Abstract:
Water availability is facing intricate climate change and anthropogenic risks that have
worsened the deteriorating water resource quality and quantity. Long term streamflow
and groundwater monitoring in various catchments has shown seasonal and long-term
variation in water levels. This occurrence can be attributed to several events among
them: recession of water sources, increasing water demand, erratic changes in land use,
changing precipitation patterns and frequencies. The study assessed the current (2020)
and future (2050) spatial-temporal water variability within Thiba sub basin. The
historical trends in climate, rainfall and water demands were characterised and their
influence on water availability assessed using the Water Evaluation and Planning
(WEAP) model. Stream flow variability was evaluated in response to changes in climate,
rainfall, and water demands. Water Evaluation and Planning system with monthly
timesteps calculations was used to compute water situation, and Parameter Estimation
Tool used in calibration of the model. Model was calibrated sing; Nash-Sutcliffe
efficiency, coefficient of determination and Percent Bias, attaining values of 0.86, 0.85
and 6.64 while the validation coefficients were 0.90, 0.88 and 1.08 respectively.
Climate evaluation scenarios under Representative Concentration Pathway 4.5 and 8.5
predict mean rainfall to be 1420.1 mm and 1332.2mm and mean temperatures of 18.8ºC
and 19.1ºC respectively at the end of 2050. The results show a non-significant increase
in rainfall and significant increase in temperatures. Both pathways predict greatly
variable decreasing mean discharge by the end of 2050. This variability is attributed to
the increase in water demand and temperatures in the face of decreasing rainfall. The
best-case and worst-case scenario predicts total water demands to increase to 201,903
and 204,302 billion cubic meters per year respectively. Water consumption will increase
from 77.0 m^3 per capita per day in 2021, to 85.2 m^3 per capita per day, and a
30% increase in unmet demands in all sectors. Results indicate climate change will
significantly impact water resources under the rising water demands, change of land
uses and varying discharge. Mitigation efforts will result in better flows compared to no
intervention. Detailed study linking groundwater to surface water will help understand
better the problem. The study recommends the development of groundwater resources to take care of the expected rise in water demand in the face of dwindling surface water
