A GIS-Based Watershed Modeling and Water Resources Use Optimization under Dynamic Changing Land Use/Land Cover Conditions

Changes in land use/land cover, and land management play an important role in watershed management due to their influence on soil erosion, hydrology and water quality. Such changes have been perhaps the most pervasive and obvious form that took place within the Legedadi Reservoir watershed, located in upper Awash River Basin, Ethiopia, since the end of the 1960s. The water quality of the Legedadi Reservoir has continually decreased as a result of nonpoint source pollution from upstream sources causing a threefold rise in the cost of drinking water treatment. The objectives of this study were: 1) identify the spatial and temporal pattern of land use/land cover change dynamics within the watershed from aerial photos (1965 and 1984) and satellite images (1996 and 2002); 2) assess the impact of the changes on soil erosion, water quantity (hydrology) and quality through a modeling approach using the Soil and Water Assessment Tool; 3) identify vulnerable sites to implement selective management alternatives; and 4) optimize water resource use that match the prevalent climatic and physical limitations of the watershed. Digital elevation model, soil, and land use/land cover maps of the watershed were developed and/or digitized. Weather and soil databases were prepared. Increased deforestation and expansion of cropland were observed from 1965 to 1984. However, the deforestation trend reversed from 1984 to 1996 mainly due to a forestation program carried out during the mid 1980s. The spatial and temporal changes in land use via remote sensing were coupled with GIS analyses and SWAT modeling to assess the impacts of the land use change dynamics on hydrology (surface water quantity), water quality and soil loss. Assessment of the spatial distribution of soil erosion within the watershed were made to prioritize sites which need immediate control measure to maintain a sustainable resource use. Finally, the soil loss at the watershed outlet was calibrated against the measured reservoir siltation rate and volume within the Legedadi Reservoir. The causes for the decrease in water quality and change in hydrological regime were investigated.