Impacts of Land Use / Land Cover Changes on Stream Flow & Sediment Yield Using SWAT Model: Case Study on Hamesa Watershed, Rift valley, Ethiopia

Abstract

Improved prediction of the hydrological effects of land use changes should lead to better decision making for stakeholders. Lake levels and water quality are impacted by LULC changes and the associated trend of increasing soil erosion, streamflow volume, and sediment transport in the watershed. This study's primary goal is to use the Soil and Water Assessment Tool (SWAT) model to evaluate the effects of LULC change on stream flow and sediment yield in the Hamesa watershed. Land use and land cover change classifications were carried out using ERDAS Imagine 2015 for three distinct time periods (1998, 2008, and 2018). To evaluate the effects of changing land use and land cover, the hydrological model (SWAT) was applied to the Hamesa watershed (526.67 km2). Utilizing meteorological and geographic data, the simulation was run at the Hamesa Gauging Station, where the watershed of Hamesa was divided into 15 sub basins with 103, 76, and 67 Hydrologic Response Units during three time periods. Within the SWAT Calibration of Uncertainty Program, Sequential Uncertainty Fitting was used to accomplish model calibration for monthly flow and sediment data for the years 1990–2000 and validation for the year’s 2001–2005. Based on identified sediment-prone areas the effective sediment reduction strategy was proposed for Hamesa watershed. From land use analysis, the majority of the grassland and cropland were converted to agricultural land. The effectiveness of the model was evaluated using the coefficient of determination (R2), Nash-Sutcliffe model efficiency (ENS), observation standard deviation ratio (RSR), and percent bias (PBIAS), all of which demonstrated strong model estimate performance. During the whole 1998–2018 period, there was a 20.67% increase in cultivated area, which led to increases in stream flow and sediment output of 0.29 m3/s and 24800 tons/year, respectively. The annual average flow and sediment output of the Hamesa watershed were 5.24m3/s, 5.29m3/s and 5.53m3/s, and 24400ton/year, 25300 ton/year and 49200ton/year, respectively, based on land use periods 1998, 2008, and 2018. The simulated annual sediment output was used to determine the watershed's spatial variability, and the results reveal an average of 3.06, 3.18, and 6.49 ton/ha/yr. for the land use periods of 1998, 2008, and 2018. Above 75.1% of total watershed area identified as sediment prone areas and contributes average annual sediment yield rate of 8.035ton/ha/yr. Temporal variability of sediment yield for three decade land use land cover shown peak at April month. After applying sediment yield reduction scenarios in SWAT, terracing was a comparatively best practice for sediment reduction in Hamesa watershed. Therefore, it is recommended that sediment reduction practice such as terracing will be implemented to improve Hamesa watershed