Explain the concept of evapotranspiration (ET) and its use in irrigation scheduling.

• A. ET is water loss from soil through evaporation and plant transpiration; used to estimate irrigation requirements
• B. ET is only soil evaporation; used to calculate soil moisture
• C. ET is plant growth rate
• D. ET is rainfall rate

Answer: (A)

Evapotranspiration is the total amount of water leaving a cropping system through evaporation from the soil surface and through plant transpiration. It reflects how much water the crop is using and losing to the atmosphere under current weather and crop conditions. In irrigation scheduling, ET guides how much water to apply by translating those losses into a planned irrigation amount. A common approach is ETc = ET0 × Kc, where ET0 is the reference evapotranspiration from weather data and Kc is a crop-specific coefficient that adjusts for the crop and its growth stage. By estimating ET for a given period and accounting for rainfall and irrigation efficiency, you determine the irrigation depth needed to replace those losses, keeping soil moisture within the desired range. This method helps avoid overwatering or underwatering by aligning irrigation with actual water demand. Evapotranspiration includes both soil evaporation and plant transpiration, not just soil evaporation; it is not a measure of plant growth rate, and it is not rainfall rate.