Hydrology of Natural and Manmade Lakes (Proceedings of the Vienna Symposium, August 1991). IAHS Publ. no. 206, 1991.

Long-term changes in the water level of and possible causes

G. LUFT Landesanstalt fur Umweltschutz -Wiirttemberg, Abt. 4: Wasser, Benzstrafie 5, D-7500 21 (F.R.G.)

G. VAN DEN EERTWEGH Agricultural University Wageningen, Department of Hydrology, Soil Physics, and Hydraulics, NL-6709 PA Wageningen, formerly Landesanstalt fur Umweltschutz Baden-Wiirttemberg

ABSTRACT Lake Constance is a natural reservoir system consisting of two parts, and Untersee. River flows through the lake. At the lakeside and in shallow shore zones, and vegeta­ tion damages have been observed. Long-term changes in the water-level can be one of the causes of erosion processes and were investigated in this study. Also possible causes of these changes were taken into consideration. The results of frequency and trend analysis and Gaussian low pass filtering show that the water level of Lake Constance and its regime has changed. Mean annual water levels (Untersee), discharges (Alpenrhein and Hochrhein), and areal precipitation depths have remained nearly constant. Peak water levels and discharges have dropped, low water levels and discharges have increased. On the contrary, the mean annual water level of Obersee has dropped, low water levels have remained constant. The changes in water level have probably been caused by changes of hydraulic conditions in the outflow-regions of the lake. This process has been superposed by development and operation of storage reservoirs (hydropower purposes) in the catchment area of the Alpenrhein. Its seasonal runoff regime has been strongly influenced by an increase of low discharge, predominantly in the winter and a decrease of peak discharge in the summer.

INTRODUCTION.

Lake Constance is a natural reservoir system consisting of two parts, Obersee and Untersee (Fig. 1). The River Rhine flows through the lake. Three

31 G. Luft & G. van den Eertwegh 32

sections of the river were considered in this study: Alpenrhein (from its source down to Lake Constance), (river connection between Obersee and Untersee) and Hochrhein (between Stein and ). At the lakeside and in shallow shore zones erosion and vegetation damage has been observed (Dittrich and Westrich, 1988). Caved in shorelines occur, as well as recession of reed: in some places reed has died back over large areas (Siessegger, 1988). Interdependencies exist between the damage and the change in water level. At the time this investigation started, it was already known that the water-level of Obersee dropped during the last five decades. Possible reasons for changes in water level can be of a natural (e.g. precipitation, évapotrans­ piration) or human kind (e.g. water management, urbanization, dredging of waterways). The determination of possible long-term changes in the water- level of the Lake Constance reservoir system as a whole, including changes of in- and outflow of the lake, was the purpose of this investigation. By means of time series analysis, possible changes in water level of the lake, river discharge, and areal precipitation depth are investigated. The investiga­ tion (Luft et al., 1990) was carried out by the Landesanstalt fur Um- weltschutz Baden-Wurttemberg, (Li.U.) and coordinated with the Institut fiir Seenforschung, -Bodensee (I.f.S.). Lake Constance is an international water system. Therefore hydrological, water management, and climatological data from several Swiss and Austrian institutions were needed to investigate possible changes in water level of Lake Constance. For providing the data needed, the authors like to thank: - Landeshydrologie und -géologie (L.H.G.) der Schweiz, Bern, , - Bundesamt fur Wasserwirtschaft, Bern, Switzerland, - Tiefbauamt des Kantons , Sektion Flussbau und Wassernutzung, Switzerland, - Hydrographischer Zentralbiiro, Wien, , - Landes-Wasserbauamt , , Austria, - Vorarlberger Illwerke AG, Bregenz, Austria, - Zweckverband Bodensee-Wasserversorgung, Stuttgart, F.R.G., - Deutscher Wetterdienst, Offenbach a. , F.R.G., - Bayerisches Landesamt fur Wasserwirtschaft, Mûnchen, F.R.G., - Versuchsanstalt fur Wasserbau, Hydrologie und Glaziologie der Eidgenôssischen Technischen Hochschule, Zurich, Switzerland, - Bundesanstalt fur Gewâsserkunde, Koblenz, F.R.G..

INVESTIGATIONS

The water levels of Obersee and Untersee depend mainly on: (a) the regime of the lake inflow and outflow, (b) the water level of the transition zones, (c) the sizes of the two lake sections (volumes). Obersee has a volume of about 47.7 mio m3. Untersee is a much smaller part of Lake Constance: it only has a volume of about 0.84 mio m3. 33 Long-term changes in the level of Lake Constance

The time series used in this investigation are (analyzed periods in brackets, stations see Fig. 1): (a) water levels of: (i) Obersee at gauging stations /Bodensee ( 1817 - 1987), Rorschach/Bodensee (1817 - 1987), (ii) Seerhein at gauging station /Seerhein (1941 - 1980), (iii) Untersee at gauging station Berlingen/Bodensee (1887 - 1987), (iv) Hochrhein at gauging station Stein-Burg/Hochrhein(1887 - 1987), (b) discharges of: (i) Alpenrhein at gauging station St.Margrethen-Diepoldsau/Alpen- rhein (1919- 1987), (ii) Rheintal-Binnenkanal - Altrhein at gauging station St.Margre- then/Rheintal-Binnenkanal (1919 - 1987), (iii) other inflow-contributors to Lake Constance (Dornbirner- and Bregenzeraach, , , , and with different analyzed periods between 1920 and 1987), (iv) Hochrhein at gauging stations Neuhausen-Flurlingen/Hochrhein (1904 - 1987), Rekingen/Hochrhein (1920 - 1987), (c) areal precipitation depth for the Swiss part of the catchment area of the river Rhine at gauging station Stein-Burg/Hochrhein (1901 - 1985). River Alpenrhein (catchment area 6.119 km2) covers 52% of the catch­ ment area of Hochrhein at gauging station Neuhausen-Flurlingen/Hochrhein (11.887 km2) and is the main inflow contributor to Lake Constance. It sup­ plies about 62% of the inflow (mean discharge 230 m3/s) of Lake Constance referring to the amount of water leaving the lake through the discharge of the Hochrhein (mean discharge at gage Neuhausen-Flurlingen 372 m3/s). River Bregenzeraach supplies 12.6%, Argen 5.2%, Schussen 2.9%, Dornbir- neraach 1.3%, Seefelder Aach 0.8%, and Rotach 0.5% (Fig.l). Additionally time series concerning land evaporation near Lake Con­ stance, water transport to and from other basins and Karst water phenomena were considered. All water level data were based on the same geodetic height level (NN = sea level) and converted to cm above NN + 390m. The time series used were extensively checked up on consistency, e.g. compared to the raw data and to each other. Intentionally time series for periods, different in length, were used. Only in this way it was possible to investigate changes in the water level of Lake Constance. The investigations consist of: (a) analysis of regime (duration curve), (b) analysis of periodicity and cycles (Gaussian low pass filter method), (c) homogeneity tests (mass curve procedure); trend analysis (linear regression). The significance of a linear trend has been determined by means of the Student's T-test. G. Luft & G. van den Eertwegh 34

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o te 35 Long-term changes in the level of Lake Constance

RESULTS AND DISCUSSION

An analysis of the duration curves of daily water levels for three time periods shows (Fig. 2): (a) an overall drop (low, mean and peak) of the water level of Obersee, (b) an almost unchanged mean water level of Untersee (increase of low water levels and decrease of peak water levels in the most recent time period). The duration curves of the discharge of Alpenrhein and Hochrhein are almost similar to those of Untersee. The investigations of mean monthly water levels and discharges on regime for two periods of 50 years each show (Fig.3): (a) general drop of the water level of Obersee, except in February and March, (b) falling peak water levels (summer) and rising low water levels (winter) of Untersee, (c) drop of peak discharge and rise of low discharge of Alpenrhein and Hochrhein, similar to changes in the water level regime of Untersee.

0 6,0 K> 15 20 25 30 35 40 -<5 60 55 60 65 70 7g SO S5 80 «5 »9,08 V»

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n0 5.0 10 15 20 35 30 35 40 45 50 65 60 65 70 76 GO 83 80 65 BB.OD .V.H.

FIG. 2 Duration curves of daily Obersee water levels for three time periods at (a) the Konstanz/Bodensee gauging station, and (b) the Berlingen/Bodensee gauging station. G. Luft & G. van den Eertwegh 36

MW Icm 0. NN + 3901 f a) 640T Konstanz/Bodensee (Obersee)

If/ \\\ m [ cm/Jahr 1

\\ 560-- V o OJ + 0,2 .>, MW 540-• \ ,7(10/1887-9/1987) H. E 520-

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m 1 cm/Jahr 1

E

a E

O N D J FM AM J J A S

10/1887-9/1987 Trend:

—10/1887-9/1937 stgnrfikanl nicht signifikant

10/1937-9/1987

FIG. 3 Mean monthly water levels of Obersee for three time periods at (a) the Konstanz/Bodensee gauging station, and (b) the Berlingen/Bodensee gauging station. Linear trends of mean monthly water levels for the period 1887-1987. 37 Long-term changes in the level of Lake Constance

The regime of the areal precipitation depth has almost not changed. In order to demonstrate periodicities or cycles, several time series were filtered by means of the Gaussian low pass filter method (Schônwiese, 1983). Using this method, long-term changes and/or periodicities can become visible because short-term changes in the time series are 'filtered out'. The time series of the annual areal precipitation depth and water-level of Obersee were filtered over periods of 13 and 30 years after analysis of the variance spectrum (Fig.4). While the mean annual areal precipitation depth remains almost constant, the mean water level of Obersee shows a strong fall during the period 1938-1948, using a filter over a period of 30 years. Since about 1918 the mean water level of Obersee has started to fall, it shows long-term changes. The water level of Obersee shows cycles between 11 and 16 years. The filtered discharge time series of the Alpenrhein, as well as the time series of the water level of Untersee and of the Hochrhein show almost similar changes to the filtered time series of the areal precipitation depth. Tests on homogeneity by the mass curve procedure on the time series show inhomogeneities in the periods 1910-1920 and 1935-1960, both for the water level of Lake Constance and the discharge of Alpenrhein. During these periods, changes in water level and discharge started. Trend analysis of minimum, mean, and maximum annual water levels of Obersee during the period 1817-1916 does not show long-term changes or inhomogeneities. Therefore, this time period was not investigated any further. Trend analysis of minimum, mean and maximum annual water levels (Obersee and Untersee: period 1887-1987) and discharges (Alpenrhein: period 1920-1987, and Hochrhein: period 1905-1987) results in (Fig. 5): (a) insignificant trends in mean annual water levels and discharge, except for Obersee (falling), (b) significant trends in falling maximum annual water levels and dis­ charge, and in rising minimum annual water levels and discharge, except for Obersee (constant). Other austrian and german inflows to Lake Constance, measured over shorter time periods as mentioned above show no significant trends in minimum, mean and maximum annual disharge. The time series of differences between the daily water level of Obersee and Untersee (averaged on a yearly basis) show that the mean annual differences decrease since about 1917 (Fig.6). The mean annual differences between the water level of Untersee and Hochrhein (near the outflow of the Untersee) have not changed. The discharge of the Alpenrhein, the main inflow-contributor to Lake Constance, shows remarkable changes, mainly due to the development (Fig.7) and operation of storage reservoirs for hydropower purposes in its catchment area (human influences; Bundesamt fur Wasserwirtschaft, 1988; Link, 1970). During the winter, water is stored in reservoirs; in the summer, 80% of the amount of water stored is released (Schàdler, 1985). The mean annual discharge of the Alpenrhein has stayed almost constant and so did the mean annual areal precipitation depth. Both time series have an insignificant positive trend. The amount of water extracted from the Alpenrhein and the water pumped out of Lake Constance for long G. Luft & G. van den Eertwegh 38

A) monthly areal precipitation depth at gauging station Stein-Burg/Hochrhein

A average: 118 mm filter-period: 360 months=30 years US -

1902 ' ' '1912 ' ' 'l922 ' ' 'l932 ' ' 'l942 ' ' 'l952 ' ' 'l96a '1972 '1982

average: 118 mm filter-period: 156 months=13 years

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WMK^^kkJâ, *^_ 1888 '1898 ~,'190 8 '1918 '192B '193MQlfl8 '194MQ, B '1958 '196U 8 '1970

average: 533 cm filter-period: 156 months= 13 years

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leae' ' 'îirae' ' 'îso'o' ' 'îbiV ' 'isàs' ' 'îgia' ' 'îs^'s' ' 'ibsa' ' 'ibda' ' '1978^ FIG. 4 Application of Gaussian low pass filter method to: A) monthly areal precipitation depth of the Swiss part of the catchment area of Lake Constance at gauging station Stein- Burg/Hochrhein, B) mean monthly water level of Obersee at gauging station Konstanz/Bodensee. 39 Long-term changes in the level of Lake Constance

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Harta MeBstelle : Differenzen HH(m) KONSTANZ-BER — Mlttalaort 22.70 HeBgrBBe : HASSERSTAND HUtal Obar 10 Korta foptachr. Mlttel Zeitspanne: OKT 1867 - SEP 1987 ~ Hnearer Trend -0.1412 / Jahr Datenart : JAHRESWERTE aignifikant 89.80 [J] (a)

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Herts MeBstelle Differenzen MW(s) BEBLINGEN-ST Hlttalnart 27.31 MeBgrflBe WASSERSTAND lllttol Obor 10 Hepte Zeitspanne; fortachp. Mittel OKT 1887 - SEP 1987 linsarer Trend 0.4839E-O2 / Jahr Datenart JArRESHERTE aignifikant bel 60.00 [«

(b)

m=0,5cm/100 years AHW = 27.3cm

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FIG. 6 Mean annual differences (cm) in water level between (a) Obersee at the Konstanz/Bodensee gauging station and Untersee at the Berlingen/Bodensee gauging station; and (b) the Untersee at the Berlingen/Bodensee gauging station and Hochrhein at the Stein-Burg/Hochrhein gauging station. 41 Long-term changes in the level of Lake Constance

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) M 0(0 10-43- 0 3 II 1948- 7 Ills ii il t 1 • • 1 lJ ;- r __ 1 1920 1930 1940 1950 1960 1970 1980 1990 Jahre FIG. 7 Development of hydropower in the River Alpenrhein catchment area: capacity of storage reservoirs in mio m3 (Luft and Vieser, 1990). distance water supply (drinking water) are smaller than the amount of water transferred from other river basins (e.g. the river 111). The discharge of the Hochrhein is constant on a long-term basis. The potential évapotranspiration at gauging station Konstanz/Bodensee has remained almost constant (insig­ nificant falling trend). Probably the evaporation of Lake Constance has also not changed (no data available). These results together show that the in- and outflow of Lake Constance have remained almost constant. Still, the mean annual water-level of Obersee is falling. There are two probable causes for the changes in the water-level of Lake Constance, especially of Obersee: (1) the natural and/or human causes of changes to the hydraulic conditions in the outflow reach of Obersee. Erosion processes in the inner bay of Konstanz and in the Seerhein may have resulted in an overall decrease of the water-level of Obersee. These processes have proba­ bly started in 1917, (2) alterations of the runoff-regime of the Alpenrhein, caused mainly by the development and operation of storage reservoirs within its catchment area. This effect has been evident since about 1960.

CONCLUSIONS The present results show that the alterations of the hydraulic conditions in G. Luft & G. van den Eertwegh 42 the outflow reach of Obersee are progressive. A natural threshold on the bottom of the inner bay of Konstanz has been lowered. As a result, the mean annual water-level of Obersee has dropped. If the presumption of alterating hydraulic conditions proves correct, measures to stabilize and protect the bed of the inner bay of Konstanz and of the Seerhein and measures to increase the low water-level at the outflow reach of Obersee have to be considered. The peak water-level of Lake Constance has dropped because of changes in the outflow-behaviour of Obersee and the operation of storage reservoirs in the catchment area of the Alpenrhein. If the peak water-level data of Obersee would be adjusted by elimination of trend, the threshold value for damages of 396.89m+NN (corresponding to a water-level of 5.0m at gauging station Konstanz/Bodensee) would be attained or exceeded 26 times instead of only 14 times in the present situation (Fig. 8). All these impacts influence indirectly flood protection at Lake Constance. Further conceptions concerning flood protection see Vischer (1989).

Herts MeSstelle : KONSTANZ/BODENSEE ib- Mttelwert HW 6SS.0 MeBgro8e : HASSERSTANO Hittei Ober 10 Herts Zeitspanne: OKT 1887 - SEP 1987 •fortschr. Hlttel Datenart : MAXIMALWERTE JAHR Ilnearer Trend -0.2662 / Jahr signifikant be; 90.00 [s] adjusted demcge causing tim water level

1987 (726cm)

-threshold value for W=Sm a. P.KN elimination of trend

'isaa' ' 'l'eàs' ' 'igce' ' 'ibis' ' ' 192B' ' '1933' ' 'i948' ' 'issa' ' 'iséa' ' '197a'

FIG. 8 Highest annual daily Obersee water level at the Konstanz/Bodensee gauging station. Trend analysis and trend adjustment by elimination of trend of the time series. 43 Long-term changes in the level of Lake Constance

PROSPECTIVES

At the moment several studies are being performed to explain the possible changes in hydraulic conditions of the outflow reach of Obersee (historical cross section measurements of Seerhein and inner bay of Konstanz); e.g. measurements to obtain the water level of the inner bay of Konstanz more accurately are currently carried out. Furthermore, a hydraulic model to compute the water level of the inner bay of Konstanz is to be developed.

REFERENCES

Bundesamt fur Wasserwirtschaft, Bern (1988) Speicherseen im schweizerischen Alpenrhein-Einzugsgebiet. Statistik- Auszug. Dittrich, A. and D. Westrich (1988) Bodensee-Ufererosion:Bestandsaufnahme und Bewertung. Mitteilungen des Instituts fur Wasserbau der Universitât Stuttgart. Heft 68. August 1988. Eidgenôssisches Verkehrs- und Energiewirtschafts-departement (1968) Naturliche und durch Ableitungen beeinflusste Wasserfiihrung schweiz- erischer Seewâsser. Mitteilungen des Eidgenôssischen Amtes fur Wasserwirtschaft. Bern. Nr. 45. Eppinger, R. (1989) Massnahmen gegen das Schilfsterben am Bodensee. Wasserwirtschaft 79 (1989) 3. Gasser, O. (1957) Die Wasserspiegelschwankungen des Bodensees und ihren meteorologi- schen Grundlagen. Berichte des Deutschen Wetterdienstes Nr. 35. Hamblin, P.F. and E. Hollan (1978) On the gravitational seiches of Lake Constance and their generation. Schweizerische Zeitschrift fur Hydrologie. 40. 1978, 119-154. Link, H. (1970) Speicherseen der Alpen. Sonderheft Wasser- und Ener- giewirtschaft/WEW Nr. 9. 1970. Schweizerischer Wasser- wirtschaftsverband Baden. Luft, G., H.-J. Vieser and G. van den Eertwegh (1990) Handbuch Hydrologie Baden-Wùrttemberg: Verânderung der Boden- see-Wasserstânde von 1887 bis 1987. Ministerium fur Umwelt und Landesanstalt fur Umweltschutz Baden-Wùrttemberg. Abteilung 4: Wasser, Stuttgart und Karlsruhe. Luft, G. and H.-J. Vieser (1990) Verânderung der Bodensee-Wasserstànde von 1887 bis 1987. Deutsche Gewàsserkundliche Mitteilungen. 1990. Heft 5/6. Schâdler, B. (1985) Der Wasserhaushalt der Schweiz. Bundesamt fur Umweltschutz, Landeshydrologie und -géologie der Schweiz, Bern. Mitteilung Nr. 6. G. Luft & G. van den Eertwegh 44

Schônwiese, Chr.-D. (1983) PROMET. Metenrologische Fortbildung. Jg. 13. Heft 1/2 (1983V Statistische Methoden der Klimatologie. Hrsg.: Deutscher Wetterdienst, Offenbach a. Main. Siessegger, B. (1988) Dokumentation zu den Schilfgebieten, zum Schilfriickgang sowie zu den erfolgten Schilfsanierungsmassnahmen am deutschen Ufer des Bodensee-Obersees. Landesanstalt fur Umweltschutz Baden-Wurttem- berg, Institut fur Seenforschung, Langenargen. Vischer, D. (1989) Ideen zur Bodenseeregulierung - Ziele, Altes und Neues. Vermessung Photogrammetrie Kulturtechnik. Jg. 87. 1989. Heft 1.