Leaching Fraction, Soil Salinity, and Drainage Efficiency

Leaching fraction, soil salinity, and drainage efficiency M. E. Grismer Measuring the efficiency of a sub- apply irrigation water in excess of crop wa- salt balance seldom occurs in the field, but it surface irrigation drainage system ter requirements. Some install subsurface helps us define LF and illustrates some basic drainage systems that collect and remove processesinvolved in rootzoneleaching. For is complicated by a number of fac- part of the excess water once it has leached the salt balance approach to succeed, the tors, including the irrigation the rootzone, preventing already shallow salts applied in irrigation water must be water's salinity, the soil's inherent water tables from rising any nearer to the removed from the rootzone by various salt salinity, and the degree to which soil surface. "sinks" (e.g., chemical precipitation, collec- saline drainwater migrates laterally To maintain favorable rootzone salinity, tion of excess irrigation by a subsurface growers depend on a combination of pro- drainage system, and deep percolation of in a shallow water table. cesses, including rootzone leaching and rootzone drainage). chemical precipitation. Leaching involves The balance of salts is often determined Salinity in the soil rootzone is a major con- applyingenough excess water to translocate on the basis of the salt load of the applied cern for farmers of irrigated crops in arid some of the salts out of the rootzone. The water and that of the subsurfacedrainwater regions. Typically, the irrigation water amount of excess water required depends discharge. But besides saline rootzone wa- available in such regions contains measur- partly on the chemical composition of the ters, subsurfacedrainage systemsin the San able, sometimessubstantial amounts of salts water, insofar as that influences salt pre- Joaquin and Imperial valleys collect saline that must be leached from the soil after cipitation and the water's ability to carry waters from deeper in the soil. The total salt irrigation. The soil profile may also contain salts. load leaving the Broadview and Imperial soluble minerals that contribute both to the Put simply, a grower can maintain the Valley water districts in drainwater is salinity hazard to crops and to the salt load rootzone salt balance by applying enough roughly twice the load applied in irrigation of agricultural drainwater. Irrigation water excess water to carry the same amount of salt water, despite subsurfacedrainage systems salts aren't always carried away in out of the soil as the water itself brings in. that collect only part of the rootzone drainwater, either; some of those salts may Here, the ratio of the rootzone drainage drainwaterafter anirrigation.Thesubsurface be deposited in the soil. All plants require volume totheappliedwatervolumeissimilar drainagesystems tend to collect any available some salt-borne nutrients (i.e., Ca, K, etc.), to the ratio of applied water salinity to groundwater, making the rootzone salt but not at the levels considered here. To drainwater salinity, otherwiseknown as the balancedifficultif not impossibleto calculate control salinity in the soil profile, farmers leaching fruction (LF). Such a simple case of without additionaldata. This report presents three soil-water flow factors that affectsalt balance determinations:LF, soilsalinity,and Applied Scenario water Comments the drainage efficiency (DE) of subsurface volume systems. The soil profile salinity is constant from one growingseason tothe next. Rootzone,drain, Salt leaching and drainage and deep percolation water salinity are all the same. Applied salt mass is the same as Figure 1summarizes thesoil-water processes rootzone drainage salt mass and the sum of involved in rootzone leaching and drainage drainfiow anddeep percolationsalt masses as required by salt balance. Chemical pre- shownschematicallyinfigure2,givingthree cipitation is balanced by dissolution. simplified scenarios that describe the d.p: volume leaching process. Other possible scenarios The soil profile salinity may be augmented combine elements of these three. II by mineral dissolution in excess of chemi- In scenarioI, salt balance is maintained in cal precipitation and salt may accumulate (mineral deeper in the rootzone and shallow the rootzone even though salts may accu- groundwater. Augmentation of the applied mulate in the shallow groundwater, de- salt mass results in larger drain and deep pending on the drains' efficiency and the percolation salt masses. d.p. salt rate of lateral movement for the shallow The soil profile and rootzone salinity, groundwater. In this case, salt accumulation 111 I rootzone drainage and deep percolation may occur intheshallowgroundwaterwhen salt mass, and deep percolationvolume are the salts in the applied water translocate to thesame as in Scenario I. Rootzone salinity may be aggravated by upward flowof saline the groundwater. (The salinity of shallow groundwater. Drainwater volume and salt groundwatermay also increase as a result of Increased mass are augmented due to collection of evaporation at the water table.)For this case, 'A. W. =appliedwater Saline shallow drain the saline shallow groundwater. d.p. =deep percolation groundwater salt mass LF can be based on the ratio of the salinityof the applied water to the salinity of the Fig. 1. Three simplified scenarios of the saltwater processes involved in rootzone leaching and rootzone drainage, or the rootzone drainage drainage. volume to applied water volume. 24 CALIFORNIA AGRICULTURE, VOLUME 44, NUMBER 6 drainage volume. Estimates based on the ratioof drainwatersaltmasstoappliedwater salt mass would overestimate the perfor- mance of the subsurface drainage system. Shallow groundwaterquality in scenarios IandIIcanbedegradedbyasoilorsubsurface drainage system with a poor DE, sigruficant lateralmovementof groundwater,andwater table evaporation.Often, the poor quality of a groundwater is a result of these factors. Scenario 111 is similar to scenario I, but withgreaterdrainflowsand salt loads caused by salinegroundwater. Estimates of LF based on drainwater salinity would be artifiaally low, and DE estimates based on drainflows would be too high. Our field measurements from the Imperial Valley illustrate scenario 111, and the difficulty of determining LF and DE on the basis of salt balance concepts. Field measurements The persistent salinity problems of heavy Imperial Valley soils have long made them Fig. 2. Schematic illustration of the flow of water through soil with respect to salt leaching and rootzone drainage. the objects of study. A 10-yearstudy (1939 to 1948 inclusive) on 18 acres of what would become the Imperial Valley Research and Rootzone drainage volume is then the soil’s dissolved salts and the salts in the Extension Center led researchersto conclude product of LF and the volume of applied applied water. Partial leaching, involving that continuous ponding was necessary to water. The drainage efficiency can be de- less water than the salt balance calculations adequately leach excess salts from the termined from the ratio of the collected would imply, may suffice, dependingon the rootzone. In 5 of that study’s 10 years, the 6- drainwatersalt mass to the applied salt mass, crop’s salt tolerance. As minerals continue foot-deep subsurface drainage system re- or the ratio of collected drainwater volume to dissolve and leach, the degree to which moved more salts than were applied (table to rootzone drainage volume. A DE of less soil salts can be extracted will decrease 1).In 2 years of those 5, the drainage system thanloo%resultsintheadditionofrootzone throughout the soil profile. If rootzone collected nearly three times the salt mass drainage water and salt load to the shallow drainage is insufficientto leach the dissolved applied, even though the same amount of groundwater. Depending on the flow pat- salts that have not precipitated, they will water was applied as in years when the terns of the shallow groundwater, the ad- accumulate deeper in the rootzone. drainage systemcollectedless salt mass than ditionalsalinitymay requireremovalatsome The salinity ratio from scenario I gives was applied. The lack of additional infor- time. too small an LF value for scenario 11, un- mation about the salinity of the soil and of In scenario 11, dissolved minerals add to derestimatingthe rootzone drainagevolume the shallow groundwater prevents any the rootzone’s salinity and to the potential and the volume of excess irrigation needed quantitative evaluation of the benefits to for saltto accumulate deeperin therootzone. to leach the salts, depending on the crop‘s reduction of soil salinity of the 10-year Increased rootzone salinityalso puts greater salt tolerance. Other, more involved calcu- leaching period. salt loads in drainwater and in deep-perco- lations are needed. In scenario 11, we can Data in table 1 clearly show that even lated water. A grower can balance the soil only determine the correct DE by measuring during the 10 years of leaching, the salts by applying enough water to leach the the drainwater volume and the rootzone drainwater‘s net salt load exceeded that of the appliedwater, so steady-statesalt balance conditions do not appear to exist. For com- parative purposes, however, we calculated the leaching fraction for each year of the study under the assumption that a rootzone salt balance did exist. ThemeanLFwas about 9%:the minimum (about 7%) occurred when the drainage system carried an excess salt load, and the maximum (about 11%) when the drainage system

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