Accra has a population of about 2.3 million and is supplied with water from both the Kpong and Weija Water Works. Water from the Weija treatment plant is taken from the Weija Reservoir which is fed by Rivers in the Densu River Basin (DRB) that flow into the Reservoir at Weija. With increasing annual population growth of Accra at 4.4% and inadequate water supply to it, this study has examined the hydrological data available on the Weija Reservoir from 1980 to 2007 in an attempt to estimate runoff into the Reservoir with the view of determining whether water is available to meet its present and future demands. Results show that even though water abstraction from the Reservoir has increased almost four times since 1980, to more than 67 million m³/year in 2007, and a maximum runoff of 7.97 ± 0.21 × 10-² km³/year was estimated in 2005, this value is less than the true runoff into the Reservoir. It was also observed that potential evapotranspiration has increased by 0.14% while precipitation has decreased by 0.93% in the DRB, indicating that runoff from the Basin into the Reservoir is probably decreasing, albeit slowly. Additionally, fishing and waste disposal methods are poor; land use practices and other anthropogenic activities in the DRB pose a threat to the sustainability of the Reservoir. Serious educational programmes and enforcement measures need to be urgently adopted to safeguard continuous water flow into the Reservoir. Proper hydrological data collection and data management practices are recommended for the Reservoir and Densu River Basin if detailed planning of the water resources of the Reservoir are to be achieved.
