Glossary
Penman-Monteith Equation
A physically based equation that combines an energy balance with an aerodynamic (vapor-transfer) term to estimate evapotranspiration. The FAO-56 form (Allen et al. 1998) is the international reference standard for reference-crop ET.
Penman-Monteith needs several inputs: net radiation, air temperature, humidity, and wind speed. Because the FAO-56 form is built for a reference crop, it must be adapted for open-water bodies, where it would otherwise over-predict loss. For moderate conditions, common methods typically agree within roughly 10–20%, with about ±10–20% uncertainty versus observed values.