Effect of Soil Erosion on Surface Water Quality in Upper Walawe River Basin

Abstract

Walawe River Basin, the fourth-largest in Sri Lanka, has emerged as a focal point for the management of water quality because of its size and population density. The catchment is highly agricultural and the land use activities in the catchment have the potential of deteriorating the surface water quality. However, very little research has been carried out to evaluate the effects of catchment land use on surface water quality in the upper Walawe basin. Therefore, this study aims to investigate the connection between soil erosion in the catchment and water quality in the Upper section of the Walawe River basin. Integrated Valuation of Ecosystem Services and Tradeoffs (InVEST) Sediment Delivery Ratio (SDR) model was used to assess the soil erosion in nine microcatchments within the upper Walawe River basin. Water quality parameters were recorded in streams draining through selected micro catchments and the association between soil erosion and water quality was analyzed using the Pearson correlation test at a 0.05 significance level. The results of the study indicated mean soil erosion of the microcatchments ranging from 0.84 to 18.1 t ha-1 yr-1 with an annual average of 11.75 t ha-1 yr-1, which is ∼2.35 times greater than the soil erosion tolerance (5 t ha-1 yr-1) in Sri Lanka. Pearson correlation test was conducted to assess the relationship between soil erosion of the microcatchments and water quality variables. The results indicated a significant positive correlation between soil erosion and potassium (K) levels in the water. In conclusion, this study provides valuable insights into the complex interactions between soil erosion, and water quality in the study catchment.