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Reservoirs play an important role in storing water for various uses. This storage is affected by rising demand due to changes in economic pattern and demand distribution. Added to these is the effect of evaporation especially in semi-arid areas with limited suitable reservoir sites. Botswana is one such country with flat terrains, ephemeral rivers with sandy beds and often lacks well defined channels for potential dam sites. These hydrographic factors lead to the development of shallow and large reservoirs, which lose more water due to evaporation. In assessing performance of Botswana's major reservoirs, a modified sequent peak algorithm has been used to study the effects of evaporation on reservoir sizing and operation, and how that affects the yield. The study concludes that evaporation is very critical for reservoir planning in semi-arid regions, because for every storage there exists a range of yield to be supplied. Outside this range, the effects of evaporation are more difficult to control.