Long-Term Changes in the Water Level of Lake Constance and Possible Causes
Total Page:16
File Type:pdf, Size:1020Kb
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 Lake Constance and possible causes G. LUFT Landesanstalt fur Umweltschutz Baden-Wiirttemberg, Abt. 4: Wasser, Benzstrafie 5, D-7500 Karlsruhe 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, Obersee and Untersee. River Rhine flows through the lake. At the lakeside and in shallow shore zones, erosion 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), Seerhein (river connection between Obersee and Untersee) and Hochrhein (between Stein and Basel). 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, Langenargen-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, Switzerland, - Bundesamt fur Wasserwirtschaft, Bern, Switzerland, - Tiefbauamt des Kantons Schaffhausen, Sektion Flussbau und Wassernutzung, Switzerland, - Hydrographischer Zentralbiiro, Wien, Austria, - Landes-Wasserbauamt Vorarlberg, Bregenz, Austria, - Vorarlberger Illwerke AG, Bregenz, Austria, - Zweckverband Bodensee-Wasserversorgung, Stuttgart, F.R.G., - Deutscher Wetterdienst, Offenbach a. Main, 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 Konstanz/Bodensee ( 1817 - 1987), Rorschach/Bodensee (1817 - 1987), (ii) Seerhein at gauging station Gottlieben/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, Argen, Schussen, Rotach, and Seefelder Aach 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 X!»j •n a cd O« a m V3 on O en »~cd< ^ •3 <*-! TJ U G m m e ar > *,,.* 1 ED cd ,HC £ o N ' o G u (I) -*-cd> •5 W5 tH ftft o c eu -»-u,* 3 ca crt Ci, to +~* c c u, in a> Xi, t-, r u p No im 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» b) Berlingen/Bodensee (Untersee) •1 '. I0/1887-9H937 - 10/1937-8/1987 j • 2 <^" ^«*ïSÏ^~«~«««~~«««~o~o<. Nï-509cm ^ means - - ^js**"*""' -• " •I 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- 500--+-'\\\ .* s -0,4 3 OJ 480- - "s>^s t X=K:; 1 III CrNDJ F M A M J ' J ' A ' S • 10/1887-9/1987 Trend: + 10/1887-9/1937 signifikanl nicht signifikant 10/1937-9/1987 MW [cm 0. NN + 3901 b) Berlingen/Bodensee (Untersee) 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.