FO-03875 Reviewed 1991
Allowable soil water depletion limits specify the maximum amount of soil water the irrigation manager chooses to allow the crop to extract from the active rooting zone between irrigations. Only a portion of the available water capacity is easily used by the plant before crop water stress develops.
This depletion limit differs among crops and should be varied with crop growth stages. That reduces the probability of moisture stress during critical growth periods and the leaching potential when mild stress can be tolerated or there is a high probability of rainfall.
Historically, irrigations have been planned to prevent the soil water deficit from exceeding 50 percent of the total available water capacity in the rooting zone. But recent research states that the depletion limit can be varied to optimize the field's production depending on the crop, stage of growth, soil water capacity, and the irrigation system's pumping capacity. Specific recommendations for some Minnesota crops are discussed in more detail later.
Allowable depletion is usually expressed as a percentage of the total available water capacity in the rooting zone. It needs to be converted to inches of soil water for a specific crop and soil situation. To convert depletion percentage to inches of water, multiply the given depletion percentage by the total available water in the root zone. For example, if a 30 percent depletion limit is desired for a soil holding 3.50 inches of water, the depletion level in inches of soil water would be 1.05 inches (.30 x 3.50 = 1.05 inches).
This material is based upon work supported by the U.S. Department of Agriculture, Extension Service, under special project number 89-EWQI-1-9180.
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