Evapotranspiration, the movement of water from land and plants into the air, is a key part of the water cycle. Accurately measuring it is important for understanding drought, water supply, and how land and climate interact. In this study, we compared two widely used methods for estimating evapotranspiration at over 90 research sites across North America. One method (Penman–Monteith) uses detailed climate and surface data, while the other (Priestley–Taylor JPL) is a simpler approach. Both methods were tested and fine-tuned against real-world measurements from AmeriFlux towers, which directly observe water and energy exchange between land and atmosphere. We found that the Penman–Monteith method generally performed better, especially in drier climates, while the Priestley–Taylor approach showed reduced accuracy as aridity increased. This work helps improve our ability to track water use and availability in different environments, supporting better planning for agriculture, ecosystems, and water resources under changing climate conditions.