What is the purpose of ET-based irrigation scheduling, and what is a practical field method to estimate daily evapotranspiration?

• A. To water on a fixed calendar schedule that ignores weather data
• B. To apply water only after severe wilting is visible
• C. To irrigate only when the irrigation system is manually turned on regardless of weather
• D. To match irrigation to plant water use and avoid over/under-watering; practical method: use weather data to estimate ET0 or a Class A evaporation pan to estimate ET

Answer: (D)

ET-based irrigation scheduling centers on delivering water when the crop actually needs it, rather than on a fixed program. Evapotranspiration represents the amount of water the crop and soil surface lose to the atmosphere each day, so estimating ET tells you how much water the root zone should receive to meet plant demand. A practical field method is to use daily weather data to estimate ET0 (reference evapotranspiration) with a standard approach like FAO Penman–Monteith, then adjust by a crop coefficient for the specific crop and growth stage. Another commonly used field method is to measure daily evaporation from a Class A pan and apply a pan-to-ET correction to approximate the crop’s daily water use. By basing irrigation on the estimated ET (and allowing for a controlled soil water depletion), you irrigate to match plant water use and avoid both over- and under-watering. This contrasts with fixed calendars, waiting for severe wilting, or turning the system on manually without regard to weather.