Evaluating the impact of land use and land cover changes on sediment yield dynamics in the upper Awash basin, Ethiopia the case of Koka reservoir

Land Use and Land Cover changes (LULC) are the driving forces to change the hydrological response of the watershed. In this study, the Quantum Geography Information System Interference Soil and Water Assessment Tool Plus (QSWAT-PLUS) model was applied to evaluate the effects of LULC on sediment load at the Upper Awash River Basin (UARB) which are causing sedimentation problems in Koka reservoir. The LULC data for 2005, 2010, and 2015 were obtained from historical satellite images using Earth Resources Observation and Science (ERDAS) 2014. The classification of LULC changes showed that the agricultural practice, and the settlement land both increased by 6.7 % and 6.3 %, respectively. In contrast, the forest area, woodland, shrubland, and water bodies decreased by 5.47 %,0.93 %,0.96 %, and 1.34 % from 2000 to 2015 respectively. The model evaluation results were satisfactory for the three LULC scenarios. The average annual surface runoff volume for the 2005 LULC data was 182.2 mm, which increased to 193.29 mm in 2010 and 205.3 mm in 2015. Similarly, the average annual sediment yield that would enter to the Koka reservoir under the 2005, 2010, and 2015 LULC scenarios were 26.03 t/ha/yr, 26.34 t/ha/yr, and 28.33 t/ha/yr respectively. In general, streamflow, surface runoff, and sediment output increased by 4.55 %, 12.68 %, and 8.84 %, respectively due to the rapid change of LULC from 2000 to 2015. Temporarily, the sediment load at the upstream side of the Koka Dam watershed was 60.8 % during the wet season. The southwest direction of the watershed was identified as the primary erosion-prone area. Based on the simulation results, the filter strip, contour, and terraces reduced the watershed sediment yield by up to 60 %, 65 %, and 80 %, respectively. Therefore, the selected best management practices are highly effective in reducing silt along the entire upstream side of the Koka Dam watershed.

Evaluation of semi-distributed hydrological models performance in borkena watershed; upper awash basin, Ethiopia

Flood is one of the most significant disasters in human life and economic destruction. To challenge this disaster, the use of models is very important to predict the magnitude and impact of river flow and to find a solution of the problems. This research is aimed to compare the performance of semi-distributed hydrological models in the Borkena watershed. The selected semi-distributed hydrological models were soil and water assessment tool (SWAT), hydrological engineering center-hydrologic modeling system (HEC-HMS), hydrologiska byråns vattenbalansavdelnin (HBV), and parameter efficient distribution (PED). The models were calibrated from 1999 to 2009 and validated from 2010 to 2015 using daily data. Based on validation results; The Nashsutclif (NSC) output of the SWAT, HEC-HMS, HBV, and PED models were 0.68. 0.66, 0.65, and 0.65, coefficient of determination (R²) 0.69, 0.67, 0.71, and 0.70, percentage of bias (PBIAS) -6.5, 0.6, 27.34, and 10.28, and root mean square error (RMSE) 14.24, 17.45, 17.63 and 0.91, respectively. Based on the models' performance results in Borkena watershed, the first effective model was SWAT and the second one was HEC-HMS. The HBV and PED models took third and fourth places respectively. The overall results show that the two infiltration excess models (SWAT and HEC-HMS) were performed in a better way than the two saturation excess models (HBV and PED) on this watershed. Therefore, according to the model output, the Borkena watershed is an infiltration excess area.