Submitted to: Agricultural Water Management
Publication Type: Peer reviewed journal
Publication Acceptance Date: 2/15/2007
Publication Date: 3/19/2007
Citation: Corwin, D.L., Rhoades, J.D., Simunek, J. 2007. Leaching requirements for soil salinity control: steady-state versus transient models. Agricultural Water Management. Vol 90: 165-180 Interpretive Summary: The increased occurrence of drought conditions in the arid and semi-arid southwestern USA creates pressure to use frugally water resources. Irrigated agriculture is the primary user of water in the Southwest. To prevent the deleterious buildup of salts in the root zone of crops, additional water beyond that needed by the plant is required to leach salts from the soil profile. The leaching requirement (LR) is a concept that establishes the minimum amount of water that is needed to remove salts from the root zone and yet maintain crop productivity at a high level. In the past, the means of estimating the LR was based on a set of conditions, referred to as steady-state conditions, which rarely actually exist in the real world. The real world is more dynamic and transient-state conditions predominate. The objective of this study was to evaluate various steady-state and transient-state models for estimating LR using California’s Imperial Valley as a test site. Simulations from the models show that the traditional method of estimating LR produces LR values that are higher than necessary, resulting in the over irrigation of crops and the loss of large volumes of water. The estimates from two models (WATSUIT and UNSATCHEM) suggest that the LR for the entire Imperial Valley can be reduced significantly (from 0.13 to 0.089) with no buildup in root zone soil salinity and with a considerable savings in water. The reason that the traditional model resulted in an over irrigation is due to the fact that it does not account for salt precipitation.
Technical Abstract: Water scarcity and increased frequency of drought conditions, resulting from erratic weather attributable to climatic change or alterations in historical weather patterns, have caused greater scrutiny of irrigated agriculture’s demand on water resources. The traditional guidelines for the calculation of the crop-specific leaching requirement (LR) of irrigated soils have fallen under the microscope of scrutiny and criticism because the commonly used traditional method is believed to over-estimate LR due to its assumption of steady-state flow and disregard for salt precipitation and preferential flow. Over-estimation of the LR results in the application of excessive amounts of irrigation water and increased salt loads in drainage systems, which detrimentally impact the environment and reduce water supplies. The objectives of this study are (i) to evaluate the appropriateness of traditional steady-state methods of estimating LR in comparison to transient-state methods, (ii) to recommend the most suitable practical method, and (iii) to discuss the implications of the findings with respect to California’s Imperial Valley. Steady-state models for calculating LR including the traditional model of the U.S. Salinity Laboratory, WATSUIT model, and water-production-function model were compared to transient-state models including TETrans and UNSATCHEM. The calculated LR was lower when determined using a transient-state approach than when using a steady-state approach. Transient-state conditions and the influence of preferential flow did not have as significant an effect on lowering the LR as salt precipitation for a representative study of the Imperial Valley using Colorado River water (EC=1.23 dS/m) for irrigation. A valley-wide LR of 0.08 for a crop rotation of alfalfa/alfalfa/alfalfa/alfalfa/wheat/lettuce, as calculated by both WATSUIT and UNSATCHEM, was concluded to be the most reasonable estimate for the entire Imperial Valley as compared to a LR of 0.13 by the commonly used traditional method. The reduced LR for the Imperial Valley would result in tremendous water savings and diminished salt loads.