Browsing by Author "Gemoraw, Getu"

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    Climate Change Impact Assessment of Dire Dam Water Supply
    (Addis Ababa University, 2015-03) Gemoraw, Getu; Fentaw, Abegaz (PhD)
    Climate change is a major development challenge to Ethiopia. Developing countries are likely to be affected most, and Ethiopia is one of the most vulnerable countries. The issue of climate change and its impacts on water resources is not new but planners are increasingly concerned about the possible negative impacts of climate change on the utilizable yields from dams and water resource systems. Accordingly, making impact assessment can provide information of the situation for which the corresponding solutions to be addressed. Addis Ababa with its ever-increasing population, has reached a state of critical water shortage. To satisfy the rapidly increasing water demands of the city, the Addis Ababa Water and Sewerage Authority, has done Dire Dam rehabilitation works, Dam raising, to provide additional water security by increasing the storage capacity of the reservoir. The aim of this study is to assess the climate change impact and verify previous hydrological studies carried out to determine the capacity of the reservoir as well as evaluation of supply and demand criteria of Dire Dam Water Supply, which is found in the Northern part of Addis Ababa Metropolitan area. Projection of the future climate variables were done using General Circulation Model (GCM) which is considered as the most advanced tool for estimating the present and future climatic conditions. Statistical DownScaling Method (SDSM) was applied in order to downscale the climate variables at catchment level. A hydrological model, HBV (the latest version 6.3) was utilized for runoff simulation and hydrological forecasting. The reservoir capacity was assessed based on the detail design reports and re-designing works. Redesigning of the reservoir capacity and comparisons with the results used for the original design purpose was also carried out. In addition, evaluation of demand and supply criteria was also carried out using the newly obtained hydrological results, design criteria set for the original design purpose and national standards. Accordingly, the projected future climate change shows an increasing trend for both minimum and maximum temperature. Their mean increment reaches by 1.78°C for minimum and 1.98°C for maximum temperature at the end of 21st century. Precipitation variable showed a various trend by an average of 17.2% and 10.25% increment annually for both A2a and B2a scenarios respectively. HBV model was calibrated and validated with the historic data to make water balance of the catchment. As it resulted with R2=0.78 for the calibration period while R2 =0.69 was obtained for the validation. Future runoff generation was made by simulating the model with downscaled climatic variables as input. The projected run off had mean annual increasing trend by 21.1%, 21.8% and 19.1% for 2020s, 2050s and 2080s respectively. The current status of the reservoir capacity was assessed and it was found that there is excess surface water potential for Legedadi River (57.54 Mm3) than previously estimated during detail design phase (47.75Mm3). As per this study, the anticipated populations to be served from this source were estimated at 949,087, 759,269 and 759,269 in the years 2014, 2020 and 2030 respectively, based on the national standard criterion. Therefore, the runoff faced variability for all three future time periods. There were increasing trends of temperature, precipitation and runoff, which indicates climate change. Key words: Climate Change, GCM, SDSM, HBV, Reservoir Capacity, Dire Dam.