Global soil moisture dry-down analysis based on SMAP retrievals

The rate at which soil moisture depletes following a precipitation event (the soil moisture dry-down) encodes information on several hydrological parameters, most prominently vertical drainage and evapotranspiration. The initial phase of the dry-down process is usually controlled by vertical drainage and soil hydraulic properties. The second phase is usually controlled by the evapotranspiration rate. Better insight into dry-down processes at global scales will enhance our understanding of soil moisture's role in the terrestrial water cycle. The launch of SMAP gives us unprecedented opportunities for studying dry-down processes globally. In this study, an algorithm for dry-down event detection was developed. Dry-down events are identified globally using one year of SMAP data from April 2015 to March 2016. Three sites spanning different climate zones, including Texas in the USA, southern China, and central Australia, were selected for testing and validation of the algorithm. Results in these sites show that the algorithm can efficiently identify dry-down events. The algorithm was then applied globally. The number of dry-down events, average dry-down duration, and average dry-down slope were estimated based on one year of SMAP data at each grid point. We relate the rate of the dry-down at each location to the time-history of precipitation, seasonal climate and the relative dominance of drainage and evaporation loss processes. The rate of dry-down under difference hydrologic and hydroclimatic regimes is proposed to be a rigorous test of how well land surface models capture and incorporate physical processes and land memory.