ESTIMATION OF SEDIMENT YIELD AND PRIORITIZATION OF SUB-WATERSHEDS FOR SOIL CONSERVATION PLANNING IN GILGEL GHIBE-I CATCHMENT, SOUTH WEST ETHIOPIA

Abstract

Onsite and off-site effects of soil erosion is the biggest global environmental problems. Some of the offsite problems are silting of dams and reservoir, disruption of lake ecosystems, contamination of drinking water and increased downstream flooding. The Gilgel Ghibe-1 catchment have been also subjected to sedimentation and other problems. This study was aimed to estimate soil loss and sediment yield of Gilgel Ghibe-1 catchment by GIS based RUSLE and SEDD models, and prioritize sub watersheds for soil conservation planning. For this study both primary and secondary data from different sources were used. The amount of soil loss from the catchment was estimated using GIS based RUSLE model which uses the rainfall-runoff erosivity, soil erodibility, topographic effect, soil/surface coverage and land management practice of the catchment as an input. SEDD model was implemented to estimate Sediment Delivery Ratio (the ratio of sediment yield to soil loss). The result of this study showed the mean annual soil loss of the catchment was found 62.98 t ha-1year-1 which is about 26.56x106 t year-1. The mean sediment delivery ratio is found 0.1367 from SEDD model. The mean weighted annual sediment yield of the catchment was then found to be 8.61 t ha-1year-1 and the overall catchment area contributes about 3.63x106t year-1sediment to the reservoir at the dam site. The result of these models were evaluated for its performance and performed well with the value of measured sediment yield and resulted with the value of R2 = 0.684, NSE =0.72 and PBIAS =-24.45 at calibration level and R2 =0.902, NSE =0.6055, PBIAS =-14.52 and RRMSE = 0.807 at validation level. On the bases of soil loss and sediment yield erosion 'hotspot' areas were identified and prioritized. SW4, SW3, SW5, SW6, SW9, SW8, SW7, SW2 and SW1 got 1 to 9 priority level respectively on the bases of mean annual soil loss; while SW4, SW5, SW3, SW6, SW7, SW8, SW9, SW2 and SW1 got 1 to 9 priority level respectively on the bases of mean annual sediment yield. Based on mean annual soil loss 35.2% of the catchment area was covered by slight soil loss class (0-5 t ha-1year-1) and about 56 % of the catchment have soil loss greater than tolerable level which is 11 t ha-1year-1. Sediment yield >10 t ha-1year-1 covers 15.75% of the catchment area and 0-5 t ha-1year-1 covers large area, 78.22%, of the catchment. The result of this study indicated that the catchment and reservoir were under a problem of soil loss and siltation. Therefore, this study recommended immediate action, soil and water conservation measures, should be taken in the catchment according to the priority