Irrigation management needs to be geared to leave room in the active root zone for precipitation. The philosophy that irrigation is supplemental to rainfall is key in reducing pollution potential. During the crop growing season, crop water needs must be balanced with soil water availability to minimize both leaching potential and crop water stress. With irrigation to supplement rainfall, water can be managed to achieve this goal. However, striking this balance is easier said than done.
Crops respond to many factors as they determine their daily water requirements.
Weather factors, soil water availability, and growth stage are important determinants of water use. Crop water use for well watered crops can be predicted from weather factors and these predictions are often reported in irrigated areas. However, each field or parts of a field have different amounts of stored soil water. The amount of soil water in storage affects not only the crop water use but also how soon the crop may go under stress and how much room there is to store subsequent irrigation or rainfall events. Knowledge of the amount of plant available soil water on any day during the growing season is key in keeping the crop from going into stress and minimizing the leaching from over irrigation that can cause pollutants to migrate into ground water.