Abstract:
The Aswa catchment is currently facing significant challenges due to water shortages. This is due to the increasing water demands which is because of industrial growth, increased population growth rate, and the changing land use in the catchment. This study aimed to assess the future water demand and supply under the influence of climate and land use changes in the Aswa sub-watershed. The study applied the SDSM software to project climate in the catchment. The projection was done for the period of 2030,2050,2070 and 2090 under RCP4.5 and RCP8.5. The land use projection was done in QGIS using the MOLUSCE plugin. This software uses the Cellular Automata for the prediction of land use changes, and this was done for the years 2030, 3050, and 2070. The kappa value obtained during the projection was 0.931 which is close to the standard value of 1.
The study used the SWAT model and the GMS model to assess the impacts of climate and land use changes on surface and groundwater respectively. The hydrological modelling was simulated from 1990 to 2015. It was calibrated from 2000 to 2004. The NSE value in calibration was 0.56 and the R2 value was 0.62. The model was validated from 2005 to 2009. The NSE value obtained was 0.72 and the R2 value was 0.78. The model showed a good performance. The GMS model revealed a notable decrease in groundwater levels of 13.1Mm3 due to reduced recharge rates and increased human activities. The available surface water in the catchment was 8.784 m3/s.
The Water Evaluation and Planning (WEAP) was used to forecast water demand in the sub-watershed. In this study, 2015 was taken as the base year and the reference period was from 2016-2070. The study employed different scenarios such as the reference scenario and the high population growth rate scenario. The water demand for the catchment is expected to increase due to the high population growth rate in the catchment. Under the reference scenario, WEAP stimulated that the water demand increased by 9.87 % in 2020 compared to that of 2015. The water demand is expected to rise by 12.71% in 2070. This is from 251.69MCM in 2015 to 967.98MCM in 2070.
The high population growth rate was predicted to increase the water demand from 251.69Mm3 in 2015 to 10839Mm3 in 2070. The reuse of water scenario was found by the model to be the most effective approach to unmet demands and thus the best mitigation measure for water scarcity in the sub-watershed.
