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N 00 J-> ....;J Examples of the first category include weather modLfication to enhance stream flow, evaporation suppre$sion, and phreatophyte control. Many of these approaches are more costly and difficult to apply than is justified by the salinLty control achieved and are therefore not considered in this work. In the second cate- gory, such measures as saline flow collection and treatment, reduction in agricultural return flows, and land use regulation can be used to reduce the volume of salinLty entering receiving waters. In this report, only $alLne flow collection and treat- ment and i~rigation return flow management are evaluated. Under these assumptions, salinity control becomes a mutually exclusive problem that allows addition of indLvidual solutLon$ to derive larger solutions. By letting the spatLal scale of the problem correspond to successive layering or additions, the multilevel approach is congruent to the subbasin breakdown of major hydrologic areas. The smallest spatial scale considered in this analysis Ls that of a subbasin containing an irrigated valley or stream segment delineated by inflow-outflow data. In a major river basin, a number of river systems may combine to form the basin itself so there are actually three subdivisions in a river basin.. Thus, vertical integration of subbasLn$ yields river subsystems and integration of river subsystems yield the aggregate river basin. In this analysis the river basin, river subsystem, and subbasin divisions have been designated as levels 4, 3, and 2, respectively. Level 1 will also encompass the $ubbasLn scale as will be described shortly. Associ~ted with each level of the model are cost- effectiveness functions describing each alternative for controlling salinity. The structure of the cost-effectiveness functions includes two parts. The first is the function itself. In order to compare the respective feasibility among varLous salinity control measures at each level, the mathematical description of each alternative must be in the same format. Since this study involves evaluating the mLnimal cost strategy for reducing salt loading, each salinity control measure's feasibility for being included in the eventual strategy Ls based on the relationship between the costs of improvement and the resulting r~duction in salt loading. The second part of the cost-effectiveness functions is what might be called a "policy space". To appreciate this aspect of the model Lt is probably necessary to first discuss the determination of the optimal basin-wide strategy. Evaluating the Optimal Strategy-- Suppose the optimal policy for controlling salinity in a river basin had been determined wLth a minimum cost decisLon criterion. Such an analysis would provide two pieces of information. First, it would detail the cost associated with a 19 . ':", o ,ow( "