ASSESSMENT OF NONPOINT SOURCE POLLUTION LOADS FROM KETAR WATERSHED INTO LAKE ZIWAY USING WATERSHED MODELLNG APPROACH

Abstract

The long term impact of nonpoint source pollution in Ketar watershed has largely affecting both the river and the receiving lake water quality. In particular Non-point source (NPS) pollution, including soil erosion and nutrient leaching from Ketar River, has mainly accounted for a recent loss of Lake Ziway storage and eutrophication problems. However, the rate of pollution loading of Ketar Rivers is not well assessed. Hence, the main objective of this research to assess the rate of sediment yield and nutrient loadings of the Ketar watershed. In efforts to do that watershed modeling, found to be the most efficient approach owing to complexity and uncertainties related to NPSP. The Proved model efficiency and wide application have singled out SWAT as a modeling tool for this study. In addition, the rating curve Load Estimator (LOADEST) simulates Nutrient loads including total Phosphorus (TP) and Total Nitrogen (TN) from limited grab. Initially the simulated model result of flow and sediment was evaluated through precise calibration and validation for defined flow year using Sequential Uncertainty Fitting (SUFI-2) within SWAT Calibration of Uncertainty Program (SWAT-CUP). Model performance efficiency was then checked and verified by the coefficient of determination (R2), Nash-Sutcliffe model efficiency (ENS), observation Standard Deviation Ratio (RSR), and percent bias (PBIAS). The simulated model result indicated that the mean annual sediment loading of the ketar watershed is 2.5M ton/year and the areal sediment yield of 6.76 ton/ha/yr. The LOADEST model simulated mean annual TP and TN loadings as 536.05 and 5427.65 tons/year respectively over the year 2014 to 2021. The watershed modeling was successfully identified 9 sub-basins that export large amount of sediment. Finally, on these identified sub-basins efficiency of three commonly used BMPs namely terracing, contouring, and filter strips were assessed in both individually and in optimal combination. Accordingly the performance evaluation of simulated BMPs demonstrated that optimal combination of individual BMPs is most efficient mitigation means possessing of an average 87.2% sediment reduction followed by 82.48, 77.85%, and 45.85% the introduction of individual BMPs namely terracing, contouring and filter strip. Finally, it can be concluded that massive amount of a pollution is being exported into Lake Ziway from Ketar watershed and optimal combination of terracing, contouring and filter strip as single conservation unit is evaluated as best option to reduce it.