Evapotranspiration (ET) – the process by which water is returned to the atmosphere from evaporation and plant transpiration – is the largest outgoing water flux in the hydrologic cycle. In arid and semi-arid regions, ET can account for nearly all the incoming precipitation and is thus critical for land and water managers to quantify accurately. In this study, we assess the accuracy of NASA’s ECOSTRESS satellite in southern Arizona by comparing it to ground-based measurements from six scientific monitoring stations. Results indicate that ECOSTRESS performs best when ET estimates are aggregated by season or by year, rather than daily estimates. We also found that the OpenET product is more closely aligned with field data than ECOSTRESS. To improve ET estimates, we developed a local correction using surface characteristics derived from Sentinel-2A satellite imagery. These local adjustments significantly improved ET estimates, highlighting the need for site specific, higher resolution data inputs for measuring ET in arid and semi-arid regions. In regions where ET dominates the water balance, accurate ET estimates enable land and water managers to make better informed decisions about water use and conservation.