River discharge estimation from satellite observations. Application in the Congo river basin

Spatial altimetry allows to complete in-situ hydrometric data through the establishment of "virtual stations", at the crossing of the satellite ground track with a watercourse. Elevation measurements of water bodies and rivers are available on the Hydroweb-NG website (http://hydroweb.theia-land.fr). The SWOT satellite, scheduled for launch mid november 2022 by the French Centre National d'Etudes Spatiales (CNES) and the U.S. National Aeronautics and Space Administration (NASA), should further improve accuracy, thanks to innovative technology. Other types of multi-sensor space data are also useful in hydrology. A project to support the International Commission of the Congo-Ubangi-Sangha river basin (CICOS) developed since 2016 with funding from the French Development Agency (AFD) has promoted space hydrology through a group of French institutions supporting CICOS. Various activities have been developed including the development of a spatial database, comparison with in-situ data and the development of an operational Hydrological Information System within CICOS, integrating both spatial and in-situ data.
In the framework of the Space Hydrology Group and the CICOS support project, an innovative methodology has been developed to estimate flows from currently available satellite data (Envisat, Jason, Sentinel, etc.), transforming altitudes into flows at virtual stations. These satellite data can be complemented by global databases (width databases with GWD-LR or Sword, mean flow databases with WBM, or Digital Terrain Model databases with SRTM Mission), as well as in-situ data on the studied area (2 hydrological stations in Kinshasa East on the Congo and Bangui on the Ubangi river). This methodology, tested on these 2 rivers, allowed the generation of a hydraulic model of the Saint-Venant 1D type, allowing the generation of Q(Z) calibration curves at any point of the river. Powered by satellite altimetry data, these calibration curves provide flow rates. A web-based interface has been developed providing this information in real-time. The comparision of the bathymetry obtained with this method with ADCP measurements, the analysis of the rating curve at two in-situ stations, and the time delay of the hydraulic model, proved to be very satisfactory, having into consideration all the hypothesis made in this methodology.