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O)[jj3C Arkansas River, An electric-analog model was developed to simulate the stream and aquifer, including the transmissivity of the aquifer and the hydrogeologic boundaries such as the Arkansas River and the bedrock- alluvial contact. Applied irrigation water, precipitation, evapo- transpiration, and well withdrawal were the hydrologic stresses to the model. The model was checked for accuracy by comparing the model results with field measurements of change in streamflow' and water level. The analog model was stressed at 266 points to obtain response curves showing the effect of aquifer stress on streamflow. The response curves, summarizing differences in aquifer transmissivity, specific yield, and boundaries, were then prepared as a "stream-depletion factor" map (Moulder and Jenkins, 1969). The lines of equal stream-depletion factor connect points where the hydrologic stresses on the aquifer have the same effects on the streamflow. The map was the basis for constructing a digital model of the stream-aquifer system that was used to analyze and optimize water-management plans for any given set of management objec- tives, The model was designed to predict the availability of surface water at successive diversion points downstream on a month-by-month schedule, and to show change in ground-water storage. Data obtained by the model of the Arkansas River valley have been used for planning optimum water use within the legal, economic, environmental, and hydro- geologic constraints. The results have been used by the Colorado State Engineer for administration of the water supply, and by the State Legislature for evaluation of proposed changes in water law. Recently, programs have been written for digital models that incorporate certain water-quality parameters. Using these newer programs, it is possible to predict changes in water quality that might result from changes in water-management practices or to predict the fate of liquid contaminants intentionally or accidentally released to the environment. A pilot project now underway in the Arkansas River valley of Colorado will soon demonstrate the use of this model to describe and predict changes in salinity in the alluvial aquifer and in the adjacent stream on a monthly basis. The sophisticated field of systems analysis has been applied successfully in analyzing ground-water management alternatives. Models developed by staffs of universities and governmental agencies have shown that water-management objectives can be optimized using this technique. The method permits consideration of physical, economic, and social constraints and analyzes effects of alternative water-management plans, and selection of those that most closely achieve certain specified objectives. For example, such a model describing a stream-aquifer system was developed by the Geological Survey to show how ground and surface water can be distributed for irrigation to satisfy the greatest number of water rights in their order of priority, 5