Leaching requirement: Steady-state versus transient models

Authors: Corwin, Dennis; RHOADES, J - AGR SALINITY CONSULTING; SIMUNEK, J - UC, RIVERSIDE

Submitted to: Book Chapter
Publication Type: Book / Chapter
Publication Acceptance Date: 12/31/2007
Publication Date: 12/5/2011
Citation: Corwin, D.L., Rhoades, J.D., Simunek, J. 2012. Leaching requirement: Steady-state versus transient models. In: Wallender, W.W. and Tanji, K.K. (eds.) ASCE Manual and Reports on Engineering Practice No. 71 Agricultural Salinity Assessment and Management. 2nd Edition. ASCE, Reston, VA. p. 801-824.

Interpretive Summary:

Technical Abstract: When water is extracted by plants, salts are left behind. These salts can accumulate in the lower portion of the root zone if insufficient water is applied to remove the salts by leaching. Under these low leaching conditions salts can accumulate to levels that reduce plant productivity. The concept of leaching requirement (LR) was developed as an irrigation management strategy to control soil salinity. The LR was originally defined by the U.S. Salinity Laboratory in 1954 as the minimum fraction of the irrigation water that is needed to leach salts to a level that will not significantly reduce crop yield and was quantitatively formulated as the salinity level of the irrigation water divided by the maximum permissible salinity level of the drainage water. The original LR model is based on steady-state conditions (i.e., water content and salinity do not change over time), which do not generally exist, and does not account for a variety of processes that influence LR including irrigation non-uniformity, precipitation-dissolution reactions, transient root water uptake distributions, preferential flow, climate, runoff, extraction of shallow groundwater, and leaching from effective precipitation. Even though there are numerous weaknesses in the original LR model, it is still widely used by farmers, extension specialists, irrigation district managers, and even research scientists. Information is presented that brings the use of the original LR model into question, suggesting that a new paradigm is needed. Simulations using transient models, which account for processes influencing LR, show that the use of the original LR model results in the over application of irrigation water and increased drainage volumes. The use of more sophisticated transient LR models that account for influencing processes will significantly reduce irrigation water needs and drainage water volumes. It is estimated that the valley-wide LR for Imperial Valley can be reduced from 0.13 to 0.08, resulting in an estimated reduction in drainage volume of 1.23 x 108 m3 or 100,000 ac-ft.