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UNESCO-IHE Institute for Water Education, Delft (2014)

The effect of soil erosion on reservoirs : a case study on Belbela reservoir, Ethiopia

Teka, M.T.

Titre : The effect of soil erosion on reservoirs : a case study on Belbela reservoir, Ethiopia

Auteur : Teka, M.T.

Université de soutenance : UNESCO-IHE Institute for Water Education, Delft

Grade : Master of Science (MS) 2014

Résumé
Soil erosion is an immense problem that has threatened water resource development in Ethiopian watersheds. The Belbela watershed is located in central highlands of Ethiopia. This watershed is one of the watersheds where sediment yield is an issue for reservoir management. Due to the increasing cultivation practice, the problem of land degradation and reservoir sedimentation is apparent in this watershed. The outlet of Belbela watershed is dammed to formulate Belbela reservoir. This reservoir is the main water source for more than 700 ha cultivation areas. It is known that reservoirs will lose their capacity due to sedimentation, and are therefore seriously threatened in their performance. Due to this, an insight in to Soil erosion and sedimentation mechanisms and mitigation methods plays a crucial role for sustainable water resource development in the catchment. More specifically, understanding of the quantity of soil erosion in the watershed and its deposition in the reservoir is necessary for effective watershed and reservoir management. The main objective of the research is to model the spatially distributed soil erosion processes in the watershed of the study area in order to predict their influence on the reservoir life expectancy. It is also to assess the impact of different watershed best management practice (BMP) scenarios (i.e. stone line, ploughing on contour and strip cultivation) on sediment yield for sustainability of the reservoir. An integrated hydrological and sediment yield model called Soil and Water Assessment Tool (SWAT) was used. The datasets that were used in this research are digital elevation model (DEM), land use map, soil map, weather data and hydrology data.The SWAT model was calibrated and validated based on stream flow data from gauging station at the outlet of the watershed. In general, the results of the model performance evaluation analysis showed well performance of the model for our study. The calibration monthly based result shows that PBIS 4.11%, r² >0.72 and ENS > 0.72 and for the validation, PBIS -7.74 %, r² > 0.90 and ENS > 0.87.Due to unavailability of sediment data, we couldn’t validate the model for sediment yield. From the sediment yield analysis, it is found that Belbela watershed generates about 4.3 ton/ha/year sediment yields, which falls in high range of sediment yields. With this sediment yield, the life expectancy of Belbelareservoir which was 30 years will be reduced to 23 years (a reduction by 23%).We predicted with our simulation model the possible impact of the different BMP elements on the life span of Belbela reservoir. If stone line is built (scenario-1a : reducing the farm slope length by 75%) in the agricultural areas with slope over 6%, up to 46% reduction in sediment yield can be obtained in the watershed. Besides, on average 18% reduction of the sediment yield can be achieved simply by application of strip cultivation (scenario-3). The reduction in the amount of sedimentation showed an impact on the life expectancy of the reservoir. Stone line (scenario-1a) alone could improve the life of the reservoir to 43 years (an increase by 43%) where as scenario-1b shows no increment but can achieve the design period of the reservoir. However, ploughing on contour (scenario 2) and strip cultivation (scenario 3) conservation method will not even achieve the design period of the reservoir.In general it is observed that reducing the farm slope length by 75% (scenario 1-c) shows the best result as compared with the rest scenarios, in reducing the sediment yield and increase the life expectancy of the reservoir. Thus technically it could be recommended for the watershed as best conservation measures. Nevertheless, this study has not incorporated the response of the BMP options to agricultural productivity in the area. Thus, the effect of this management option on agricultural productivity needs to be studied. The one that have considerable productivity change due to its implementation can be practiced as it sustain the reservoir life which is one of the main water sources for agriculture.

Sujets  : soil erosion reservoirs case studies Ethiopia

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