Modeling runoff-sediment influx responses to alternative BMP interventions in the Gojeb watershed, Ethiopia, using the SWAT hydrological model

Abstract

The Soil & Water Assessment Tool (SWAT) has been calibrated over a 33-year period to evaluate the Gojeb watershed's hydrological processes, sediment yield with downstream loading to the Gibe III dam, and erosion hotspot locations. Best management practices (BMPs) were run through the model to simulate the effects of watershed intervention scenarios on sediment yield and runoff. Simulation results of BMP intervention were compared with the reference and worst-case scenarios. The simulation of sediment production indicates a clear growing trend. Temporally, the maximum amount of sediment transported out of the watershed is experiential from June to September, and the minimum is in February. A plainly defined similar orientation is observed between precipitation, surface runoff, and sediment load in the landscape. Spatially, the maximum sediment transported out of the watershed is from agricultural landscape units with a slope of over 50%, annual precipitation above 1592 mm, and surface runoff over 151 mm. This signifies that the watershed is under serious threat from erosion due to vegetation loss, steep slope farming, and high surface runoff. Gibe III is a 243-m high roller compacted gravity dam built on the Omo-Gibe River basin in Ethiopia for hydroelectric power and downstream flood control. It is one of Africa's tallest dams, with an annual electric output of 1870 MW that began operation in 2016. Thus, Gibe III could see a loss of storage capacity due to higher-than-expected sedimentation resulting from worsening environmental degradation, which implies that the beneficial uses that depend on this dam - electricity production, regulated irrigation water supply, and flood control - will decline with significant economic losses. Despite that, selected sustainable land management interventions and the application of BMPs to critical erosion-prone hotspot areas can support the overall reduction in total sediment yield and surface runoff.

Keywords: BMP intervention; Gojeb; Hydrosediment; Management scenario; Runoff discharge; Watershed.