Impact analysis of surface water level and discharge from the new generation altimetry observations
PD Dr.-Ing. L. Fenoglio
University of Bonn | +49 228 73-3575 |
Surface water level and river discharge are key observables of the water cycle and among the most sensible indicators that integrate long-term change within a river basin. The new generation of spaceborne altimeters includes Delay Doppler, laser and bistatic SAR altimeter techniques. The central hypothesis of B01 is that these new observations outperform conventional altimetry (CA) and in-situ measurements providing (a) surface water levels and discharge of higher accuracy and resolution (both spatial and temporal), (b) new additional parameters (river slope and width) and (c) better sampling for flood event detection and long-term evolution, providing valuable new information to the CRC.
Phase 1 addresses two research questions: „How can we fully exploit the new missions to derive water level, discharge, and hydrodynamic river processes“ and „Can we separate natural variability from human water use“?
For this, B01 monitors with space observations water height change and water exchange between rivers, lakes and reservoirs and the impact of natural and human disturbances. River discharge Q is the primary product of B01, that is made available in the CRC modelling and for assimilation in the IMS.
Figure 1. Graphical Summary of B01
Contribution to CRC
The project contributes to the CRC’s key objectives in that it addresses the surface water compartments with improved observations types to be included in the modelling. Our main contribution to the CRC is our expertise in Satellite Altimetry to derive water surface height and water surface extent from satellite techniques. Combining space techniques and in-situ data we derive river discharge and water storage change in time and we provide these two quantities to the CRC, together with their accuracy. Moreover, we prepare for assimilation of discharge data in the land model. We provide tempo-spatial observations of surface water and discharge changes, both indicators of climate change.
Main results in 2022
We have designed a processing scheme to derive water level heights at the intersection of the altimeter tracks with the river, the so called virtual gauges and to validate them with in-situ stations. We have analysed SAR altimetric data of the CryoSat-2, Sentinel-3 and Sentinel-6 missions produced by an Unfocused SAR processing. The validation against in-situ data shows a STDD smaller than 20 cm most virtual gauges (Figure 1). We have simulated SWOT data with the CNES hydrology simulator to experiment with data of the SWOT mission (Figure 2). Region of analysis is the river Rhine (ROI1).
Main results in 2023
We have extended an existing FF-SAR Processor to process all altimeter SAR mission. With dedicated processing we have increased the along-track resolution of the standard data and the quality of the data. The long computation time needed in FFSAR processing with the Back Projection algorithm has been reduced by 80% by implementing as second algorithm the omega-kappa algorithm. The radargram in FFSAR along the ROI is shown in Figure 4.
For the calibration and validation of the SWOT mission, four GNSS-R instrumentations and 4 Vortex stations have been installed in the river Rhine between Mannheim and Andernach. First evaluation of the in-situ data are on going, the comparison with SWOT data is planned in October.
We have derived discharge from space data in the ROI using simplified empirical hydrodynamic relationship.
Figure 2. Time-series of water surface height at Virtual Gauge (VG) in Mainz (northern red point in Fig. 3) and STDD of differerence with Real Gauge (RG) in Mainz.
Figure 3. Time-series of water surface height at Virtual Gauge (VG) in Mainz (northern red point in Fig. 3) simulated in CNES hydrology simulator.
Figure 4. Sentinel-3A radargramm and along-track power.