A MODEL FOR RESERVOIR YIELD UNDER CLIMATE CHANGE SCENARIOS FOR THE WATER STRESSED CITY OF BULAWAYO, ZIMBABWE *

Abstract

The City of Bulawayo, the second largest city in Zimbabwe (population 981,000), although located in the Zambezi Basin, sources its water mainly from five reservoirs located in the Upper Mzingwane Subcatchment, Limpopo Basin. The reservoirs have failed to store their expected 4 % yields. The city has frequently implemented water rationing or lesser restrictions. The city is currently under stringent water rationing since the total runoff into the water supply reservoir was low. A study was undertaken to determine if reservoir yields were falling, and possible causes thereof. Examination of 30-year rainfall and runoff records showed declining precipitation and runoff on the five year moving averages. Dam yields, determined by Yield 200 model, have been declining over the same period, from 131.3 Mm 3 in 1980 to 67.90 Mm 3 in 2005. The phenomenon, which might influence such a trend, is global climate change. Future yields of the reservoirs were estimated by two methods. The yield was projected from the 30-year trend to year 2030 using the best-fit line. This formed one scenario, while two additional projections were determined using two scenarios of runoff and precipitation decline, predicted from IPCC SRES emissions scenarios. Both of these predicted less reduced yields by 2030 (45.04 Mm 3 and 41.72 Mm 3 ) than the yield projection from current data (67.90 Mm 3 ), suggesting the possibility that the impact of climate change in southern Zimbabwe may be higher than predicted by global models. Unrestricted water demand for the city is projected to 83.70 Mm 3 of raw water by year 2030. This is far above even the most optimistic yield projection. It can therefore be concluded that the City should have determined additional sources of water already. The state-owned Mtshabezi dam and the Nyamandlovu aquifer connection could provide an increased total water supply in the short-term, but longer-term solutions, water demand management or additional sources, or both are required.