Characterization and Modeling of watershed Processes using SWAT and GIS in Gumara Watershed, Upper Blue Nile, Ethiopia

Abstract

Soil erosion and Land degradation is a major problem throughout the Blue Nile Basin, Ethiopian highlands. Poor land use practices and improper management systems have significant role in causing high soil erosion rates, sediment transport and loss of agricultural nutrients. Gumara watershed is located in the south Gondar zone, North West part of Ethiopia in Amhara Regional State. There is fast growing population and the density of livestock in the basin and also lack of awareness of the watershed management strategies and agricultural practices. The main objective of the study is to Characterize and Modeling of Gumara Watershed Processes with respect to Watershed management and Agricultural practices and also delineate the sub-watershed. The area of river basin was discretized into 7 sub- basins using Soil and Water Assessment Tool (SWAT) interface of the model. The semi-automated Sequential Uncertainty Fitting (SUFI2) calibration process built in SWAT calibration and uncertainty program (SWAT-CUP) were used to calibrate the model parameters using time series of flow and sediment load data of 2004 to 2010 and validated with the observed data from years 2011 to 2014. Model performance on monthly time step reviled that (R2= 0.90, NSE = 0.84, PBIAS = 29.3 and RSR = 0.41) and (R2= 0.84, NSE = 0.71, PBIAS = 38.9 and RSR = 0.54) for flow calibration and validation, respectively. Similarly SWAT-CUP (SUFI2) performed well with (R2= 0.86, NSE = 0.71, PBIAS = 46.4 and RSR = 0.53) and (R2= 0.85, NSE = 0.65, PBIAS = 52.9 and RSR = 0.59) for Sediment calibration and validation respectively. This calibrated model was used to predict sediment yield, identify spatial distribution of sediment, and to test the potential of watershed management interventions in reducing sediment load. In this research, two mitigation measures to reduce the sediment inflowing to the Lake Tana from Gumara watershed, these were: (i) Applying area enclosure (afforestation) or Land-Use Redesign of any land use on steep slope (greater than 30%) in the whole Gumara watershed and (ii) Implementation of Parallel terrace (stone bund) in the agricultural HRUs of potential sub basins. The Soil and Water Assessment Tool (SWAT) was used to model soil erosion, identify soil erosion prone areas and assess the impact on BMPs on sediment reduction. Use of terrace in the agricultural HRUs and afforestation of steep slope areas reduced sediment yield (SYLD) inflowing to the Lake Tana from Gumara by 32% and 24%, respectively