This paper presents results of a study conducted to evaluate the possible impacts of climate change caused by the doubling of atmospheric carbon dioxide (CO2) on the reliability of the Mazowe reservoir in Zimbabwe. The reservoir supplies most of its water to citrus plantations. Thirty years (1961-1990) of hydrological data (reservoir inflows) and meteorological data were collected from the Zimbabwe National Water Authority and Department of Meteorological Services, respectively. Outputs from the Canadian Climate Centre model for the doubling of CO2, temperature and rainfall scenarios were used in the study. The Penman model was used to estimate potential evapotranspiration, while reservoir catchment runoff was simulated using the Pitman lumped conceptual model.