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.
