Partitioning Evapotranspiration Over the Continental United States Using Weather Station Data

Accurately characterizing evapotranspiration is critical when predicting the response of the hydrologic cycle to climate change. Although Earth system models estimate similar magnitudes of global evapotranspiration, the magnitude of each contributing source varies considerably between models due to the lack of evapotranspiration partitioning data. Here we develop an observation-based method to partition evapotranspiration into soil evaporation and transpiration using meteorological data and satellite soil moisture retrievals. We apply the methodology at 1,614 weather stations across the continental United States during the summers of 2015 and 2016. We evaluate the method using vegetation indices inferred from satellites, finding strong spatial correlations between modeled transpiration and solar-induced fluorescence (r(2) = 0.87), and modeled vegetation fraction and leaf area index (r(2) = 0.70). Since the sensitivity of evapotranspiration to environmental factors depends on the contribution of each source component, understanding the partitioning of evapotranspiration is increasingly important with climate change. Plain Language Summary Water moves from the land surface to the overlying atmosphere by evaporation. The two main sources of evaporation include (1) evaporation from soils and (2) evaporation from pores on plants, called transpiration. Although methods exist to measure total evaporation over an ecosystem, it is challenging to observe soil evaporation and transpiration separately over an ecosystem. Consequently, the amount of estimated soil evaporation and transpiration varies considerably across models. In this study, we develop an observation-based method to estimate the fraction of water moved from the land to the atmosphere by plants, or the fraction of total evaporation that comes from transpiration. The method primarily relies on weather station data and soil moisture estimates from a recently launched satellite. We apply the method across the continental United States during the summers of 2015 and 2016 and evaluate it using observations of plants inferred from other satellites. Looking toward the future, it is important to estimate transpiration and soil evaporation correctly because they respond differently to changes in climate.