LEACHING REQUIREMENT

(1) The amount of excess irrigation water passing through the Root Zone to reduce the salt concentration in the soil for reclamation purposes. (2) That fraction of irrigation water (Crop Water Requirement) that must be leached through the root zone to control soil salinity at a specified level. The extra water is used to dissolve the salts and move them from the soil root zone and out into a drainage tile or channel where they can be removed from the area entirely. Extra water can be added by irrigating more or by natural precipitation. The amount of water needed is governed by the amount of salt that the crop can tolerate in its root zone. As a general equation, this amount of water can be defined (U.S. Bureau of Reclamation, 1993) as a leaching requirement (LR): LR = ECiw/ECdw X 100; where: LR = leaching requirement in percent; ECiw = electrical conductivity of irrigation water in millimhos per centimeter (mmhos/cm); ECdw = electrical conductivity of soil drainage water at the bottom of the root zone (mmhos/cm). Both natural precipitation (rainfall) and conservation efforts affect the leaching requirement. Rainfall that enters the soil in sufficient quantity to create deep percolation will help move the salts down through the soil. As irrigators increase conservation efforts and use less water, there will be a point at which the deep percolation requirements for soil salt balance will not be met, soil salinity will increase, and crop production will decrease.