Surface water demand and supply of Gaborone city and surrounding areas under climate change and population growth

The adequate supply of the ever-increasing demand of fresh water continues to be a global

challenge due to increase in population. 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 anticipated population growth and climate change using

the Water Evaluation and Planning (WEAP) model for future periods. The model was run

with the current accounts and reference period set at 2014 and 2015-2100 respectively to

predict their possible impacts on the water balance and allocation of the region due to varied

water demands. Moreover, the study includes analysis of population trends, water

production and consumption rates, hydrological information as well as projected rainfall

over the catchment supplying water to Gaborone Dam. The rainfall data over the catchment,

simulated in the frame work of Coupled Model Intercomparison Project Phase 5 (CMIP5)

by Max Planck Institute Earth System Model Mixed Resolution (MPI-ESM-MR) for

scenario periods, are 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 downscaled rainfall data are then employed in WEAP

model and climate trend analysis. The stream flow of Notwane River has decreased from

0.65 MCM in 2006 to 0.53 MCM in 2016. The change is consistent with decrease in rainfall

in the area. The WEAP simulated flow of Notwane River station with the observed flow

gives an EF (Coefficient of Efficiency) of 0.91 which is a good correlation. The analysis

shows that the projected population of Gaborone, Mogoditshane and Tlokweng using the

high population growth rate of 3.4% will be about 4106670, 1029877, and 644092 by the

year 2100 respectively. Under both RCP (Representative Concentration Pathway) 4.5 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 2080-2098 period.

The unmet water demand of the whole study area will be 88.04 MCM (Million Cubic

Meters) in 2050 as compared to 3666 MCM in 2100 under RCP 8.5 climate and high

population growth scenarios. However, the unmet demand under RCP 4.5 climate and high

population growth scenarios will be 84.65 MCM in 2050 as compared to 3569 in 2100. The

climate under RCP 8.5 emission scenario will be drier than that of the RCP4.5. On the other

hand, the unmet water demand will be 25.9 MCMin 2050 as compared to 355 MCMin 2100

under the RCP4.5 climate with low population growth rate of 2.2% scenario. In contrast, the

v

unmet demand is as high as 26.8 MCM in 2050 and 373 MCM in 2100 under the RCP 8.5

climate with low population growth rate. The unmet water demand in both high and low

population growth and the dry climate of RCP8.5 climate scenario will lead to shortage of

water in the city. These changes in water supply and demand of the city under various

scenarios show that there is a need for various forms of water loss control interventions. For

example, if the current 39% loss through leakage is reduced to 0% by 2100, the unmet water

demand will reduce from 3569 to 422 MCM under RCP4.5 climate change scenario.

Similarly for the RCP 8.5 climate change scenario, the unmet water demand will reduce

from 3666 MCM to 436 MCM. These estimates show that mitigation of impact of climate

change on water resources is possible but needs aggressive and robust interventions.