Monitoring water level and lake extent change with nadir-altimeters and SWOT

Water level and lake extent are estimated from combined in-situ, Sentinel-3 and Sentinel-6 nadir altimeters and SWOT-altimetry. Accuracy and precision of the various techniques are compared, the goal is the validation of the new SWOT data in the river Rhine and in Swiss lakes.

 

The main challenge is the interference among multiple water surfaces which contaminate the signal. Fully-focus and Unfocused SAR and individual echoes processed data have a different sensitivity to the signal coming from non-nadir targets. For nadir-altimeters the accuracy and precision of water level depend on the frequency selected for the low level processing. The accuracy ranges from 10-30 cm and depends on the location. The precision is of few centimeters at 80-140 Hz and decreases with increasing frequency selected in low level processing.

 

SWOT derived parameters are validated against the nadir derived equivalent. A more accurate river slope parameter is expected from the SWOT high spatial resolution data. Water extent is another new parameter from SWOT, which is used to derive river discharge and water storage change. In rivers, Sentinel-3A pass 156, that is parallel to the river centerline for about 30 km, is a test area for a direct comparison of water height, slope and discharge parameters from nadir-altimeters and SWOT.

 

In lakes, SWOT water level and water extent are validated against in-situ lake bathymetry, water area extent from Sentinel-1 and Sentinel-2 satellite imagery and water level from nadir-altimeters. In its 21-day phase, SWOT is used to monitor storage change of lakes and reservoirs.