Sediment Yield Source Identification in Gilgel Gibe-I Catchment Using GIS-based RUSLE and SEDD Models for Soil Conservation Planning, South West Ethiopia

Gizaw Tesfaye

doi:10.7176/jees/9-2-04

Abstract

Onsite and off-site effects of soil erosion are the biggest global environmental problems. Some of the offsite problems are silting of dams and reservoir, disruption of lake ecosystems, and increased downstream flooding. Gilgel Gibe-1 catchment has been also subjected to sedimentation and other offsite problems. This study was aimed to identify the source of sediment yield in Gilgel Gibe-1 catchment using GIS-based RUSLE and SEDD models for soil conservation planning. Primary and secondary data from different sources were used to estimate soil loss from the catchment using GIS-based RUSLE model. Rainfall-runoff erosivity, soil erodibility, topographic effect, surface coverage, and land management practice of the catchment were used as an input to RUSLE model. Sediment Delivery Ratio was estimated using the SEDD model. The result of this study shows the mean annual soil loss of the catchment is 62.98 t ha -1 year -1 which are about 26.56x106 t year -1 and the mean sediment delivery ratio is 0.1367. The mean annual sediment yield of the catchment is 8.61t ha -1 year -1 and the overall catchment area contributes about 3.63x106t year -1 sediment to the reservoir at the dam site. On the bases of mean annual sediment yield, a source of sediment yield areas are identified and prioritized. Accordingly, SW4, SW5, SW3, SW6, SW7, SW8, SW9, SW2, and SW1 got priority level in the order of 1 to 9. Sediment yield >10 t ha -1 year -1 covers 15.75% of the catchment area and 0-5 t ha -1 year -1 covers 78.22%, of the catchment. The result of this study indicates the catchment and reservoir are under the problem of soil loss and siltation, respectively. Therefore, it is recommended that immediate action, soil, and water conservation measures, should be taken in the catchment according to the priority of sub-watersheds and further detail study is suggested with the support of experimental test plots. Keywords: GIS, RUSLE, SEDD, Sediment Yield, Soil Loss, Sub-watershed. DOI : 10.7176/JEES/9-2-04

Highlights

Soil erosion is one of the biggest global environmental problems resulting in both on-site, and off-site effects (Niu et al, 2003)
Revised Universal Soil Loss Equation (RUSLE) is used to estimate mean annual soil loss (A) and Sediment Distribution delivery (SEDD) is used for estimation of sediment delivery ratio (SDR)
Determination of Revised Universal Soil Loss Equation (RUSLE) Model Factors Five major factors are used in RUSLE for computation of average annual soil erosion expected on the field slopes and are represented in the equation given by (Renard et al, 1997): A = RKLSCP

Summary

Introduction

Soil erosion is one of the biggest global environmental problems resulting in both on-site, (loss of topsoil, minimize water holding capacity of the soil, pollutants carried off by water), and off-site effects (silting of dams, disruption of lake ecosystems, contamination of drinking water and increased downstream flooding) (Niu et al, 2003). Soil erosion is a serious problem in the Ethiopian highland areas that increased sedimentation of reservoirs and lakes (Bezuayehu, 2006). The Koka reservoir, supplied by the Awash and the Modjo rivers, was formed by the construction of the Koka dam in 1959 (with an original storage capacity of 1650 Mm3) for developing hydroelectric power for domestic use (Musa et al, 2005). The mean annual sedimentation rate of this reservoir has been estimated or cited by several authors: 2302 t km-2 year (Devi et al, 2008); 13-20 Mm3 year-1 (Musa et al, 2005). It is estimated that the transboundary rivers that originate from the Ethiopian highlands carry about 1.3 billion tons/year of sediment to neighboring countries whereas the Blue Nile alone carries 131 million tons/year (Kidane and Alemu, 2015)

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Conclusion