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.


INTRODUCTION
Climate change is associated with global warming that is induced by the increase in carbon dioxide and other radioactive trace gases in the atmosphere. This was the focus of scientific investigations due to the fact that climate change has significant implications for the environment, ecosystems, water resources and virtually every aspect of human life (IPCC, 2001). The most important and immediate effects of global warming would be the changes in local and regional water availability, since the climate system is interactive with hydrologic cycle. Such effects may include the magnitude and timing of runoff, the frequency and intensity of floods and droughts, rainfall patterns, extreme weather events, and the quality and quantity of water availability; these changes in turn influence the water supply system, power generation, sediment transport and deposition, and ecosystem conservation. Some of these effects may not necessarily be negative, but they need to be evaluated as early as possible because of great socio-economic importance of water and other resources (ARORA and BOER, 1996) Ethiopia is vulnerable to climate change since the economy of the country mainly depends on agriculture, which is very sensitive to climate change and variations. In Bilate watershed the increase in population growth, economic development together with climate change is expected to cause an increase of water demand for different purposes. Bilate River basin is among the major sub basins that are part of the Abaya-Chamo basin, which is the sub basin of the Rift Valley Lake Basin with high population pressure, and high variation of climatic parameters. This increase in population growth, economic development together with climate change is expected to cause an increase of water demand for different purposes such as irrigation, hydropower, industrial and domestic needs (AWULACHEW, 2002). Due to climate change, water and its availability and quality will be the main pressures on, and issues for, societies and the environment. Climate change also increase the vulnerability of ecosystems due to temperature increases, changes in precipitation patterns, frequent severe weather events such as flooding and prolonged droughts (IPCC, 2007). Owing to rapid growth in population, the need for the optimum development of water resources has become more urgent than ever. Population distribution, irrigated agriculture, urban and industrial water use are major factors deriving water availability issues. The impact of climate change on water availability of Bilate watershed is not well investigated in terms of both quantity and quality.
The study was conducted to assess impacts of climate change on water availability of the watershed by developing temporal climate change scenarios (precipitation and temperature) for the future periods of 2011-2099 and to quantify the possible impacts of the climate change on the available water for the period of 2011 till 2070 using SWAT model.

METHODOLOGICAL APPROACH
The general methodology consists of using climatic output data from general Circulation models (GCMs) to retrieve climate scenarios. The weather generator was then used to produce daily temperature and precipitation data to serve as an input data for the SWAT hydrological model to simulate stream flow. The future simulated results were then compared with the base line period as a means of obtaining the change caused by climate change. The detail methodology of the study is clearly stated below.

Statistical Downscaling Model (SDSM)
Climate change scenario The climate change scenarios produced for this study were based on the outputs of GCM results that are established on the SRES emission scenarios. The outputs of HadCM3 GCM model for the A2 and B2 emission scenarios were used to produce the future scenarios. As it is easy to implement, and generation of the downscaled values involves observed historic daily SDSM was adopted to downscale the global scale outputs of the HadCM3 model outputs into the local watershed scale. The future time scales from the year 2011 until 2099 were divided into three periods of 30 years and their respective changes were determined as deltas (for temperature) and as percentages (for precipitation) from the base period values. HadCM3A2a and HadCM3B2a were the two GCM output files used for the scenario generation and the regression weights produced during the calibration process were applied to the time series outputs of the GCM model. The final product of the SDSM downscaling method was then found by averaging the twenty independent stochastic GCM ensembles and comparison was done between observed and downscaled base line future data's. The future scenarios were developed by dividing the future time series in to three equal periods of 30 years: 2011-2040, 2041-2070, 2071-2099.

Soil and Water Assessment Tool (SWAT) hydrological model
For this study SWAT hydrological model was selected for the impact assessment as the model is physically based, spatially distributed, and it belongs to the public domain and (LENAHART et al, 2002), and computationally efficient hydrological model, which uses readily available inputs. Watersheds were subdivided into sub watersheds and further into hydrologic response units (HRUs) to account for differences in soils, land use, crops, topography, weather, etc. The model has a weather generator sub routine that generates daily values of precipitation, air temperature, solar radiation, wind speed, and relative humidity from statistical parameters derived from average monthly values. For estimating potential evapotranspiration Hargreaves method was selected at it requires air temperature only. SWAT splits hydrological simulations of a watershed into two major phases: the land phase and the routing phase. The difference between the two lies on the fact that water storage and its influence on flow rates is considered in channelized flow (NEITSCH et al, 2002). Two weather generator stations such as Bilate Farm and Hossana were selected to generate missing weather data in SWAT simulation model. For data generation, weather parameters were developed using the weather parameter calculator WXPARM (WILLIAMS, 1991) and Dew point temperature calculator DEW02 (LIERSCH, 2003).
Determination of Impacted Future Flow rate SWAT was applied to simulate the impacts of climate change on river flow by assuming that change in precipitation, temperature and land use in the future from base line period affects flow volume. 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 assumed change in land use were given directly as an input to the SWAT model. Other climate variables such as wind speed, solar radiation, and relative humidity were assumed to be constant throughout the future simulation periods. The future two periods, 2011-2040 and 2041-2070 of both Bilate Farm and Hossana station data's obtained from SDSM output and change in land use were used as an input in SWAT model to determine the impact of climate change on future stream flow volume. The SWAT simulation for the 1991 to 2010 period was used as a baseline period against which the climate impact was assessed. The SWAT model was then re-run for the future periods with the downscaled output of climate variables and assumed land use changes to compare them with the baseline period model results.

Scenarios developed for base period
Even though the SDSM model performs reasonably well in estimating the mean monthly precipitation at Bilate farm in many months but in some months especially in rainy seasons such as April, May, and July there is a relatively large model errors. But at Hossana station SDSM doesn't able to simulate except for some months such as January, February and October. This is to mean that, the model underestimates in months of April to September. But the model overestimates in months of March, November and December. The result, however, can be taken as satisfactory in such a way that precipitation downscaling is necessarily more problematic because of daily precipitation amounts at individual sites are relatively poorly resolved by regional scale predictors than temperature.

Flow Calibration and validation
The calibration results show that there is a good agreement between the simulated and gauged monthly flows at the out let of the watershed i.e. at Bilate Tena gauging station. This is demonstrated by the correlation coefficient (R 2 =0.79 and R 2 =0.76 during calibration and validation period), the Nash-Suttcliffe (1970) simulation efficiency (E NS =0.77and E NS =0.75 during calibration and validation period) and percent deviation (D=1.83 and D=-2.48 during calibration and validation period) The results fulfilled the requirements suggested by (Santhi et al, 2001) for R² >0.6 and E NS > 0.5.

Climate Change Impact on Future Flow
In both A2a and B2a scenarios there is a slight increase of average monthly flow in months of September, October, November, December and January. On the other hand, there is a slight decrease of average monthly flow in other months. Generally, the decreasing and increasing pattern seems to be higher in the time period of 2011-2040. In general there is a decrease of base flow during dry months but during the rainy months there is an increase of peak surface flow. In general the average total seasonal flow shows decreasing pattern in month of March to September for A2a scenario but for B2a scenario it is almost constant. But in months of October to February there might be an increasing pattern of average total seasonal flow in both scenarios. In 2011-2040 period, the average total annual flow might decrease by 3.7% and 1.5% for A2a and B2a scenarios respectively. But in 2041-2070 period, it might increase by 2.6% and 3.7% for A2a and B2a scenarios respectively. This decrease of average total annual flow in both scenarios in 2011-2040 period might be due to the fact that decrease of a total average annual precipitation in Hossana station is higher than the increase of precipitation in Bilate farm station and slight increase of annual minimum temperature in both scenarios of Bilate station in SDSM out puts results.
Climate change adaptation strategies In general adaptation to climate change problems causing reduction in the total water yield can be the range of actions taken in response to changes in local and regional climatic conditions. These responses include autonomous adaptation, i.e., actions taken by individual actors such as single farmers or agricultural organizations, as well as planned adaptation, i.e., climate-specific infrastructure development, regulations and incentives put in place by regional, national and international policies in order to complement, enhance and/or facilitate responses by farmers and organizations. For the 2011-2040 periods, watershed based integrated water resource management approach is the possible adaptation options to be implemented to overcome the reduction of average monthly flow in some months of the watershed.

CONCLUSIONS AND RECOMMENDATIONS
The SDSM has good ability to replicate the historical maximum and minimum temperatures than precipitation. A SWAT hydrological model simulation has shown that the model is able to simulate the observed stream flow in the watershed reasonably. This is proved during calibration and validation period of the model performance criterion such as regression coefficient and Nash-Sutcliffe used to evaluate the model.
The change in climate variables such as decrease in precipitation and increase in temperature thereby increase in evapotranspiration which is very sensitive parameter that can be affected by changing climate than any other hydrological component are likely to have significant impact on Stream flow. There is an increase of water demand due to an increase of human need of water for different purposes. This in combination of the future climate change impact on reduction of the available water in the watershed causes a water stress within and around the watershed.
ACKNOWLEGMENT Above all I thank the almighty of GOD for his mercy and grace upon me during all my works and in all my life. Very special thanks to my supervisor Dr. Ing. Kassa Tadele for his valuable advice, guidance, encouragement and critical comment during the research work. His contribution was really appreciable. Even with a lot of work engaged we have discussed many times at his work and rest time. He devoted his time from the beginning of the research title to the completion of the thesis work. Without him this research would not have been realized. I would like to express my appreciation to Arba Minch University School of Graduate Studies staff members and libraries for their provision of the necessary reference books and assistance during my study. I am extremely grateful for all my friends', class mates for the challenges, knowledge sharing and amazing time we spend together during my stay at Arba Minch University.
Last but the bests, I would like to thank my wonderful family especially MOM, for their ultimate support throughout my life.