ESTIMATION OF RUNOFF AND SEDIMENT YIELD OF SEDE WATERSHED FOR CONSERVATION DESIGN AND PLANNING IN BLUE NILE BASIN, ETHIOPIA

Abstract

Soil erosion is the major cause of land degradation and reservoir sedimentation. Therefore, modelling of runoff and sediment yield at a watershed level is very important. A conceptual, distributed and continuous time, Soil and Water Assessment Tool (SWAT Model) was selected for the simulation of the runoff and sediment yield from Sede watershed, in Blue Nile Basin. The main objective of the study was to determine the magnitude and extent of runoff and soil erosion of Sede watershed. To set up the model for simulation a 30×30 m DEM (Digital Elevation Model), land use map and soil map were used. The daily record weather data from 1996 to 2015 was used as input to the model. Daily stream flow record data and two to four days record sediment data within the year was available from 2002 to 2015. In this study both local (one-at-a-time) and global sensitivity analysis were performed and the ranking of the parameters in both cases compared. The model was calibrated by using both automated calibration and manual calibration for monthly to stream flow and daily for both steam flow and sediment by using the flow and sediment data from 2002 to 2011 and validated for 2012 to 2015. The performance of the model was evaluated by using coefficient of determination (R2), the Nash-Sutcliffe efficiency (NS) and percent bias (PBIAS). The statistical analysis of calibration results for Sede watershed showed good agreement between observed and simulated values in monthly and daily flow simulation, with an R2 (0.81 and 0.71), NS (0.76 and 0.70) and PBIAS (9.5 and 20.5); and for daily sediment simulation R2 value of 0.75, NS of 0.73 and PBIAS of 21.5. The R2, NS and PBIAS value for validation period was (0.90 and 0.75), (0.80 and 0.76) and (10.5 and 18.6) in monthly and daily flow simulation; and 0.76, 0.75 and 19.1 in daily sediment simulation respectively. The estimated soil loss rate from different subwatersheds had ranged from 0.4 t/ha/year to 98.35 t/ha/year, whereas the annual weighted average soil loss rate from the watershed was estimated 22.5 t/ha. According to the class assigned to the annual sediment yield, map of the study area reclassified into five major categories of soil erosion hazard area. Based on this classification about 8.6 % of the watershed area is under none to slight, 24.4% under slight, 25.2% under moderate and 41.8% under sever degree of erosion. Among all sub-watershed SW34, SW35, SW36, SW33, and SW31 need immediate intervention to minimize soil losses from the watershed. After identifying the critical erosion areas, different soil and water conservation measures were recommended and designed. Because of heavy rainfall in the study watershed it is generally recommended to apply graded structures (graded soil bund, cutoff drain and grass waterway), in order to drain excess surface runoff and to conserve soil within the watershed. Structural soil and water conservation practices should be laid out and designed based on estimated peak runoff rate in the catchment and with proper integration of measures and maintain soil and water conservation measures every year in order to sustain their benefits.