Assessing the Potential of the Surface Water and Ocean Topography Mission for Reservoir Monitoring in the Mekong River Basin

The planned Surface Water and Ocean Topography (SWOT) mission, scheduled to launch in 2021, is designed to significantly improve on current lake and reservoir remote monitoring capacity. These improvements are especially important for the Mekong River Basin (MRB), where rapid dam construction puts downstream food security at risk and upstream dam operations are largely unknown to downstream water managers due to lack of upstream hydrologic information. This study aims to answer: How accurately will SWOT estimate reservoir storage change in the MRB? A physically based simulation tool developed by NASA Jet Propulsion Laboratory to mimic SWOT-like observations of water bodies was applied to 20 reservoirs in the MRB and for 17 of those reservoirs, synthetic SWOT-based storage change estimates demonstrated high skill, with errors typically less than 8%. The remaining three reservoirs showed higher errors due to complex neighboring topography or long narrow reservoir shape. In comparison with current satellite observations of six reservoirs in the MRB, the synthetic SWOT observations showed between 4% and 10% lower storage change normalized root-mean-square error. Finally, the simulated elevation and surface area was successfully used to estimate the area-elevation relationships for each reservoir, with a median percent difference of 6.9%. These relationships are a potential avenue for multisensor cooperation toward improved understanding of reservoir dynamics. Such cooperation, where observations from many difference remote sensors are synthesized to provide a more complete view of human interactions on the river system, is vital for the MRB as more dams as human influence over the system increases. Plain Language Summary The Surface Water Ocean Topography (SWOT) is a satellite mission scheduled for launch in 2021. It will provide the hydrology community with simultaneous observations of width and height of water bodies that are currently not available. Such unprecedented capability is expected to be very useful in reservoir monitoring. In this study, we assess how useful SWOT data are likely to be for the Mekong River Basin that is now undergoing rapid dam development. We find that SWOT, with its wide spatial coverage, can effectively monitor most of the reservoirs of its storage change and outflow. We also find that a one-year period of observation is typically sufficient to build an effective area-elevation relationship for a reservoir which can then be useful for predicting reservoir dynamics. Overall, our study indicates that SWOT combined with other satellite sensors can make upstream and transboundary reservoir operations much more transparent for the downstream water manager.