Mapping Evapotranspiration and Recharge from Near Surface State Observations and Quantifying Their Relationship with the Atmospheric and Environmental Variables

Evapotranspiration and recharge fluxes are key components of the global water-energy-carbon cycles, thus have major impact on agriculture, water supply, climate etc. These fluxes are interconnected and depend on the soil moisture content. Sequence of land surface state observations of moisture (SM) and temperature (LST) contain implicit information that can be used to estimate evapotranspiration and recharge fluxes. A Variational data assimilation framework is implemented in large areas of the Southern great plain (SGP) and Oklahoma Panhandle region to estimate the optimal parameters (evaporative fraction, bulk heat transfer coefficient, Brooks-Corey parameter) of evapotranspiration and recharge fluxes by assimilating Geostationary Operational Environmental Satellite (GEOS) land surface temperature and Soil Moisture Active Passive (SMAP) surface soil moisture observations into a coupled water and energy balance model. Comparison with in-situ observations from the USCRN and the Mesonet sites showed data assimilation has improved the accuracy of simulated soil moisture at both near surface and root zone compared to the open loop estimates and the simulated soil moisture profiles were able to replicate the temporal dynamics of in-situ observations with sufficient accuracy. The evapotranspiration estimates showed good agreement with the in-situ data from Atmospheric Radiation Measurement (ARM) sites at different locations and the estimated annual recharge flux values are within the range as suggested in the literature. In the end, Genetic Expression Programming (GEP) is used to find the explicit equations between the evapotranspiration and recharge fluxes based and their major atmospheric and land/environmental drivers. These easy-to-use equations will aid the water resource managers and decision makers to better monitor these fluxes and predict the impact of climate change and land use change on these fluxes.