Science Requirements for Hydrologic Storage Change from the SWOT Mission

The Surface Water Ocean Topography (SWOT) satellite mission has been selected by the NRC Decadal Survey for launch between 2014 and 2016. NASA and CNES have jointly endorsed SWOT and provided encouragement via the formation of a Science Working Group. It is critical to study the Level 1 SWOT hydrologic science requirements, which drive the mission design. Hydrologic requirements include estimates of surface water discharge and changes in storage. This paper focuses on describing current results of simulation studies aimed at quantifying specific SWOT science requirements on global hydrologic storage changes. The objectives of the study include the optimal spatial and temporal sampling of storage changes and the related height accuracies and radar pixel sizes for the SWOT instrument. (1) Storage changes in the Amazon and Siberian Arctic have been estimated from existing satellite measurements, in-situ data, and model outputs. (2) The changes in water surface elevations and areas are calculated by dividing the storage changes from Task 1 by classifications indicating water body locations (i.e., water masks). (3) The desired level of storage change accuracy needed for hydrologic science descriptions of the Level 1 requirements have been incorporated. (4) Orbital tracks with differing spatial and temporal samplings have been studied using the various storage change maps (generated from Task 1) to determine percentages of the total that are or are not measured. We report the results of weekly, monthly, and seasonal variations in water surface elevations and areas (from Task 2) to determine the required SWOT instrument accuracies.