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000299 For Irrigated agriculture to be sustainable, salt balance must be achieved so that salts added equals salts removed. (Jacobsen and Adams 1958). Declining yields and shifts toward cultivating more salt-tolerant crops paralleled in- creasing salinity. Meanwhile, cities were devastated by yield reductions, and the resulting inability to meet the food needs of a considerable super- structure of priests, admi'nistrators, merchants, soldiers and craftsmen. The southern part of the alluvial plain never recovered and many great Sumerian cities dwindled into villages or were left in ruins. The story of Mesopotamia is an- cient, but it could be repeated in Cali- fornia. Approximately 4.5 million acres of irrigated croplcmd - prima~ rily on the West Side of the San Joaquin Valley - are affected in Cali- fornia by saline soils or saline irriga- tion wnter. Predictions vary for the rate at which salinization is occurring, but tens of thousands of productive agricultural acres are clearly at risk (CAST 1996). Irrigation in sem.iarid regions such as CCllifornia invariably lends to soil saUnization unless mitigated by ap- propriate management. . Three factors contribute to salinization: . All irrigation waters contain dis- solved salts, with concentrations varying considerably according to the origin of the water. . Plants extract negligible amounts of s.1lt, so the soil solution becomes concentrated as water is removed by evapotranspiration. . Water quantities in excess- of evapotranspiration must be appliedto.leach salts beyond 'the root zone to prevent reduced crop yields. Saline water leaving a plant's root zone may also percolate down until it reaches groundwater, result~ ing in degraded ,vater qual- ity. Or, it may migrate and seepto a lower land surface. When the iatter occurs, the seepage area becomes salin!~.~d as water ' evaporates and the slI1ts concentrate. Water percolating below the root zone can cause the water table to rise. Without the ability to leach salt from the root zone, concentrations rise to detrimental levels, Additionally, high water content interferes with the oxy. gen supply to the roots, resulting in further crop damage. When a water table reaches the root zones, yields can be sustained only by draining to re- move excess salt, water and other dis: solved compounds. Sustainable irrigated agriculture in areas where salinity poses a problem can be achieved by adhering to two basic principles: (1) salt balance, de- fined as salts removed must equal salts added, must be achieved, and (2) systems must be installed to remove drainage water and di"solved salts when the water table reaches the root zone. These principles can be applied on a field, farm or regional basis. Most surface'wate" supplies in Cali- fornia are relatively low in salinity. Inw deed, some irrigation waters derived predominantly from snowmelt and rainwater from the Sierra Nevada con~ tain extremely low concentrations of dissolved salts. In contrast., irrigation water from the Colorado River is the most saline. Irrigation wa ter pumped from wells varies considerably in sa- linity, depending upon the geologic strata from which it is extracted. Although salinity is a concern in isolated areas, the Imperial Valley and the western San Joaquin Valley are of 44 CALIFORNIA AGRICULTURE. VOLUME 54. NUMBER 2 particular concern to agricultural pro- ductivity, each with its own uniqlle set of features. Imperial Valley and Salton Sea The Imperial Valley, located in southeast California, has a hot desert climate, allowing some agricultural productivity year-round. Very little precipitation occurs, and the evapo- transpiration rate is high. An elaborate irrigation and drainage system has been implemented in the Imperial Val- ley. Crops are irrigated with Colorado River water and drainage water is then discharged into the Salton Sea, which is fed predominantly by this source. The 3S-mile-long, lS-mile-wide sea initially came into existence when, at the turn of the 20thcentury, Colo- rado River waters were diverted to the flood-prone region for agriculture. An irrigation <;:anal subsequently broke and flooded the basin for 2 years. While those waters would have e"apo- rated years ago, the sea has since been fed by drainage wafer, sewage and other sources of water. With no outlet, the Salton Sea's salinity has continu- ously increased as the water evaporates. The Salton Sea area serves as an im- portant link on the Pacific Flyway, North America's West Coast route for migrating birds. The sea and adjacent wetlands, rivers and croplands pro- vide foraging and nesting for more Agriculture has a history of increasing productivity per unit of land area so that a growing human population can be fed without large expansions of acreage. Attention should now be directed to- ward increasing wildlife production per unit of habitat area.