Farmers and water managers in California face the challenge of making irrigation decisions in a state with limited water. This study looks at how to better estimate two key indicators of crop water use: reference evapotranspiration (ETo), which shows how much water crops need under ideal conditions, and actual evapotranspiration (ET), which reflects how much water crops are actually using in the field.

We tested four methods for estimating ETo against field measurements from well-irrigated alfalfa sites. One method called the Priestley-Taylor method, stood out for its consistent performance across different climates and when using weather forecast data. Other methods, including the widely used Penman-Monteith approach, tended to overestimate water needs in drier areas, especially when forecasts were used.

We also worked to improve a satellite-based model (GEESEBAL) used to estimate actual ET. By adjusting how the model calculates energy components like radiation and heat, we were able to bring estimates closer to field measurements. Using data from multiple satellites helped improve daily estimates by filling in gaps between satellite overpasses.

These results can help improve irrigation planning and support more efficient water use in California’s agriculture.