Identification of soil erosion hot-spot areas for prioritization of conservation measures using the SWAT model in Ribb watershed, Ethiopia

Abstract

Soil erosion is one of the pressing environmental problem in Ethiopia Thus, identifying soil erosion hot-spot areas of a watershed/basin and isolating best management practices (BMPs) for soil erosion-prone sub-watersheds is imperative for effective soil loss reduction and lessening its on-site and off-site effects. The present study aimed to identify erosion hot-spot areas for prioritization of conservation measures and evaluate BMPs for reducing soil erosion using the Soil and Water Assessment Tool (SWAT) model in Ribb watershed, Ethiopia. The SWAT model was calibrated (1995–2002) and validated (2003–2008) using flow and sediment data in the SWAT Calibration and Uncertainty Program (SWAT-CUP). The NSE (Nash–Sutcliffe efficiency), R² (Coefficient of determination), PBIAS (Percent bias) and RSR (Root Mean Square Error — observations standard deviation ratio) values during flow and sediment calibration and validation periods ranged from 0.7 to 0.9, 0.71 to 0.92, -0.4% to 15.2% and 0.32 to 0.55, respectively, indicating that SWAT is able to simulate streamflow and sediment with sufficient accuracy. The calibrated and validated SWAT model was used to evaluate the effectiveness of three BMPs against the baseline conditions for reducing soil erosion, such as filter strips Scenario (Scenario F), stone/soil bunds Scenario (Scenario S) and reforestation Scenario (Scenario R). The result indicated that about 95% of the watershed are indentified as soil erosion hot-spot areas, which experienced sediment yield > 11 t ha ${}^{-1}$ yr${}^{-1}$. The mean annual sediment yield (1995–2008) of the watershed at the baseline conditions was nearly 29 t ha ${}^{-1}$ yr${}^{-1}$. The implementation of Scenario F, Scenario S and Scenario R reduced the baseline sediment yield by 28%, 76% and 62% at the watershed scale, respectively. At the sub-watershed levels, Scenario S and Scenario R reduced the mean annual sediment yield from 45%–90% and 0%–90%, respectively. Therefore, the study suggests implementations of Scenario S and Scenario R for effective soil erosion reduction in the study watershed in particular and to other watersheds in Ethiopia in general which have similar environmental settings.