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The accumulation of excess soluble salts in the root zone of arid and semiarid irrigated soils is a widespread problem that seriously affects crop productivity throughout the world. Squires and Glenn (2009) estimated the global extent of saline soils to be 412 Mha. The estimate of Szabolcs (1989) is more conservative at 352 Mha. Of the estimated 230 Mha of irrigated land worldwide, 20 to 50% may be salt affected (Szabolcs 1992; Ghassemi et al. 1995; Flowers 1999). Ghassemi et al. (1995) estimated that salinization of irrigated soils causes an annual global income loss of $12 billion. Recent estimates of income loss due to salinity within California alone are $3.7 billion for 2014 (Welle and Mauter 2017). All irrigation waters contain salts and, except for nutrients and some specific elements, crop roots take up nearly pure water for transpiration, causing most of the remaining salts to concentrate in the root zone. Therefore, the predominant mechanism causing the accumulation of salt in irrigated agricultural soils is evapotranspiration. Periodic leaching by water to move excessive salts downward below the root zone is required to avoid reduced crop yields. Thus, a combination of irrigation and rainfall is required to meet surface evaporation, crop transpiration, and salt leaching needs. Application of water greater than the amount required for leaching salts and meeting evapotranspiration needs is not desirable because water is used inefficiently and nutrients and pesticides are also being leached. The fraction of the amount of applied irrigation and precipitation that drains beyond the root zone is defined as the leaching fraction (LF). Under steady-state conditions the leaching fraction is approximated by ECiw=ECdw, where ECiw is the electrical conductivity of the irrigation water (dS m−1) and ECdw is the electrical conductivity of the drainage water (dS m−1); consequently, the LF is an indication of the degree to which salts are leached from the root zone (US Salinity Laboratory Staff 1954). A leaching fraction equal to 1 indicates a uniform soil salinity profile through the root zonewhere sufficient leaching has occurred so that the drainage water has the same salinity level as the irrigation water. Leaching requirement (LR) is defined as the minimum LF that is required over a growing season for a particular quality of water to maintain crop yield at or near maximum. However, often a yield decrement of no greater than 10% frommaximum yield is regarded as permissible due to the spatial variation that crop yield exhibits. Leaching requirement is a specific quantitative value that can be determined and is not simply an abbreviated way of stating that leaching is required. Clearly, an accurate and reliable method of calculating the LR is important for the efficient utilization of irrigation water. An underestimate would result in salt accumulation in the root zone and yield reduction. An overestimate would result in excessive water utilization and nutrient removal producing detrimental environmental impacts on groundwater or degraded drainage waters. For instance, the importance of having correct information on the LR is critical in California where salinity (and Se or other harmful solutes) in agricultural drainage waters in the western San Joaquin Valley results in drainage water disposal problems. The amount of drainage water produced relates directly to the LF: as the LF increases, the drainage volume increases. Irrigation practices using a low LF would be a positive approach to mitigate partially the impact of salinity on drainage waters. At a very low LF, the precipitation of salts occurs below the root zone, which lowers the total amount of salts in the drainage water. A low LF also lowers total drainage volumes. However, this approach could salinize soils to levels that would reduce crop yields if the proposed LF was too low. 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 methods or guidelines for the calculation of the crop-specific leaching requirement of irrigated soils have fallen under the microscope of scrutiny and criticism because they are believed to erroneously estimate LR due to the assumption of steady-state conditions and disregard for processes such as transient conditions, precipitationdissolution reactions, preferential flow, and rainfall. The goal of this paper is to stimulate discussion and rethink the current approach for determining LR. To achieve this goal, there are two objectives: (1) to evaluate the appropriateness of the traditional steady-state method for estimating LR in comparison to the transient method; and (2) to discuss the implications that these findings could have on irrigation guidelines and recommendations.

Are Existing Irrigation Salinity Leaching Requirement Guidelines Overly Conservative or Obsolete?