Assessing Impact of Land Use/Cover Practices on Reservoir Capacity Through Erosion and Sedimentation in Gilgel Gibe III Dam, Southwestern Ethiopia
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Date
2021-06-25
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Addis Ababa University
Abstract
Land use/cover change practice and the conversion of the natural vegetation cover into cropland, in particular, are major human-induced problems in Ethiopia. This has played a substantial role in increasing the rate of soil erosion, sediment yield, and reducing the designed reservoir storage capacity of hydroelectric power in the country. The study evaluates various land cover change and their impact on the reservoir capacity, through erosion and sedimentation. Time series Landsat images of (2001−2015) and Sentinel-2A of (2020) besides Sentinel-1 data were used to assess land cover change and reservoir area dynamics. Both supervised and unsupervised land use classification technique; Kappa statistics assessment for accuracy of the classification; post-classification method for change evaluation; Integrated model of the revised universal soil loss equation (RUSLE) and the sediment delivery ratio (SDR) along streamline to estimate soil erosion and sediment yield; threshold approach of Radar and normalized difference water index (NDWI) delineated water spread area for the water year of 2016-20; the Trapezoidal and Mid Area system was applied to estimate the live storage capacity of a reservoir with the elevation. Five land cover maps (2001-2020) were undertaken and six classes were classified. Over all accuracy and Kappa coefficient were above eighty percent agreed with classified land cover. Land cover change assessment resulted that 15 percent change took place between grassland and cropland which is the highest conversion. The mean rate of soil erosion (t/ha/yr) rises from 1093.9 in 2001 to 4222.7 in 2020 while the corresponding sediment yield along streamlines increased from 573.3 to 609t/ha/yr for the same period. Grassland accounts for the highest soil erosion (t/ha/yr.) 1085 and 770.5 in 2001 and 2020. The area (sq. km) of Gilgel Gibe III reservoir augmented starting from 121.5 in 2016 to 195.8 in 2020. The original live storage capacity (2015) was 11750 Mm3 at a normal operating level of 855m. The estimated live storage capacity was 9511.4 Mm3 at 889. 9 m and has reduced from 11750 Mm3 (2015) to 9511.4 Mm3. Finally, the percentage coverage of cropland area is 43 following grassland 41.8 in 2020 and the mean rate of soil erosion and analogous sediment yield in the same year of the catchment were 4,222.7 and 609 t/ha/yr. respectively. The study advocates watershed management by planting tries and land controlling practices from erosion.
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Land Use/Cover, Revised Universal Soil Loss Equation, Normalized Difference Water Index, Trapezoidal, Gilgel Gibe III Dam, Radar