Abstract:
The Nyangores River watershed, located in Kenya's upper Mara Basin has challenges of rivers degradation, where increased non-point source pollution has caused water quality issues. The objectives were identify the key sources of pollutants, calibrate and validate the SWAT model for streamflow, sediment, and nutrient transport and to assess the impact of various Best Management Practices (BMPs) on nutrient and sediment loads. Data collection followed: gathering hydro-climatic data for model calibration, field observations, collecting samples and laboratory analysis. Impacts of various BMP scenarios on water quality were modelled, focusing on riparian buffer zones, reforestation, and contour farming. Calibration and validation with indicators such as the Nash-Sutcliffe Efficiency (NSE) and R² was done for reliability. Parameters tested, Ph., Ec, Nitrates, Phosphates and Total suspended solids (TSS).Nitrogen levels in the river exceeded safe limits by 50%, with average concentrations of 3.2 mg/L, while phosphorus levels were recorded at 1.1 mg/L, surpassing the eutrophication rate which is TP 0.05mg/L and TN 1.0mg/L. Sedimentation contributed to a 35% increase in total suspended solids (TSS), with average TSS levels measured at 210 mg/L, particularly in areas with steep slopes and poor soil conservation practices. The SWAT model was calibrated using streamflow data from 2003-2008, and the calibration process achieved satisfactory results, with a Nash-Sutcliffe Efficiency (NSE) of 0.72 and an R² value of 0.75. During validation, the model performance well with an NSE of 0.70 and an R² of 0.73 for the validation period (2009-2013) demonstrating the effectiveness of the model. Nitrogen and phosphorus was reduced by 38% and 42% in simulation. In conclusion, this research demonstrates the SWAT model's efficacy in Nyangores watershed, advocating for sustainable land management practices to mitigate water pollution and emphasizing the importance of preserving natural ecosystems for water quality protection. The study recommends preserving natural forests as they effectively control pH, electrical conductivity, nitrates, and phosphates compared to grasslands. Management practices, such as filter strips and contouring, resulted in reduced sediment yields. Public awareness on the importance of management practices and riparian vegetation is paramount.
