ASSESSING THE EFFECT OF RAINFALL VARIABILITY ON STREAMFLOW IN RIVER MPOLOGOMA CATCHMENT

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Abstract

River Mpologoma catchment is one of Uganda’s water resources that is vastly relied on for supporting agricultural livelihoods among other activities. Stream flow is an important feature used to determine availability of water in surface water resources and is partly formed from rainfall. This study sought to assess the effect of rainfall variability on streamflow behaviour in Mpologoma catchment and to project future streamflow behaviours. Daily rainfall and discharge data for four stations located in the catchment were obtained for the period 1981 – 2015. Totals (for rainfall) and averages (for discharge) were generated for the annual period as well as for the major wet seasons March-May and September-November after which low flow, average flow and peak flow statistics were computed. The Mann-Kendall test was used to assess trends in streamflow while the Pearson correlation and time-series regression analysis were used to investigate the relationship between rainfall and streamflow. Future rainfall data (2021 - 2040) were down scaled using the CORDEX program for 2 climate scenarios (RCP 4.5 and RCP 8.5) after which streamflow were forecasted from the fitted regression equations by extrapolation. The Mann-Whitney U-test was used to examine if future changes in streamflow were significant. Results reveal that over time, low and peak flow within Mpologoma catchment are highly variable (CV > 60%) especially during the wet seasons. Significant negative and positive trends were observed in the MAM low flows and SON peak flows respectively. The results further show that low flow, peak flow and average flow are positively correlated (p < 0.05) with MAM, SON and annual rainfall respectively. Time series regression shows that rainfall has a significant influence on streamflow where a 1 mm increase in rainfall at a given time resulted in a 0.2%-0.7% instant rise in streamflow. Future projections suggest that low flow during the MAM season is expected to decline by 66%-77% whilst annual average flow is expected to fall by 58%-64%. On the contrary, peak flow for the SON season is not expected to change in the near future. The study concludes that annual and seasonal rainfall variability threatens water availability in the Mpologoma river catchment. The projected decline in streamflow is expected to bring about water scarcity in the area which would threaten agricultural production and livelihood in general. The study recommends that adaptation measures be developed to mitigate the anticipated impacts of water shortages.

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