Impacts of land use/ land cover Change on sediment yield and stream flow to Reservoirs. Case Study on Finchaa Hydropower Reservoir, Ethiopia.

Abstract

Throughout the world, Land use/ land cover change impact on the reservoir is the main concern for sustainability of water management and water use activities. Changes in land use/ land cover is a fundamental variable that impacts on, and links many parts of the global environment. The effects of land use/ land cover changes and improper management systems have played a significant role in causing high soil erosion rates, sediment transport and affects life expectancy of the reservoir and have an impact on the water balance of the catchment by changing the magnitude and pattern of runoff, peak flow, sediment yield and ground water levels. The Finchaa watershed has an area of about 2863km 2 . For the analysis of land use/ land cover change in the Finchaa watershed the Geographic Information System (GIS) version based Soil and Water Assessment Tool (SWAT) has been used to evaluate sensitivities and patterns of land use/ land cover changes in the Finchaa sub-basin. The aim of this study is to evaluate the possible impacts of land use/ land cover Changes on sediment yield and stream flow to Finchaa hydropower reservoir, located in western Oromia Regional state, Ethiopia, Upper Blue Nile Basin. The required input data for this study were Digital Elevation Model, Land use/ land cover map, soil map and data, stream flow data and weather data. After the data was collected, an analysis of all the collected data was made. Model calibration and uncertainty analysis were performed with sequential uncertainty fitting (SUFI-2) that is linked with SWAT in calibration Uncertainty program known as SWAT-CUP. The calibration process was used to calibrate the model parameters using time series data from 1990 to 2002 and data from 2003 to 2011 were used to validate the model using the input parameters. Twelve flow parameters were the most sensitive parameters for the stream flow of the study area and used for the model calibration and validation. Calibration and validation of the SWAT against streamflow in the Finchaa reservoir attained a coefficient of determination (R 2 ) and Nash- Sutcliffe (NS) were used to evaluate the performance of the model monthly. Flow calibration gives coefficient of determination and Nash-Sutcliffe simulation efficiency 0.83 & 0.74 respectively. Flow validation gives 0.86 & 0.83 for coefficient of v determination and Nash-Sutcliffe values respectively. This result indicates that the observed values show good agreement with simulated value for stream flow. Land use change scenarios were generated for four scenarios based on socio-economic data and physical factors influencing the land use. The results of the Evaluation of land use/land cover changes in Finchaa watershed show that land use changes can have significant impacts on the Finchaa reservoir. From scenario simulation it was observed that extreme deforestation (0% forest cover) likely due to growth in urbanization, agriculture etc. exhibited an increase of about 0.210% in the water yield and 57.361% in the sediment yield from base conditions. In case of afforestation (100% forest cover) scenario the sediment yield decreased by about 16.207% and 0.160 % water yield. Therefore, various land use mitigation measures were further evaluated based on economic analysis as adaptation options to mitigate the land use/ land cover change impacts and appropriate soil conservation measures based on suitable afforestation techniques can prove influential in mitigating the risk of soil erosion in this Finchaa watershed. Understanding how changes in individual land use types influence the dynamics of streamflow and sediment yield would greatly improve the predictability of the hydrological consequences of land use changes and could thus help stakeholders to make better decisions