Quantifying the level of improvement in discharge estimation from the SRTM-era to the proposed Surface Water Ocean Topography (SWOT)-mission era

Abstract:

It was almost 15 years ago, when the Shuttle Radar Topography Mission (SRTM) flew for a few days to map the elevation of earth’s surface. SRTM has since become the community standard for a global digital elevation model (DEM) and has triggered numerous studies that require elevation information. One particular avenue that has benefited the hydrologic community is the space-borne discharge estimation using water slope information that is afforded by the spatial imaging concept of SRTM. Numerous feasibility studies involving SRTM data for discharge estimation in rivers have led to adopting a similar concept for the proposed Surface Water and Ocean Topography (SWOT) mission (launch date: 2020). Because SWOT is expected to have significantly higher accuracy and smaller spatial scale in resolving the elevation characteristics of a water surface, it is important to understand the extent of improvement that SWOT will afford for discharge estimation once it is launched. In this study, we explored geophysical sources of uncertainty of satellite interferometric-based discharge estimation in Bangladesh delta of the Ganges, Brahmaputra and Meghna (GBM) river basins. This exploration was carried out for two scenarios: A) using SRTM elevation data and B) using SWOT-simulated elevation data. We contextualized the improvement in accuracy as a function of river’s geophysical characteristics (river width, reach averaging length, bed/water slope) and also to explored a pragmatic approach to further uncertainty reduction using water level climatology. The discharge was estimated according to the slope-area (Manning’s) method using elevation data assuming availability of in-situ river bathymetry (in order to remove uncertainty due to river cross section data). A high resolution hydrodynamic model was accurately calibrated (against in-situ water level data) to simulate water level and flow dynamics along the entire river reaches of the GBM river network and served as reference for comparison with satellite-based estimates. For simulation of space-borne water elevations expected during the SWOT-era, we used a comprehensive simulator that can mimic all the likely measurement issues related to SWOT’s planned orbit, inclination and sensor characteristics.