LANDSCAPE-SCALE HYDROLOGIC BUDGET USING REMOTELY-SENSED EVAPOTRANSPIRATION IN THE COLUMBIA PLATEAU, WASHINGTON, OREGON, AND IDAHO

As part of a U.S. Geological Survey Groundwater Resources Program study, a landscape-scale hydrologic model has been constructed to estimate irrigation water demand, and groundwater withdrawals and groundwater recharge within the 160,000 km² Columbia Plateau regional aquifer system in Washington, Oregon, and Idaho. Monthly evapotranspiration (ET) is estimated using remotely sensed temperature and model-assimilated weather fields with a simplified surface energy balance (SSEB) method. The SSEB method uses 1-km thermal data available since 1979 from the AVHRR and MODIS sensors, to reconstruct a historical time series of ET at landscape scales. The resulting ET flux is specified in a distributed SOil WATer balance model (SOWAT) that accounts for precipitation, soil moisture storage, and irrigation efficiency to estimate monthly irrigation water demand and groundwater flux (recharge and discharge). The SOWAT model computes soil water deficit or surplus using precipitation data from PRISM (Parameter-elevation Regressions on Independent Slopes Model), ET from SSEB, and available soil moisture storage. The only additional data requirements are: land cover type (irrigated, native vegetation, or developed/open water), available soil water capacity from the STATSGO (State Soil Geographic) database, runoff fraction, minimum soil moisture for irrigated land, and irrigation efficiency. The coupled energy and soil water balance methods are being used to estimate hydrologic fluxes between the groundwater system, vadose zone, and atmosphere, at spatial and temporal scales appropriate for evaluation of the effects of anthropogenic and climate driven changes on water resources in the Columbia Plateau.