This depends on rainfall and on soil type, as well as stage of corn growth. One simple way to decide on how much water to use is called the “checkbook method,” in which water use by the crop is estimated and rainfall is subtracted, giving the amount needed from irrigation. Daily water use is estimated based on “potential evapotranspiration (PET)” numbers—calculated from temperature, humidity, sunlight, and wind speed—times a “crop coefficient,” which is the percentage of PET that the crop uses on a given day. The crop coefficient is based on crop growth stage. It is low when the crop plants are small (much of the water loss early in the season is evaporation from the soil since the leaf area is so small). It rises to 0.8 to 0.9 when the crop has a full, active canopy, due to loss of water as water vapor through tiny holes in the leaf surface. Water loss from crop plant leaves can total up to one-third of an inch per day when the temperature is 100 degrees or more, the humidity is low, the wind is blowing, and when the crop can take up enough water to supply the leaves fast enough.

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