Assessment of Water Availability using Water Evaluation and Planning (WEAP) model in Namatala River Catchment.

Abstract

Trends in global population growth coupled with effects of climate change are a threat to water availability as experienced in Namatala River Catchment (NRC), Mbale, Uganda. This study was conducted to quantify and characterize water availability and demand using the Water Evaluation and Planning (WEAP) model over a 20-year period to guide water resources management in NRC. WEAP model was used to perform quantification and characterization of water availability under the following scenarios: I) reference or current conditions of climate, population and land-use/land cover, II) increased population of 3.0% year-1 and climate change (increased mean temperature of 8.5% and rainfall of 9.8%), III) land-use/land-cover changes (converting 10% agricultural land into agroforestry). WEAP model sensitivity analysis and calibration were performed using hydrometeorological data of 2016-2018 and validated using data of 2019. WEAP was sensitive to crop coefficient (Kc) and runoff resistance factors (RRF) out of nine parameters. The model’s performance during validation produced Nash-Sutcliffe efficiency (NSE) value of 0.7, Normalized Roof Mean Square Error (nRMSE) of 0.6%, and Percent Bias (PBIAS) of 7.4%. Model scenario analysis showed that: under reference scenario I, NRC yields 247 MCM year-1; under scenario II, NRC yields 200 MCM year-1; and under scenario III, NRC yields 257 MCM year-1. Model scenario analysis results showed that NRC has sufficient water resources of 284 million m3 year-1 available to satisfy an average water demand 92 million m3 year-1 over the study period of 2020 to 2040. However, fluctuations in quantities of available water, lack of water reservoirs and inadequate abstraction capacity result in low water demand satisfaction of 36.6%. Therefore, this study recommends that the potential of runoff storage should be explored together with improvement in water supply system capacity to enhance access to the available water. To mitigate the threat of water stress, a supply prioritisation should be done such that domestic and industrial water supply are allocated water before providing water for agriculture. In addition, catchment management to increase ground water storage is vital to improve water availability for socio-economic and environmental wellbeing of the catchment.