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The cost of constructing storage reservoirs in Cholistan was studied using a mathematical model. The variables studied were reservoir side slope and depth, seepage, evaporation and capacity of the reservoir. After computing the reservoir dimensions to store the required 15 140 m(3), a full reservoir was assumed starting from 1 August of the year. The depth and quantity of water stored were then depleted by use, evaporation and seepage by the month. In this manner, the length of time that water would be available was computed. The study showed that under general conditions a 3 m deep reservoir would be depleted by December or January, a 4.6 m deep reservoir would be depleted by March or April and a 6 m deep reservoir would be depleted by April or May. Only a 7.6 or 9 m deep reservoir would contain water for a whole year provided that both seepage and evaporation are controlled. The recommended reservoir dimensions are 6 m deep with 1 : 2 or 1 : 3 side slopes, as these give minimum cost plus better water availability. The cost with the recommended optimal dimension was US$6 per m(3) with good seepage and evaporation control. Copyright (c) 2008 John Wiley & Sons, Ltd.