Assimilation of high-resolution node averaged water surface elevations from the SWOT mission towards improve discharge estimates.

Abstract

Comprehensive and accurate quantification of inland surface water dynamics is vital to our understanding of terrestrial water cycle. Declining trend in availability of in-situ gauge stations elevates a need to switch to alternate measurement sources. In this context, satellite radar altimetric observations of Water Surface Elevations (WSE) offer vast possibilities, especially in poorly gauged basins.  The potential of altimetry is expected to escalate with the availability of high-resolution point cloud measurements of surface waters from the recently launched Surface Water and Ocean Topography (SWOT) mission. In the proposed research, we evaluate the potential of node averaged vector product of river WSE from SWOT to improve discharge estimation through assimilated hydrodynamic modelling over an entire river basin in India. The study uses proxy SWOT river products generated using an Observing System Simulation Experiment (OSSE), the CNES Large Scale SWOT Hydrology Simulator (Elmer et al., 2020; Nair et al., 2022) and RiverObs software. Here, we use the state-of-the-art CaMa-Flood (Catchment-based Macro-scale Floodplain Model) hydrodynamic model (Yamazaki et al., 2011) and the Local Ensemble Transform Kalman Filter (LETKF) assimilation algorithm (Hunt et al., 2007) with a physically based empirical localization approach (Revel et al., 2019). Normalized assimilation approach is adopted to handle the bias between modelled WSE and observed WSE from SWOT. The integration of SWOT altimetric observations in river modelling presents a promising avenue, considering its unprecedented spatiotemporal resolution and accuracy. The research addresses the challenges associated with the terrestrial water cycle, acknowledging the limitations of hydrodynamic modelling and uncertain space-borne observations. Results provide valuable insights into the potential of node averaged products of WSE from the SWOT mission in enhancing discharge estimation in the context of Indian river systems. The study is highly beneficial to sparsely gauged or ungauged basins, which are very common in India.