What is Evapotranspiration and Why is FAO-56 Important?
Evapotranspiration (ET) combines evaporation from soil and water surfaces with plant transpiration into the atmosphere. It is the primary hydrological variable for agricultural irrigation, reservoir management, and climate modeling.
The universally recognized FAO-56 Penman-Monteith equation computes reference ET (ET₀) by integrating air temperature, relative humidity, wind speed, and solar radiation.
Practical meaning: An ET₀ value under 2 mm/day indicates low water loss (light irrigation); 3–5 mm/day reflects moderate seasonal demand; values above 6 mm/day indicate intense moisture loss requiring aggressive watering cycles to protect crops.
Across the American Southwest (such as California's Central Valley and Arizona), daily summer ET₀ rates frequently reach 8–10 mm/day. This tool provides land managers and growers with science-based moisture estimates.