Estimating evapotranspiration and water use in Columbia Basin

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Estimates of groundwater pumpage and surface water diversions for irrigation and recharge were made for the period of interest (1985-2007) using a spatially distributed soil water balance model. The model uses simple relations among weather, soils, land cover, and irrigation data, to estimate monthly irrigation requirements and surplus moisture available for recharge. The model also uses landscape actual evapotranspiration (ETa), as an indicator of vegetation biomass accumulation which is directly associated with water use. ETa was calculated using the Simplified Surface Energy Balance Method (SSEB), which assumes that latent heat flux, a surrogate for actual evapotranspiration, varies linearly with near-surface temperature differences. One kilometer land-surface temperature data from the Moderate Resolution Imaging Spectroradiometer (MODIS) sensor were used to identify “cold pixels” in heavily irrigated areas and “hot pixels” in dry barren or fallow areas where ETa is near zero.