Assimilation of SWOT discharge versus water level into CTRIP-12D over the Congo Basin

Land Surface Models are key tools to study the continental water cycle and can be used to better understand the main hydrological processes and their sensitivity to climate change. Yet, they are subject to potentially large errors, especially over ungauged basins where they cannot be calibrated or validated. The Surface Water and Ocean Topography (SWOT) satellite mission will provide unprecedented measurements of water elevation for all rivers wider than 100 m worldwide. Many recent studies focused on the assimilation of such observations into global hydrologic models, including ISBA-CTRIP developed at Météo-France, and they have demonstrated its added value. The SWOT mission will also provide discharge estimations derived from observed water elevation, river width and slope. The algorithms require ancillary data, such as the roughness coefficient, which needs to be estimated empirically at the global scale, potentially resulting in large errors in the discharge estimation. Yet, it is still unclear whether assimilating discharge instead of water level (or water level anomalies) would lead to better performances in terms of simulated discharge along the river network. In this study, we extended the assimilation of water elevation to river discharge into the CTRIP river routing model. We used the new version of the model at a 1/12 degree spatial resolution, which is more compatible to the resolution of the SWOT discharge product (reach length of about 10 km). The Congo river basin is chosen as a test case. SWOT-like river elevations and discharges are constructed by adding realistic errors to elevations and discharge provided by an independent river routing model, MGB. Also, a realistic satellite orbit is used to provide times and locations of available SWOT observations. The impact of observation errors on the assimilation is analysed, as well as the propagation of discharge corrections through the river network. Finally, model performances using discharge assimilation are compared to those using water level assimilation.