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Agricultural drainage water management in arid and semi-arid areas 13SEEKING SOLUTIONSStep 4. On the basis of the root definitions, conceptual models need to be constructed. Thesemodels include all the probable activities and measures that the system needs toimplement to achieve the root definition. In other words, alternative scenarios need tobe formulated. This involves doing sufficient work on the technical and other details,which need to be defined, in order to enable sound decision-making. Moreover,measurable indicators need to be established to compare the results of the conceptualmodels with the analysed situation or base case. As the formulation of scenarios isbased on a thorough analysis of the systems, it should take into consideration systemobjectives, possibilities and constraints.Improvements needed to minimize or correct a particular drainage water managementproblem may consist of physical structures, non-physical improvements or both. Physicalimprovements could involve using irrigation water conservatively by on-farm watermanagement practices along with regional drainage practices such as recirculating usabledrainage water to meet waste discharge requirements. Non-physical improvements mayinclude implementing tiered water pricing to encourage growers to use water wisely, i.e.charging a penalty for overuse.Step 5. The next step is to compare the scenarios or conceptual models with the situationanalysis. The idea is to test the scenarios and decide whether the implementation of ascenario would resolve the defined key issues.Step 6. If it would, it needs to be investigated and there needs to be debate as to whether thechanges proposed, resulting from implementation of the scenario, are both desirableand feasible. What is desirable and what is feasible might clash as a result of systemobjectives, possibilities and constraints.Step 7. The final step is to define the measures and changes to be implemented.SPATIAL ISSUESDrainage water problems vary in space and time due mainly to soil heterogeneity and watermanagement practices. The following example illustrates how spatial variability needs to betaken into account in the problem analysis, and also how it influences the options for drainagewater management.Environmental problems related to agricultural drainage water disposal on the western sideof the San Joaquin Valley, California, the United States of America, have created a need forimproved irrigation water and salt management (SJVDP, 1990). The presence of harmful traceelements, mainly selenium, in the drainage water is of major concern (Tanji et al., 1986) and hasled to limitations on drainage water disposal to rivers and impoundments such as agriculturalevaporation basins. Where water districts in the problem area fail to meet selenium and salt loadtargets, they risk monetary penalties and loss of access to disposal sites. Young and Wallender(2000a) raised the question of whether the constraints raised by current or future regulationswill reduce drainage to the point where salt accumulation would occur and in which areas thismight occur first. Second, they raised the question of the spatial distribution of drainage waterdisposal costing strategies. To answer these questions, they developed a methodology for thePanoche Water District to calculate the spatial distribution of water-, salt- and selenium-balance
14Defining the problem and seeking solutionscomponents using data collected by thewater district. Furthermore, theydeveloped and evaluated spatiallydistributed drainage water disposalcosting strategies. The following is a briefoverview of their research findings.FIGURE 5Topography and boundaries of the Panoche WaterDistrictThe Panoche Water District, situatedin the western side of the San JoaquinValley, is a typical example of a waterdistrict that needs to cope with disposallimitation in the form of selenium loadtargets. Figure 5 shows that the districtlies on two alluvial fans and is generallyflat with slopes of not more than 1 percenttrending in a northeasterly direction.The groundwater in the LittlePanoche Creek alluvial fan containssodium-chloride type water, relatively lowin salt content, and with seleniumconcentrations ranging from 1 to 27µg/litre (Young and Wallender, 2000b).In contrast, the groundwater in thePanoche Creek alluvial fan containssodium-sulphate type water, relativelyhigh in salt content, and with seleniumconcentrations ranging from 20 to400 µg/litre. Due to overirrigation sincethe introduction of surface water deliverysystems in the 1950s, the water tablerose to within 1–3 m of the groundsurface. Subsurface drainage wasinstalled which maintained successfullythe water table at an acceptable level foragriculture. However, due to irrigationand drainage practices the naturallyoccurring salts and selenium in theregion’s soils are mobilized and enter intothe subsurface drainage system as wellas into the shallow groundwater.Calculation of a spatially distributedwater balance revealed that downslopeareas with shallow water tables receivegroundwater discharge. Drains in theseareas intercept lateral and verticalupward flowing groundwater, while theupslope undrained areas recharge to thegroundwater (Figure 6). The highest saltload in the collected drainage waterSource: Young and Wallender, 2000a.FIGURE 6Annual recharge to groundwater in Panoche WaterDistrictNmmSource: Young and Wallender, 2000a.1 0 1 2 3 KilometresSource: Young and Wallender, 2000a.-1200 - -800-800 - -400-400 - 0mm0 - 400400 - 800800 - 12001200 - 1600
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