Jiaming Chen (University of Bonn, Germany)

DETECT-REDS investigates in its second phase the impact of water use change on the water cycle in different climate zones in Europe between 2016 and 2025. We consider as sub-regions the Alps, Southern-, Central-, Northern- and Eastern-Europe. Water surface elevations from swath-, SAR nadir- and icesat altimetry cross-validated and validated against in-situ, are merged in a time-series for each lake. Seasonal and intra-annual variability of surface water storage are investigated for lakes of area larger than 0.5 km**2.

Nadir altimetry data are processed with Fully Focused SAR algorithm (FFSAR) at 80 Hz for Sentinel-3 and 140 Hz for Sentinel-6. For Swath-altimetry we choose the HR products PICX of 60 meters resolution and the Raster products. We observe that 1/8 of the water bodies observed by SWOT are seen from nadir altimetry. The sub-regions Alps and Northern-Europe are the most reach in water, with natural lakes and man-made reservoirs.

Reservoirs for hydropower generation have the highest variation in water level.
The accuracy of water heights is assessed against in-situ, bathymetry and Sentinel-1 images and found to be higher with SWOT that with nadir-altimetry. Is better than 15 cm in all water bodies and the surface area has a mean accuracy of 10%. In the Alps the water surface elevation (WSE) varies by more than 10 meters, with changes up to 100 m in hydroelectric and up to 10 m in irrigation reservoirs. Switzerland has more than 70 reservoirs for hydropower of area larger than 0.5 Km**2 and with annual minima in April. Annual minima is in November for irrigation reservoirs and annual maxima in summer for natural lakes. In hydroelectric reservoirs the seasonal amplitude of storage change is 70% higher than in irrigation reservoirs and 80% higher than in natural lakes. The water storage change (WSC) is estimated from area and height change and is combined to precipitation and evaporation and runoff to verify the water balance. Runoff and snowmelt from a land model that does not include irrigation and hydropower are compared with.
In Northern Europe the variation of water surface elevation in reservoirs is smaller than in Switzerland and varies by less than 10 meters in all the lakes.
This study highlights the relevance of the new satellite altimeter observations to study the effect of water use. Similar methodology can be applied to monitor lakes in areas where in situ measurements are not available and to improve water management.

This study is partly funded by the German Research Foundation (DFG) as sub-project B01 of CRC 1502 (https://sfb1502.de).