Secondary desertification due to salinization of intensively irrigated lands: The Israeli experience.

Secondary salinization of intensively irrigated lands is an increasingly alarming redesertification process experienced in many irrigated regions of the developed countries. The major cause is a profound interference in the geochemical/salt balances of irrigated regions. A case-in-point is the recent salinization of the Yizre'el Valley, a 20,000 ha intensively irrigated region in Israel. The extremely intensive and advanced 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 flood impoundment in reservoirs for summer irrigation, and cloud seeding to enhance rainfall. Modern irrigation methods were applied, including sprinkler, trickle, moving-line, and center-pivot systems. Water use efficiency at any level was very high. Nevertheless, large-scale salinization of regional water resources and many fields had developed in the mid-1980s. Reconstructing and evaluating the water and salt balances of the Yizre'el Valley (using Cl as the representative salt constituent) shows that as water use in the valley increased to about 60 million 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 impounded surface water and applied to fields - increased significantly and in the late 1980s amounted to more than 9,000 tons Cl per year. The source of recirculated salts was the accumulated salts in soils and in the shallow aquifer in the valley, which were leached by floodwater or drained or infiltrated into reservoirs, grossly and adversely affecting 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 geochemical balance by the following means: limiting the extent of irrigated areas, developing a well-maintained drainage system that drains tail-water and salinized shallow-aquifer water, and devoting a significant portion of water for regional leaching. The sustained long-term productivity of irrigated lands in arid zones crucially depends on correctly managing water and soil resources. Regional management of irrigated lands to prevent secondary desertification will be aimed at carefully balancing the undisputed benefits of irrigation with the long-term (on time scales of 10 to 100 years) detrimental processes set in motion when irrigation is introduced to arid and semiarid zone soils.