Surface water demand and supply of Gaborone city and surrounding areas: Response to climate change and population increase

The adequate supply of the ever-increasing demand of fresh water continues to be a challenge in parts of the globe. This challenge has been aggravated due to increasing population and climate change. The anticipation for better lifestyles and improved water supply has resulted in an increase in migration from rural settlements leading to an increase in the populations of many cities globally. This study therefore investigates the variability and trends in the surface water demand and supply of the city of Gaborone and surrounding areas in response to population growth and climate change using the Water Evaluation and Planning (WEAP) model for future scenarios. The study includes analysis of population trends, water production and consumption rates, hydrological aspects of the study area as well as projected climate data at a high spatial resolution of 1 km2. The current General Circulation (GCM) or Regional Climate (RCM) models are not able provide such data. Therefore, the climate data for existing GCMs is statistically downscaled using the high resolution Worldclim data to spatial resolution of 1 km2 and bias corrected against Global Climatology Precipitation Center (GPCC) precipitation. The GCM data for the mid-range Concentration Representative Pathways (RCP4.5) and high emission RCP 8.5 future scenarios of Coupled Model Inter-comparison Project Phase 5 (CMIP5) are employed in the study. Under both RCP4.5 and RCP8.5 scenarios, the reservoir inflow indicates that the level of reservoirs at Foresthill, Diremogolo, Gabane hill, Oodi hill and Mabutswe will be reduced during 2081-2097 period. The unmet water demand of the whole study area will be 52.5 million m3 in 2050 as compared to 1490 million m3 in 2100 under RCP 8.5 climate and high population growth scenarios. However, the unmet demand under RCP4.5 climate and high population growth scenarios will be 51.14 million m3 in 2050 as compared to 1450 million m3 in 2100. On the other hand, the unmet water demand will be reduced by as much as 50% under both scenarios if low population growth rate of 2.2% is assumed. As an option of water management, increasing water loss reduction by 3% every year could drastically reduce the unmet water demand.