Assessment of climate change impact on water availability of bilate watershed, ethiopian rift valley basin

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 water availability of Bilate watershed, Ethiopian Rift Valley Basin. The watershed is situated in Ethiopian Rift Valley Basin and covers an area of about 3643 km² in which Bilate river is the main river flowing throughout the year. To project the probable impact of climate change on the available water, Hadley Centre Coupled Model, version 3 (HadCM3) atmosphere-oceans Global circulation Model (GCM) was used since it is the only GCM model that has grid box containing the study area for Statistical Downscaling Model (SDSM). The output of HadCM3 coupled atmosphere-ocean GCM model for the A2a and B2a Special Report on Emission Scenarios (SRES) 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 and their respective changes were determined as deltas (for temperature) and as percentages (for precipitation) from the base period values. The future climate variable such as daily precipitation, maximum and minimum temperature found as an output from the GCM model and downscaled by the SDSM model and the likely change in land use were given directly as an input to the Soil and Water Assessment Tool (SWAT) model. The SWAT simulation of future average annual flow shows a decreasing trend in 2011–2040 periods and an increasing trend in 2041–2070 periods. The average total annual flow at outlet of the watershed might decrease up to 3.7% for A2a scenario and 1.5% for B2a scenario for the 2011–2040 periods but for 2041–2070 periods it might increase up to 2.6% for A2a scenario and 3.7% for B2a scenario. The decrease in the future flow of 2011–2040 periods might be insufficient in some months to meet future demands for water of the ever increasing population with in and around the watershed. Adopting water storage options to store the excess water flowing during the rainy period is crucial.