER image to be water was already stored in the Kolka glacier, over 400,000 m2 in area,and close to 10 million reconstruction of the avalanche dynamics. 3 A few minutes after initiation and 18 km possibly facilitating its disintegration. Most m in volume, posed a dramatic danger of downvalley from the Kolka glacier,the gigantic likely,water played an important role in sub- lake outburst and catastrophic flooding of mass overran the lower parts of the village of stantially reducing basal friction and allowed downvalley areas (Figure 2,top).The lake level Karmadon (Figure 2, top), killing dozens of the high avalanche speed and reach.Conversion started to slowly sink after the 22 October 2002, inhabitants. Shortly beyond this point, the ava- of gravitational potential energy into frictional possibly due to the development of channels lanche was abruptly stopped by the narrowing heating and enthalpy of fusion of ice may within or under the ice.A new critical stage is, valley flanks of the Karmadon gorge (Figure have changed as much as 6% of the avalanche however,expected in springtime with the onset 2, top) and roughly 80 million m3 of ice and mass to liquid water.Unusually low basal friction of snow melt. Due to their high mass and ther- debris were deposited. Large amounts of mud and small ratios between vertical drop and mal content, the ice avalanche deposits at were suddenly pressed out of the mass.The horizontal runout zone have also been reported Karmadon will persist and represent a hazard resulting mudflow,up to 300 m wide, ravaged for other rock/ice avalanches in glacial environ- over many years. Urgent mitigation work and the valley bottom below Karmadon, traveling ments [e.g.,Bottino et al., 2002]. a more thorough documentation are underway. for another 15 km down from the gorge (Figure 1). The avalanche deposits at Karmadon soon In total,the avalanche and subsequent mudflow started to block the rivers entering the gorge Rapid Development of a Glacier Lake, killed over 120 people. (Figure 2 top, blue arrows). Over a period of Italian Alps No unusually high runoffs were registered approximately one month, the dammed river at the Gisel bridge some 20 km downvalley of water progressively flooded the populated Whereas satellite imagery is often the only Karmadon, which suggests that the mudflow areas near Karmadon that had not been directly available data source for remote high mountain had a low water content.At present, evidence affected by the avalanche.Water stored in the regions, airborne and terrestrial measurements is missing on whether the water content of avalanche or produced from its melt might, to may be applied to regions with comparably easy Eos,Vol. 84, No. 13, 1 April 2003 access such as the European Alps. However, routinely taken satellite images might still be the only available data source when it comes to reconstructing past developments. In summer 2001, the Belvedere glacier (Figure 3) underwent a surge-type movement, with speeds of up to 200 m a-1 as opposed to the approximately 30 m a-1 measured for the mid-1980s and 1995–1999 [Haeberli et al., 2002]. Detailed observations of the Belvedere glacier started in the mid-1980s in connection with a catastrophic outburst and subsequent artificial drainage of the moraine lake Lago delle Locce in 1979. In autumn 2001, a small lake developed at an altitude of approximately 2150 m asl.on the flat Belvedere glacier tongue at the foot of the east face of the Monte Rosa. The lake formation was attributed to enhanced englacial water pressure and/or ice compression from the surge-type movement. Development of a large topographic depression on the gla- cier surface and the rapid evolution of a supraglacial lake are rare on temperate glaciers, and have not been previously observed in the Alps in this dimension. It was with great surprise, therefore, that the first control visit in mid-June 2002 encountered an exceptionally large lake of 150,000 m2 with a volume of 3 million m3 (the lake area at that time was comparable to that on the 19 July 2002 ASTER image; Figure 3).The lake level was rising at up to 1 m per day and had only a few meters of freeboard left.The Italian Civil Defense Department and the scientists involved initiated emergency actions.This included continuous lake level monitoring, evacuation of certain parts of the village of Macugnaga, an automatic alarm system, the installation of Fig. 3.ASTER false color imagery documenting the fast development of a 3-million-m3 glacier lake pumps, and detailed scientific investigations. on Belvedere glacier,Italian Alps. In spring 2002, the lake provoked a sudden flood risk for the vil- A cold spell in early July 2002 significantly lage of Macugnaga.The red area on the map shows the location of the image. Lake boundaries reduced melt water input, and together with are highlighted by blue lines.The yellow dotted line indicates a zone of presently increasing pumping and naturally occurring, sub-glacial rock/ice avalanche activity. drainage, helped to stabilize and then lower the lake level. By the end of October 2002, the global consortium that is working to establish References a remote sensing-derived digital baseline lake area had decreased to a size comparable Bottino,G.,M.Chiarle,A.Joly,and G.Mortara,Modelling to that seen on the 24 August 2001 ASTER inventory of global land-ice extent for climate rock avalanches and their relation to permafrost image (roughly 2000–3000 m2; Figure 3). monitoring purposes.The GLIMS data base degradation in glacial environments, Permafrost will be hosted at the National Snow and Ice ASTER imagery formed the base for a rapid, and Periglacial Proc., 13, 283–288, 2002. Data Center in Boulder,Colorado.The GLIMS Haeberli,W.,A. Kääb, F.Paul,M. Chiarle, G. Mortara, first-order assessment of the surge-type glacier coordination center at the U.S. Geological Sur- A. Mazza, P. Deline, and S. Richardson,A surge-type dynamics,in that it detected enhanced glacier vey in Flagstaff,Arizona, also provides satellite movement at Ghiacciaio del Belvedere and a speed and crevassing.The 30 May 2002 ASTER data acquisition support to help scientists developing slope instability in the east face of image,in which an open water surface of roughly Monte Rosa, Macugnaga, Italian Alps, Norwegian 2 and responsible authorities in dealing with 20,000 m2 is visible (or roughly 40,000 m if the J. of Geography,56, 104–111, 2002. glacier hazards and disasters. Kääb,A., Monitoring high-mountain terrain deforma- snow and ice-covered area in the lake center tion from repeated air- and spaceborne optical is included) represents the only data source Acknowledgments data: examples using digital aerial imagery and documenting the development of this supraglacial ASTER data, ISPRS J. Photogrammetry and Remote lake during spring 2002.These findings illus- Rapid ASTER imaging was provided by Anne Sensing, 57, 39–52, 2002. trate the need for continuous monitoring of Kahle and Leon Maldonado working with Kieffer,H. H., et al., New eyes in the sky measure the area over the winter of 2002–2003 in order NASA/GSFC/METI/ERSDAC/JAROS, the U.S./ glaciers and ice sheets, Eos Trans.,AGU,81, 265, Japan ASTER Science Team,and the ASTER 270–271, 2000. to facilitate an early warning if the lake depression Wessels,R., J. S. Kargel, and H. H. Kieffer,ASTER should start to refill.This would allow a quick expedited data processing team at the USGS measurement of supraglacial lakes in the Mount implementation of mitigation measures.Lastly, EROS Data Center.Colleagues at NSIDC and Everest region of the Himalaya, Ann. Glaciology, in view of the Kolka- Karmadon event,special INSTAAR provided initial site coordinates and 34, 399–408, 2002. attention will be given to an increasingly active useful background information.The Kolka- rock and ice fall zone in the permafrost on the Karmadon results are based on ASTER imagery Author Information Monte Rosa east face,with its steep hanging and preliminary in-situ assessments in close glaciers (Figure 3,yellow arrow). collaboration with the North Ossetian Ministry Andy Kääb,Rick Wessels,Wilfried Haeberli,Christian The applied rapid ASTER imaging has been of Environment, the Russian Academy of Sci- Huggel,Jeffrey S.Kargel,and Siri Jodha Singh Khalsa ences, and the Swiss Humanitarian Aid Unit. For additional information, contact Andy Kääb, performed within the Global Land Ice Mea- Department of Geography,University of Zurich, surements from Space (GLIMS) program,one The Belvedere glacier work was performed by Switzerland; E-mail: [email protected]; or Rick of the science projects connected to the the Italian Civil Defense Agencies, together with Wessels,U.S. Geological Survey,Flagstaff,Ariz.; ASTER sensor [Kieffer et al., 2000]. GLIMS is a local scientists and authorities. E-mail: [email protected]. Monte Rosa Ostwand – Geologie, Vergletscherung, Permafrost und Sturzereignisse in einer hochalpinen Steilwand

Luzia Fischer1

Stichworte

Hochalpine Steilwand, Gletscherrückzug, Permafrost, Klimaerwärmung, Felssturz, Eislawine, Sturzmodellierung

Zusammenfassung

Die Monte Rosa Ostwand, Italienische Alpen, ist eine der höchsten Steilwände in den Alpen (2200 – 4500 m ü.M). Sie weist eine starke Bedeckung von steilen Gletschern und grossflächige Permafrostvorkommen auf. In letzten zwei Jahrzehnten allerdings konnte ein starker Rückgang der Eisbedeckung beobachtet werden. Sehr offensichtlich ist auch die stark erhöhte Massenbewegungsaktivität. An verschiedenen Stellen in der Wand entwickelten sich Instabilitäten in Fels und Eis und es ereigneten sich vermehrt Felsstürze, Murgänge und Eislawinen. Im Rahmen dieser Arbeit wurden drei verschiedene Faktoren, die Geologie, die Gletscherveränderungen und die Permafrostverteilung der Monte Rosa Ostwand Wand untersucht. Es zeigte sich, dass sich viele der neu entstandenen Anrisszonen an Stellen befinden, die vor kurzer Zeit eisfrei geworden sind. Dazu liegen die meisten Anrisszonen in der Höhenstufe des Permafrostvorkommens, viele davon nahe der modellierten und abgeschätzten untere Grenze des regionalen Permafrostvorkommens. Aufgrund der fortschreitenden Entwicklungen der Klimaerwärmung und damit zusammenhängenden Veränderungen werden Instabilitäten Monte Rosa Ostwand auch weiterhin eine kritische Gefahrenzone darstellen. Modellierungen von Felssturz-, Eislawinen- und Murgangereignissen zeigen, dass diese eine Gefährdung für besiedelte und touristisch genutzte Gebiete darstellen können.

Abstract

The Monte Rosa east face, Italian Alps, is one of the highest flanks in the Alps (2200 – 4500 m a.s.l.). Steep hanging glaciers and permafrost cover large parts of the flank. During recent decades, the ice cover of the Monte Rosa east face experienced an accelerated and drastic loss in extent. The develop- ment of new slope instabilities rock and ice and enhanced gravitational mass movements such as rock falls, debris flows and ice avalanches was observed. This study is based on multidisciplinary investiga- tions on the geology, glacial changes and permafrost distribution in the flank. It reveals that most of the new detachment zones are located in areas, where the surface ice disappeared recently. Further- more, most of the active detachment zones are located in permafrost zones, for the most part close to the modelled and estimated lower boundary of the regional permafrost distribution. In the view of ongoing or even enhanced atmospheric warming and associated changes it is therefore very likely that the slope instabilities in the Monte Rosa east face will continue to represent a critical hazard source. Modelling of mass movement events has shown that large magnitude events may endanger populated and touristic used areas.

1Glaziologie und Geomorphodynamik Gruppe, Geographisches Institut, Universität Zürich, Winterthurerstr. 190, 8057 Zürich, [email protected]

1 1. Einleitung

Gletscher- und Permafrostvorkommen in Hochgebirgsregionen reagieren sehr sensibel auf Änderungen der atmosphärischen Temperatur, da sich die Eistemperaturen oft nahe am Schmelzpunkt befinden (Haeberli & Beniston, 1998; Harris et al., 2001). Daher verursacht die klimatische Entwicklung seit dem zwanzigsten Jahrhundert starke Auswirkungen in den glazialen und periglazialen Gebieten der Alpen (IPCC, 2001; Harris et al., 2003). Die Veränderungen sind deutlich sichtbar durch den Rückzug der Gletscher und das Abschmelzen von Firnfeldern. Weniger offensichtlich, jedoch sehr signifikant sind Änderungen in Permafrosttemperaturen und -ausdehnung.

Hochalpine Steilwände, die von Hängegletscher und Firnfeldern bedeckt sind und Permafrostvorkommen aufweisen, sind stark betroffen von diesen Veränderungen im Oberflächen- und Untergrundeis. Die Stabilität solcher Steilwände hängt von vielen verschiedenen Faktoren ab. Das Temperatur- und Spannungsfeld in Fels und Eis sowie die hydrologischen Verhältnisse haben allerdings einen sehr grossen Einfluss (Davies et al. 2001). Durch starken Gletscherschwund und Permafrostdegradation im Hochgebirge infolge der Klimaerwärmung verändern sich diese Faktoren zum Teil extrem, und vor allem in Kombination mit ungünstigen geologischen Faktoren – Trennflächensystem und Lithologie – können sich Instabilitäten entwickeln und verstärkte Massenbewegungen wie Felsstürze, Murgänge oder Eislawinen zur Folge haben (Haeberli et al. 1997; Ballantyne, 2002). Die Zusammenhänge und Prozesse bei solchen Veränderungen sind jedoch sehr komplex und das Forschungsfeld zu diesem Thema ist noch jung.

Die Monte Rosa Ostwand (Fig. 1) ist ein aktuelles Beispiel für solche Veränderungen und Entwicklungen. In den letzten zwei Jahrzehnten hat sich die Wandvereisung stark verändert und die Massenbewegungsaktivität hat deutlich zugenommen. In der vorliegenden Arbeit geht es in einem ersten Schritt darum, den aktuellen geologischen und glaziologischen Zustand der Monte Rosa Ostwand aufzuzeigen und die Entwicklungen im letzten Jahrhundert zu rekonstruieren. Dafür wurden drei Faktoren - die Geologie, die Vergletscherung und die Permafrostverteilung - mit verschiedenen Methoden untersucht. Zuerst wurden die drei Faktoren einzeln analysiert. Danach wurden sie miteinander verglichen, um die Zusammenhänge zwischen Gletscheränderungen und Permafrostdegradation und der erhöhten gravitativen Massenbewegungsaktivität in der Monte Rosa Ostwand zu untersuchen.

2. Untersuchungsgebiet

2.1 Monte Rosa Ostwand

Die Monte Rosa Ostwand (Fig. 1) liegt in Norditalien, im Valle Anzasca, südwestlich von Domodossola. Ihre Gipfel - Signalkuppe, Zumsteinspitze, Dufourspitze und Nordend - sind alle über 4500 m ü.M und bilden die Grenze zwischen der Schweiz und Italien. Die Wand ist mit ihren über 2300 m Höhendifferenz eine der höchsten Steilwände Europas. Sie weist eine starke Eisbedeckung und grossflächige Permafrostvorkommen auf.

In den letzten zwei Jahrzehnten fanden in der Monte Rosa Ostwand sehr starke Veränderungen statt. Der Rückzug von Hängegletschern hat sich massiv verstärkt, Firnfelder sind teilweise vollständig abgeschmolzen. Markant ist auch die stark erhöhte Massenbewegungsaktivität. An verschiedenen Stellen in der Monte Rosa Ostwand entwickelten sich Instabilitäten in Fels und Eis und es ereigneten sich viele kleinere bis mittlere Felsstürze, Murgänge und Eislawinen. In den letzten Jahren fanden auch einzelne ausserordentlich grosse Eislawinenereignisse statt.

2

1: Ghiacciaio del Belvedere 2: Lago Effimero 3: Canalone Imseng 4: Canalone Marinelli 5: Canalone Zapparoli 6: Parete Innominata 7: Signalkuppe 8: Jägerhorn 9: Ghiacciaio del Signal 10: Rifugio Zamboni 11: Ghiacciaio del Monte Rosa

Fig. 1: Monte Rosa Ostwand vom Monte Moro Pass her gesehen (Aufnahme September 2004).

2.2 Ghiacciaio del Belvedere

Am Fusse der Monte Rosa Ostwand liegt der Ghiacciaio del Belvedere (Fig. 1, Nr. 1). Der Gletscher hat eine zirka 3 km lange, flache und stark schuttbedeckte Gletscherzunge. Er wird von steilen Gletschern, sowie von Eis- und Schneelawinen aus der Monte Rosa Ostwand genährt. Zwischen 2000 (möglicherweise schon im Herbst 1999 beginnend) und etwa 2003 wies der Ghiacciaio del Belvedere starke Veränderungen im Fliessverhalten auf und zeigte ein surgeartiges Verhalten (d.h. stark erhöhte Fliessgeschwindigkeiten). Die Oberflächengeschwindigkeiten, berechnet aufgrund von photogrammetrischen Messungen in Luftbildern, vervierfachten sich zwischen 2000 und 2001 (Kääb et al., 2004). Infolge des veränderten Fliessverhaltens kam es auch zu starken Veränderungen der Gletscheroberfläche. Von September 1999 bis Oktober 2001 erhöhte sich die Gletscheroberfläche praktisch über die ganze Gletscherzunge zwischen 10 bis zu 25 m und wurde durch viele Gletscherspalten aufgerissen. Der Gletscher überragte die Moränen an gewissen Stellen um mehrere Meter, sodass Wanderwege wegen Stein- und Eisschlag aus Sicherheitsgründen gesperrt werden mussten. Im obersten Teil des flachen Gletschers allerdings, am Fusse der Monte Rosa Ostwand se nkte sich die Gletscheroberfläche um etwa 15 - 35 m und eine tiefe Senke entstand. Während der Schneeschmelze im Frühling 2002 entwickelte sich in dieser Senke auf dem Gletscher ein supraglazialer See, der Lago Effimero (Fig. 1, Nr. 2). Die Fläche des Sees vergrösserte sich sehr schnell und erreichte ein kritisches Volumen von rund 3 Millionen m3 (Tamburini et al., 2003). Dank künstlicher Massnahmen und dem natürlichen Abflusssystem entleerte sich der See im Verlaufe des Sommers praktisch vollständig. Seit 2002 begannen sich das Fliessverhalten und die Oberflächenhöhe des Ghiacciaio del Belvedere wieder zu normalisieren. Die Senke am Wandfuss blieb jedoch bestehen. Im Frühling 2003 füllte sich der Lago Effimero erneut und entleerte sich im Juni 2003 durch einen subglazialen Ausbruch, bei welchem talabwärts glücklicherweise keine Schäden entstanden. Seither hat sich der Lago Effimero nicht mehr in grossem Masse gebildet, nur kleine Wassertümpel sind jeweils im Sommer zu beobachten.

3 2.3 Geologie

Während der Feldarbeit im Sommer 2003 wurde Geologie und Geomorphologie des Talkessels des Ghiacciaio del Belvedere und der Monte Rosa Ostwand untersucht und kartiert (Fischer, 2004). Grundlage dazu bildeten der Geologische Atlas der Schweiz im Massstab 1:25'000 mit den Kartenblättern Zermatt und Monte Moro und die Untersuchungen von Bearth (1953).

Die Monte Rosa Ostwand wird aus Orthogneis und Paragneis aufgebaut (Fig. 2). Die zwei Lithologien entstammen dem Kristallin der penninischen Monte Rosa-Decke. Ortho- und Paragneise lassen sich im Feld sehr gut anhand des Mineralbestandes und Gefüges unterscheiden. Die Paragneise fallen allgemein durch ihre rötlich-braune Anwitterungsfarbe auf. Die mineralogische Zusammensetzung der Orthogneise ist variabel. Nebst Quarz, Kalifeldspat, Plagioklas, Muskowit und Biotit treten kleine Anteile an Apatit, Zirkon, Turmalin und sehr spärlich Erz auf (Bearth, 1953). Die Ausbildung der Orthogneise variiert zwischen Gneis und Granitgneis, das Gefüge geht von massig (Granitgneise) bis paralleltexturiert (Gneise). Der Paragneis entstammt einem vorgranitisch metamorphen Gneis- und Schieferkomplex. Die meist feinkörnigen, biotit- und granatführenden Muskowitschiefer weisen allgemein eine ausgeprägte Paralleltextur auf. Die Ausbildung variiert zwischen Schiefer und plattigem Gneis.

Die Monte Rosa Ostwand ist von verschiedenen Kluftscharen durchzogen, die zusammen ein komplexes Trennflächensystem bilden. Die Hauptklüftung im zentralen Bereich der Wand fällt mit 70 bis 85° in Richtung NE ein. Eine weitere ausgeprägte Kluftschar fällt ebenfalls sehr steil (75-85°) gegen ESE. Diese Einfallrichtung entspricht vielerorts der Orientierung der Felsflanken in der Monte Rosa Ostwand. Zwei weitere Kluftscharen fallen deutlich flacher (zwischen 20-40°), ebenfalls Richtung NE ein. Aufgrund der Orientierung dieser Kluftscharen ergibt sich bereichsweise eine starke Durchtrennung der Felsflanken. Auch die Schichtflächen, die in beiden Gesteinen vorkommen, bilden wichtige Trennflächen, ihre Orientierung variiert allerdings sehr stark und kleinräumig aufgrund der alpinen Verfaltung.

Fig. 2: Die Geologie der Monte Rosa Ostwand, zusätzlich markiert sind die Anrisszonen von beobachteten Massenbewegungsereignissen. Die Nummerierung entspricht jener in Figur 1.

4 3. Felssturz-, Murgang- und Eislawinenaktivität in der Wand

3.1 Aktuelle Massenbewegungsaktivitäten

Aufgrund der Höhe und Steilheit der Monte Rosa Ostwand finden hier seit jeher und häufig gravitative Massenbewegungen in verschiedenen Formen statt. In den letzten zwei Jahrzehnten jedoch hat sich die Massenbewegungsaktivität stark erhöht, und neue Anrisszonen haben sich gebildet. Die Untersuchungen der Massenbewegungsaktivitäten basieren auf Beobachtungen während der Feldarbeit im Sommer 2003 und 2004, Erfahrungen und Beobachtungen von Einheimischen sowie der Analyse von Fotos und Luftbildern.

Die vorherrschenden Massenbewegungsprozesse in der Monte Rosa Ostwand sind Steinschlag, kleinere Felsstürze, Murgänge und Eislawinen. Die Anrisszonen der Steinschlag- und Felssturzereignisse befinden sich in sehr steilen Wandpartien über 3500 m ü.M. Die aktiven Anrisszonen sind über die ganze Wandbreite verteilt (Abb. 2). Am häufigsten sind Sturzereignisse aus der Parete Innominata (Fig. 1, Nr. 6) und im Bereich des Canalone Imseng (Nr. 5). Auch in den Felsflanken der Signalkuppe (Nr. 7) und des Jägerhorns (Nr. 8) finden in gewissen Zonen regelmässig Steinschlag und kleinere Felsstürze statt. Das Sturzmaterial lagert sich in den Couloirs oder auf Hängegletschern in der Monte Rosa Ostwand ab, keine der beobachteten Felssturzereignisse haben den Talboden erreicht. Vor allem im Frühling, Sommer und Herbst kommt es wiederholt zu Sturzereignissen, die in ihrer Grösse und Reichweite sehr unterschiedlich sind. Während der Feldarbeit im ausserordentlich heissen Sommer 2003 konnten beinahe täglich Sturzereignisse beobachtet werden. Auch im Winter finden Sturzereignisse aus dem Fels statt, wenn auch seltener. Dies weist auf stellenweise grössere Veränderungen in der Felsstabilität hin und zeigt, dass die Aktivitäten nicht nur durch die Schmelze in den Frühlings- bis Sommermonaten, sondern vielmehr auch durch die veränderten Gletscher-, Permafrost- und Felszustände beeinflusst werden.

Die Murgänge ereignen sich im Canalone Imseng (Nr. 3) und im Canalone Marinelli (Nr. 4), teilweise auch im Canalone Zapparoli (Nr. 5). Diese Murgänge bilden sich aus Schuttmaterial, das durch die Steinschlagereignisse und durch Felsverwitterung entstanden ist und sich in den Couloirs angesammelt hat. Oftmals bestehen die Murgänge aus einem Gemisch aus Lockermaterial und Eis. Sie können auch durch Eislawinen ausgelöst werden. Die Murgänge treten vor allem in den Sommermonaten auf, wenn viel Schmelzwasser in der Monte Rosa Ostwand vorhanden ist. Während dem heissen Sommer 2003, aber auch im Sommer 2004 ereigneten sich viele Murgangereignisse – manchmal mehrere pro Tag – die in Grösse und Anzahl der Murgangpulse sehr stark variierten. Die beobachteten Murgangereignisse erreichten meistens den Ghiacciaio del Monte Rosa (Nr. 11) und bildeten dort ausgeprägte Schuttfächer.

Die Eislawinen, die im Sommer 2003 beobachtet wurden, waren relativ klein. Sie können als natürliche Ablation der Hängegletscher betrachtet werden und weisen keine sichtbar erhöhte Aktivität auf. Vor allem vom Ghiacciaio del Signal (Nr. 9) lösten sich oft Eislawinen. Die Reichweiten betrugen meistens zwischen 500 und 1000 m, jedoch nie mehr als 1500 m. Im August 2005 ereignete sich allerdings ein sehr grosses Eislawinenereignis, bei dem mit einem Volumen von 1.1 Mio. m3 (pers. Mitteilung A. Tamburini) ein grosser Teil des Hängegletschers oberhalb des Canalone Imseng (Nr. 3) abbrach. Das Sturzmaterial wurde grösstenteils in der – zum Glück kaum mit Wasser gefüllten – Senke des Lago Effimero abgelagert. Der Staubanteil der Eislawine dagegen überfuhr die Seitenmoräne des Ghiacciaio del Belvedere und bedeckte die ganze Ebene oberhalb des Rifugio Zamboni (Nr. 10) mit Eis- und Gesteinsbruchstücken und traf auch die Hütte, wo jedoch kein grosser Schaden entstand. Glücklicherweise ereignete sich diese Eislawine nachts, als sich niemand in der touristisch sehr stark genutzten Gegend rund um das Rifugio aufhielt. Bei einem solchen Ereignis tagsüber wäre es mit grosser Wahrscheinlichkeit zur Gefährdung von Menschenleben gekommen.

5 4. Entwicklung der Wandvereisung

4.1 Veränderungen der Gletscherfläche

Die Veränderung der Vergletscherung der Monte Rosa Ostwand wurde anhand von Fotos, orthorektifizierten Luftbildern, alten topographische Karten und Feldbegehungen analysiert. Verschiedene Fotografien der Wand seit 1885 wurden zusammengestellt. Italienische und schweizerische Luftbilder sind seit 1956 vorhanden, dazu kommt eine italienische topographische Karte aus dem Jahre 1924. Die Gletscherstände von 2003 wurden während der Feldarbeit kartiert.

Beim letzten Gletscherhöchststand, am Ende der kleinen Eiszeit etwa 1850, war die Wand bis auf wenige markante Felsrücken und Spitzen zu grossen Teilen eisbedeckt. Seit dieser Zeit hat die Ausdehnung der Eisbedeckung - wie in praktisch allen Gebieten der Alpen - abgenommen. Die Untersuchung der alten Fotografien zeigen, dass seit dem Gletscherhöchststand um 1850 bis etwa 1980 ein schwacher Rückgang der steilen Gletscher und Hängegletscher in der Monte Rosa Ostwand sichtbar ist. Die Ausdehnung der Hängegletscher blieb in diesem Zeitraum relativ konstant, es kann aber eine Abnahme der Eismächtigkeit beobachtet werden. Der Rückgang der Eisbedeckung äussert sich in erster Linie durch das Abschmelzen von geringmächtigen Firnfeldern. Bei den Firnfeldern kann jedoch nicht von einem kontinuierlichen Rückgang gesprochen werden. Vor allem auf den Luftbildern ist gut sichtbar, dass sie zeitweise auch an Fläche zugelegt haben. Die Firnfelder reagieren schnell auf Niederschlags- und Temperaturänderungen und haben in kälteren, niederschlagsreichen Jahren Eis akkumuliert.

Seit etwa zwei Jahrzehnten kann allerdings ein deutlich beschleunigter Gletscherrückzug, zum Teil sogar ein regelrechter Gletscherzerfall, beobachtet werden (Fig. 3 und 4). Die Gletscher beginnen sich oft auch von oben und von dünnen Stellen innerhalb des Gletschers her zurückzubilden oder verkleinern sich durch den Abbruch von grossen Eislawinen. Auch die Murgang- und Steinschlagaktivität im Bereich der Hängegletscher und Firnfelder kann den Eisrückgang zusätzlich verstärken, indem Eis mitgerissen wird.

In Figur 3 sieht man im unteren Teil der Markierung 1 den Canalone Imseng. Zwischen Herbst 1999 und Sommer 2001 kam es hier zu einer Verkürzung des Hängegletschers um rund 350 m. Es ist jedoch nicht bekannt, ob sich dieser Abbruch in einem einmaligen Ereignis oder in mehreren kleineren Eislawinen ereignete, wahrscheinlicher sind allerdings mehrere kleine Ereignisse. Der Canalone Marinelli ist in der Markierung 2 zu sehen. Da die Eisbedeckung in diesem Couloir nie sehr mächtig war, bildete sie sich durch die verstärkte Schmelze innerhalb von kurzer Zeit sehr stark zurück. Im ausserordentlich heissen Sommer 2003 existierten hier perennierende Eisflächen nur oberhalb von 3800 m ü.M. In den letzten zwei Jahren haben sich wieder einige Firnfelder gebildet. In Figur 4 ist die Zone mit den stärksten Veränderungen der Wandvereisung sichtbar, wo durch das Verschwinden eines ganzen Hängegletschers die Steilwand Parete Innominata freigelegt worden ist.

Fig. 3: Entwicklung der Wandvereisung zwischen 1983 (links, W. Haeberli) und 2003 (rechts, L. Fischer). 1: Parete Innominata und Canalone Imseng, 2: Canalone Marinelli. 6

Fig. 4: Vergleich der Vergletscherung im obersten Teil der Monte Rosa Ostwand zwischen 1986 (links, H. Röthlisberger) und 2002 (rechts, W. Haeberli). Die Markierung zeigt die Parete Innominata, wo ein ganzer Hängegletscher und die darunter liegenden Felsplatten verschwunden sind.

Aufgrund der Analyse des Bildmateriales wurden die Gletscherstände für verschiedene Jahre rekonstruiert und im GIS (Geographisches Informationssystem) digitalisiert. Die Analyse der Orthofotos und der georeferenzierten Karte konnte direkt im GIS vorgenommen werden und die Gletscherumrisse digital gezeichnet werden. Die Abschätzungen anhand von Fotos wurden auf der Basis der topographischen Karte von Hand digitalisiert. Die Überlagerung dieser Gletscherstände bringt die Veränderungen deutlich zum Vorschein (Fig. 5).

Fig. 5: Rekonstruktion der Gletscherstände verschiedener Jahre. Rot markiert sind Bereiche mit starken Veränderungen in der Vergletscherung, in denen sich zugleich auch neue Anrisszonen von Felssturz- und Murgangereignissen gebildet haben. Die Veränderungen der Gletscherzunge des Ghiacciaio del Belvedere wurden in dieser Arbeit nicht rekonstruiert.

7 4.2 Permafrostverbreitung

Die Permafrostverbreitung in der Monte Rosa Ostwand wurde mittels zwei verschiedenen Permafrostmodellen abgeschätzt. Die Modellierungen wurden im Rahmen eines Auftrages der Regione Piemont und des Italienischen Zivilschutzes von der Gruppe für Glaziologie und Geomorphodynamik der Universität Zürichs gemacht (Huggel et al., 2004b). Sie wurden für diese Arbeit übernommen.

Das Programm PERMAKART (entwickelt durch Keller, 1992) basiert auf Faustregeln, die von Haeberli (1975) zur Abschätzung der Permafrostverbreitung in den Schweizer Ostalpen aufgestellt wurden. Basierend auf einem DHM (Digitales Höhenmodell) bestimmt PERMAKART die topographischen Parameter Höhenlage, Exposition, Neigung und die Schneeakkumulation am Hangfuss (länger liegenbleibender Schnee von Lawinenablagerungen) und berechnet aufgrund der Faustregeln die Höhe der Permafrostgrenze. Für die Berechnungen wurde das DHM25 der Swisstopo verwendet.

Das Modell ROCKFROST basiert auf Felstemperatur-Berechnungen. Gruber et al. (2004) berechneten Oberflächentemperaturen von Felswänden in allen Expositionen auf der Basis von Meteodaten für die Regionen Corvatsch (Engadin) und Jungfraujoch (Berner Alpen) für die Zeitperiode von 1982 – 2002. Die über diese Zeitperiode gemittelte Höhe der 0°C-Isotherme der Oberflächentemperatur entspricht etwa der Untergrenze des Permafrostvorkommens. Basierend auf dieser 0°C-Isotherme wird in ROCKFROST die Höhe der Permafrostgrenze in Betrachtung der Hangneigung, Exposition und Höhenlage berechnet. Auch diese Modellierung wurde mit dem DHM25 der Swisstopo durchgeführt.

Die beiden Modellierungen ergeben eine Übersicht über die Permafrostverbreitung im Fels der Monte Rosa Ostwand, in Figur 6 stellvertretend gezeigt an der Modellierung mit ROCKFROST. Die oberen zwei Drittel der Monte Rosa Ostwand weisen gemäss den Modellierungen Permafrostvorkommen auf, was aufgrund von direkten Felstemperaturmessungen in der Wand auch bestätigt werden konnte. Bei beiden Modellen liegt die Permafrostuntergrenze je nach Exposition und Neigung der Wand zwischen 3100 und 3600 m ü.M. Diese grosse Variation der Untergrenze ist vor allem aufgrund der stark variierenden Expositionen in der Monte Rosa Ostwand, die praktisch von SSE bis NNE reichen, zu erklären.

Fig. 6: Modellierung der Permafrostverbreitung mit ROCKFROST. In hellblau die Resultate der Berechnungen basierend auf Klimadaten von der Region Jungfrau, in dunkelblau basierend auf jenen von Corvatsch. Im hellblauen Gebiet liegt der Schwankungsbereich des Permafrostvorkommens, im dunkelblauen Bereich ist kontinuierlicher Permafrost vorhanden.

8 5. Felssturz-, Murgang- und Eislawinenmodellierung

5.1 Datengrundlagen

Für die Beurteilung der Sturzbahnen und Reichweiten von Sturzereignissen wurden für verschiedene Anrisszonen Sturzmodellierungen mit dem Modified Single Flow Direction-Modell (MSFD-Modell) gemacht. Das MSFD-Modell ist ein einfaches, GIS-basiertes Modell (Huggel et al., 2003). Es beruht auf dem Prinzip des Pauschalgefälles, und die Berechnungen werden basierend auf einem DHM gemacht. Das Resultat der Modellierung zeigt für jede Zelle des DHM eine qualitative Wahrscheinlichkeit, bei einem gravitativen Massenbewegungsereignis durchflossen zu werden. Die Berechnungen werden beim Erreichen des zu Beginn definierten Pauschalgefälles, das für Extremereignisse gilt, gestoppt. Daher sind die Reichweiten eher qualitative Abschätzungen für grosse Ereignisse. Für Murgänge wurde ein minimales Pauschalgefälle von 11° (19%) angenommen, für Eislawinen 17° (31%) und für Felsstürze 20° (36%). Das Modell wurde bereits erfolgreich angewendet für Gletscherseeausbrüche (Huggel et al., 2003), Murgänge (Huggel et al., 2004a) und grosse Fels-/Eissturzereignisse (Noetzli et al., 2006).

Als Eingabedaten für die Sturzbahn-Modellierungen werden ein DHM des Gebietes und die genaue Lage der Anrisszonen benötigt. Für die Modellierungen wurden zwei verschiedene DHM verwendet. Das erste DHM ist das DHM25 der Swisstopo und zeigt im Bereich des Ghiacciaio del Belvedere die Topographie vor dem surgeartigen Verhalten (vor 2000). Das zweite DHM, hergestellt aus Luftbildern von 2001, zeigt für den Ghiacciaio del Belvedere die Topographie mit erhöhter Oberfläche der Gletscherzunge und tiefer Senke am Wandfuss. Die Startzonen wurden aufgrund von Beobachtungen der Anrisszonen im Sommer 2003 und 2004 definiert. Im Rahmen dieser Arbeit wurden Sturzmodellierungen für vier verschiedene Startzonen gemacht: Zwei Modellierungen für Felsstürze (Canalone Imseng, Fig. 1, Nr. 3 und Signalkuppe, Nr. 7) und je eine für Murgänge (Canalone Marinelli, Nr. 4) und Eislawinen (Ghiacciaio del Signal, Nr. 9) gemacht. Für jede Startzone wurden die zwei oben beschriebenen DHM angewendet, um die Sturzbahnen für die Topographie vor und während dem surgeartigen Verhalten des Ghiacciaio del Belvedere zu modellieren.

5.2 Resultate und Gefahrenpotential

Die beobachteten Steinschlag-, Murgang- und Eislawinenereignisse der Ostwand bewegten sich– mit Ausnahme der Eislawine 2005 – in Grössenordnungen, die aufgrund ihrer Volumina und Reichweiten für Siedlungsgebiete und touristische Infrastrukturen ungefährlich sind. Die alten Felssturzablagerungen im Bereich des Rifugio Zamboni zeigen jedoch, dass es in diesem Gebiet im Spät- und Postglazial schon zu Felsstürzen mit grösseren Volumina und Reichweiten gekommen ist, und auch die Eislawine im August 2005 zeigt das Gefahrenpotential auf. Solche grosse Ereignisse können auch für die Zukunft weder für Eis noch für Fels ausgeschlossen werden.

Alle Modellierungen zeigen, dass grosse Sturzereignisse Reichweiten aufweisen, eine Gefährdung für besiedelte und touristisch genutzte Gebiete im unteren Bereich des Ghiacciaio del Belvedere darstellen (Fig. 7, 1). Die Resultate werden anhand der Sturzmodellierungen von einer Startzone, der heute sehr aktiven Zone im Bereich des Canalone Imseng, vorgestellt (Fig. 7).

9

Fig. 7: Sturzmodellierung von der Anrisszone im Canalone Imseng, links mit der Topographie vor 2000, rechts mit der Topographie in 2001 mit Senke am Wandfuss. Hinterlegt ist ein Luftbild aus dem Jahr 1999. Diese Sturzmodellierung wurde für ein Felssturz gemacht, die Modellierung wiedergibt jedoch auch sehr gut die Sturzbahn der beobachteten Eislawine im August 2005 (blaue Markierung) und jene von regelmässigen mittleren Felssturz-, Murgang- und Eislawinenereignissen (grüne Markierung).

Ein Vergleich der Modellierungen mit den zwei verschiedenen DHM zeigt, dass unterschiedliche Topographien im Bereich des Wandfusses und des Ghiacciaio del Belvedere einen grossen Einfluss auf den Verlauf der Sturzbahn haben (3). Die rechte Seitenmoräne des Ghiacciaio del Belvedere hat ihre kanalisierende und schützende Ablenkwirkung verloren, da die Gletscheroberfläche seit 2001 infolge des surgeartigen Vorstosses des Gletschers höher liegt als die Moränen (im zweiten DHM). Die Sturzbahnen überqueren die Moräne und verlaufen in Richtung des Rifugio Zamboni und nicht mehr über den Gletscher (2). Daher entsteht eine erhöhte Gefahr für das Rifugio Zamboni und das touristisch stark genutzte Gebiet rund um die Hütte. Andererseits kann die gebildete Senke im Bereich des Lago Effimero (3)- solange der See nicht gefüllt ist - für kleine bis mittlere Ereignisse als Auffangbecken dienen, was teilweise auch beim Eislawinenereignis im Sommer 2005 geschehen ist. Bei einem gefüllten supraglazialen See kann ein grosses Sturzereignis aus der Monte Rosa Ostwand jedoch verheerende Kettenreaktionen auslösen. Praktisch alle Sturzbahnen der modellierten Ereignisse verlaufen durch den Bereich des Lago Effimero. Bereits mittelgrosse Sturzereignisse können den See erreichen. Grössere Sturzereignisse (100‘000 m3 und mehr, Abschätzungen von Huggel et al., 2004c) könnten den gefüllten See zum Ausbrechen bringen. Durch die freigesetzten Wassermassen kann in dem reichlich vorhandenen Schuttmaterial (Moränenbastion, Seitenmoränen) ein Murgang ausgelöst werden, der besiedelte Gebiete talwärts stark gefährden kann. Murgänge direkt aus der Monte Rosa Ostwand hingegen werden nicht als gefährlich eingestuft, da das Wasser und Lockermaterialangebot in der Wand nicht ausreicht, um ein grosses Ereignis auslösen zu können.

Die markierten Sturzbahnen und Ablagerungsgebiete von regelmässigen mittelgrossen Murgang- und Eislawinenereignissen (Fig. 7, grüne Markierung) und dem ausserordentlich grossen Eislawinenereignis im August 2005 (blaue Markierung) zeigen, dass die Sturzmodellierungen sehr gut mit den beobachteten Sturzbahnen übereinstimmen. Der Vergleich der Modellierungen mit den zwei verschiedenen DHM zeigt sehr deutlich, dass man für die Modellierung von Sturzbahn und Ablagerungsgebiet immer die aktuelle Topographie berücksichtigen muss, um eine präzise Aussage machen zu können.

10 6. Diskussion

Um die Zusammenhänge zwischen Gletscheränderungen, Permafrostdegradation, Geologie und der erhöhten gravitativen Massenbewegungsaktivität in der Monte Rosa Ostwand zu untersuchen, wurden die drei untersuchten Faktoren und die beobachteten Anrisszonen miteinander verglichen.

In der geologischen Karte (Fig. 2) sind mit grünen Punkten die während der Feldarbeit beobachteten Anrisszonen von Felsstürzen markiert, in braun jene von Murgängen und in blau jene von Eislawinen. Auffallend ist, dass viele Anrisszonen von Felsstürzen und eventuell auch von Murgängen im Übergangsbereich zwischen Paragneis und Orthogneis liegen. Dies deutet darauf hin, dass diese Lithologieübergänge Instabilitäten begünstigen können.

Der Vergleich der Gletscheränderungen in der Wand mit den beobachteten Anrisszonen (Fig. 5, rote Markierungen) zeigt, dass viele der aktiven Zonen in Gebieten liegen, die erst vor kurzer Zeit eisfrei geworden sind. Die Zonen mit dem stärksten Rückgang der Eisflächen befinden sich im Canalone Imseng, Canalone Marinelli und der Parete Innominata. Genau in diesen Zonen kann auch die höchste Massenbewegungsaktivität beobachtet werden. Durch den Rückzug von Gletschern kommt es zu veränderten Temperatur- und Spannungsverhältnis- sen in den Felsflanken. Des Weiteren können sich auch die hydrologischen Bedingungen stark verändern. Der Fels ist nun vermehrt Niederschlag und Schmelzwasser ausgesetzt, was verstärkte Verwitterung und auch veränderte Poren- und Kluftwasserdrucke zur Folge haben kann. All diese Veränderungen können dazu führen, dass es zu Instabilitäten im Fels kommt. Bei den Anrisszonen von Eislawinen kann kein direkter Zusammenhang mit der Geologie beobachtet werden. An einigen Stellen jedoch kann die Felssturz- und Murgangaktivität eine erhöhte Eislawinenaktivität auslösen. Zum Beispiel indem instabiles Felsmaterial unterhalb eines Hängegletschers auch den darüber liegenden Gletscher destabilisiert (Canalone Zapparoli).

In Figur 6 sind die aktiven Anrisszonen von Felssturz-, Murgang- und Eislawinenaktivitäten mit roten Kreisen in der modellierten Permafrostverbreitung markiert. Es ist erkennbar, dass praktisch alle Anrisszonen in der Höhenlage des Permafrostvorkommens liegen und dass sich viele dieser Anrisszonen im unteren Bereich der Permafrostverbreitung befinden, im sensitiven Bereich des warmen Permafrostes. Dies bestärkt die Annahme, dass sich die Instabilitäten unter anderem im Zusammenhang mit dem Aufwärmen des Permafrostes und dem Ansteigen der unteren Permafrostgrenze bilden. Vor allem Steinschlag- und kleinere Felssturzereignisse, wie sie in der Monte Rosa Ostwand stattfinden, können durch das Auftauen der obersten Permafrostschichten entstehen. Tiefgründige Veränderungen im Permafrost durch langfristige Temperaturänderungen können auch zu grossen Sturzereignissen führen. Temperaturveränderungen im Untergrund können auch auf Hängegletscher einen grossen Einfluss haben. Diese kalten Hängegletscher sind meistens – zumindest stellenweise – am Untergrund angefroren und durch eine Erwärmung kann sich die Stabilität vermindern.

Zusammenfassend können folgende Schlüsse gezogen werden:

• Viele der aktuellen Anrisszonen von Steinschlag, Felsstürzen und Murgängen liegen in Gebieten, die erst vor kurzer Zeit eisfrei geworden sind.

• Praktisch alle aktiven Anrisszonen befinden sich in der Höhenstufe des Permafrostes. Viele liegen in der Nähe der Untergrenze des Permafrostvorkommens.

• Viele der aktiven Anrisszonen befinden sich im Bereich von lithologischen Übergängen.

In dieser Arbeit wurden die Veränderungen und Zusammenhänge der verschiedenen Faktoren vor allem aufgrund von Bildvergleichen beurteilt. Es wird deutlich, dass alle drei der untersuchten Faktoren einen grossen Einfluss auf die Wandstabilität haben, vor allem auch das Zusammenspiel der verschiedenen Faktoren.

11 7. Ausblick

Aufgrund der Entwicklungen der globalen Klimaerwärmung ist auch in Zukunft mit weitergehender Permafrostdegradation und Gletscherschwund zu rechnen. Die Entwicklung von Instabilitäten in Fels und Eis, neuer Anrisszonen und erhöhter gravitativer Massenbewegungsaktivität wird in der Monte Rosa Ostwand voraussichtlich weitergehen und kann auch in anderen periglazialen Steilwänden der Alpen stattfinden.

Daher ist es notwendig, die zum Teil noch wenig bekannten Faktoren und Prozesse im Zusammenhang mit der Stabilität von periglazialen Steilwänden eingehender zu untersuchen. Damit könnten sowohl weitere wissenschaftliche Erkenntnisse gewonnen als auch bessere Abschätzungen der hochalpinen Naturgefahren gemacht werden.

Literatur Ballantyne, C.K. 2002: Paraglacial geomorphology. Quarternary Science Reviews, 21, 1935-2017. Bearth, P. 1952: Geologie und Petrographie des Monte Rosa. Beiträge zur Geologischen Karte der Schweiz, Bern. Davies, M.C.R., Hamza, O. & Harris, C. 2001: The effect of rise in mean annual temperature on the stability of rock slopes containing ice-filled discontinuities. Permafrost and Periglacial Processes, 12, 137-144. Fischer, L. 2004: Monte Rosa Ostwand – Geologie, Vergletscherung, Permafrost und Sturzereignisse in einer hochalpinen Steilwand. Unveröffentlichte Diplomarbeit, ETH Zürich. Gruber, S., Hoelzle, M. & Haeberli, W. 2004: Rockwall temperature in the Alps: modeling their topograhic distribution and regional differences. Permafrost and Periglacial Processes 15, 299-307. Haeberli, W. 1975: Untersuchungen zur Verbreitung von Permafrost zwischen Flüelapass und Piz Grialetsch (Graubünden). Zürich: Mitteilung Nr. 17 der Versuchsanstalt für Wasserbau, Hydrologie und Glaziologie der ETH Zürich. Haeberli, W. & Beniston, M. 1998: Climate change and its impact on glaciers and permafrost in the Alps, Ambio 27/4, 258- 265. Haeberli, W., Wegmann, M. & Vonder Muehll, D. 1997: Slope stability problems related to glacier shrinkage and permafrost degradation in the Alps. Eclogae geologicae Helvetiae 90, 407-414. Harris, C, Davies, M. & Etzelmüller, B. 2001: The assessment of potenential geotechnical hazards associated with mountain permafrost in a warming global climate. Permafrost and Periglacial Processes 12, 145-156. Harris, C., Vonder Muehll, D., Isaksen, K., Haeberli, W., Sollid, J.L., King, L., Holmlund, P., Dramis, F., Gugliemin, M. & Palacios, D. 2003: Warming permafrost in European mountains, Global and Planetary Change, 39, 215-225. Huggel, C., Kääb, A., Haeberli, W. & Krummenacher, B. 2003: Regional-scale GIS-models for assessment of hazards from glacier lake outbursts: evaluation and application in the Swiss Alps. NHESS, 3 (6), 647-662. Huggel, C., Kääb, A. & Salzmann, N. 2004a: GIS-based modeling of glacial hazards and their interactions using Landsat-TM and IKONOS imagery. Norsk Geografisk Tidsskrift-Norwegian Journal of Geography, Vol. 58, 61-73. Huggel, C., Nötzli, J., Kääb, A. & Haeberli, W. 2004b: Permafrost modeling studies for the Monte Rosa east face, Macugnaga. Glaciology and Geomorphodynamics Group, Dep. of Geogr., Universität Zürich, unpublished. Huggel, C., Kääb, A. & Haeberli, W. 2004c: Rock/ice avalanche study, Monte Rosa East face, Macugnaga. Glaciology and Geomorphodynamics Group, Dep. of Geogr., Universität Zürich, unpublished. IPCC, 2001. Climate Change 2001: The scientific basis. Contribution of working group 1 to the third assessment report of the intergovernmental panel on climate change. Cambridge University Press, Cambridge. Kääb, A., Huggel, C., Barbero, S., Chiarle, M., Cordola, M., Epifani, F., Haeberli, W., Mortara, G., Semino, P., Tamburini, A. & Viazzo, G. 2004: Glacier hazards at Belvedere glacier and the Monte Rosa east face, Italian Alps: Processes and mitiga- tion. International Symposium, Interpraevent 2004 – Riva/Trient. Keller, F. 1992: Automated mapping of mountain permafrost using the program PERMAMAP within th geographical infor- mation system ARC/INFO. Permafrost and Perigacial Processes, 3(2), 133-138. Noetzli, J., Huggel, C., Hoelzle, M. & Haeberli, W. 2006: GIS-based modelling of rock/ice avalanches from Alpine perma- frost areas. Computer and Geosciences 10, 161-178. Tamburini, A., Mortara, G., Belotti, M. & Federici, P. 2003: The emergency caused by the “short-lived lake” of the Belve- dere Glacier in the summer 2002 (Macugnaga, Monte Rosa, Italy). Terra glacialis, 6, 37-54. 12 WATER RESOURCES RESEARCH, VOL. 48, W02521, doi:10.1029/2011WR010733, 2012

Projections of future water resources and their uncertainty in a glacierized catchment in the Swiss Alps and the subsequent effects on hydropower production during the 21st century David Finger,1,2 Georg Heinrich,3 Andreas Gobiet,3 and Andreas Bauder4 Received 31 March 2011; revised 29 December 2011; accepted 5 January 2012; published 18 February 2012.

[1] Hydropower accounts for about 20% of the worldwide electrical power production. In mountainous regions this ratio is significantly higher. In this study we present how future projected climatic forcing, as described in regional climate models (RCMs), will affect water resources and subsequently hydropower production in downstream hydropower plants in a glacierized alpine valley (Vispa valley, Switzerland, 778 km2). In order to estimate future runoff generation and hydropower production, we used error-corrected and downscaled climate scenarios from regional climate models (RCMs) as well as glacier retreat projections from a dynamic glacier model and coupled them to a physically based hydrological model. Furthermore, we implemented all relevant hydropower operational rules in the hydrological model to estimate future hydropower production based on the runoff projections. The uncertainty of each modeling component (climate projections, glacier retreat, and hydrological projection) and the resulting propagation of uncertainty to the projected future water availability for energy production were assessed using an analysis of variance. While the uncertainty of the projections is considerable, the consistent trends observed in all projections indicate significant changes to the current situation. The model results indicate that future melt- and rainfall-runoff will increase during spring but decline during summer. The study concludes by outlining the most relevant expected changes for hydropower operations. Citation: Finger, D., G. Heinrich, A. Gobiet, and A. Bauder (2012), Projections of future water resources and their uncertainty in a glacierized catchment in the Swiss Alps and the subsequent effects on hydropower production during the 21st century, Water Resour. Res., 48, W02521, doi:10.1029/2011WR010733.

1. Introduction production, as they can react to the electricity consumption variations within seconds and thus ensure sufficient energy [2] Although about 20% of the gross electricity produc- tion worldwide comes from hydropower plants, the eco- supply for adequate network stability. Nevertheless, hydro- nomic potential of hydropower has not been exploited power production depends on local precipitation, snow completely [Sternberg, 2010]. In mountainous regions, the coverage, and glacier ice melting, and is therefore subject ratio of hydropower production is significantly higher, e.g., to important interannual fluctuations [Bartolini et al., in Austria and Switzerland hydropower accounts for more 2009; Lambrecht and Mayer, 2009]. than 50% of the national electrical power production [Zim- [3] Throughout the last century hydropower plants have mermann, 2001]. In these countries, the hydropower poten- been adapted and improved to optimize power production and tial is nearly exploited completely or will not be further other services [Alfieri et al., 2006; Egre and Milewski, 2002]. developed for various environmental reasons [Finger et al., Nevertheless, climate change remains one of the biggest chal- 2006; 2007; Jager and Smith, 2008; Miranda, 2001]. In lenges for the hydropower sector [Lehner et al., 2005], in par- particular, reservoirs are important constituents of power ticular in Switzerland, where reservoirs are frequently glacier fed [Hauenstein, 2005]. Water resources in mountain regions are particularly sensitive to climatic forcing as glacier retreat 1Institute of Environmental Engineering, ETH Zurich, Switzerland. is already affecting hydropower production across the Alps 2Now at Institute of Geography and Oeschger Centre for Climate Change [Hock, 2005]. Although increasing water runoff has been Research, University of Bern, Bern, Switzerland. observed in many mountain streams below glaciers in recent 3 Wegener Center for Climate and Global Change (WEGC) and Institute years, discharge is expected to decrease in the future, when for Geophysics, Astrophysics, and Meteorology/Inst. of Physics (IGAM/ IP), University of Graz, Graz, Austria. glaciers will have shrunk below a threshold extent [Huss, 4Laboratory of Hydraulics, Hydrology and Glaciology (VAW), ETH 2011; Huss et al., 2008; Stahl and Moore, 2006]. According Zurich, Switzerland. to the IPCC report [IPCC, 2008], the hydropower potential is expected to decline by 6%, with strong regional variations Copyright 2012 by the American Geophysical Union from a 20%–50% decrease in the Mediterranean region to a 0043-1397/12/2011WR010733 15%–30% increase in Northern and Eastern Europe. These Errors to the author affiliations were introduced during the production predictions will doubtlessly affect energy production and of this article. The PDF version is correct here. The article as originally flood control, so that water management measures will have published appears in the HTML. to be adapted accordingly [Xu and Singh, 2004].

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[4] In Switzerland the Federal Office of Energy (SFOE) runoff and hydropower production was assessed using an estimates that in the long term 2000 hydro GWh a1 will analysis of variance (ANOVA analysis). This approach be lost due to climate change and 1900 GWh a1 due to allows us not only to quantify the uncertainty of our projec- legal residual water flows [OFEN, 2004]. Such estimations tions, but also to determine which modeling component is rely, however, on empirical linear projections and provide responsible for the uncertainty. Finally, we compare the only limited information for future water management projected changes in water availability with the expected strategies. Projections performed with hydrological models, natural variability of the annual cycle. This allows us to using state of the art climate projections, can help in devel- identify which projected changes are statistically stronger oping adequate water management strategies to anticipate than the natural variability of future climate patterns. This expected changes. Efforts to assess such changes have been identification is a prerequisite for establishing adequate made by Mimikou and Baltas [1997] using three global cir- water management policies for future periods. culation model (GCM) outputs to evaluate the reliability of [7] We chose the Vispa valley, a mountain valley in the a hydropower scheme, by Bergström et al. [2001] to assess Swiss Alps, for this study as its setting with two hydro- the potential of future power plants in Sweden, and several power companies exploiting the natural runoff using com- authors in the western United States have used different plex operational rules is representative for typical modern GCM and downscaling techniques to assess future water hydropower storage plants. The study concludes by assess- resources [Christensen et al., 2004; Payne et al., 2004], to ing the projected impact and its uncertainty of climatic name just a few. While the use of different climate scenar- change on future water resources and energy production in ios reveals the uncertainty of climate projections, it is also the valley. Our approach opens new possibilities to water essential to quantify and discuss the uncertainty propagated resources managers to develop specific strategies to antici- to the future hydrological projections. For example, Min- pate climate change impacts on hydropower production. ville et al. [2008] and Kay et al. [2009] investigated differ- ent sources of uncertainty and demonstrated that the 2. Study Site and Data uncertainty of future hydrological projections is primarily [8] The Vispa River is situated about 200 km east of due to uncertainties within the choice of a global climate 0 0 model (GCM). Schäfli et al. [2007] were the first to present Geneva (7 50 E, 46 08 N) in the southern part of a thorough approach to compare modeling uncertainties with projected changes due to climate change for a small case study in Switzerland. Stahl et al. [2008] incorporated periodic changes in glacier cover in a conceptual model to project future runoff generation. However, all these previ- ous studies do not account for dynamic glacier retreat and hydropower operational rules simultaneously, nor do they employ the newest state of the art error-corrected and downscaled climate scenarios. This makes their result less reliable and their modeling approaches can only be applied to study sites without river diversion. Furthermore, the propagation of the specific uncertainty from climate models and dynamic glacier models to final projections of future water resources has never been quantified. [5] Motivated by the ongoing discussion about the impacts of climate change and its uncertainty on hydro- power production [SGHL, 2011], our study presents an inte- grative modeling chain in which we couple error-corrected and downscaled scenarios from regional climate models (RCMs), glacier retreat from a dynamic glacier model, and a hydrological model which accounts for hydropower opera- tional rules. For this purpose we integrated all relevant hydropower operational rules (diversion of mountain stream, storage in reservoirs, and routing of pressurized water to the turbines) in a physically based, fully distributed hydrological model and coupled the model to a dynamic glacier model and seven error-corrected and downscaled A1B forced RCM climate scenarios. This modeling chain allows us to investi- Figure 1. Vispa valley including the watershed of Matt- gate the impacts of climate change on complex hydropower marksee. Dots in circles illustrate location of important installations which collect and divert water from several landmarks, double circles are indicating hydropower facili- subcatchments with different degrees of glaciation. ties, and white circles locate water intakes. The color bar [6] Furthermore, we quantify the uncertainties of climate on the left side indicates the retreat of glacier extent in 10 change projections by assessing the variability in an ensem- year intervals. Arrows on the map illustrate water flow in ble of realistic projections of each modeling component. pipelines of the hydropower companies. Values in paren- Subsequently, the propagation of the variability of each theses indicate altitude in m asl. Details about each labeled modeling component to the global uncertainty of projected symbol are listed in Table 4.

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Switzerland. The gauging station near the village of Visp of the annual discharge in the entire Vispa valley is used (location V in Figure 1) drains a total area of 778 km2,which for hydropower production by KWM, altering the runoff includes the Matter valley on the western part and the Saas regime at the gauging station in Visp significantly. Before valley on the eastern part. The average annual runoff volume hydropower operations started in 1964, natural runoff in of 0.53 km3 a1 (equivalent to 689 mm a1) measured in Visp reached average monthly discharge of 75 m3 s1 in Visp represents about 9.3% of the total runoff from the summer and about 5 m3 s1 in winter, while in recent years Rhone valley as measured at the outlet into Lake Geneva. runoff during summer reaches about 35 m3 s1 compared The catchment elevation ranges from 659 m asl in Visp to to 10 m3 s1 in winter (Figure 2). This is a direct effect of 4634 m asl at the Dufourspitze. About 29% of the area is the hydropower operations, revealing the practice of KWM covered by glaciers, containing seven larger (>5km2)val- to store water during summer and release it during winter ley glaciers. The seven major glaciers are all located at alti- when energy prices are highest. tudes between 2287 and 4408 m asl, varying in size from [11] An overview of all available observational data in 4.6 km2 (glacier in the Schalibach watershed) to 51 km2 the catchment is given in Table 2. The Federal Office of (Gornergletscher, the second largest glacier system in Swit- Meteorology and Climatology (MeteoSwiss) operates two zerland). Icefields, firns, and snow fields smaller than 4.6 automatic weather stations, one close to the village of Visp km2 were modeled as permanent snow fields (see section 3). and one close to Zermatt (location V and Ze in Figure 1, Based on the simulations of Farinotti et al. [2011] we esti- respectively), where mean daily air temperature (T), precip- mate the mean annual glacier retreat to vary between 0.05 itation (P), and global radiation (I) recordings are available (glacier in the Schalibach watershed) and 0.48 km2 a1 since 1980. KWM provided 9 years of daily lake level (Gornergletscher). Further topographic information on the recordings (L), natural runoff in m3 d1collected in Matt- major glaciers is summarized in Table 1 and provided in marksee (QM) and in Zermeiggern (QZ at location Z in Farinotti et al. [2011]. Figure 1), as well as water release rates in the generating [9] The natural runoff in Vispa valley is exploited by two plant in Stalden (QS at location S in Figure 1). Snow cover hydropower companies, which divert parts of the runoff extent (SC) was derived from daily satellite images into two hydropower reservoirs, Mattmarksee (storage vol- recorded by the moderate resolution imaging spectroradi- ume of 101 Mio m3; surface: 1.76 km2; depth: 96 m), oper- ometer (MODIS) continuously operational since 2001 ated by the Kraftwerke Mattmark (KWM), and Lac de Dix [Hall et al., 2002]. In this study we used the MODIS prod- (storage volume of 400 Mio m3; surface: 3.65 km2; depth: uct MOD10A1.5 (see http://nsidc.org/). Daily discharge 227 m) operated by Grande Dixence (GD). While GD has data from the gauging station at Visp (QV) were provided diverted water to the Lac de Dix reservoir outside the study by the Swiss Federal Office of Environment (FOEN) start- site since 1964, Mattmarksee has been operational since ing from 1903. 1965 and is located in the eastern valley of the catchment [12] A digital elevation model (DEM) with a 250 m spa- (location M in Figure 1). KWM operates nine water intakes, tial resolution from the Swiss Federal Office of Topography draining a total area of 162 km2, diverting the water into (Swisstopo) was used as catchment topography. The 250 m Mattmarksee or down to the power plant at Stalden (loca- resolution appears to be a good compromise between the tion S in Figure 1). GD disposes of 19 water intakes, divert- sparse spatial resolution of observational data and the high ing over 400 M m3 a1 of natural runoff from a total area of resolution of local topographic settings. Soil and geology 241 km2 to Lac de Dix. Two further hydropower compa- properties, as well as land cover information, were obtained nies, Energy Zermatt and EnAlpine operate river power from digital thematic maps available from the Swiss Fed- plants, which do not enact storage capacities and therefore eral Statistical Office (FSO). do not affect the temporal runoff of the Vispa. [10] While GD diverts all water collected to the Lac de Dix situated outside the Vispa valley, water storage in 3. Methods Mattmarksee leads to a seasonal shift of the discharge dy- [13] In order to assess the impact of climate change on namics downstream of the hydropower plant. About 41% hydropower production, we present an integrative modeling

Table 1. Characteristics of the Individual Glacierized Subcatchments in the Vispa Valley

2010 2050 2090 Retreat Rate

Aa LEb Aa LEb Aa LEb Rc Catchment (km2) (m asl) (km2) (m asl) (km2) (m asl) (km2 a1) HPCd Methode

Mattmark 18.9 2733 6.6 3202 0.6 3343 0.23 KWM M Feevispa 13.9 2880 7.6 3298 2.1 3762 0.15 KWM E Riedbach 7.5 2399 4.1 3356 1.1 3558 0.08 KWM E Findelbach 18.3 2627 12.3 3115 3.1 3520 0.19 GD M Gornera 51.0 2287 31.6 2941 12.8 3407 0.48 GD M Schalibach 4.6 2382 2.5 2835 0.7 3406 0.05 GD E Zmuttbach 12.8 2402 6.9 2908 1.9 3444 0.14 GD E aApproximate glacierized area (A) based on 250 m grid resolution, given by the model resolution. bAltitude of the lowest glacierized cell (LE) based on 250 m grid resolution, given by the model resolution. cMean annual glacier retreat (R) based on 250 m grid resolution, given by the model resolution. dHydropower company (HPC) which captures glacier runoff for hydropower production. eMethod used to make glacier extend projections: M: modeled by Farinotti et al. [2011]; E: extrapolated as described in the text.

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scenarios used in the IPCC AR4 report [Solomon et al., 2007] (B1 at the lower bound and A2 at the upper bound). [15] De´que´ et al. [2011, 2007] showed that the choices of the GCM as well as the RCM are major sources of uncertainty. To account for this uncertainty, we selected seven RCMs from a set of 15 available simulations that are forced by four distinct GCMs (Table 3), adequately accounting for uncertainty in boundary conditions and RCM model formulation as suggested by van der Linden and Mitchell [2009]. We excluded all simulations driven by the GCMs HadCM3Q3 and HadCM3Q16 because they are regarded as not realistic [van der Linden and Mitchell, 2009]. From the remaining RCM simulations, five simula- tions were forced by the global climate model ECHAM5-r3. In order to avoid an artificial bias toward the climate response of ECHAM5-r3, we limited our selection to two ECHAM5-r3 simulations. Therefore our final selection of Figure 2. Monthly average discharge patterns in the all climate scenarios (Table 3) represents an ensemble of Vispa River at Visp before 1964 without hydropower oper- state of the art climate projections. ations, in 1965 when water diversion to Lac de Dix started, [16] In order to correct RCM errors in representative and after 1965 when Mattmarksee was fully operational. present day climate and for further downscaling [Frei et al., 2003; Hagemann et al., 2004; Suklitsch et al., 2008; approach, by coupling a hydrological model with down- Suklitsch et al., 2010], we used an innovative approach in scaled and error-corrected RCM projections, glacier retreat this study, combining dynamical and statistical downscal- projections, and hydropower operational rules. In section 3 ing with observations, as proposed by Themeßl et al. we describe the individual components of this model chain, [2011a]. This quantile based error correction approach the uncertainty they introduce into hydrological projec- (quantile mapping; QM) is based on the observations of tions, and how uncertainty is analyzed in this study. the two meteorological ground stations, Visp and Zermatt, in order to produce error-corrected and downscaled RCM 3.1. Climate Model Projections projections for daily mean air temperature, daily precipita- [14] In order to drive our hydrological model we use the tion sum, and daily mean global radiation. most comprehensive RCM ensemble available from the EU [17] In a first evaluation step we checked the perform- FP6 Integrated Project ENSEMBLES. ENSEMBLES is re- ance of QM using the 28 years (1980–2008) of observatio- stricted to the A1B emission scenario, which is character- nal data from Visp and Zermatt. Mean differences (MD) ized by global economic growth and a balanced emphasis and the ratio of the standard deviation (SD) between simu- on all energy sources [Nakicenovic et al., 2000]. However, lated and observed daily values (R ¼ (sim)/(obs)) for all since the emission scenarios have only a small impact on three meteorological variables are summarized in Table 3. climate change until the middle of the 21st century [Prein As can be seen from Table 3, the error-corrected RCM sim- et al., 2011] this is only a minor restriction to the results of ulations adequately reconstruct mean and variability of the this study until 2050. For the projections until the end of the local weather patterns. This is particularly true in moun- 21st century, the global mean temperature increase due to tainous regions, where QM proved to improve the skill of the A1B emission scenario is between the other two major RCM simulations significantly, as discussed in detail by

Table 2. Overview of Available Data Sets

Parameter Unit Period Locationa Resolution

Used to Drive the Model T Mean air temperature C 1980–2009 V, Ze daily P Precipitation mm d1 1980–2009 V, Ze daily I Global radiation W m2 1980–2009 V, Ze daily L Lake level of Mattmarksee m asl 2000–2009 M daily

Used to Calibrate the Modelb c 3 1 QM Total input to Mattmarksee (Q1 þ Q2 þ Q3 þ Q5 þ Q6 þ QZ)md 2000–2009 4 daily d 3 1 QZ Total discharge collected at Zermeigern (QZer þ Q7 þ Q8 þ Q9)md 2000–2009 Z daily SC Snow cover extent derived from satellite images binary 2001–2009 entire catchment daily

Only Used to Evaluate the Model 3 1 QV Discharge in Visp m d 1903–2009 V daily 3 1 QS Hydropower discharge release at Stalden m d 2000–2009 S daily aLocations are marked in Figure 1. bThese data have been used for calibration (2004) and evaluation (2001–2008) cSum of natural discharge collected at locations 1, 2, 3, 5, 6, and total runoff collected at Z. dSum of natural discharge collected at locations Z, 7, 8, and 9.

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Table 3. Performance of the Seven Selected Climate Scenarios in Representing Present-Day Climate Mean and Variability

a 1 2 Nr GCM Institute RCM MDP (mm d )RP (–) MDT ( C) RT (–) MDI (W m )RI (–) At Zermattb 1 ARPEGE CNRM-ALADIN5.1 0.003 1.006 0.006 0.997 0.21 1.001 2 ARPEGE DMI-HIRHAM5 0.02 1.004 0.003 0.997 0.131 0.999 3 HadCM3Q0 ETHZ-CLM 0.02 0.984 0.017 0.996 0.232 0.998 4 HadCM3Q0 HC-HadRM3 0.009 1.009 0.011 0.996 0.003 1 5 ECHAM5-r3 KNMI-RACMO2 0.024 0.995 0.005 1 0.048 1 6 ECHAM5-r3 MPI-REMO 0.017 0.999 0.007 1 0.032 1 7 BCM SMHI-RCA 0.036 0.973 0.003 0.998 0.221 0.998

At Vispb 1 ARPEGE CNRM-ALADIN5.1 0.01 1 0.009 0.999 0.049 1.001 2 ARPEGE DMI-HIRHAM5 0.019 1.016 0.019 0.998 0.283 0.998 3 HadCM3Q0 ETHZ-CLM 0.025 0.992 0.03 0.998 0.013 0.998 4 HadCM3Q0 HC-HadRM3 0.016 1.004 0.027 0.999 0.128 1 5 ECHAM5-r3 KNMI-RACMO2 0.01 0.993 0.009 1.001 0.077 1 6 ECHAM5-r3 MPI-REMO 0.011 1.001 0.014 1.001 0.087 1 7 BCM SMHI-RCA 0.01 1.005 0.008 0.998 0.203 0.998 aRCM scenarios for Europe are available from the ENSEMBLES project (http://ensembles-eu.org/). bValues in the table indicate mean difference (MD) and the ratio of SD of simulated and observed climate variables (R) during the reference period 1980 to 2008. Index P stands for precipitation, T for air temperature, and I for global radiation.

Themeßl et al. [2011a, 2011b]. Furthermore, these tests Nevertheless, some scenarios also predict an increase of confirm that the QM method can also be applied to global mean radiation of up to þ9.3 W m2 (Figure 3). radiation, a finding that was not described by Themeßl et al. [2011a]. Nevertheless, these results do not represent 3.2. Glacier Model Projections the quality of QM applied to future climate simulations, [19] The hydrological model used in this study can only since evaluation and calibrations periods are identical. A account for glacier ice melt and snow accumulation, disre- more stringent evaluation of the methods is given in garding glacial dynamics and retreat. We therefore rely on Themeßl et al. [2011b]. externally determined evolution of glacier masks for model [18] The variability of the projected climate change sig- projections up to the year 2100. The ice volume changes of nals within the seven error-corrected RCM scenarios reveals the biggest glaciers in the Vispa catchment have been the uncertainty of present state RCM projections for the monitored throughout the last century [Bauder et al., Vispa valley. Climate change signals between the long-term 2007]. Farinotti et al. [2011] used a glacier evolution period (2069 to 2098) and the representative reference pe- model [Huss et al., 2008] that includes all important proc- riod of the current climate (1992 to 2019; see section 3.3.4) esses of accumulation and melt of snow and ice as well as vary between þ3.2 and þ4.7C warming during the melt the dynamic response of the ice masses to evaluate the tran- season (May to September) and þ1.6 and þ3.0C during sient evolution of the ice volume, glacier covered area, and the low flow season (January to April; see Figure 3). Since the distributed mass balance until 2100 of the three highly the RCMs have a grid spacing of about 25 km and an effec- glacierized subcatchments, Gornera, Findelbach, and Matt- tive resolution of at least four times this value, spatial differ- mark (Table 1). The future projections are based on ten re- ences in the projections for the two weather stations, which gional climate change scenarios derived from the are only about 35 km apart, can only be achieved by apply- ENSEMBLES project [van der Linden and Mitchell, 2009] ing empirical-statistical methods like QM, which introduce as described by Bosshard et al. [2011]. Although the cli- local climate information from the stations. For example, mate scenarios used to drive the glacier model in the study the weather station at Visp records typical weather patterns of Farinotti et al. [2011] slightly differ from those used to for the lower valley (e.g., stratus formations), and at Zermatt drive the hydrological model in this study, they can be con- typical weather patterns for the upper valley are recorded sidered as largely consistent since mostly the same climate (e.g., adiabatic winds) [Rüedlinger, 2010]. On average, the simulations are used, all driven by the A1B emission seven scenarios project slightly higher temperature increase scenario. during the melt season in Zermatt (mean increase: þ4.3 C) [20] Projections of the remaining smaller glaciers in the than in Visp (mean increase: þ3.7C). During the low flow catchment, for which detailed simulations were not avail- season, the multimodel average warming is quite similar able due to lack of observational data, we used an elevation with þ2.2C in Zermatt and þ2.3C in Visp. Fairly similar dependent linear retreat as projected for the three simulated signals can be observed for precipitation. In Zermatt, pre- glaciers. We justify this simplification for the smaller gla- cipitation is projected to increase by an average of þ11.5% ciers by the fact that the three biggest glaciers, for which during low flow season but decrease during the melting sea- explicit modeling is available, account for over 69% of the son by 13.4%. In Visp, an increase of precipitation during glacierized area (Table 1). The results of Farinotti et al. low flow of þ14.8% and a decrease during melt season of [2011] demonstrate that the glacier retreat is a continuous 3.5% is projected. On average, the selected scenarios pro- gradual process, rendering our approach to use externally ject a decrease in mean global radiation of up to 4.2 W m2. determined glacier extents with a 250 m spatial resolution,

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Figure 3. Climate change signals in mean air temperature, precipitation, and global radiation between the reference period (1992–2019) and the end of the century (2071–2098) during low flow (January to April) and melt season (May to September). Labels indicate specific climate scenarios as listed in Table 3.

valid. The evolution of the projected glacier extents are [22] For the model setup of the Vispa valley study site, illustrated in Figure 1 and summarized in Table 1. the DEM with 250 m grid resolution provided by Swisstopo was used. According to the digital soil and land use maps from the FSO, 97% of the watershed can be characterized 3.3. Hydrological Modeling and Projections as glaciers, ridges, steep slopes, or high alpine, with the 3.3.1. Model Setup according soil characteristics (Table 5). Similarly, four [21] The modified topographic kinematic approximation land use types were defined, assigning each one according and integration model (TOPKAPI) is a physically based Manning’s coefficients and crop factors (Table 5). The rainfall-runoff model [Todini and Ciarapica, 2001], devel- channel geometry in the entire catchment is assumed to be oped from the conceptual ARNO model [Todini, 1996]. In rectangular with decreasing width according to the distance this study we used a modified version of TOPKAPI almost from the outlet of the catchment. For calibration purposes identical to the version used by Finger et al. [2011]. The the glacier extent of 2010 was used, which seems appropri- modified version contains the enhanced temperature index ate as glacier retreat within one decade is below the spatial (ETI) model, which computes ice and snow melt rates grid resolution of our model (Figure 1). based on ambient temperature and solar radiation using a [23] The model is driven with daily meteorological data constant temperature factor (TF) and short wave radiation (P and T) recorded at the MeteoSwiss station at Visp, factor (SRF) [Pellicciotti et al., 2005]. The water routing in located at 640 m asl, and at Zermatt at 1626 m asl (Figure 1). TOPKAPI is based on the kinematic wave concept, which Data from the meteorological station of Zermatt was used to can lead to numeric problems when fluctuation is very extrapolate P and T above 1600 m asl and data from Visp large. While in this study this was never the case for simu- was used to extrapolate P and T below 1600 m asl. Radia- lation of the present climate, about 1.6% of simulated tion is computed by the hydrological model, accounting for future time periods revealed numeric problems and were daily cloud cover according to the parameterization pro- hence not considered in the results. posed by Pellicciotti [2004], which relies on daily

6of20 W02521 FINGER ET AL.: PROJECTIONS OF FUTURE WATER RESOURCES W02521 maximum and minimum air temperatures available at stopped by setting Qres to 0. This constraint prevents Qres Zermatt and Visp. from becoming negative and allows the model to compen- 3.3.2. Modeling Hydropower Operations With sate short-term underestimations of simulated inflow to the TOPKAPI reservoir as discharge from the reservoir is stopped until the simulated lake level reaches the observed lake level [24] The main modification to the TOPKAPI version used by Finger et al. [2011] is the implementation of a res- again. ervoir module [Todini and Mazzetti, 2008], to account for [25] Water extraction, diversion of mountain streams, storage in reservoirs, water abstraction, diversion of natural and routing of pressurized water to the turbines were imple- mountain streams and routing of pressurized water to tur- mented by subtracting the amount of water diverted from bines. The discharge released from the reservoir is deter- the river cell flowing through a water frame and adding it mined by the means of user provided lake level time series, to the target river cell. The hydropower operational rules in as originally described by Todini and Mazzetti [2008]: the Vispa valley can be limited within our study to the activities of GD and KWM, as other hydropower activities 3 1 QresðtÞ¼QMðtÞþPresðtÞAres ÁVresðhÞðm d Þ ; (1) (e.g., Energy Zermatt and EnAlpin) are irrelevant for our study since they do not affect natural runoff beyond a daily where Qres denotes the discharge from the reservoir, QM time scale. All the operational rules for the specific case of stands for total natural runoff and diverted water input to GD and the KWM are summarized in Table 4 and visual- the reservoirs, Pres stands for the precipitation above the ized in Figure 1. KWM collects water at five intakes (loca- reservoir area Ares, and ÁVresðhÞ represents the observed tions 1, 2, 3, 5, and 6) and diverts it to the Mattmarksee volume change depending on the lake level observations (M). Water collected at locations 7, 8, 9, and in Zermeig- and the bathymetric map, between the present and the pre- gern (Z) is stored in our model in one single reservoir, the vious time step. If ÁVresðhÞ becomes greater than the input Mattmarksee. This is a simplification, as in reality water into the reservoir the discharge from the reservoir is can be released directly to Stalden (S), or be pumped up

Table 4. Overview of Water Intakes of KWM and GD

a b b 3 1 Loc. Name X (m) Y (m) H (m asl) Qmax (m s ) Diversion Saas Valley 1 Trift 640,467 109,477 2291 5.5 diverted to 4 2 Almageller/Rottal 642,911 105,545 2280 4.6 diverted to 4 3 Furg 643,225 101,306 2282 3.8 diverted to 4 4 Inflow Mattmarksee – – 2197 – inflow to lake 5 Hohlaub 639,624 100,796 2219 3.5 diverted to 4 6 Allalin 639,729 100,292 2255 3.5 diverted to 4 7c Saas-Fee 637,747 106,569 1770 14.0 diverted to 4 8c Schweiben 635,482 113,739 1772 1.5 diverted to 4 9 Riedbach 630,376 113,087 1823 3.0 diverted to 4 Zc Zermeiggern 640,055 102,997 1770 3.0 diverted to 4 S Stalden – – 715 – outlet to river Md Mattmarksee – – 2197 – released to S V Visp 634,150 124,850 658 – gauging station

Matter Valley 10 Hohberg 629,295 106,863 2534 2.4 diverted to LdD 11 Festi 628,833 105,351 2548 2.3 diverted to LdD 12 Kin 629,393 103,730 2764 1.9 diverted to LdD 13 Rotbach 629,866 101,453 2548 1.5 diverted to LdD 14 Alphubel 630,535 99,525 2571 1.3 diverted to LdD 15 Mellichen 631,150 98,372 2637 6.0 diverted to LdD 16 Längflue 630,822 97,817 2598 1.3 diverted to LdD 17 Findelbach 629,087 94,919 2508 17.7 diverted to LdD 18 Obertheodul 621,631 92,814 2486 4.5 diverted to LdD 19 Furgg 620,781 92,823 2487 6.6 diverted to LdD 20 Bis 624,851 106,159 2168 3.4 diverted to Zm 21 Schalibach 624,004 102,869 2128 10.0 diverted to Zm 22 Edelweiss – – 2078 5.0 diverted to Zm 23 Trift 621,469 98,095 2442 7.0 diverted to LdD 24 Arben 617,554 96,190 2554 2.3 diverted to LdD 25 Hohwäng 616,065 95,388 2612 2.0 diverted to LdD Ge Gornera 622,600 93,211 2007 12.0 diverted to LdD St Stafel 618,125 94,900 2212 10.0 diverted to LdD Zme Z’mutt 620,990 95,179 1970 7.0 diverted to LdD residual to G aLocations are illustrated in Figure 1. bSwiss coordinate system using the geodetic datum CH1903. cMathematical simplification 1: water collected at these locations is diverted to 4, as M in the model acts as reservoir including all water stored in the entire system. dMathematical simplification 2: water from M is directly released to S, as M accounts for all water stored in the system. eMathematical simplification 3: at Z’mutt 7 m3 s1 are directly pumped to Lac de Dix (LdD) the rest is diverted to G. At G 12 m3 s1 is diverted to GD the rest is released as residual flow.

7of20 W02521 FINGER ET AL.: PROJECTIONS OF FUTURE WATER RESOURCES W02521 from Z to M. As storage volume in the pipes and at Zer- only outline the specific adaptations and extensions neces- meiggern is negligible, mathematically it does not matter if sary for the present study. water is stored in the pipes, at Z or in M. Therefore we sim- [28] We first generated 10,000 random parameter sets plify the hydropower installations with one single reservoir, from a uniform and physically plausible constrained range the Mattmarksee. The water released from this reservoir is indicated in Tables 5 and 6. Although 10,000 parameter defined by lake level observations (equation (1)). KWM sets cannot sample the entire parameter space, many also operates historic turbines in Saas Fee, but these are authors have demonstrated that this number is sufficient to being neglected in our modeling approach, as they are obtain adequate model efficiency [Beven and Binley, 1992; rarely used and do not affect runoff beyond a daily basis. Finger et al., 2011; Seibert and Beven, 2009; Uhlenbrook [26] GD collects water at 13 water intakes (at location and Sieber, 2005]. Furthermore, as only the headwaters are 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 23, 24, and 25) and diverted into Mattmarksee, the relevant areas of the Vispa diverts it directly to the Lac de Dix outside the catchment. valley are comparable to the Rhonegletscher study site At Stafel, water is collected and diverted (through pumps) investigated by Finger et al. [2011], making their approach to GD. The pumps can only reach a capacity of 10 m3 s1. also suitable for the Vispa valley. In accordance with Fin- The rest is released downstream, but collected at Z’mutt ger et al. [2011], the saturated hydraulic conductivity pa- (location Zm). Water collected at 20, 21, and 22 is diverted rameter Ksh was generated from a log-uniform distribution. to the cell upstream of Z’mutt. At Z’mutt water coming The initial range of Ksh was defined to vary over several from 20, 21, 22, and from the upstream basin is pumped to magnitudes to allow the model to mimic runoff through 3 1 GD with a Qmax of 7 m s . The remaining water is coarse gravel as well as through glacier pulp (Table 5). diverted to Gornera (G). This is again a simplification, as in Although TOPKAPI disposes of the option for a second reality no water is diverted from Zm to G. At Gornera (G) soil layer, we refrained from using this option to avoid 3 1 water is being diverted to GD with a Qmax of 12 m s . overparameterization of the model. All initial parameter This is also a simplification, as in reality water is being ranges were defined based on experience and experimental diverted to Zm. However, at Zm there are two pumps which data as described by Finger et al. [2011]. 3 1 have a Qmax of 12 m s . Consequently, first water from [29] Efficiency criteria were used to quantify the model the upstream basin is pumped to GD, before water from G performance regarding total natural and diverted runoff is pumped. Accordingly, we simplify this by first diverting into Mattmarksee (QM), total discharge from the lower 3 1 all surplus water (>7m s ) from 20, 21, and 22 to G and intakes collected at Zermeiggern (QZ), and correctly pre- limiting the diversion of water at G to GD to 12 m3 s1. dicted snow cover area (SC). Performances of all simulated discharges were surveyed using the Nash-Sutcliffe coeffi- 3.3.3. Stochastic Model Calibration cient (EQ)[Nash and Sutcliffe, 1970]: [27] We use a stochastic model calibration technique proposed by Finger et al. [2011] which identifies physi- Xn cally plausible parameter sets, resulting in good model per- 2 ðQ ; ; Q ; ; Þ formance regarding all available observational datasets. i j obs i j sim E 1 i¼1 ; (2) Using this technique, Finger et al. [2011] demonstrated Q;j ¼ Xn 2 that the combination of satellite snow cover images and ðQi;j;obs Qi;j;obs Þ discharge data leads to a high internal consistency, making i¼1 it suitable for climate change predictions. In the present study we extended the technique to also assess the uncer- where index i represents the time steps, index j stands for tainty of model projections. The technique has been pre- the respective location, Qobs stands for observed discharge, sented in detail by Finger at al. [2011], therefore here we and Qsim depicts simulated discharge. Furthermore, the

Table 5. Summary of the Soil Parameters

Soil Parameters Unit Glacier Ridges Steep Slopes High Alpine

a Soil depth, d1 ir m 0.25–1.5 0.74; 0.31 1–6 2.9; 1.25 0.75–4.5 2.22; 0.93 1.5–9 4.4; 1.87 m; a Horizontal saturated hydraulic ir m h1 1e12–1 e 8 5e9;3e9 1e5–1 e 1 5e2;4e2 1e5–1 e 1 5e1;4e1 1e5–1 e 1 5e2;4e2 b conductivity, Ksh m; 1 Vertical saturated hydraulic mh 0.2 Ksh1 0.2 Ksh1 0.2 Ksh1 0.2 Ksh1 c conductivity, Ksv Saturated soil moisture content, s ir – 0.2–0.6 0.35; 0.13 0.1–0.3 0.18; 0.07 0.15–0.45 0.27; 0.1 0.1–0.3 0.18; 0.07 m; d Residual soil moisture content, r const. – 0.2 0.2 0.2 0.2

Surface and Vegetation Parameters Unit Alp. Pastures Unprod. Veg. Surf. Without Veg. Slightly Dev. Lands Manning’s coeff. – 0.35 0.03 0.025 0.03 Crop factor – 0.4–1 0.4–1 0.4–1 0.4–1 air stands for initial range; m stands for mean of 10 best parameter sets, and stands for SD of the respective mean. b Ksh values were generated from a log-uniform distribution for Monte Carlo runs. c In order to minimize the number of parameters to be varied r1 was kept constant and Ksv was made dependent on Ksh. d r was kept constant to minimize varying model parameters.

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Table 6. Summary of the Melt Parameters Required By the Melt Model Component of TOPKAPI

Parameter Unit Value Range ma b

Tt threshold air temperature for melt C 2toþ2 0.926 0.474 Tt,P threshold air temperature for precipitation state transition (solid/fluid) C 2toþ2 0.926 0.474 c Tonset threshold temperature for Nday C 2toþ2 0.926 0.474 1 Tgrad temperature gradient with elevation Cm 0.002–0.007 0.006 0.0006 Tmod temperature decrease over glacier area C 0–2 1.231 0.7059 SRF shortwave radiation factor m we m2 W1 h1 0–14 0.0122 0.0012 TF temperature factor m we C1 h1 0–400 0.1046 0.0463 p1 first albedo factor – 0.7–1 0.9377 0.0510 p2 second albedo factor – 0.1–0.2 0.1334 0.0272 G glacier ice albedo – 0–0.4 0.1221 0.1109 ground average basin wide ground albedo – 0.2–0.3 0.2512 0.0358 Ksnow storage constant for snow melt and rain on glaciers h 5–100 51.97 26.35 Kice storage constant for ice melt and channel inflow of glaciers h 5–100 51.35 21.76 su 1 Pprec precipitation gradient with elevation from May to Oct m 0.01–0.06 0.0484 0.0072 wi 1 Pprec precipitation gradient with elevation from Nov to Apr m 0.055–0.33 0.2663 0.0395 am stands for the mean of the 10 best parameter sets. b stands for the SD of the mean of the 10 best parameter sets. c Tt, Tt,P, and Tonset were set equal to minimize numbers of parameters to be varied. Nday was kept constant at 1 day. accumulated volume error of total input into Mattmarksee during an 8 year evaluation period (2001–2008), including (EVE) was computed for each run as follows: the extreme heat wave of the year 2003 [Schär et al., Xn 2004]. In order to initialize the water content in soil and streams, as well as the seasonal snowpack, we included in EVE ¼ jQi;j;obs Qi;j;simj: (3) i¼1 each calibration run a 12 month spin up period.

[30] Predicted snow cover was evaluated by determining 3.3.4. Climate and Glacier Forced Hydrological the daily ratio of correctly predicted snow cover area Projections and Their Uncertainty (ECPSC) as follows: [34] Future hydrological projections are subject to uncer- tainty due to (i) natural climate variability, (ii) uncertainty ccorr ECPSC ¼ ; (4) in anthropogenic climate forcing by the emission of green- c c tot missing house gases, (iii) uncertainty in the formulation of climate where ccorr stands for the number of correctly predicted models, (iv) parameter uncertainty (calibration and setup) cells, ctot stands for the total of cells in the catchment, and in hydrological models, and (v) uncertainty in the future cmissing accounts for the number of cells with no data due to projection of glacier extent. cloud cover. [35] As argued in section 3.1, emission scenario uncer- [31] Based on these efficiency criteria, the 10,000 Monte tainty is not a focus of our study, as only the A1B emission Carlo runs were ranked regarding the performance of the scenario is used [Nakicenovic et al., 2000]. Uncertainties in four above mentioned efficiency criteria. For this purpose the formulation of global and regional climate models are i we defined a ranking value Pr for each run r and each effi- regarded by applying seven different GCM-RCM combina- ciency criterion k as tions (Table 3). Uncertainties due to internal variability of the GCMs and RCMs are not regarded separately, but are to ðN þ 1ÞRankk some degree implicitly included in the different GCM-RCM Pk ¼ r ; (5) r N combinations. The seven climate scenarios adequately rep- resent the uncertainty of state of the art RCM projections, as where N is the total number of runs performed and R stands discussed in section 3.1. Natural climate variability is for the rank of the considered run with regard to the effi- intrinsically sampled by assessing the interannual variability ciency criterion k. Thus, ranking values were defined for within the projected 28 year time periods (section 3.4.2). As EQ E each data set used for calibration, namely Pr and P VE of E r TOPKAPI computes distributed global radiation internally, ECPSC Q QM, Pr in the entire catchment and Pr of QZ. Aggre- with a user defined daily cloud factor, we projected cloud gated ranking values for the best run with respect to EQ,M factors for future time periods based on the projected daily and EQ,Z, lowest EVE and best ECPSC, were determined by mean global radiation divided by the 28 year maximum of averaging the corresponding ranking values according to the specific month global radiation up to the end of the 21st equation (6): century. 1 [36] Concerning the uncertainty in future projection of POA ¼ ðPEQ;M þ PEQ;Z þ PEVE þ PECPSC Þ: (6) r 4 r r r r glacier extent, the glacial retreat is predicted by simulations presented by Farinotti et al. [2011]. The average horizontal OA [32] The runs with the highest Pr were defined as the retreat in most areas is less than 25 m per year, making it runs with the best overall performance only possible to account for glacier retreat in decadal inter- [33] To keep computational time at a reasonable level vals, as the spatial resolution of our model is 250 m. we limited the calibration period to one representative aver- Accordingly, in order to account for uncertainty in future age year (1 January 2004 to 31 December 2004), only in projections, we force the hydrological model for the order to challenge the model by surveying its performance medium-term time period with the extents projected by

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Farinotti et al. [2011] of the years 2040, 2050, and 2060 glacier extents. Based on the 210 projected monthly and for the long-term time period with the extents of 2070, changes for each of the two future periods and the three 2080 and 2090 (Figure 1). As standard deviations of the ten uncertainty components, monthly sum of squares for each projected glacier extents for a particular decade are smaller component were determined. Furthermore, monthly sums than the 28 year change of glacierized area [Farinotti et al., of squares concerning the nonlinear interaction between 2011], it can be assumed that the three glacier extents for factors and the residual error are computed. In order to each time period adequately account for uncertainty in the determine the contribution of each specific component, glacier extent projections. interaction term, and residual error to the global uncer- [37] Parameter uncertainty in the hydrological model is tainty, the corresponding ratios between component spe- sampled by using the 10 best model parameter sets, defined cific sum of squares and total sum of squares are OA by the highest Pr (equation (6)). Finger et al. [2011] dem- calculated. This allows us to allocate the monthly uncer- onstrated that reducing the number of considered best pa- tainty propagated to the end result by each uncertainty gen- rameter sets to 10 increases the uncertainty of parameter erating component, the interaction of components, and the estimation. By using only the 10 best parameter sets, we residual error. are assessing a higher uncertainty as if we considered more parameter sets, making the uncertainty due to parameter 3.4.2. Uncertainty of Discharge Due to Natural estimation rather large. Uncertainty due to the specific Climate Variability structure of TOPKAPI could not be assessed, as this would [41] Natural runoff in Alpine regions is characterized by require the application of hydrological models with differ- high interannual variability, as each year is characterized ent structures, which is not the objective of the present by different climatic patterns [Bartolini et al., 2009; Lam- study. brecht and Mayer, 2009]. Furthermore, some state of the [38] The 7 climate scenarios, 10 plausible hydrological art RCMs indicate increasing interannual variability for the model parameter sets, and 3 glacier extents were used to Alpine region with ongoing climate change [Heinrich and perform hydrological projections for (i) a current reference Gobiet, 2011]. In order to demonstrate how long-term time period which covers the decade before and after the changes in the mean compare to the interannual variability evaluation period, namely from 1992 to 2019, (ii) a me- we compare the climatological mean of the two future peri- dium-term future time period, from 2037 to 2064, and (iii) ods with their interannual variability. Similar to the uncer- a long-term future time period from 2071 to 2098. Alto- tainty analysis described in section 3.4.1, we first computed gether we performed for each of the two future time periods the climatological monthly mean values of the reference 210 projections, which allow us to perform a comprehen- period for each simulation. Subsequently, interannual vari- sive uncertainty analysis as discussed in section 3.4.2. Sim- ation of the monthly DQM was determined by subtracting ilarly to the calibration procedure, all runs were started 2 the climatological monthly mean values of the reference years earlier to obtain an adequate spin up period. period from the 28 monthly multisimulation (all 210 simu- lations) mean QM of the respective future period for each 3.4. Uncertainty Analysis of the Model Chain simulation year. Interannual variability is then defined as 3.4.1. Uncertainty of Mean Discharge the square root of the 28 multisimulation averaged variance [39] The uncertainty analysis focuses on the change of of the DQM values. future total inflow into Mattmarksee compared to present total inflow (DQM ¼ QM,future time period QM,present time 4. Results period) since it is the most important result of our study with regard to future hydropower production. The global uncer- 4.1. Evaluation of Hydrological Model Performance tainty of DQM was assessed by first computing climatologi- [42] The performance of the hydrological model is cal monthly averages for the two projected future time assessed during an 8 year period from 2001 to 2008, includ- periods (medium term and long term) of QM for all 210 ing the calibration year 2004. The ensemble of the best OA projected simulations, defined thereafter as the multisimu- 10 simulations (out of the 10,000) with the highest Pr lation mean. Likewise, multisimulation climatological (equation (6)) corresponds to those runs which reveal best monthly averages for the reference time period of QM were performance regarding EQ of total water inflow to Matt- computed, using the observed glacier extent from 2010. marksee (QM) and water collected at Zermeiggern (QZ), Subsequently, monthly DQM for each of the future time pe- lowest EVE of QM and best daily snow cover extent in the riod and simulation is computed by subtracting the climato- entire catchment. In Figures 4 and 5 the model performance logical monthly mean QM of the current reference period regarding lake level of Mattmarksee (L), natural and from the 28-year monthly average QM of the respective diverted inflow to Mattmarksee (QM), natural discharge col- future time period. Hence, global uncertainty for each lected at location Z (QZ), total water released from the res- month in future time period is given by the variance of the ervoir (QS), accumulative water released from reservoir 28 year multisimulation average DQM. (QSacc), discharge observed at Visp (QV) and correctly pre- [40] In order to partition the global uncertainty into the dicted snow cover ratio (CPSC) are illustrated for the cali- uncertainty generating components, we performed an anal- bration period and the evaluation period, respectively. ysis of variance (ANOVA) of DQM for the two future time Numerical values of the efficiency criteria are summarized periods, as described by Storch and Zwiers [2003]. The in Table 7, distinguishing between calibration period and three-way ANOVA analysis was performed regarding the validation period. three components of uncertainty, namely (i) the 7 climate [43] As the model is driven by observations of L, simu- scenarios, (ii) the 10 model parameter sets, and (iii) the 3 lated L almost perfectly matches observed L. Accordingly,

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OA Figure 4. Model performance of the 10 best MC simulations with the best overall performance Pr during the calibration year 2004. (a) Illustrates lake level of Mattmarksee, (b) shows total water amount available for hydropower, (c) depicts runoff collected at location Z, (d) depicts water released at Stalden, (e) demonstrates the accumulative error of simulated water release in Stalden, (f) pictures discharge in Visp, and (g) illustrates the ratio of correctly simulated snow cover in the entire catchment. Grey area indicates the range of the 10 best MC runs. Error bars in (g) show the standard deviation from the mean correctly predicted snow cover if it is greater than 0. the Nash values for L are close to 1 (Table 7). More impor- ods in summer. As a consequence of the good agreement of tantly, the total inflow to Mattmarksee is also very well pre- simulated total water input into Mattmarksee, discharge dicted by the model. During the calibration year 2004 and released by the hydropower company at Stalden (QS)is during the entire evaluation period, mean Nash values of also simulated adequately (Figures 4d and 5d). Deviations 0.81 and 0.78 are obtained, respectively. As illustrated in of simulated QS from observed QS are a direct consequence Figures 4 and 5b, efficiency is particularly reduced during of discrepancies in the input to Mattmarksee. Accordingly, and after heavy precipitation events. We discuss possible QS is slightly overestimated in May and June, but underes- reasons for this discrepancy in section 5. However, the sea- timated in August and September. Although the accumula- sonal dynamics are predicted very well. Total water col- tive errors of QS may become important for short time lected at the pumping station Zermeiggern (QZ) is also periods (Figure 4e), they are averaged out over the course predicted reasonably well, revealing mean Nash values of of several years (Figure 5e). While the mean volume error 0.83 during calibration and 0.72 during the entire evalua- of predicted QS during the calibration year 2004 is 12% of tion period. In particular, QZ is overestimated during heavy the observed QS, it is significantly less over the entire 8 precipitation events as well as during strong melting peri- year evaluation period, accounting for about 3% of

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OA Figure 5. Model performance of the 10 best MC simulations with the best overall performance Pr during the entire evaluation period (2001–2008; striped area marks the calibration period). (a) Illustrates lake level of Mattmarksee, (b) shows total water amount available for hydropower, (c) depicts runoff col- lected at location Z, (d) depicts water released at Stalden, (e) demonstrates the accumulative error of simulated water release in Stalden, (f) pictures discharge in Visp, and (g) illustrates the ratio of correctly simulated snow cover in the entire catchment. Grey area indicates the range of the 10 best MC runs. Error bars in (g) show the standard deviation from the mean if it is greater than 0. observed QS (Table 7). Hence, short term errors in QS are change projections, as also discussed by Finger et al. averaged out by the model over longer time periods, reveal- [2011]. The mean values of the 10 best parameter sets ing the robustness of the model calibration. Efficiency of (Tables 5 and 6) turn out to be similar to those values discharge at Visp (QV) shows similar model efficiency as obtained by Finger et al. [2011], indicating that the deter- observed for QS. Errors revealed in QS during the summer mined values are physically plausible. In particular, the val- time are propagated down to the gauging station in Visp, ues of SRF and TF are different from the values found by leading to an overestimation during May and June and an Finger et al. [2011], but are very similar to the values underestimation in July and August. Generally, QV is determined deterministically by Carenzo et al. [2009] for underestimated by the model, indicating inadequate model the adjacent study site Haut Glacier d’Arolla. parameters for the western side valley, which was, how- ever, not subject to model calibration. [44] Finally, snow cover extent is predicted with a mean 4.2. Climate Change Impact on Hydrology efficiency of over 0.83 throughout the calibration and eval- [45] In Figure 7 climatological monthly mean values of uation period. The good performance of snow cover and relevant projected variables are illustrated for the three natural discharge during the entire evaluation period con- time periods: (i) reference period from 1992 to 2019; (ii) firms that the calibration is adequate for both wet and dry medium-term projection from 2037 to 2064, and (iii) long- years. This supports the suitability of the model for climate term projection from 2071 to 2098.

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Table 7. Efficiency of the Ensemble of Plausible Hydrological Simulations

Evaluation Period (2000–2008) Calibration Year (2004) (Excluding Calibration Year 2004)

Criterion Variable Location Mean Max Min Mean Max Min

Used for Calibration EQ,M (–) QM 4 0.81 0.87 0.81 0.78 0.86 0.71 EQ,Z (–) QZ Z 0.83 0.93 0.69 0.72 0.84 0.45 ECPSC (–) SC Entire site 0.84 0.84 0.84 0.83 0.83 0.82 a EL (–) L M111111 6 3 1 EVE (10 m a ) QM 426 3

Only used for evaluation 6 3 1 b EVE (10 m a ) QS S 26.8 45.5 12.0 7 20.5 25.8 c RVE (–) QS S 0.12 0.21 0.05 0.032 0.09 0.12 6 3 1 d EVE (10 m a ) QV V 73.5 29.4 138 100 56.2 170.4 c RVE (–) QV V 0.14 0.06 0.26 0.19 0.11 0.32 aL was used to drive the model (equation (1)), accordingly it was not used for calibration and gives a perfect fit as long as there is enough water in the reservoir. b 6 3 1 3 1 Observed QS in 2004 was 221 10 m a and 218 m a for the entire calibration period. cRVE is the annual ration of simulated accumulated discharge and observed accumulated discharge. d 6 3 1 6 3 1 Observed QV in 2004 was 486 10 m a and 533 10 m a for the entire calibration period.

[46] Based on our hydrological projections, the seasonal rules, defined in the model by the water level in Mattmark- run off generation dynamics will significantly change in the see (Figure 7h). The projections indicate that runoff water Vispa valley. Glaciers will continuously decrease until they will be sufficient to fill Mattmarksee until the middle of the almost disappear by the end of the 21st century (Figure 1). century, but water deficiencies will occur by the end of the While temperature and global radiation are expected to century. These simulations rely on mean lake level obser- increase throughout the entire year (Figures 7a and 7c), it is vation as illustrated in Figure 6. In order to obtain similar difficult to define distinct trends in precipitation patterns lake level as today, future hydropower production will have (Figure 7b). As a consequence of warmer temperatures, to be significantly reduced during the second half of snow melt will occur earlier in the year and generate less summer, while more power can be produced during spring runoff in the second half of the year (Figure 7d). The most (Figure 7i). This will of course significantly impact the eminent changes will be driven by the shrinking of the gla- river flow at Visp, leading to an additional shift of summer cier extents in the entire catchment, which will lead to a runoff to the winter months (Figure 7j). decrease of glacier runoff throughout the 21st century [47] Table 8 summarizes the expected changes based on the [Farinotti et al., 2011; Huss et al., 2008]. This projection is presented projections. The greatest change over an entire in accordance with the observations throughout the last 10 hydrological cycle is expected in QIM, which is expected to be years, which show an average decline of QM and QZ of 102 reduced by one third by the mid century and halved by the end 3 1 3 1 km a and 4000 m a , respectively (Figures 5b and 5c). of the century (Table 8). Slightly lower QSM and the reduction Based on the model projections, the decline of QM and QZ of QIM is expected to reduce QM and subsequently QS by will continue, as illustrated in Figures 7f and 7g. While the about 17% by midcentury and 33% by the end of the century. increased snow and ice melt in spring will lead to enhanced In turn, snow melt is expected to increase by one third by the runoff in spring, runoff in the second half of the year will middle of the century and half by the end of the century during be significantly lower. Subsequently, this seasonal shift the low flow season. Nevertheless, all modeled hydrological will impact hydropower production. The water released at variables are expected to decrease during the melt season. The Stalden entirely depends on the hydropower operational reduction of QIM by one third by the middle of the century and

Figure 6. Observed lake level and daily mean lake level used for future hydrological projections.

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4.3. Uncertainty Analysis of the Hydrological Projections 4.3.1. Uncertainty of Mean Discharge [48] The multisimulation monthly mean DQM (mean of all 210 simulations) and its standard deviation for the medium- and long-term future period are illustrated in Figure 8a. Global uncertainty, namely the standard devia- tion of all 210 projections, remains below 3 m3 s1 in the middle of the century and below 2.5 m3 s1 at the end of the century, which is during July and August, when melt reaches its maximum, clearly less than the mean projected change DQM. As expected, uncertainty of the climate pro- jections generates the largest uncertainty during the low flow season (October to April), as illustrated in Figures 8b and 8c, which is based on the ratio of the sum of squares between specific uncertainty sources and the total sum of squares. Up to midcentury, uncertainty due to the glacier extent is the largest source of uncertainty during the ice melt season (July and August). At the end of the century, when glacier extents have retreated drastically, the uncertainty generated by hydrological model parameters becomes domi- nant during the melt season, indicating that the equifinality problem becomes only relevant once glacier extents have retreated significantly. Variance contribution due to the non- linear interaction between the different uncertainty sources and nonexplained variance is very low throughout the year. [49] Moreover, in May, July, August, and September the uncertainty produced by the 210 simulations remains below Figure 7. Monthly mean values of 10 selected variables the absolute value of the projected differences (Figure 8a), for the three investigated time periods: (i) current reference indicating that our projected changes are significant. time period (1992–2019), (ii) medium-term future time pe- 4.3.2. Uncertainty of Discharge Due to Natural riod (2037–2064), and (iii) long-term future time period Climate Variability (2071–2098). [50] Meteorological and hydrological long-term trends are only significant if the trends are stronger than the natu- one half by the end of the century will directly affect QS and ral variability of the respective annual cycle. In Figure 9a subsequently QV. According to these results, hydropower pro- multisimulation monthly mean DQM of all 210 simulations duction is expected to decrease by over half by the end of the is visualized (same as in Figure 8a) including error bars century during the summer months. illustrating the respective standard deviation of 28 years of

Table 8. Multisimulation Mean of the Relative Changes of Selected Variables Compared to the Present Reference Time Period

Variable Mean Annual Change Mean Low Flow Change Mean Melt Season Change

Medium Term Projections (2037–2064) T (C)a 1.81 1.27 2.41 P (–) 0.01 0.05 0.13 I (–) 0.09 0.09 0.09 QSM (–) 0.05 0.33 0.11 QIM (–) 0.34 0.01 0.33 QM (–) 0.17 0.75 0.23 QZ (–) 0.05 0.76 0.14 QV (–) 0.07 0.15 0.25 QS (–) 0.17 0.08 0.38

Long Term Projections (2071–2098) T (C)a 3.25 2.24 4.36 P (–) 0.01 0.12 0.12 I (–) 0.12 0.14 0.11 QSM (–) 0.11 0.64 0.24 QIM (–) 0.67 0.22 0.67 QM (–) 0.33 1.45 0.45 QZ (–) 0.19 1.40 0.33 QV (–) 0.25 0.26 0.54 QS (–) 0.33 0.14 0.62 aFor air temperature the absolute change is given, while all other values are relative changes compared to average of the present time period (1992–2019).

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Figure 8. In (a) the multisimulation mean projected change in total input into Mattmarksee, DQM, based on the 7 climate scenarios, 10 plausible parameter sets of the hydrological model, and 3 glacier extents of the relevant time periods are illustrated. The whiskers represent the standard deviation of the 210 projections. In (b) and (c) the relative contribution of the individual modeling components to the global uncertainty for the medium- and long-term future time period is illustrated. interannual variability. While the small changes during low expected to increase with climate change [Luterbacher flow season are smaller than the respective standard devia- et al., 2004; Milly et al., 2002]. This is especially important tion, reduced ice melt in July, August, and September leads for hydropower production, as water intakes in the Vispa to significant changes in the runoff. As expected, largest valley are constructed to meet present runoff (Table 4). interannual variability is projected in spring and fall, when Particular attention should be given to the water intake ice melt and precipitation events lead to high variability in close to the village Saas Fee (Figure 1), as even today it is runoff generation (Figure 9b). Toward the end of the 21st continuously overstrained during melt season (K. Sarbach, century, variability is expected to increase particularly in personal communication, KMW). The mean amount of spring and decrease in July and August, when glacier melt excess water which cannot be collected at the water intake will be significantly reduced. We will discuss these circum- at Saas Fee, is illustrated in Figure 10. Today, most of the stances and relate them to the different components of the water is lost during the melt period, when ice and snow hydrological cycle in section 5. melt generate peak runoff. The annual amount of excess water is not expected to change significantly during the 4.4. Impacts of High Water Events on Hydropower course of the century, however, a seasonal shift is expected. Production By midcentury, most of the excess water is expected during [51] Particular attention should be given to the frequency heavy precipitation events in October and November (Fig- and the intensity of heavy precipitation events, as these are ure 10). Heavy precipitation events in fall are expected to

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Figure 9. Interannual variability, illustrated as the standard deviation of the mean of 28 monthly aver- ages of DQM each one representing one particular year in the future time period. In (a) the mean pro- jected change in total input into Mattmarksee, DQM, is illustrated. The whiskers represent the standard deviation of the mean of the 28 year period. In (b) the standard deviation of the mean for the medium- and long-term time period are visualized. become even more frequent at the end of the century (Fig- projections remain the only possible way of projecting ure 7b), while air temperature is expected to increase by future changes. By using a stochastic model calibration, we almost 3C (Figure 7a). As a consequence, projected runoff take into account that the calibration of our physically is expected to increase drastically during fall, as shown for based hydrological model is susceptible to the well-known the illustrative example at the water intake of Saas Fee equifinality problem [Beven, 2006; Beven and Binley, (Figure 10). This result indicates that in the future peak 1992], as different parameter sets reveal similar performan- runoff due to heavy precipitation events will become more ces for the evaluation period, but lead to different projec- important, while the rather constant runoff of ice melt dur- tions under a changing climate. ing summer will decrease significantly. [53] Nevertheless, our model setup leads to some dis- crepancies from observed data, in particular to overestima- tion of runoff generation in spring and underestimation 5. Discussion during high summer (Figures 4 and 5; subplots b and c). [52] Numerical projections based on future climate sce- The most eminent explanation for the discrepancies comes narios will always remain a partial description of natural from the multivariable calibration technique: we calibrate processes influenced by climate change. Nevertheless, such our model with four criteria (EQ,M, EQ,Z, EVE, and ECPSC).

Figure 10. Multisimulation monthly mean runoff exceeding maximum capacity of the water intake in Saas Fee.

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Accordingly model efficiency of one variable is lost in a of the art RCMs [Heinrich and Gobiet, 2011]. Climatologi- tradeoff with model performance of other criteria. Further- cal and hydrological trends over several decades can there- more, as already discussed by Finger et al. [2011], one fore only be of statistical nature, making a deterministic weak point of TOPKAPI is the constant temperature gradi- projection for a specific year impossible. Nevertheless, the ent, which is in contradiction with observed seasonally projected trends during glacier melting months indicate varying temperature gradients in the region [Rüedlinger, that mean DQM is larger than its respective standard devia- 2010]. Indeed, in reality temperature gradients are rather tion of interannual variability (Figure 9a). This suggests lower in spring and higher in summer, which partially that the projected changes are strongly significant and that explains the overestimation in spring and underestimation glacier retreat will drastically impact the runoff in July, Au- in summer of melt runoff. A further source of error could gust, and September. also emerge from the tipping-bucket precipitation gauge [57] As all our simulations rely on the A1B emission sce- used in Visp and Zermatt which underestimates solid pre- nario, our results do not take uncertainty due to different cipitation by 20% compared to electronic weighing emission scenarios into account, so that the overall uncer- gauges [Savina et al., 2011]. Finally, high water events are tainty due to climate projections is underestimated. Given difficult to simulate, particularly because it is practically that future greenhouse gas emissions are the product of impossible to determine the maximum capacities of the very complex dynamic systems, depending on socio- water intakes. During heavy precipitation events, sediment economic development, and technological change, their and gravel can partially block the intakes while elevated future evolution is very uncertain [Nakicenovic et al., hydrologic pressure may increase the theoretical water 2000]. Nevertheless, the A1B scenario describes a world flow, making an exact determination of the maximum that has rapid economic growth, quick spreading of new and capacity impossible. Nevertheless, an error within the max- efficient technologies, and a global population that reaches imum capacities of water intakes is directly propagated to 9 billion by the midcentury and then gradually declines, it simulated water input to Mattmarksee. A further possible seems to be a realistic scenario for future projections. source of error is the rarely applied procedure of the hydro- [58] Considering that our projections are based on state power company to release water from Saas Fee and Ried- of the art RCM scenarios and accounting for the discussed bach directly to Stalden (Table 4) instead of first diverting uncertainties, we could demonstrate that the presented it to Zermeiggern. future changes in runoff generation are statistically signifi- [54] Even with a perfectly calibrated hydrological model, cant. Nevertheless, our results remain projected trends and projections of future water resources rely on the accuracy not deterministic forecasts. It is very probable that increas- of the applied climate scenarios. A comparison of the seven ing air temperature will lead to enhanced snow melt in selected climate scenarios used in this study reveals a large spring and drastic reduction of ice melt, due to a drastic uncertainty within the projected changes (Figure 3). Evi- retreat of the glaciers. Based on these projections up to the dently, this uncertainty is propagated to the results of the end of the 21st century, it is expected that water supply will hydrological model, as discussed with the uncertainty anal- increase during winter but will be insufficient to completely ysis (Figure 8). The uncertainty of the selected scenarios is fill Mattmarksee during the melting season. Accordingly, particularly important in spring during snow melt and in hydropower companies will have to adapt their energy pro- fall when heavy precipitation events lead to runoff. Never- duction to the altered hydrology. theless, the climate scenarios used in this study are repre- [59] Particular attention should be given to the peak flow sentative for the state of the art of regional climate change frequencies, during which many water intakes are over- projections. strained. For example, today the water intake at Saas Fee is [55] In this study projections of glacier extent rely on the continuously overstrained during the strong melt season in results presented by Farinotti et al. [2011], as TOPKAPI summer. In the future, the water intake is expected to be cannot account for glacier dynamics and retreat. Although rather overstrained during heavy precipitation events in these results were obtained with a different modeling fall. In order to anticipate such changes in the runoff gener- approach, the continuous and gradual retreat of the glacier ation, the infrastructure of hydropower companies should extent, along with the coarse spatial resolution of 250 m of be adapted to heavy precipitation events, rather than strong our model, make this approach valid. As expected, the melt periods. uncertainty of the glacier extent is the major uncertainty [60] Overall, our integrative modeling approach—com- source during July and August when primarily ice melt bining state of the art climate projections, dynamic glacier leads to runoff generation. By the end of the century, this modeling, and hydrological modeling accounting for water uncertainty will become less important, as ice melt contri- storage in reservoirs and river diversion—proved to be a bution will have diminished due to the retreat of the gla- powerful tool to project future water resources for hydro- ciers. Furthermore, the retreat of the glacierized area will power production. In view of an expected intensification of also affect the suspended sediment loads eroded beneath hydropower production in the future, such modeling the glaciers leading to siltation of reservoirs, as investigated approaches can provide helpful information for water by Finger et al. [2006] and Haritashya et al. [2010] in strategists to anticipate the impacts of projected climate other glacierized study sites. change. [56] Interannual variability of the climate is independent of modeling performance, as it is simply due to the chaotic nature of the climate system. However, future climate pro- 6. Conclusions jections indicate increasing interannual variability for the [61] The impacts of climate change on future water Alpine region with ongoing climate change for some state resources and its subsequent effects on hydropower

17 of 20 W02521 FINGER ET AL.: PROJECTIONS OF FUTURE WATER RESOURCES W02521 production during the 21st century were assessed using an [66] 5. The seasonal shift of the hydrological cycle and integrative modeling approach which allows simulating the reduced ice melt generation will very likely force complex hydropower operations in an alpine catchment. hydropower companies to adapt new water management For this purpose hydropower operational rules were imple- strategies. The new strategies have to take into account that mented in the fully distributed, physically based hydrologi- ice melt in summer will be drastically reduced, but the fre- cal model TOPKAPI and future projections of rainfall and quency of heavy precipitation events during fall will melt runoff generation were performed by coupling glacier increase. Accordingly, the current practice of hydropower retreat projections from a dynamic glacier model and A1B companies, of producing maximum energy during winter forced error-corrected and downscaled RCM climate sce- and relying on ice melt to fill the reservoirs, might be jeop- narios to TOPKAPI. Based on 7 future RCM climate pro- ardized by the end of the century. jections, 10 parameterizations of the hydrological model, [67] 6. By midcentury, high water events due to heavy and 3 projected glacier extents, the uncertainty of projec- precipitation events are expected to become more frequent tions for two future 28 year time periods were analyzed and than today, leading to an increase of water loss due to over- the source of uncertainty was quantified. Based on the pre- flow at some water intakes during fall. While today most sented projections we could observe (1) a significant tem- water is lost during strong melt periods in summer, the poral shift of the seasonal runoff generation to earlier future dynamics of hydrology will lead to overflows in par- months in the year, (2) reduced runoff generation during ticular during heavy precipitation events in fall. This repre- summer, and (3) the necessity to adapt future hydropower sents new challenges for hydropower companies to adapt production to the projected future runoff dynamics. Fur- their infrastructures accordingly. thermore, the results of the uncertainty analysis reveal that, depending on period and season, any of the three regarded [68] Acknowledgments. The present study is part of the ACQWA uncertainty components (climate projection, hydrological Project (assessing climate impacts on the quantity and quality of water), funded within the seventh Framework Program of the European Union simulations, and glacier modeling) may propagate the larg- (grant agreement no. 212,250). The ENSEMBLES data used in this work est uncertainty contribution to the projected total water was funded by the EU FP6 Integrated Project ENSEMBLES (contract availability for hydropower production. The main conclu- 505,539) whose support is gratefully acknowledged. Data on glacier evolu- tion was established in the projects WWK (Department of Energy and sions can be summarized as follows: Hydro Power, Canton Valais, Forces Motrices Valaisannes), CCHydro [62] 1. Although future climatic and hydrological projec- (Swiss Federal Office for the Environment, BAFU), and FUGE (Swiss tions are subject to large uncertainties, the consistent trends National Science Foundation, NFP 61). In addition, we would like to observed in all 210 projections are significant in May, July, express our sincere thanks to C. Abgottspon, T. Bilgischer, and K. Sarbach (all KMW) for the presented hydropower data about the Kraftwerke Matt- August, and September, as mean projected changes are mak SE and to C. Constantin (GD) for detailed information about the infra- greater than their variability. The projected changes in July, structure of Grande Dixence SE. Furthermore, we are thankful to M. Huss August, and September are even greater than projected and T. Bosshard for valuable discussions, D. Farinotti for providing glacier extent masks, S. Rimkus for advice on the TOPKAPI coding, and S. Bauer uncertainty due to interannual climate variability, indicat- for proofreading the manuscript. Finally, we would like to thank the three ing the projected changes will not be overtopped by natural anonymous reviewers who gave valuable comments on an earlier version climate variability. of this manuscript. [63] 2. The uncertainty analysis revealed that the largest fraction of uncertainty is propagated to the end result (total available water runoff) in spring and fall by climate model References uncertainty and in summer by glacier extents till the middle Alfieri, L., P. Perona, and P. Burlando (2006), Optimal water allocation for of the century, and by hydrological model parameters at the an alpine hydropower system under changing scenarios, Water Resour. end of the century. Although the absolute values of our pro- Manage., 20(5), 761–778, doi:10.1007/s11269-005-9006-y. Bartolini, E., P. Claps, and P. 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20 of 20 Geomorphology 68 (2005) 224–241 www.elsevier.com/locate/geomorph

Analyzing rockfall activity (1600–2002) in a protection forest—a case study using dendrogeomorphology

Markus Stoffela,*, Dominique Schneuwlya, Michelle Bollschweilera, Igor Lie`vrea, Reynald Delaloyea, Moe Myintb, Michel Monbarona

aGroupe de Recherches en Ge´omorphologie (GReG), Department of Geosciences, Geography, chemin du Muse´e4, University of Fribourg, 1700 Fribourg, Switzerland bRemote Sensing and GIS Unit, Department of Geosciences, Geography, University of Fribourg, 1700 Fribourg, Switzerland Received 28 October 2003; received in revised form 19 November 2004; accepted 20 November 2004 Available online 6 January 2005

Abstract

For the first time, dendrogeomorphology has been used to investigate spatial and temporal variations of rockfall activity in a protection forest. We report results of 564 cores from 135 severely injured Larix decidua Mill. trees on the west-facing Ta¨schgufer slope, Swiss Alps. While trees sampled reached an age of 297 years on average, the oldest one attained breast height in AD 1318. For reasons of sample depth, the analysis was limited to the period 1600–2002. In total, we reconstructed 741 growth disturbances (GD) during the last four centuries. Impacts were most commonly found in trees located in the southern part of the slope, where GD recurred more than once per decade. In contrast, trees in the northern part were less frequently disturbed by rockfall and define recurrence intervals of more than 150 years. Throughout the last four centuries, rockfall has caused GD to the trees sampled on the Ta¨schgufer slope, most frequently in the form of low magnitude–high frequency events. In addition, we identified one high magnitude–low frequency event in 1720, which displaced the forest fringe of the northern sector a considerable distance downslope and eliminated an entire forest stand. To analyze past rockfall activity, we introduce a brateQ defined as the number of impacts per meter width of all tree surfaces sampled per decade. Results clearly demonstrate that this rockfall brateQ continually decreased in both sectors after the large 1720 rockfall event. Significantly low rockfall bratesQ can be observed during the 1850s, 1960s and 1970s in the northern and during the 1820s in the southern sector. In contrast, high rockfall bratesQ were identified during the 1870s and 1990s in the northern, and during the 1770s in the southern sector. Reconstructed data further show that the forest recolonizing the southern sector after the 1720 event gradually improved its protective function, reducing bratesQ by a factor of 13 between the 1740s and the 1990s. In the recent past, bratesQ oscillated around 0.7 GD 1 meter widthÀ1 (10 years)À1. In the well-established forest of the northern sector, the efficacy of the protective forest was temporarily reduced by the rockfalls in 1720, resulting in increased rockfall bratesQ. Since then, the protective

* Corresponding author. Tel.: +41 26 300 90 10; fax: +41 26 300 97 46. E-mail address: [email protected] (M. Stoffel).

0169-555X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.geomorph.2004.11.017 M. Stoffel et al. / Geomorphology 68 (2005) 224–241 225 function of the forest stand has increased again, resulting in a rate of 0.4 GD 1 m widthÀ1 (10 years)À1 during the late 20th century. D 2004 Elsevier B.V. All rights reserved.

Keywords: Dendrogeomorphology; Rockfall; Growth disturbances; Frequency; Magnitude; Protection forest; Swiss Alps

1. Introduction scree give a reasonable overview of long-term fluctuations, but they lack more detailed data on Rockfall represents the most intensely studied decadal or even yearly variations in rockfall activity geomorphic process of the cliff zone in mountainous on a slope. In contrast to their use in other mass areas (Luckman and Fiske, 1995). Nevertheless, little movement processes (e.g., Shroder, 1980; Wiles et al., information exists on how rockfall frequencies and 1996; Fantucci and Sorriso-Valvo, 1999; Solomina, magnitudes vary over time. So far, studies have mainly 2002, Stoffel et al., in press), tree-ring analyses have been based on short-term observations of contempo- been used only rarely in rockfall research, where the rary rockfall activity in the field (e.g., Luckman, 1976; focus in these few cases has been on sedimentation Douglas, 1980; Gardner, 1980, 1983), rendering it rates, the dynamics of forest fringes on scree slopes difficult to estimate long-term accretion rates. Long- (Lafortune et al., 1997) or the seasonal timing of past term estimates of rockfall accumulation rates have, in rockfall activity (Stoffel et al., in press). contrast, been derived from accumulated talus vol- It is therefore the purpose of the present paper to umes (e.g., Rapp, 1960). But such rates may, as use tree-ring analysis in order to: (1) analyze the Luckman and Fiske (1995) pertinently object, neither magnitude and frequency; (2) determine spatial be representative of the present-day rockfall activities variations; (3) derive decadal and yearly variations nor of those that prevailed in the past. He´tu and Gray in rockfall activity on a forested slope. In it, we use a (2000) managed to avoid this problem by combining rockfall brateQ to reconstruct and evaluate fluctuations present-day processes and stratigraphic data to study in rockfall activity. Results were obtained from two the dynamics of scree slopes throughout the post- neighboring sectors differing in both their age glacial period. On slopes composed of siliceous structure and recurrence intervals. In total, 564 lithologies, lichenometry has repeatedly been used to increment cores of 135 European larch trees (Larix evaluate the mean age or activity of talus surfaces decidua Mill.) were analyzed, documenting decadal (Andre´, 1986, 1997) or to estimate 50-year and long- and yearly variations in rockfall activity on a forested term rates of rockfall accretion (Luckman and Fiske, slope at Ta¨schgufer (Valais, Swiss Alps) over more 1995). Finally, McCarroll et al. (1998) combined a than 400 years. lichen-based analysis of spatial and temporal patterns of past rockfall activity with a modeling approach. Simultaneously, investigations of forest–rockfall 2. Study area interactions have tended to evolve towards the analysis of mountain forests as a means of protection The area investigated was the west-facing Ta¨sch- against rockfall (e.g., Bebi et al., 2001; Berger et al., gufer slope. As can be seen from Fig. 1,the 2002). He´tu and Gray (2000) indicated that forest Ta¨schgufer slope descends from the Leiterspitzen cover would provide effective protection in the case of summit (3214 m a.s.l.) in the Siviez–Mischabel nappe, low magnitude–high frequency rockfall events, but southern Swiss Alps. Rockfall frequently occurs on could not prevent the devastating effects of high- the slope, originating from the heavily disintegrated magnitude events. gneissic rockwalls below the Leiterspitzen summit. However, quantitative information on the effect of Layers of this weak bedrock generally strike SSW and forest stands on the downslope movement of rockfall dip WNW with angles of 40–808 (Lauber, 1995), thus boulders remains insufficient. The previously men- containing joints dipping out of the slope. The main tioned studies on rockfall activity or the distribution of rockfall source areas on the slope are located between 226 M. Stoffel et al. / Geomorphology 68 (2005) 224–241

Fig. 1. Location of the study site. (A) The Ta¨schgufer slope is located in the southern Swiss Alps, northeast of the village of Ta¨sch. (B) The detailed sketch illustrates the rockfall slope with the main rockfall source areas and trajectories, the rockslide deposits and the dams built after 1988 (Dams 1–7).

2300 and 2600 m a.s.l. (Rockfall Source Area 2) and lichenometry to be at least 600 years BP (Joris, above 2700 m a.s.l. (Rockfall Source Area 1), where 1995). The spatial extent of the rockslide deposits bedrock is highly fractured with many joints. As can be seen in Fig. 1B. Furthermore, small debris suggested by a locally calibrated permafrost distribu- flows may occur at Ta¨schgufer. Single events tion model (Gruber and Hoelzle, 2001), Rockfall generally amount to a few cubic meters and move Source Area 2 would be located on the borderline downslope in well-defined channels. In contrast, between seasonal frost and permafrost environments. snow avalanches have never been witnessed on the The presence of contemporary permafrost is con- slope. firmed on the southern edge of the slope between The forest at Ta¨schgufer predominantly consists of 2400 and 2500 m a.s.l., where ground ice was L. decidua Mill. trees, accompanied by single Picea encountered during construction work (Haeberli, abies (L.) Karst. and Pinus cembra ssp. sibirica trees. 1992; see Fig. 2). Although the regional timberline is located at approx- On the upper slope, mean slope gradients locally imately 2300 m a.s.l. in the immediate neighborhood, rise up to 488 and gradually decrease to 208 near the continuous forest cover reaches only 1780 m a.s.l. in valley floor. The volume of single rockfall fragments the most heavily affected areas of the active rockfall normally does not exceed 2 m3. Besides frequent slope. As shown in Fig. 1B, surfaces are sparsely rockfall activity, one rockslide is noted in chronicles wooded in this part of the slope and large areas remain (Zurbriggen, 1952). Its age was estimated with free of vegetation. M. Stoffel et al. / Geomorphology 68 (2005) 224–241 227

Leiterspitzen

1 2

Dam 6

Dam 7

Fig. 2. View of the upper Ta¨schgufer slope and the Leiterspitzen summit (3214 m a.s.l.): Note the main Rockfall Source Areas (1, 2), the main rockfall trajectories (arrows) and the zone where ground ice has been encountered (star) during construction works (Photo: D. Schneuwly).

In the recent past, rockfall regularly caused damage north of Ta¨sch (1440 to 1760 m a.s.l.). Within the to roads and hiking trails. For example, on October 6, study area covering approximately 39 ha, virtually all 1985, single blocks reached the valley floor, damag- trees (L. decidua Mill.) show visible damage related ing agricultural buildings in the village of Ta¨sch to rockfall activity (i.e., broken crowns or branches, (Valais). According to Lauber (1995), rockfall activity scars, tilted stems). We sampled severely affected at Ta¨schgufer apparently increased in the 1980s and trees with obvious signs of growth disturbances (GD) again after 1993. These observations have recently from both the rockslide deposits (northern sector) and been confirmed with dendrochronological analysis the southern sector of the slope. The separation of the (Stoffel et al., in press), which further indicated that two parts of the slope (Fig. 3) is based on the different almost 90% of the intra-annual rockfall activity at origins of rockfall material. As indicated in Fig. 1B, Ta¨schgufer occurs in April and May. As a response to trees located in the northern sector are normally the increased rockfall activity, five deflection dams subject to rockfall fragments from the Rockfall Source were erected in 1988 (Dam 1) and 1989 (Dams 2–5; Area 1, while the samples growing in the southern see Fig. 1B). In the late 1990s, two large protection sector are predominantly influenced by boulders dams completed the construction works on the slope originating at Rockfall Source Area 2. We therefore (Dams 6–7, see Fig. 2). analyze the two parts of the slope separately and refer to them as bnorthern sectorQ and bsouthern sectorQ.In addition, we excluded a well-defined area in the 3. Material and methods southern sector, which is influenced by both rockfall and debris flow activity (Fig. 3). 3.1. Sampling strategy In this investigation, at least 4 cores were extracted per tree using increment borers. Fig. 4 illustrates that As shown in Fig. 3, dendrogeomorphological one core was taken upslope (core c), one downslope investigations were conducted in the protection forest1 (core d) and two cores perpendicular to the slope (cores a, b). In the case of visible scars, further increment cores were extracted from the wound and 1 Protection forest: forest stand which directly protects people, buildings and infrastructure against the impacts of mass wasting the overgrowing callous tissue. In addition to the events such as landslides, rockfall or snow avalanches (see Brang, disturbed trees sampled at Ta¨schgufer, we sampled 2001). undisturbed reference trees from a forest stand located 228 M. Stoffel et al. / Geomorphology 68 (2005) 224–241

NorNorthern sector

1900 SouS uthern sector

1800 18000 11700

1600 1700

(adapted after Schneuwly 2003) 15 00 1600

15005 N Täsch 0 500m

Forest reference trees area with debris flow deposits rockslide deposits selected trees

Fig. 3. Distribution of the 135 L. decidua Mill. trees sampled on the Ta¨schgufer slope (1440 to 1780 m a.s.l.). The area influenced by both debris flow and rockfall activity (hatched surface) has been excluded. south of the rockfall slope, indicated in Fig. 3.For fall of 2002: 135 trees (564 cores) from the rockfall every reference tree, two cores per tree were extracted slope and 17 trees (34 cores) from the undisturbed perpendicular to the slope (cores a, b). In total, 154 L. reference site south of the rockfall slope. Increment decidua Mill. trees were sampled (598 cores) in the cores of the reference trees were extracted at breast height (c130 cm), whereas those from the disturbed trees were–whenever possible–taken at the height of upslope the visible damage. c Data recorded for each tree sampled included (1) determination of its 3D-position on the slope with a compass and an altimeter (x- and y-values, elevation a.s.l.); (2) sketches and position of visible defects in the tree morphology such as scars, broken crowns or branches, candelabra trees and tilted stems; (3) the a b position of the sampled cores (i.e., a, b, c, and d); (4) diameter at breast height (DBH) derived from circum- ference measurements; (5) data on neighboring trees, micro-topography and/or rockfall deposits.

3.2. Tree-ring analysis and rockfall d downslope Samples were analyzed and data processed Fig. 4. Core sampling procedure for individual trees. following the standard procedures described in M. Stoffel et al. / Geomorphology 68 (2005) 224–241 229

Bra¨ker (2002). Single steps of sample analysis Judging the frequency or magnitude of single event included surface preparation, skeleton plots as well years or decadal fluctuations in the number of GD as ring-width measurements using digital LINTAB proved to pose a considerable problem: So far, positioning tables connected to a Leica stereomicro- dendrogeomorphology has mainly involved the anal- scope and TSAP 3.0 (Time Series Analysis and ysis of impacts by geomorphic processes that have a Presentation) software (Rinntech, 2003). Growth relatively large volume or surface area, such as debris curves of the disturbed samples were then crossdated flows, snow avalanches or floods. In contrast, rockfall with the reference chronology (1596–2002) con- consists of single falling, bouncing or rolling stones structed from 17 undisturbed L. decidua Mill. trees. which may only disturb trees along their trajectories This procedure allowed differentiation of climatically and within a range defined by the size of the clast. driven fluctuations in tree growth within the study Further, only the effects of a single boulder are area from GD caused by rockfall activity (Cook and recorded, whereas the fall itself may be composite. Kairiukstis, 1990). Finally, important differences emerged as to the age of Growth curves were then used to determine the the individual trees, their DBH and their decadal DBH initiation of abrupt growth reductions or recovery increment which rendered absolute comparisons of (Schweingruber, 1996). In the case of tilted stems, past rockfall activity difficult. both the appearance of the cells (i.e., structure and We therefore use a rockfall brateQ2 expressed as color of the reaction wood cells) and the growth curve number of rockfalls (GD) per meter width of all tree data were analyzed (e.g., Shroder, 1980; Braam et al., surfaces sampled per decade instead of analyzing 1987; Fantucci and Sorriso-Valvo, 1999). Further absolute values. The brateQ is based on the idea that focus was placed on the visual analysis of callous thick stems expose a larger target (DBH) to falling tissue overgrowing rockfall scars and traumatic rows rocks than thin ones and so large trunks are more of resin ducts (Larson, 1994; Schweingruber, 2001). likely to be subject to GD. We then determined the As resin ducts may result from causes other than yearly DBH increment of every individual tree by rockfall (e.g., climate, wind, insects, fraying or dividing its DBH by the number of rings between pith browsing by ungulates), thresholds were needed to and sample year (2002) at breast height. In a further determine bresin duct eventsQ caused by rockfall step, values were multiplied by the number of years a activity. Criteria were defined using 270 stem discs tree had existed at the beginning of a particular sampled on the same slope (Stoffel et al., in press) that decade. The DBH values of all trees existing at the showed both tree damage (scars, decapitation) and beginning of a particular decade were then summar- resin ducts. As a result, resin ducts were only ized to comprise what we refer to as exposed diameter considered the product of rockfall activity if they (ED; in m). To obtain the rockfall brateQ, the decadal formed (a) traumatic, (b) extremely compact and (c) sum of GD was finally divided by the ED, indicating continuous rows. Both the presence of resin ducts on the number of GD recorded per meter ED and per multiple radii in a single year and the occurrence of decade. Within this study, increment rates in trees multiple consecutive years with traumatic rows of were considered to remain constant, disregarding resin ducts were used as further indicators but not a juvenile growth or ageing trends. compulsory criterion. In contrast, neither dispersed In addition to the decadal rockfall bratesQ, the trees resin ducts within a single tree ring nor the presence of sampled in the two sectors were classed according to traumatic rows of resin ducts occurring in the years their age at breast height (oldest to youngest) and split following an event represents bresin duct eventsQ. into two samples each. In both cases, Sample 1 was Characteristic examples of disturbed stem discs and formed with the uneven samples (i.e., oldest, 3rd respective growth curves are illustrated in Fig. 5. oldest, 5th oldest, ...), while Sample 2 contained the Reconstructed GD of individual trees were then even ones (i.e., 2nd oldest, 4th oldest, 6th oldest, ...). compiled in a database (Schneuwly, 2003). For every individual tree, we determined recurrence intervals by 2 The term drockfall brateQT used in this paper is only a proxy for dividing its age at breast height by the number of the true rate. We therefore keep the quotation marks whenever using dated GD. this term. 230 M. Stoffel et al. / Geomorphology 68 (2005) 224–241

Stem discs Ring width measurements

abrupt growth decrease

A decapitation

2 Ring width 1 1 2 Years

injury callous growth + resin ducts B (scar)

C eccentric growth tilting 2 Ring width D 1 1 2 Years

abrupt growth release

1 elimination of neighboring tree(s) 2 Ring width

1 2 Years

Fig. 5. Evidence used to infer rockfall events.

We then calculated rockfall bratesQ for these samples Finally, we used yearly rockfall bratesQ (real as well. values) to analyze short-term fluctuations in rockfall In a further step, the rockfall bratesQ of the entire activity for the period 1950–2002. This use of yearly populations as well as those of Samples 1 and 2 were resolved data on GD also allowed estimation of the converted from real into logarithmic values, a power influence of anthropogenic rockfall triggering during regression trend line calculated and residuals from the the periods of dam construction work in 1988/89 regression model determined. (Dams 1–5; see Figs. 1B, 2) and 1996–1998 (Dams 6 Due to the difficulties of dendrogeomorphological and 7) as well as the distribution of GD thereafter. analysis in rockfall research and in order to analyze variations in decadal rockfall bratesQ, we arbitrarily 3.3. Spatial visualization of single-tree data chose a range of F1.15 standard errors on the regression fit, assuming a normal distribution of the Data on single trees included their position, their residuals from the regression model, as a criterion for age at breast height, and the number as well as the identifying exceptional rockfall bratesQ. Indeed, dec- years of GD. Tree coordinates were transformed into adal rockfall bratesQ located within these F1.15 geo-objects and information from the database linked standard errors (called Var75%) were considered to as attributes to the single trees, allowing spatial be noise and therefore insignificant. In contrast, the visualization of the data in a Geographical Informa- 25% of rockfall bratesQ lying beyond these limits were tion System (GIS). Data were investigated with the subject to analysis in greater detail. ArcGIS Geostatistical Analyst software (ESRI, M. Stoffel et al. / Geomorphology 68 (2005) 224–241 231

2003a) in order to examine spatial relationships are, on the other hand, considerably younger and between all sample points. Following the procedure generally started to (re-)colonize the slope in the early described in Johnston et al. (2001), skewed data were 18th century. The youngest trees are concentrated first transformed to make them normal. Trend alongside the rockfall couloirs underneath Dam 7 and analyses were then used to identify directional at the southeastern edge of the study area. Even so, influences (global trends), and data detrended using younger trees are repeatedly located close to the forest second-order polynomials. In a next step, spatial fringes on the valley floor, where anthropogenic autocorrelations were analyzed using spherical semi- activity markedly influenced tree growth and succes- variogram models and covariance clouds. As a result, sion rates (i.e., farming activities, extraction of fire- the numbers of lags and bin sizes have been adapted. and construction wood). Finally, cross-validation of measured with predicted The major differences in the age structure between points allowed determination of mean prediction the two sectors also appear in Table 1. While the trees errors of the interpolations. After the exploration of sampled in this study averaged 362 years in the the data, the Ordinary Kriging model (Johnston et al., northern sector, they have an average age of only 212 2001) was chosen for the visualization of continuous years in the southern sector. Similarly, the oldest surfaces. Interpolations were performed including sample in the northern sector attained breast height in data from five neighboring trees. In angular sections, AD 1318 and the youngest tree here reached breast at least two trees were taken into consideration. height in 1941. In contrast, the oldest tree from the Results were edited using ArcView 8.3 software southern sector dates to 1596 and the youngest to (ESRI, 2003b). Within this study, interpolations were 1957. performed to visualize the age structure of the forest stand and the spatial distribution of recurrence 4.2. Visible defects and growth reactions to rockfall intervals. impacts

Investigations permitted identification of 236 4. Results visible defects on the 135 L. decidua Mill. stems chosen for analysis. As illustrated in Table 2, 4.1. Age structure of the forest stand candelabra trees (=broken crown) largely predomi- nated among the visible defects. This growth feature Data on the pith age at breast height indicate that occurs after the decapitation of the crown or part of the 135 sampled L. decidua Mill. trees are, on the stem and could be identified in 151 cases (64%). average, 297 years old.3 Over the centuries, sampled Based on observations in the field, we believe that trees gradually (re-)colonized the slope to build up the candelabra trees at Ta¨schgufer were largely due to current forest at Ta¨schgufer. However, we observe that the propagation of impact energies from the lower- 25% of the sampled trees–mostly those located in the most part of the trunk to the apex of the stem, southern sector–reached breast height between 1725 resulting in crown breakage between 2 and 13 m and 1759. above the ground. In contrast, the decapitation of Fig. 6 illustrates the spatial distribution of the age tree crowns was only rarely caused by high bounces structure at Ta¨schgufer, indicating that old trees are of rockfall fragments. Recent or (partly) overgrown largely concentrated on the northern part of the study injuries (scars) were observed in 58 cases (25%). area. In this sector, 78 trees were considered. The Finally, 27 trees (11%) were obviously tilted by boundaries of age classes broadly correspond with the rockfall impacts. outer limits of the rockslide deposits mentioned in The analysis of the 564 cores allowed identifica- Section 2. The 57 trees sampled in the southern sector tion of 786 GD attributed to rockfall activity on the slope. In some cases, impacts caused GD in more than one core of the same tree, reducing the number 3 All of the numbers given below refer to age at breast height and of different rockfall events to 761. Table 2 shows do not indicate germination or inception dates. the predominant occurrence of traumatic rows of 232 M. Stoffel et al. / Geomorphology 68 (2005) 224–241

Age of trees at breast 1900 height (in years) Dam 7 45 - 100 18000 18000 101 - 175 11700 176 - 250

1700 251 - 325 1600 326 - 400

401 - 475 150 161600 0 476 - 550

1500 N Täsch 0 500mm

Fig. 6. Mean age of L. decidua Mill. trees sampled on the Ta¨schgufer slope. The patterns have been generalized based on interpolations. Ages are for tree at breast height. resin ducts. This feature was used in 675 cases thereafter, while tree-ring formation remained unaf- (86%) to determine GD. Abrupt growth suppression fected on the uphill side (core c). In this case, occurred in 50 samples (6.5%), whereas abrupt eccentric growth lasted until the core was extracted growth release and reaction wood could only be in 2002. Finally, Tree C was apparently scarred in found in 24 cores each (3%). Finally, callous tissue 1970: While the callous tissue had not yet recovered in cores proved to be rather scarce, occurring in only the injury on the uphill side (core c), it overgrew the 13 cases (1.5%). wound at core b–c by 1981. Characteristic increment curves of cores showing compression wood, growth suppression and missing 4.3. Spatial distribution of growth disturbances rings after scarring are illustrated in Fig. 7: Tree A displays an abrupt growth reduction in 1908. As the In the investigated forest stand at Ta¨schgufer, indexed reference chronology–marked as REF in Fig. reconstructed rockfall activity varied greatly across 7–does not show this sudden change in yearly the slope. The spatial distribution of interpolated increment, the growth suppression in Tree A was not recurrence intervals are given in Fig. 8, and show that driven by climatic variations but by stem breakage due to rockfall. The increment curves of Tree B illustrate that this tree was tilted in 1982 and started to Table 2 produce reaction wood on the downhill side (core d) Different types of damage observed in the field (left) and growth reactions reconstructed from increment cores (right) at Ta¨schgufer (adapted after Schneuwly, 2003) Table 1 Visible defects Growth reactions Pith age at breast height of the trees sampled for analysis (in years) Broken crown 151 (64%) Growth release 24 (3%) Pith age at breast height Northern sector Southern sector Tilted stem 27 (11%) Growth suppression 50 (6.5%) Mean 362 212 Injuries (scars) 58 (25%) Reaction wood 24 (3%) Standard deviation 141.4 75.6 Callous tissue 13 (1.5%) Maximum 684 406 Traumatic resin ducts 675 (86%) Minimum 61 45 Total 236 (100%) Total 786 (100%) M. Stoffel et al. / Geomorphology 68 (2005) 224–241 233

1890 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000

200 b-c core b-c c Tree C 100 core c d core d 0

600 500 c core d 400 Tree B 300 200 d 100 core c 0 Ring width (1/100mm) c 200 Tree A 100 core c d 0 core d

+50

0 REF

-50 indexed ref. 1890 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 (Data source: Schneuwly 2003)

Fig. 7. Examples of growth disturbances produced by rockfall events. Tree A shows an abrupt growth reduction as a result of stem breakage in 1908. Tree B was tilted in 1982 and initiated reaction wood on the downhill side (core d) thereafter. Tree C was scarred in 1970: While the scar is not yet overgrown at core c, callous tissue overgrew the wound at core b–c by 1981. Part of the reference ring width chronology (REF) is also shown.

Recurrence intervals 1900 Dam 7 between two GD 18000 18000 > 150 years 11700 100 - 149 years 17001 80 - 99 years 1600 60 - 79 years

1500 161600 40 - 59 years

20 - 39 years 1500 < 20 years N Täsch 0 500m

Fig. 8. Recurrence intervals of GD for the forest stand investigated. Intervals designate the number of years passing between two reconstructed growth disturbances on a single tree. 234 M. Stoffel et al. / Geomorphology 68 (2005) 224–241

GD abundantly occurred in the trees located above 4.4. Rockfall magnitudes and frequencies 1700 m a.s.l., where surfaces gradually become more sparsely forested. Similarly, GD have repeatedly been Throughout the last four centuries, rockfall frag- reconstructed in trees growing in the rockfall couloirs ments have continuously caused GD to the trees of the southern sector below the recently built Dam 7. sampled for analysis. There seems to have been no Here, trees were regularly disturbed by rockfall period since AD 1600 without rockfall, and activity fragments and recurrence intervals were locally b10 most commonly consisted of low magnitude–high years. In a few century-old trees growing close to the frequency events. In addition, tree-ring and age northern sector, dendrogeomorphological analysis structure analyses also allowed identification of one indicates that tree growth has been disturbed almost high magnitude–low frequency event, which (almost) twice a decade since the mid-19th century. In contrast, completely destroyed the forest stand in the southern low numbers of GD can be found in trees growing in sector of the slope in 1720. In the northern sector, the northern sector. In this part of the slope, recurrence trees sampled were disturbed considerably during the intervals proved to be particularly high. Here, rockfalls but largely survived. Fig. 9 shows that intervals between two GD regularly exceeded 150 according to the reconstructed data, 13 trees were years and individual century-old trees even showed no obviously injured by abundant rockfall. Simultane- GD at all. Not surprisingly, the spatial pattern of ously, 11 trees reacted to the event with an abrupt recurrence intervals largely coincides with the distri- growth release starting in 1721. These trees presum- bution of the age structure seen in Fig. 6 and oldest ably benefited from the sudden elimination of trees are commonly found in areas with relatively low neighboring trees, which improved their own growth numbers of GD. In contrast, youngest trees are largely conditions (e.g., light, nutrients, humidity). Within the concentrated in areas where rockfall repeatedly caused northern sector, GD mainly occurred in trees located GD, leading to increased mortality and subsequently in the upper part of the slope (above 1590 m a.s.l.). to higher recruitment rates by opening up sites for Here, 16 of the existing 33 trees (48%) showed GD in germination. 1720. In contrast, trees sampled in the lower part of

Dam 7 1800 1800

1700 1700 160 0

(modified after Schneuwly 2003) 1600

1500

Trees showing growth release in 1721 N Trees showing injuries in 1720 0 500m Trees reaching breast height 1725-1759

Fig. 9. Damage resulting from the 1720 rockfall. Thirteen trees have been injured (+) and 11 trees show an abrupt growth release starting in 1721 (o). The (re-)colonization of the rockfall slope (.) in the succeeding decades (1725–1759) most probably represents a reaction to the 1720 rockfall event. M. Stoffel et al. / Geomorphology 68 (2005) 224–241 235 the slope mostly remained unaffected by the 1720 important rockfalls in 1720, rockfall bratesQ indicate event and scars were only present in 3 of the 21 considerable fluctuations in reconstructed numbers of existing trees (14%). Even so, abrupt growth releases GD. As a result of the high-magnitude rockfalls in following the event are missing here. Fig. 9 also shows 1720 and the considerable damage caused to the forest that, as an indirect consequence of the high-magnitude fringe, the highest decadal rockfall brateQ in the last event in 1720, trees abundantly (re-)colonized the four centuries was noted in the 1720s, causing 1 GD slope. Between 1725 and 1759, 25% of all sampled per 0.65 m ED (i.e., a brateQ of 1.53). Since, data trees reached breast height (i.e., 34 trees). Most of the indicate that rockfall activity continually decreased successor trees were located in the southern sector of after the 1720 event (see Fig. 10A), interrupted by the slope. decades with significantly low (1850s, 1960s, 1970s) and significantly high rockfall bratesQ (1870s, 1990s). 4.5. Decadal variations in rockfall activity During the 19th century, rockfall bratesQ indicate rather low values during the 1850s (0.24) and, to a 4.5.1. Northern sector (1600–1999) minor extent, the 1810s (0.36). Thereafter, rockfall Investigations of rockfall activity in the northern caused higher bratesQ during the late 19th and the early sector start in the year 1600, when 29 of the trees 20th century (i.e., 1870s, 1890s, 1920s, 1940s). sampled were taller than breast height. We hence Within the second half of the 20th century, rockfall disregarded 12 rockfall events occurring between bratesQ indicate a period with low activity lasting from 1394 and 1599, since the sample depth appeared to the 1950s until the 1970s. For the 1970s, a decadal be too low for reliable analysis and the individual value of 0.2 was identified, meaning that only one trees were unevenly distributed within the sector. The growth disturbance (GD) occurred every 5 m of number of GD was reduced to 400 events derived exposed diameter (ED). This period of low activity from 78 trees. Among all trees sampled in the sector, was followed by an increase in the rockfall brateQ the DBH averaged 47.97 cm in 2002. As indicated in during the 1980s (0.54) and, even more, the 1990s Table 3, the largest and smallest trees were 76.4 and (0.57). The importance of this increase as well as the 17.51 cm DBH in 2002. As a result, the decadal DBH influence of dam construction works on the decadal increments varied between 0.6 and 6.29 cm, with an ratios of the 1980s and 1990s will be analyzed below average increment of 1.56 cm (10 years)À1 treeÀ1. in Section 4.6. While the exposed diameter (ED) of the trees sampled within the northern sector only totaled 3.9 m at the 4.5.2. Southern sector (1740–1999) beginning of the investigated period in 1600, it Due to the different age structure and the large gradually rose to 37.8 m by 1990. Table 3 further number of trees sampled that had grown after the illustrates that the decadal rockfall bratesQ averaged high-magnitude rockfalls in 1720 in the southern 0.65 GD 1 m EDÀ1 (10 years)À1. sector, the analysis of rockfall bratesQ only starts in the The reconstructed rockfall bratesQ of the entire 1740s, when 20 of the trees sampled were greater than population as well as the bratesQ from Samples 1 and 2 breast height. We disregarded 8 reconstructed GD are illustrated in Fig. 10A. For the period before the occurring between 1657 and 1734, since the sample depth appeared to be too low for reliable analysis. In total, 341 GD were dated in the 57 trees sampled. Table 3 Among the trees sampled, the DBH averaged 48.3 cm Size statistics of the trees sampled in the northern sector; DBH: diameter at breast height as measured in 2002 (in cm), 10-yr DBH: in 2002. As illustrated in Table 4, the largest tree had 10-year increment of the DBH (in cm), and rockfall bratesQ [GD 1 m 79.5 cm and the smallest only 25.4 cm DBH in 2002. EDÀ1 (10 years)À1] As a result, the decadal DBH increment greatly varied Northern sector DBH 10-yr DBH Rockfall bratesQ between 0.78 and 8.59 cm, with an average DBH À1 À1 Mean 47.97 1.56 0.65 increment of 2.70 cm (10 years) tree . At the Standard deviation 12.62 0.97 0.34 beginning of the investigation in 1740, the ED of all Maximum 76.40 6.29 1.53 trees sampled totaled 0.9 m and gradually rose to 27.7 Minimum 17.51 0.61 0.20 m by 1990. 236 M. Stoffel et al. / Geomorphology 68 (2005) 224–241

-1 100 Northern sector (1600-1999) A R2 = 0.6331

1720 rockfalls (10 years) 10 -1

1

0.1 Number of GD 1m ED 1600s 1620s 1640s 1660s 1680s 1700s 1720s 1740s 1760s 1780s 1800s 1820s 1840s 1860s 1880s 1900s 1920s 1940s 1960s 1980s

-1 100 Southern sector (1740-1999) B R2 = 0.8599 ? (10 years) 10 -1 1720 rockfalls

1

0.1 Number of GD 1m ED

"Rate" Sample 1 Power Regression (Rockfall rate) Rockfall "rate" Var 75% (± 1.15 standard deviation) "Rate" Sample 2

Fig. 10. Reconstructed rockfall activity. Rockfall activity is measured by the rockfall brateQ (for explanation see text). (A) In the northern sector, analyses cover the last four centuries (1600–1999), indicating that the large 1720 rockfalls temporarily reduced the protection afforded by the forest. (B) After the elimination of the forest in the southern sector by the 1720 event, the recolonizing trees permanently improved their protective function, reducing the number of rockfall impacts on the trees sampled by almost 13 times since the 1740s.

Results on decadal rockfall bratesQ in the southern that of the northern sector. Reconstructed rockfall sector averaged 2.36 GD 1 m EDÀ1 (10 years)À1, bratesQ of the entire population as well as the bratesQ of which represents a value almost four times higher than Samples 1 and 2 are shown in Fig. 10B. During the early decades following the high-magnitude rockfalls of 1720, the successor trees sampled for analysis were Table 4 repeatedly subject to GD. As a consequence, the Size statistics of the trees sampled in the southern sector; DBH: b Q diameter at breast height as measured in 2002 (in cm), 10-yr DBH: highest rockfall rate was reconstructed for the 10-year increment of the DBH (in cm), and rockfall bratesQ [GD 1 m 1740s, when almost 11 disturbances were identified EDÀ1 (10 years)À1] per 1 m ED (i.e., a brateQ of 10.99). Similarly to the Southern sector DBH 10-yr DBH Rockfall bratesQ northern sector, reconstructed rockfall bratesQ contin- Mean 48.39 2.70 2.36 ually decreased in the southern sector after the high Standard deviation 13.89 1.58 2.70 magnitude rockfalls of 1720. Maximum 79.58 8.59 10.99 Unlike the results from the northern sector with Minimum 25.47 0.78 0.63 six decadal values located beyond the Var75% M. Stoffel et al. / Geomorphology 68 (2005) 224–241 237 boundaries, only two rockfall bratesQ lie beyond the 4.6. Yearly fluctuations in rockfall activity Var 75% limits here, namely, those of the 1770s and (1950–2002) the 1820s (Fig. 10B). We note that, after a slight decrease in the 1750s and 1760s, the brateQ indicates In complement to the decadal rockfall bratesQ, a considerable increase in rockfall activity in the yearly fluctuations were analyzed for the period 1770s. For this period, the rockfall brateQ gives a 1950–2002. In total, we identified 172 GD, 105 in value of 8.86 GD 1 m EDÀ1 (10 years)À1, the southern (61%) and 67 in the northern sector approaching that for the 1740s. This period of (39%). On average, more than 3 GD yearÀ1 could be increased rockfall activity lasted until the 1780s or identified in the 135 trees sampled. even 1790s. In contrast to the high magnitude As illustrated in Fig. 11A, the number of recon- rockfalls of 1720s, reconstructed data indicate that structed GD remained on a comparably low level in the the considerable number of GD identified for the last northern sector during the first three decades of three decades of the 18th century were not caused by investigation, when years without GD occur almost one high-magnitude event, but rather by a series of as frequently as years with GD. After 1980, rockfall years with high rockfall activity (i.e., 1772, 1774, repeatedly caused more GD to the trees sampled. 1779, 1783, 1792). Even though the number of GD Reconstructed data neatly reflect the period of also increased in the northern sector (see Fig. 10A), increased rockfall activity in the 1980s and the early significant effects remained mostly restricted to the 1990s, as described for the Ta¨schgufer slope by Lauber bratesQ of the southern sector. Between the 1870s and (1995). Between 1950 and 2002, yearly rockfall the 1940s, comparably low numbers of GD were bratesQ averaged 0.037 GD 1 m EDÀ1 yearÀ1, found in the trees sampled, resulting in a consid- indicating that approximately 1 GD was recorded for erable decrease in rockfall bratesQ.Duringthis every 27 m of ED. period, the lowest rockfall brateQ of the last 260 In the southern sector, decadal rockfall bratesQ years was derived, totaling 0.63 during the 1910s. In slightly increased in the 1940s and thereafter. Yearly contrast to the northern sector, the rockfall brateQ resolved bratesQ indicate that during the first three started to increase again in the 1940s, oscillating decades, (short) periods with increased numbers of around the calculated power regression trend line GD (e.g., 1959–1964) were followed by several years ever since. However, the changes in the rockfall with (almost) no GD (Fig. 11B). Similar to the bratesQ seem to be less marked here. Again, the northern sector, considerably increased rockfall influence of the dam construction works on the bratesQ can be discerned in 1980 and 1992. In contrast, decadal ratios of the 1980s and 1990s will be the high rockfall brateQ reconstructed for 1986 (0.28) analyzed further down in Section 4.6. was probably restricted to the southern sector. On -1 A Northern sector (1950-2002) Mean: 0.037 year

-1 0.1

0

B Southern sector (1950-2002) Mean: 0.085 0.3

0.2

0.1 Number of GD 1m ED 0 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000

Fig. 11. Annual estimates of rockfall 1950–2002. Note the important variability in reconstructed growth disturbances and different activity in the sectors. 238 M. Stoffel et al. / Geomorphology 68 (2005) 224–241 average, yearly bratesQ totaled 0.085 GD 1 m EDÀ1 construction of Dam 6 in 1996 (c2000 m a.s.l.) yearÀ1, indicating that more than 1 GD has been caused six GD. Growth reactions were even more recorded per 12 m of ED. frequent during the construction of Dam 7 in 1998 In contrast to the results from trees located in the (1780 m a.s.l.). As a result of anthropogenic heavily disturbed forest stand above 1780 m a.s.l. intervention on the slope, six GD were caused in (Stoffel et al., in press), the construction of Dams 1 to the northern and nine GD in the southern sector. For 5 (1988/89; see Fig. 1B) only slightly influenced the this reason, the rockfall bratesQ of the 1990s given in rockfall bratesQ of the present study. In the northern Section 4.5 were clearly influenced by anthropogenic sector, the two GD recorded in 1989 caused an intervention on the slope, making it difficult to increase in the decadal brateQ from 0.46 to 0.52. estimate the undisturbed rockfall frequency over this Similarly, the only GD recorded in the southern sector period. in 1989 influenced the brateQ, resulting in a value of As a further consequence, anthropogenic interven- 0.96 instead of 0.92. In contrast, reconstructed tion on the slope hindered rockfall fragments to cause rockfall bratesQ prove to be influenced by the GD to the trees sampled after 1998. While no GD construction works (e.g., excavation, blastings) of have been recorded since 1999 in the southern sector the two protection dams in the late 1990s. As can be (Fig. 11), GD could still be identified in the northern seen from Fig. 12, rockfall triggered during the sector. As illustrated in Fig. 12, rockfall fragments apparently passed north of the recently built dams in 1999 and 2001, causing GD to five trees in the northern sector. In the sparsely wooded areas between Dam 6 and 7, reconstructed data clearly indicate that 190019 Dam 6 abundant rockfall still occurs on the slope (Stoffel et Dam 7 al., in press). We therefore have to believe that the 180000 recently built dams efficiently stopped rockfall in the 170070 recent past and so further influenced the rockfall (adapted after Schneuwly 2003) bratesQ of the 1990s. 1600

1500 5. Discussion

In the study we report here, cores extracted from 135 living L. decidua Mill. trees allowed reconstruc- tion of yearly and decadal rockfall activities on the Ta¨schgufer slope. Similar to analysis used in debris flow, snow avalanche or flooding research (e.g., Schweingruber, 1996), we investigated growth dis- turbances (GD) in increment cores to analyze past

N rockfall activity. In the trees sampled, rows of

0500m traumatic resin ducts proved to be by far the most common reaction to rockfall impacts. Reaction wood, Distribution of trees with GD (1996 - 2001) growth suppression or growth release could be identified less frequently. 1996 1998 1999 2001 We identified 741 GD covering the period 1600– 2002. Impacts could be found more commonly in Fig. 12. Rockfall events during and after dam construction, 1996– trees located in the southern sector of the slope, where 2002. Growth disturbances have been recorded during construction works at Dam 6 in 1996 (+), Dam 7 in 1998 (E) and in the GD rockfall recurred locally more than once per succeeding years (.=1999; *=2001). No growth disturbances were decade. According to the results, we believe that recorded in the years 1997, 2000 and 2002. rockfall activity at Ta¨schgufer mainly consisted of low M. Stoffel et al. / Geomorphology 68 (2005) 224–241 239 magnitude–high frequency events, considered to be Table 5 typical for rockfall in alpine areas (e.g., Matsuoka and Growth disturbance data by decades for the two sites; GD=growth disturbances; ED=exposed diameter, i.e., DBH of all trees sampled Sakai, 1999; Jomelli and Francou, 2000). The only at a particular decade (in m); Tr=decadal number of existing trees, high magnitude–low frequency event occurred in rockfall bratesQ=number of GD per 1 m ED and decade; GD–Tr 1720, causing considerable damage to the forest in ratio=number of GD per tree (Tr) and decade the northern sector. As a result, the forest fringe was Decade GD ED (m) Tr Rockfall GD–Tr displaced downslope. The forest stand in the southern bratesQ ratio sector was almost completely destroyed by the NS N S NS N S N S devastating rockfalls in 1720. Data also show that 1600s 2 – 3.9 – 29 – 0.51 – 0.07 – the high rockfall frequency both inhibited recoloniza- 1610s 4 – 4.2 – 29 – 0.95 – 0.14 – tion and caused considerable damage in the juvenile 1620s 1 – 4.6 – 30 – 0.22 – 0.03 – trees of the southern sector thereafter. As a result, 1630s 4 – 5.0 – 35 – 0.79 – 0.11 – considerable differences emerge in the reconstructed 1640s 3 – 5.5 – 38 – 0.55 – 0.08 – 1650s 6 – 5.9 – 39 – 1.01 – 0.15 – numbers of GD between the northern and the southern 1660s 6 – 6.4 – 40 – 0.94 – 0.15 – sector. 1670s 2 – 6.9 – 43 – 0.29 – 0.05 – In contrast to the analysis of geomorphic processes 1680s 4 – 7.5 – 45 – 0.53 – 0.09 – involving larger volumes, such as debris flows, snow 1690s 10 – 8.1 – 48 – 1.24 – 0.21 – avalanches or flooding, results from dendrogeomor- 1700s 5 – 8.6 – 49 – 0.58 – 0.10 – 1710s 4 – 9.2 – 49 – 0.43 – 0.08 – phological investigations of rockfalls cannot immedi- 1720s 15 – 9.8 – 50 – 1.53 – 0.30 – atelybeusedtoillustrateyearlyordecadal 1730s 10 – 10.5 – 51 – 0.95 – 0.20 – fluctuations in rockfall activity. As rockfall consists 1740s 13 10 11.3 0.9 58 19 1.15 10.99 0.22 0.53 of single falling, bouncing or rolling stones, a single 1750s 13 8 12.1 1.4 62 22 1.07 5.93 0.21 0.36 event may only disturb trees along its trajectory. 1760s 10 11 13.0 1.8 66 25 0.77 6.01 0.15 0.44 1770s 11 21 13.9 2.4 68 28 0.79 8.86 0.16 0.75 Furthermore, trees at Ta¨schgufer were of uneven age 1780s 11 13 14.8 2.9 69 30 0.74 4.42 0.16 0.43 and displayed abundant differences in their DBH as 1790s 14 10 15.7 3.6 69 33 0.89 2.75 0.20 0.30 well as the decadal DBH increment. As a result, the 1800s 9 12 16.6 4.4 70 35 0.54 2.75 0.13 0.34 number of reconstructed GD steadily increased with 1810s 6 7 17.6 5.1 70 37 0.33 1.36 0.09 0.19 time, reaching maximum values for the recent past. 1820s 8 6 18.5 5.9 71 38 0.43 1.02 0.11 0.16 1830s 12 11 19.5 6.7 71 40 0.62 1.64 0.17 0.28 Even so, a ratio dividing the decadal number of GD 1840s 10 13 20.5 7.6 71 43 0.49 1.70 0.14 0.30 by the number of trees (Tr) existing at the beginning 1850s 5 9 21.5 8.7 74 46 0.23 1.04 0.07 0.20 of a particular decade cannot concisely represent the 1860s 9 14 22.6 9.7 74 48 0.40 1.44 0.12 0.29 importance of decadal rockfall activity. As illustrated 1870s 16 11 23.7 10.9 75 49 0.68 1.01 0.21 0.22 in Table 5, GD–Tr ratios regularly increased in 1880s 11 11 24.8 12.1 76 51 0.44 0.91 0.14 0.22 1890s 13 9 25.9 13.3 76 52 0.50 0.68 0.17 0.17 periods following major modifications in the sample 1900s 9 15 27.0 14.6 76 53 0.33 1.03 0.12 0.28 depth (e.g., 1690s and after 1720 in the northern and 1910s 9 10 28.1 15.9 76 54 0.32 0.63 0.12 0.18 after the 1770s in the southern sector), leading to an 1920s 14 22 29.3 17.3 77 55 0.48 1.28 0.18 0.40 overestimation of past rockfall activity. Even so, it 1930s 10 13 30.5 18.6 77 55 0.33 0.70 0.13 0.24 appears that the GD–Tr ratio may not correctly 1940s 15 21 31.7 20.0 77 55 0.47 1.05 0.19 0.38 1950s 10 18 32.9 21.5 78 57 0.30 0.84 0.13 0.32 represent the rockfall activity during the late 20th 1960s 7 22 34.1 23.1 78 57 0.21 0.95 0.09 0.39 century, when the lack of succeeding trees and an 1970s 7 17 35.4 24.6 78 57 0.20 0.69 0.09 0.30 almost constant sample depth again influence the 1980s 19 25 36.6 26.2 78 57 0.52 0.96 0.24 0.44 ratios. 1990s 21 23 37.8 27.7 78 57 0.56 0.83 0.27 0.40 The rockfall brateQ, considering the exposed Bold numbers indicate maximum, bold italic numbers minimum diameter (ED=DBH of all sampled trees within a values. sector at the beginning of a particular decade), proved, instead, to be a more reliable indicator of gradual changes of the ED with time. As a further past rockfall activity, as it takes account of the result, the rockfall bratesQ nicely illustrate the diameter exposed to rockfall fragments as well as the protective function of the two selected forest stands: 240 M. Stoffel et al. / Geomorphology 68 (2005) 224–241

After the devastating rockfalls of 1720, the recolo- long-term variations in rockfall activity or by the nizing trees could not efficiently stop rockfall frag- temporal resolution of the approach (e.g., lichen- ments in the southern sector, resulting in ometry). Although the techniques used in this considerably high decadal ratios in the early decades investigation need further refinement and minor of the analysis [e.g., 10.99 GD 1 m EDÀ1 (10 modifications in the sampling strategy, the results years)À1 in the 1740s]. Compared to the 1740s, presented above clearly show that dendrogeomor- almost 13 times fewer GD are dated per 1 m of ED phic investigations have the potential to produce today. In the forest stand of the northern sector, results on yearly fluctuations and decadal ratios of rockfall bratesQ persisted on a relatively low level rockfall activity over several centuries. Moreover, throughout the last four centuries. Nonetheless, the we have been able to determine spatial variations in high-magnitude event of 1720 also emerged from the rockfall activity, considerable differences in recur- GD–ED data, resulting in the highest decadal rock- rence intervals and the changes in the efficacy of the fall brateQ [1.53 GD 1 m EDÀ1 (10 years)À1] protective function of the two forest stands at recorded since AD 1600. As a further consequence, Ta¨schgufer. the efficacy of the protective forest was temporarily reduced, resulting in higher bratesQ until the late 18th century. Since then, the protective function has Acknowledgements further increased, resulting in a rockfall brateQ based on the linear regression trend of approximately 0.4. The authors gratefully acknowledge Sascha Negro This study represents the first attempt to analyze for assistance in the field. Simone Perret is warmly yearly and decadal fluctuations in rockfall activity acknowledged for providing helpful comments and on a forested slope. It has furthermore analyzed the Heather Murray for improving the English of this age structure of the forest stands. Nevertheless, paper. The authors also want to thank the local some issues remain to be resolved. In this study, community president Kilian Imboden and the local we tried to select severely injured trees. However, forester Leo Jfrger for coring permission and support. comparisons between visible defects with recon- Finally, the authors gratefully acknowledge the structed events indicate that scars and bcandelabra suggestions made by the reviewers Nel Caine and formQ trees may only reflect 28% of the GD (i.e., Brian H. Luckman. resin ducts, callous tissue, growth suppression) dated on the cores. We therefore stress the key role played by the sampling strategy by pleading for more References random strategies when choosing trees in heavily disturbed rockfall forests. Even so, as the results Andre´, M.-F., 1986. Dating slope deposits and estimating rates of presented above are from two sectors of one single rockfall wall retreat in Northwest Spitsbergen by lichenometry. slope, replicate studies are needed from a variety of Geogr. Ann. 68, 65–75. Andre´, M.-F., 1997. Holocene rockwall retreat in Svalbard: a triple- similar sites to place our reconstructions as well as rate evolution. Earth Surf. Processes Landf. 22, 423–440. the general applicability of the rockfall brateQ in a Bebi, P., Kienast, F., Schfnenberger, W., 2001. Assessing structures wider context. in mountain forests as a basis for investigating the forestsT dynamics and protective function. For. Ecol. Manag. 145, 3–14. Berger, F., Quetel, C., Dorren, L.K.A., 2002. Forest: a natural protection mean against rockfall, but with which efficiency? The 6. Conclusion objectives and methodology of the ROCKFOR project. Proc. Int. Congress Interpraevent 2002 in the Pacific Rim, Matsu- The approach outlined in this study proved to be moto, Japan, pp. 815–826. a useful tool for analyzing fluctuations in inter- Braam, R.R., Weiss, E.E.J, Burrough, P.A., 1987. Spatial and annual and inter-decadal rockfall activity on a temporal analysis of mass movement using dendrochronology. Catena 14, 573–584. forested slope. In contrast to former studies, this Bra¨ker, O.U., 2002. Measuring and data processing in tree-ring analysis was no longer limited by the restricted research—a methodological introduction. Dendrochronologia temporal sample available for the assessment of 20, 203–216. M. Stoffel et al. / Geomorphology 68 (2005) 224–241 241

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