River Basin Water Balance Estimates of Evapotranspiration Using GRACE and Other Observations

Evapotranspiration links the global cycles of water, energy, and carbon, thus it is integral to Earth system science. However, it is difficult to estimate on regional, climatic scales. One approach is to use a water budget equation, i.e., total precipitation minus the sum of evapotranspiration and net runoff equals the change in terrestrial water storage. Gravity Recovery and Climate Experiment (GRACE) satellite observations of Earth's gravity field are enabling closure of this equation by providing the terrestrial water storage change term, which has been even more elusive than evapotranspiration until now. Here we describe the method for estimating evapotranspiration using data from GRACE along with observation based precipitation and runoff, which takes into account the unique nature of the GRACE observations. GRACE water storage changes are first substantiated by comparison with results from a land surface model and a combined atmospheric-terrestrial water budget approach. Evapotranspiration is then estimated over the Mississippi River basin and compared with output from the land surface model and two operational atmospheric modeling systems. Results suggest that the new technique provides skill in evaluating modeled evapotranspiration, particularly in terms of bias.