Estimating Global Evapotranspiration Using Smap Surface and Root-Zone Moisture Content

Evapotranspiration (ET) is a key link between the global carbon, water and energy cycles. ET generally occurs from soil, vegetation and intercepted precipitation. ET components are commonly estimated using a combination of variables, including meteorology, vegetation, and soil moisture conditions. Although vegetation transpiration has a major effect on global ET variations, soil evaporation can also contribute significant water loss to the atmosphere. This study utilizes satellite derived soil moisture data from the NASA SMAP mission to produce global ET estimates using a modified Penman Monteith algorithm. The global ET results were assessed using other available ET benchmarks. In addition, the ET estimates were evaluated for monitoring spring onset in relation to other complementary satellite observations of vegetation phenology and landscape freeze/thaw metrics. The comparisons between global ET products showed similar latitudinal variation, but with larger differences in tropical rainforests. The spring onset results showed landscape thawing related to rising temperature facilitating the new release of plant-available soil moisture that accompanies a dramatic seasonal rise in both vegetation photosynthesis and ET.