The Leaching Requirement During Irrigation
The leaching of soluble salts from the plant rooting zone is a pivotal concern when irrigating cropland. Irrigation water is used to maintain crop productivity, so drought conditions need not occur to induce irrigation measures. Irrigation simply provides supplemental precipitation that may not be achieved through natural processes, i.e. rainfall.
Basically, leaching is described as passing additional water through a medium to remove unwanted materials. This is usually achieved through two types of ponding. Additional water is pumped onto the soil surface and allowed to accumulate until surface ponding occurs. There are two mechanisms that perform this duty, continuous or intermittent ponding. Intermittent ponding, applying the excess water in intervals, is more favorable for milder climates where evaporation rates are low (NATO, 1994). Continuous ponding, applying all the water at once, may not be appropriate given geographical, climatic, or user-related constraints.
Salts accumulate in the soil profile over time, therefore, leaching may serve as a form of soil “reclamation”. Normally, leaching curves are developed to determine the amount of water that may be actually required to reduce the initial soil salinity by a certain percentage (NATO, 1994). Although rainfall and the present soil already have saline concentrations, additional salts are added to the soil via irrigation water. Moisture is then extracted by the processes of evaporation and transpiration, and the salts begin to precipitate. Now, the salt balance of the soil profile changes: excessive salt concentrations are introduced without having adequate outlets. If located it the plant’s root zone, this unbalanced situation can be extremely detrimental to crop vitality.
The leaching requirement may be defined as the fraction of the irrigation water that must be leached through the root zone to control soil salinity at any specified level (United States Salinity Lab, 1954). Calculating the leaching requirement for a land area encompasses many dynamic processes. During salt balance techniques, these changing situations are simplified by assuming several conditions: 1) the irrigation water resembles steady-state flow characteristics over a long period of time, 2) water’s electrical conductivity is a good estimate of dissolved salt concentration, 3) the water and salt interaction are limited to the specific crop’s root zone. The water balance equation adopted by Gupta (1979) proposes the following relationship in its simplest form:
Z (precipitation) + Di (irrigation water) = E (evapotranspiration) + Dd (drainage) +/_ (moisture changes) 
*Note: keep equal units throughout the equation
Using the above equation, and following the stated assumptions, an associated salt balance equation can be...