SALT AND WATER-BALANCE IN IMPERIAL-VALLEY, CALIFORNIA

Salt balance (SB = Veff.w .cntdot. Ceff.w - Vinf.w .cntdot. Cinf.w) of the Imperial Valley (IV) has been determined annually since 1943 by the Imperial Irrigation District [California, USA]. Salinity trends in the valley are assessed from biweekly measurements of the volume V and concentration C of influent inf.w and effluent eff.w waters. The SB data are summarized, their significance evaluated and approaches suggested for assessing salinity trends in the soils of the valley. The SB data reflected the cropping and water use patterns in the valley. The data were insufficient to distinguish origin of water and salt in effluent waters or to provide information about changes in root zone salinity. In 1973 total evapotranspiration (ET) by crops in the valley was estimated to be 229 .times. 103 ha-m, equivalent to 70% of the water delivered to the farmers. Deductions as to Cl- composition of influent and effluent during 1973 suggest that the Cl- load in the effluent water was contributed as follows: 54.7% from ground water, 35.0% from root zone drainage water and 10.3% from tail water (runoff). A more definitive interpretation of the salt balance data would require more accurate evaluation of volume of tail and consumptive use waters and volume and concentration of root zone percolate. If this information cannot be obtained on a valley-wide basis, an alternative approach is to establish a representative number of soil salinity monitoring plots through the valley and assess their root zone salinity periodically.