SECONDARY DESERTIFICATION DUE TO SALINIZATION OF INTENSIVELY IRRIGATED LANDS - THE ISRAELI EXPERIENCE

Secondary salinization of intensively irrigated lands is an increasing ly alarming redesertification process experienced in many irrigated re gions of the developed countries. The major cause is a profound interf erence in the geochemical/salt balances of irrigated regions. A case-i n-point is the recent salinization of the Yizre'el Valley, a 20,000 ha intensively irrigated region in Israel. The extremely intensive and a dvanced agroecosystem developed in the region since the 1940s included pumping and importing irrigation water by the National Water Carrier, large-scale reclamation and reuse of municipal sewage water, winter f lood impoundment in reservoirs for summer irrigation, and cloud seedin g to enhance rainfall. Modern irrigation methods were applied, includi ng sprinkler, trickle, moving-line, and center-pivot systems. Water us e efficiency at any level was very high. Nevertheless, large-scale sal inization of regional water resources and many fields had developed in the mid-1980s. Reconstructing and evaluating the water and salt balan ces of the Yizre'el Valley (using Cl as the representative salt consti tuent) shows that as water use in the valley increased to about 60 mil lion m(3) per year, the importing of soluble salts by water totaled 15 ,000 tons of Cl per year. Recirculated salt - salt picked up by impoun ded surface water and applied to fields - increased significantly and in the late 1980s amounted to more than 9,000 tons Cl per year. The so urce of recirculated salts was the accumulated salts in soils and in t he shallow aquifer in the valley, which were leached by floodwater or drained or infiltrated into reservoirs, grossly and adversely affectin g water quality. Analysis of the Yizre'el Valley's case points to the utmost importance of maintaining the geochemical balances in addition to increasing irrigation efficiency. An irrigated region may achieve g eochemical balance by the following means: limiting the extent of irri gated areas, developing a well-maintained drainage system that drains tail-water and salinized shallow-aquifer water, and devoting a signifi cant portion of water for regional leaching. The sustained long-term p roductivity of irrigated lands in arid zones crucially depends on corr ectly managing water and soil resources. Regional management of irriga ted lands to prevent secondary desertification will be aimed at carefu lly balancing the undisputed benefits of irrigation with the long-term (on time scales of 10 to 100 years) detrimental processes set in moti on when irrigation is introduced to arid and semiarid zone soils.