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Show in light of the regional perspective and the multi- objective goals of this study. Actual cost rates and a cost structure for each agency could also have been used on a well by well basis if resources for collecting and documenting this information had been available. ( 6) Solve optimization problem defined for a set of demand conditions. A base optimization using historical annual groundwater demands for a 10 year period ( 1973- 1982) was first performed. The objective of this run was to compare the optimal cost solution for pumping over this period with the actual pumping cost ( both specified in terms of computed energy costs rather than actual costs incurred by each agency) for the same historical period. This was the only 10 year period for which systematic pumping records were available to us. A direct evaluation of the demand variability in wet and dry periods was not possible. Rather than adopting the demand pattern over these 10 years as representative, a number of different hypothetical demand scenarios were considered. The first critical factor considered is the spatial pattern of demand. Clearly there is no fixed spatial pattern of groundwater demand across different water agencies. The pumping rates and patterns of the agencies over the years are responsive to the particular agency demands and potential alternate sources of water supply. For the purposes of planning, rather than an actual operation of the water system, some presumptions as to the demands to be satisfied do , however, need to be made. The specification of such patterns could be made using forecasts from each agency and an evaluation of the appropriated groundwater rights of each agency. It is however, difficult to correlate and synthesize this information in a consistent manner. The investigations of Waddell et al. ( 1987) considered a fixed percentage increase in pumping from 1982 historical records for each well ( and hence for each agency). Recognizing the limitations of using the historical pumping record of just one year for specifying the spatial demand pattern, two scenarios were explored. The first was consistent with those of Waddell et al. ( 1987) - the minimum demand for each agency was a prescribed percentage of the 1982 pumping. The second scenario prescribed the minimum demand for each agency as a percentage of the average annual historical pumping in the 1969 to 1982 period. Fixed annual demands at some percentage of these base values were considered in the development of a least cost strategy. The second major factor considered for demand variability was annually increasing demand because of population growth. This was explored by considering a base level of demand prescribed as a percentage of either the 1982 or the average annual historical pumping from 1969 to 1982, for the first year of operation, followed by constant percentage increases in the base level for each of the remaining 9 years of operation considered. This case is referred to as linear growth in demand. Results for several base levels of demand and several rates of growth were obtained. The third factor considered is operation of the groundwater system during a drought. It was expected that water use would shift from surface water to groundwater sources during a drought period, posing increased demands on groundwater during a drought. The effects of a drought on the least cost pumping strategy were explored by specifying a base demand for the first year as some percentage of either the 1982 or the average annual pumping from 1969 to 1982, and then increasing it by fixed percentages up to the fifth year, and then allowing demand to decrease to the base level by the tenth year. Various drought severities were experimented with. The next section presents results of these applications and is organized to follow the categories of demand variability listed below. 56 |