River Discharge And Bathymetry Estimation From Swot Altimetry Measurements

An inverse method to estimate the discharge of rivers observed by satellite altimetry is developed and assessed. The flow model relies on the Saint-Venant equations combined with a dedicated algebraic system. The resulting hierarchical flow model combined with variational data assimilation enables estimation of the key unknown flow features: the dischargeQ(x,t), an effective bathymetryb(x) and friction coefficientsK(x). Numerical results are analyzed for three rivers in two different observation contexts (corresponding to the SWOT Cal-Val orbit and to the SWOT nominal orbit) and three different scenarios of prior information availability. These investigations cover numerous realistic worldwide applications. It is shown that the space-time variations of the river dischargeQ(x,t) and the bathymetry profileb(x) can be accurately inferred. However the inferred values ofQmay be obtained at a multiplicative factor only (a bias). This bias vanishes as soon as an accurate mean value or one reference value of one of the three inferred field is provided. Once the assimilation of the satellite measurements has been done during a sufficiently long time period representing enough flow variability (one year), the low complexity algebraic flow model provides discharge estimations in real computational time from newly acquired measurements.