Evaluation of The Impacts of Climate Change on Water Resources of oMO-Gibe River Basin (A Case Study of Gibe Iii Catchment)
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Date
2017-03
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Addis Ababa University
Abstract
use of regional climate model (RCMs) outputs in hydrological impact assessment has given due attention on the world. Studies that evaluate downscaled simulations of General Circulation Models (GCMs) by Regional Climate Models (RCMs) for African basins are noticeably missing. Recently, the Coordinated Regional Climate Downscaling Experiment (CORDEX) initiative has made multiple RCMs' outputs available for end users across the African continent. Before climate simulations receive applications in impact studies, accuracy of the simulation results has to be evaluated. In the study the impact of climate change projections on stream flow of three independent GCMs as simulated by the regional climate model COSMO-CLM (CCLM) over the Gibe III basin were evaluated. The performance of all models was assessed in several ways: At first, the performance of the RCM simulation results were evaluated in Gibe III basin using a suite of statistical measures such as bias, Root Mean Squared Error (RMSE), and coefficient of Variation (CV). The second evaluation was based on the combined influence of corrected RCM-simulated temperature and precipitation on hydrological simulations of monthly mean stream flow under current (1986–2005) climate conditions. Then, the hydrological simulations of projected future (2011–2030) and (2031-2050) climate conditions were compared with the baseline with two emission scenario projections (RCP4.5 and RCP8.5). Findings of this study indicate that the annual rainfall bias of the models varies between 9.6% and 30.2% suggesting overestimation. In many aspects the MPI climate model performed best in terms of bias and RMSE. However, it was found that the performance of the models differs subject to the performance measures used for evaluation. The use of the ensemble mean rainfall simulation improves representation of assessed rainfall characteristics in the basin. Based on the finding climate change studies may benefit from the use of multi model simulation. Biases of most of the models prove that correction for the systematic error of RCM outputs before receiving application by users. The maximum and minimum temperatures were projected to increase for both-time horizons under both scenarios with largest increase under RCP8.5 and the lowest increase under RCP4.5. An increase of mean annual maximum temperature under RCP4.5 ranges from (+0.66oC to +0.48oC) and (+1.13oC to +1.33oC) while under RCP8.5 that ranges from (+0.88oC to 1.24oC) and (+1.58oC to 1.78oC) in both future periods of (2011–2030) and (2031-2050) respectively. Similarly, the annual minimum temperature increase ranges from (+0.32oC to +0.71oC) and (+0.92oC to +1.21oC) under RCP4.5 scenario while (+0.56oC to +1.29oC) and (+1.14oC to +1.93oC) under RCP8.5 in both future periods (2011–2030) and (2031-2050) respectively. Compared to the base line period all the models indicates the total annual precipitation increases in near term period (2011-2030) ranging from (0.16% to 8.36%) compared to midterm (2031-2050) as projected under the RCP4.5. In midterm periods, climate models predicted that a decrease in rainfall in all seasons of the year except for autumn season under RCP4.5 scenario, rainfall is largely predicted to increase by (36.57%) from the ensemble mean compared to the base line period. The stream flow projections are made for both periods in the future with two emission scenarios of RCP4.5 and RCP8.5. Consequently, acceptable calibration and validation of the model performance statistics R2, NS and Pbias were obtained. Relative to the baseline period the change in mean annual stream flow from 2011–2030 are mostly positive and indicate increase in available discharge in the river under RCP4.5 and RCP8.5 scenarios and from 2031-2050 under RCP4.5 scenario. Flow duration curves showed that the probability of occurrence of high flows will be more in both future periods under RCP4.5 and RCP8.5 scenarios relative to the baseline period. The overall conclusion of the study is that Gibe III basin is likely to face more floods in the future and low flows are projected to decrease. The outcomes suggest that it is important to consider the influence of climate change on water resources to frame appropriate guidelines for planning and management.
Keywords: COSMO-CLM (CCLM), CORDEX, CV, MPI, Gibe III, RCP, RCM, RME, bias
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Keywords
Cosmo-Clm (Cclm), Cordex, Cv, Mpi, Gibe Iii, Rcp, Rcm, Rme, Bias