A regulating reservoir between the water supply source and the delivery system buffers the system from fluctuating demands. In addition, the system can reduce energy costs, by storing water pumped during off-peak energy tariff times and by improving the operation of pumping stations. A model is proposed to determine a regulating reservoir's capacity and establish an annual pumping schedule that accounts for energy costs. The optimal reservoir size and operation minimize the sum of the amortized reservoir construction and annual pumping costs (annual total cost). The solution was determined using a dichotomic searching procedure that includes one algorithm to calculate the optimal reservoir dimensions using Newton's method and another algorithm to determine the optimal annual pumping to the reservoir based on the concept of ‘emptying period’. The model was applied to a pressurized irrigation delivery system located in southern Spain. The analysis used historical hourly water demand data. A cluster analysis was used to screen non-typical demand data from the data set before extracting a typical temporal demand pattern. The analysis indicated that the addition of a regulating reservoir with optimal size and operation scheme was cost effective. Total costs can be further reduced by adjusting the demand pattern to better match with off-peak energy tariff times.

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