Are existing irrigation salinity leaching requirement guidelines overly conservative or obsolete?

Authors: Corwin, Dennis; GRATTAN, STEPHEN - University Of California, Davis

Submitted to: Journal of Irrigation and Drainage Engineering
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/31/2018
Publication Date: 6/12/2018
Citation: Corwin, D.L., Grattan, S.R. 2018. Are existing irrigation salinity leaching requirement guidelines overly conservative or obsolete? Journal of Irrigation and Drainage Engineering. 144(8):02518001. https://doi.org/10.1061/(ASCE)IR.1943-4774.0001319.
DOI: https://doi.org/10.1061/(ASCE)IR.1943-4774.0001319

Interpretive Summary: Leaching requirement (LR) is a fundamental parameter used in irrigation management to establish the amount of irrigation needed for a particular water quality to leach salinity from the root zone to maintain crop yield to no greater than a 10% yield decrement from maximum yield. Traditional means of determining LR are based on an assumption of steady-state conditions or rather conditions that remain the same over time, e.g., water content in the soil remains the same over time. Steady-state conditions rarely occur in agricultural systems; consequently, determinations of LR from the traditional approach are over estimated, resulting in the application of excessive amounts of irrigation water and increased salt loads in drainage systems. This perspective paper provides a compelling argument for the use of transient models where changes over time in soil properties influencing leaching are considered. The goal of this paper is to stimulate discussion and rethink the current methodology for determining LR. Evidence is presented that indicates crop yields can be maintained with reduction in detrimental impact to the environment and reduction in water demand. As an example, the LR determined from a transient model will reduce water demand for the Imperial Irrigation District by approximately 3.9 x 103 m3 (100,000 acre feet) annually. Soil and agronomic researchers, irrigation specialists, producers, consultants, conservation and irrigation district managers, cooperative extension, and federal and state agencies will directly benefit from the establishment of more technically and scientifically sound LR guidelines.

Technical Abstract: Water scarcity and increased frequency of drought, 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 erroneously estimate LR due to its assumption of steady-state conditions and disregard for processes such as transient conditions, precipitation-dissolution reactions, preferential flow, and rainfall. The objectives of this forum paper are (i) to evaluate the appropriateness of the traditional steady-state method for estimating LR in comparison to the transient method and (ii) to discuss the implications these findings could have on irrigation guidelines and recommendations. The goal of this paper is to stimulate discussion and rethink the current approach for determining LR. Transient solute transport models indicate that the traditional means of calculating the LR over-estimates LR. An over-estimation of the LR results in the application of excessive amounts of irrigation water and increased salt loads in drainage systems, which can detrimentally impact the environment and reduce water supplies.