Assessing the Impacts of Climate Variability on Stream Flows in the Upper Catchment of River Rwizi.

Abstract

Rwizi River is very vulnerable to climate variability because it relies heavily on rainfall as its main flow contributor. This study’s main objective was to assess the impacts of climate variabilty on streamflow of Rwizi River Upper Catchment, Uganda and it was achieved considering the RCP4.5 (low) and RCP 8.5 (high) scenarios over the horizons 2030s and 2050s and simulating the projected climate with calibrated SWAT model for the two scenarios. The trend analysis for the climate parametrs and stream flows was done by Mann-Kendall test and its magnitude was estimated using the Theil-Sen approach. The precipitation of the 2030s and 2050s horizons will be higher than that of historical period (1989-2017) by 6.176% and 7.312% under the RCP4.5. However, for the RCP8.5, the precipitation is projected to increase by 7.199% and 7.376% for the 2030s and 2050s horizons, respectively. The maximum temperature of the 2030s and 2050s horizons will be higher than that of historical period (1989-2017) by 11.961% and 12.069% under the RCP4.5. However, for the RCP8.5, the maximum temperature is projected to increase by 12.015% and 12.252% for the 2030s and 2050s horizons, respectively. The minimum temperature of the 2030s and 2050s horizons will be higher than that of historical period (1989-2017) by 7.997% and 8.007% under the RCP4.5. However, for the RCP8.5, the minimum temperature is projected to increase by 12.015% and 8.257% for the 2030s and 2050s horizons, respectively. The Soil and Water Assessment Tool (SWAT) hydrological model was calibrated and used for the impact assessment. Their difference in simulating the flows under future climate scenarios was also investigated. The performance of SWAT in terms of the Nash and Sutcliffe was 0.52 for both calibration and validation, respectively. The models performance in terms of pbias was 20.8% and -29.5% for calibration and validation respectively. The R2 performance for the model was 0.74 and 0.72 for calibration and validation respectively. This indicated that the hydrological model was satisfactory for application for climate variability impact investigation. The simulated streamflow for RCP 4.5 and RCP 8.5 scenarios over the 2030 horizon is less than the one for baseline period. The same can be said for the 2050 horizon where the simulated flows for both scenarios are less as compared to the baseline flow. These impacts of climate variability on the river flow show the need for a careful planning of relevant and appropriate adaption measures at a catchment scale