Impacts of climate change and demography on surface water demand and supply in the ungauged N’Zo river basin

This study was conducted in the N'ZO river basin. The population of this catchment area is growing significantly, and combined with the effects of climate change, which are likely to cause a decrease in streamflow, this worsening trend exacerbates the problem of surface water resource supply. The primary objective of this work is to evaluate the alignment between water demand and water supply in the city of Man and its surrounding localities. We utilized various climate scenarios, namely Representative Concentration Pathway 4.5 (RCP 4.5) and 8.5 (RCP 8.5), to determine the impact of climate change on surface water resources within the catchment. We employed a rainfall-runoff modelling approach using the GR2M model to simulate different river discharges from 1982 to 2050. Using the WEAP model, we developed scenarios for water demand and resource availability for the period 2021 to 2050. The results showed that temperatures are projected to increase at an annual growth rate of 1.95% according to RCP 4.5 and 2.28% for RCP 8.5, while rainfall is expected to decrease at rates of 0.75% and 0.33% respectively, for RCP 4.5 and RCP 8.5. Additionally, the GR2M model effectively simulated streamflow with satisfactory accuracy. We estimated that the average annual water inflows will be 63.34 million cubic meters (M.m3) under RCP 4.5 and 56.27 M.m3 under RCP 8.5 for 2050. The total annual water demand for the city of Man and surrounding localities is projected to be 9.52 M.m3 under the reference scenario (SR) and 11.03 M.m3 under the high-growth scenario (SC). Our adequacy analysis using the WEAP model indicates that demand remains below supply up to 2050, so there is no cause for concern.