PRIORITIZATION OF SUB-WATERSHEDS FOR WATERSHED MANAGEMENT USING SWAT MODEL AND GIS TECHNIQUES: A Case study on Gilgel· Abbay Watershed, Lake Tana basin, Ethiopia

Abstract

Gilgel- Abbay watershed which is one of the sub basins of the Lake Tana is known to be severely affected by water erosion. This has been resulting in decline of the land productivity in the watershed and excessive sedimentation on downstream areas that is a serious concern for the sustainability of the three large irrigation projects proposed in the watershed by huge investment. To reverse the above concerns, appropriate watershed management is urgently needed in the entire watershed. However, as it is difficult and economically not feasible to consider all areas under the threat of erosion at once, prioritization of sub watersheds is required to address few critical sub watersheds at a time. Therefore, the main objective of this study was prioritization of sub watersheds of the Gilgel-Abbay watershed on the basis of sediment yield for watershed management. Soil and Water Assessment Tool (SWAT) model that embodied with Geographic Information System (GIS) was used to simulate stream flow and sediment yield of the watershed. The simulations of the model were calibrated and validated against measured data of stream flow and sediment yield. Both the calibration and validation results showed good match between measured and model predictions. To this end, the average annual sediment yield of the watershed was estimated to be 26ton/ha/yr. However, the spatial patterns of sediment yield vary within the watershed, with maximum value of 95ton/ha/yr. Based on the category assigned to the sediment yield, sub watersheds were categorized in to five watershed management priority categories. Out of the total 49 sub watersheds, 4 sub watersheds (7. 8%) with sediment yield of 60 to 95 ton/ha/yr fall under extremely high priority category. Eight sub watersheds (18%) with sediment yield of 30 to 60 ton/ha/yr fall under very high priority category. Sixteen sub watersheds (36.6%) with sediment yield of 15 to 30ton/ha/yr fall under high priority category. Eleven sub watersheds (23. 9%) with sediment yield of 9 to15ton/ha/yr and ten sub watersheds (12 . 70%) with sediment yield of Oto 9 ton/ha/yr are categorized as medium and low priority sub watersheds, respectively. Therefore, detail watershed management plan need to be developed at manageable size of sub watersheds and immediate action is required to be taken with high emphasis to areas showing higher sediment yield to control the existing accelerated erosion in the watershed and for reducing sedimentation on reservoirs and downstream areas.