Spatial and temporal variability evaluation of sediment yield and sub-basins/hydrologic response units prioritization on Genale Basin, Ethiopia

Abstract

Erosion of the topsoil is a serious environmental problem worldwide that critically alarms agricultural and upland areas. Most wetlands, rivers, and reservoirs are losing their capacity because of sediment deposition into the water body from the upstream of watersheds, as they were not managed. Soil and Water Assessment Tool (SWAT) model is used to simulate streamflow, sediment load, & identify spatiotemporal variability of the sediment yield and sediment delivery ratio (SDR), point out erosion-prone area, and prioritize sub-basins/hydrologic response units (HRUs) for management. The model is implemented by utilizing a digital elevation model, land use, soil type, and slope of the Genale basin; 464 HRUs were created, spreading over 25 sub-basins within the drainage area 54,942Km2. SUFI 2 algorithm of SWAT calibration & uncertainty programs (SWAT_CUP) is used to calibrate (1990 – 2005) and validated (2006 – 2013) streamflow & sediment on monthly scale, and the resulted statistics are; R2 = 0.87, 0.85 and NSE = 0.81, 0.78 for flow calibration and validation respectively whereas R2 = 0.84, 0.82 and NSE = 0.79, 0.75 for sediment calibration and validation respectively. Significant spatiotemporal variability of sediment yield was observed in the watershed, as the characteristics of the terrain, land use, & soil type in each sub-basin varies. Among the total 25 sub-basins, three sub-basins produced a very high sediment rate (21–31 ton/ha/year), one produced high (16–20 ton/ha/year), one is moderate (11–15 ton/ha/year), three were low (6–10 ton/ha/year), and the rest 16 sub-basins were under very low categories. In the case of SDR, three sub-basins have very high SDR (>0.452), two sub-basins resulted in high SDR (0.326–0.451), which are located at the upland basin. Based on the model results, sub-basins 6, 8, 12, 10, and 7 were identified as sediment-prone areas. Further investigation at the HRUs scale is taken to understand critical erosion areas and minimize the cost of management practice, time & human resources. HRU analysis has revealed the immense scope of minimizing cost by concentrating management measures only in the critical HRUs. For example, Sub-basin-6 has 31 HRUs, of which only seven are assessed to have the high rates of sediment yield and thus prioritized as; HRU-159, HRU-160, HRU-161, HRU-168, HRU-170, and HRU-171, which are located on agricultural and arable lands of steep slopes.