ASSESSMENT OF CLIMATE CHANGE IMPACT ON BILATE WATERSHED WATER AVAILABILITY, ETHIOPIA

Abstract

Nowadays climate change is expected to affect society in a number of ways ranging from food security to water resources. Water plays an important role in the socio-economic development of any society. Thus, this study mainly deals with assessing climate change impact on Bilate watershed water availability, Ethiopian Rift Valley Lake Basin. The watershed is situated between 37 0 47‟6‟‟ to 38 0 20‟14‟‟ E and 6 0 33‟18‟‟ to 8 0 6‟57‟‟ N and covers an area of about 3643 km 2 in which Bilate river is the main river flowing throughout the year. To project the probable impact of climate change on the available water, HadCM3 coupled atmosphere-ocean GCM model was used since it is the only GCM model that has grid box containing the study area. The output of HadCM3 coupled atmosphere-ocean GCM model for the A2a and B2a SRES emission scenarios were used to produce future scenarios of precipitation and temperature. Climate change scenarios of precipitation and temperature were developed at two upper and lower stations of the watershed for three periods namely; 2020s (2011-2040), 2050s (2041-2070) and 2080s (2070-2099). A SDSM was used to downscale the coarse scale output of GCMs to the watershed scale. Generally, precipitation shows an increasing trend in the lower sub basin and decreasing trend in upper sub basin and in case of minimum temperature there is an increasing trend in both sub basins but maximum temperature doesn‟t show significant difference from the base period. These changes of climate variables were used as an input for SWAT hydrological model to simulate future flows. The SWAT simulation future flow volume indicated an overall trend of significant decrease in all periods under review. The total average annual flow volume at outlet of the watershed might decrease up to 18.1% and 17.7% for A2a and B2a scenarios respectively. This decrease in the future flow volume might be insufficient to meet future demands for water of the ever increasing population with in and around the watershed. The simulated results should be considered by policy makers with care to aid in water resource planning and management.