ESTIMATION OF SEDIMENT YIELD FROM BILATE WATERSHED, RIFT VALLEY LAKE BASIN, ETHIOPIA

Abstract

High sediment yield from watershed can be significantly resulted due to poor land use practices, inappropriate management systems and lack of suitable soil and water conservation measures. The main objective of this study was to estimate sediment yield from Bilate watershed using Soil and Water Assessment Tool (SWAT) model. Simulation was carried out using meteorological and spatial data by dividing watershed in to 17 sub basins with 122 Hydrologic Response Units (HRUs) at Bilate Tena gauging station and 23 sub basins with 174 Hydrologic Response Units (HRUs) at outlet of Bilate watershed. 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 good performance of model estimation. An average annual sediment yield of 5.538million ton/yr at Bilate Tena and 6.468million ton/yr at outlet of Bilate watershed was estimated with average spatial distribution of 9.99ton/ha/yr. From 23 sub basins, 11 were categorized from moderate to very high (10-26 ton/ha/yr) sediment yielding sub basins and selected for sediment management 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 54.45%, 30.13%, 63.26% and 59.56% respectively. Also, at treated sub basins level 68.04%, 38.41%, 80.58% and 77.42% of sediment reduction revealed after application of grassed waterway, filter strips, terracing and contouring respectively. It was concluded that sediment yield reduction applying terracing was more effective than other conservation measures for affected sub basins.