Accurately estimating consumptive water use in the Colorado River Basin (CRB) is important for assessing and managing limited water resources in the basin. Increasing water demand from various sectors may threaten long-term sustainability of the water supply in the arid southwestern United States. We have developed a first-ever basin-wide actual evapotranspiration (ET_a) map of the CRB at the Landsat scale for water use assessment at the field level. We used the operational Simplified Surface Energy Balance (SSEBop) model for estimating ET_a using 328 cloud-free Landsat images acquired during 2010. Our results show that cropland had the highest ET_a among all land cover classes except for water. Validation using eddy covariance measured ET_a showed that the SSEBop model nicely captured the variability in annual ET_a with an overall R² of 0.78 and a mean bias error of about 10%. Comparison with water balance-based ETa showed good agreement (R² = 0.85) at the sub-basin level. Though there was good correlation (R² = 0.79) between Moderate Resolution Imaging Spectroradiometer (MODIS)-based ETa (1 km spatial resolution) and Landsat-based ET_a (30 m spatial resolution), the spatial distribution of MODIS-based ET_a was not suitable for water use assessment at the field level. In contrast, Landsat-based ET_a has good potential to be used at the field level for water management. With further validation using multiple years and sites, our methodology can be applied for regular production of ET_a maps of larger areas such as the conterminous United States.