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""''-, -J:'<: uuu, VI... After the elements of the hydrologic system have been described and simulated the next step in the evaluation is the testing for accuracy or calibration of the model, The hydrologist selects a time period (months or years) and programs the hydrologic stresses such as recharge from precipitation and applied water, ground-water withdrawal, and evapotranspiration. Some of these stresses can be defined by direct measurement (for example the pumping history and stream diversions). However, the definition of most of the stresses requires inference by the hydrologist. Preliminary estimates are usually made of the percent- age of applied water and precipitation that recharges the aquifer and the rate of evapotranspiration where the water table is near the land surface, A comparison of the model results and field data is used to evaluate the correctness of the hydrologist's inferences or estimates. The results of the model analysis consist of a map or hydrographs showing changes in ground-water storage and variations in streamflow, The hydrologist compares these results with field data collected for the same time period. If the model does not duplicate the field con- dition, then the hydrologist must decide whether to modify the physical framework or make new estimates of the hydrologic stresses, or both. The testing is dependent on the hydrologist's interpretation of the basic hydrologic data and the final results will be dependent on his knowledge of the hydrologic system and the accuracy of the field data. The model analysis may indicate major data deficiencies and it may be necessary to collect additional field data before the model testing can continue. When the model simulates the real system it may be used to evaluate water problems and to predict future changes, These analyses will require additional information on the major constraints that will effect water- management planning, Some of the constraints that affect planning are the cost of pumping ground water, the cost of surface water, the legal controls on water use, the ground-water and surface-water quality, and the effects of the water use on the natural environment. For example, the model analysis may indicate that water management in some areas could be improved by increased use of the ground-water reservoir; this use, however, may result in long-term degradation of the water quality and eventually in reduced crop yields. Optimization models using mathematical programing and dynamic programing can be used to evaluate problems where there are a number of decision-variables and constraints. These models, when applied to existing constraints or changes in these constraints, may provide optimal solutions to water-supply problems, In summary, highly sophisticated models have been developed and are available for use in planning the development of ground-water supplies, for the conjunctive use of ground and surface water, and for optimizing management objectives within specified physical, economic, and social constraints. Because of such development, it is now possible to integrate use of ground water into water-resource planning with a high degree of certainty. 8