Modeling of Rainfall-Runoff Process and Sediment Yield using SWAT Model for Geba River Catchment, Ethiopia

Abstract

The use of rainfall-runoff modeling in the decision making process of water resources planning and management has become crucial. Extreme Runoff coupled with faulty management systems may result in high rate of soil erosion and increased sediment transport by changing the magnitude and pattern of runoff and sediment yield. The main problem in the study area is erosion, sediment transport, and sedimentations also water resource in the basin is scare due to limited rainfall, high evapotranspiration and ongoing land degradation. The aim of this study was to model rainfall runoff process and sediment yield in Geba River Watershed using Soil and Water Assessment Tool (SWAT) model. Simulation was carried out using meteorological, hydrological and spatial data that was collected from different sources. Model calibration period (2001- 2010) and validation period (2011-2015) were performed for monthly flow and sediment data using Sequential Uncertainty Fitting (SUFI-2) within SWAT Calibration of Uncertainty Program (SWAT-CUP). Model performance efficiency was checked by coefficient of determination (R2 ), Nash-Sutcliffe model efficiency (ENS), observation Standard Deviation Ratio (RSR) and percent bias (PBIAS) indicating well performance of model estimation. For flow, the values of R 2 , NSE, RSR and PBIAS were 0.71, 0.7, 0.55 and 6.7 during calibration period and 0.74, 0.7, 0.55 and 3.6 during validation period, respectively and for sediment yield 0.72, 0.66, 0.61 and -8.7 during calibration period and 0.79, 0.72, 0.53 and -11.3 during validation period, respectively. Average annual sediment yield from Geba watershed after calibration and validation at Geba gauging station was total sediment loading 18.440 ton/yr. Spatial variability of sediment yield was performed using the simulated sediment yield results. Also based on the spatial result for the critical sub-watersheds the design and development of best management practices were proposed under different scenarios. Scenarios result showed that average annual sediment yield reduction at entire watershed level after application of grassed waterway, filter strips, terracing and contouring were 20.3%, 54.7%, 78.8% and 61.57% respectively. Therefore, practicing terracing for Geba watershed should be developed and encouraged for efficient sediment reductions.