Monitoring surface water storage change in lake and reservoirs

Switzerland is today rich in water, and retreating glaciers give way to new landscapes with lakes as an important element.

As part of the Collaborative Research Center (SFB 1502) funded by the German Research Foundation (DFG), a project is being carried out to analyze surface water storage change and river discharge using data from the latest generation of satellite altimetry. The goal is to monitor the impact of land use change on the water cycle, here on the exchange of water between rivers, lakes and reservoirs.

We distinguish two groups of lakes: natural lakes and reservoirs. The first group includes both ancient large lakes of small variations related to long-term changes in temperature and small lakes formed in the deglaciated area rapidly changing and related to glacier melting. The second group includes reservoirs with large water variations related to resource management, like hydropower and irrigation. We generate a lake inventory for modern times and trace them in the nadir-altimetry and wide-swath altimetry to monitor seasonal and intra-annual variability of surface between 2016 and 2024. 

Fully Focused SAR nadir-altimeter processed data at 80 Hz, with along-track spacing of 85 meters are chosen together with SWOT swath-altimeter HR products. Lakes with area larger than 0.5 km**2 are used. Only ten of the more than eighty water bodies observed by SWOT in the region are detected by nadir-altimetry, showing that swath-altimetry is best suited for this application. Space-derived height and area time-series evaluated against in-situ, bathymetrie and Sentinel-1 images have higher accuracy in the natural Murnersee (1 cm bias and 3 cm standard deviation) than in reservoir Lake de Joux (31 cm bias  and 13 cm stdd). The surface area has mean accuracy of 10%,  highest change found is 100 m in hydroelectric reservoirs and 10 m in irrigation reservoirs. Most reservoirs are operated in a network. 

We look at 70 water bodies with variations larger than 10 m, assuming that larger variations are related to water management. Annual minima are in May for hydroelectric and in November for irrigation reservoirs, while in natural lakes the annual maximum is in Summer. The amplitude of storage change in hydroelectric reservoirs is 70% higher than in irrigation reservoirs and is 80% higher than in natural lakes.  The water budget in catchments is analysed comparing to land runoff and snowmelt from CLM model which is not including irrigation and hydropower.

This study hightlights the importance of the new satellite altimeter observations to study climate change, land and water use.