Agricultural drainage water management in arid and semi ... - FAO.org

24Framework for selecting, evaluating and assessing the impact of drainage water managementTABLE 3Reuse measures, practices and points for considerationOption Practices Points for considerationReuse inconventionalagricultureReuse in salineagricultureIFDM SystemReuse inwildlife habitatsand wetlandsReuse forreclamation ofsalt-affectedsoilsSource: SJVDIP, 1999a.Agricultural drainage water is collected andredistributed among farmers. Reuse canbe direct or in conjunction with othersources of irrigation water. Conjunctive usecan be through blending or by cyclic use ofdrainage water and other sources ofirrigation water.Moderately and highly saline drainagewater is collected and used to cultivate salttolerant shrubs, trees and halophytes.On-farm sequential reuse of agriculturaldrainage water on crops, trees andhalophytes with increasing salt tolerance.In every reuse cycle the volume ofdrainage water decreases while the saltconcentration increases. The final brine isdisposed in a solar evaporator. Saltutilization might be feasible.Where of suitable quality, drainage watermay be utilized to support wildlife, includingwaterbirds, fish, mammals and aquaticvegetation that serves as food and coverfor wildlife.Use of (moderately) saline drainage waterfor initial reclamation of saline, saline sodicor sodic soils. On sodic soils the salinewater may help prevent soil dispersion anddegradation of soil structure.The extent of reuse depends on the quality of thedrainage effluent, time of availability, croptolerance, etc. Soil quality degradation andproduction losses need to be prevented throughmitigation measures. Residuals of reusedwaters, which are often highly concentrated witha reduced volume, need to be managed.Sustainability of the saline agricultural systemsneeds to be safeguarded while the highlyconcentrated drainage effluent needs to bemanaged.To maintain adequate control of soil salinity andsodicity and to prevent a buildup of toxicelements, the leaching fraction must besufficient. The solar evaporator should bedesigned in such a manner that it will not sustainaquatic life and attract birds. The salts have tobe disposed of in a safe and sustainablemanner.Of primary concern is the possible presence oftrace elements that may be toxic to wildlifethrough bioaccumulation in the food chain e.g.selenium, molybdenum and mercury.Once initial reclamation is obtained, water ofsufficient quality and quantity needs to beavailable for desired land use. Initially, thedrainage effluent will be highly saline and/orsodic and needs to be disposed of in a safemanner.initial reclamation of salt-affected lands (SJVDIP, 1999a). Table 3 provides a brief descriptionof the alternative reuse measures.Reuse measures can be implemented in combination with conservation measures. Wherethe drainage water is of relatively good quality, its reuse potential in conventional agriculture ishigh. Where it is moderately to highly saline, its reuse may be limited to salt tolerant plants. InCalifornia, the United States of America, a new IFDM system reuses drainage water sequentiallyin high water table lands with no opportunities for disposal of subsurface drainage (SJVDIP,1999a). Freshwater is used to grow salt sensitive crops, and subsurface drainage water from itis reused to grow salt tolerant crops. Drainage water from the salt tolerant cropland is used toirrigate salt tolerant grasses and halophytes. When the drainage water is no longer usable, it isdisposed into solar evaporators for salt harvest. In this system and others, it is always necessaryto generate a minimum volume of drainage effluent to prevent the rootzone from becoming toocontaminated for any beneficial water use activity. Similar systems are being developed andtested in Australia.Treatment measuresDrainage water treatment in a drainage water management plan normally takes place onlyunder severe constraints, such as stringent regulations on disposal of saline drainage waters intostreams, or severe water shortage. The drainage water treatment options are based on physical,chemical and/or biological processes (SJVDIP, 1999b). Many of these processes are borrowed