Soil Erosion in Uma Oya Watershed and It’s Association with Water Quality

Abstract

Soil erosion and sedimentation are naturally inter-related processes occurring in the landscape. Human interventions accelerate the soil erosion in catchments increases the amount of sediment delivery to the rivers causing water quality deterioration. Uma Oya watershed is an intensively cultivated landscape with high levels of human activities contributing to land degradation. However, the empirical evidence on quantitative assessment of soil erosion and their association with surface water quality is lacking in the context of river catchments in Sri Lanka. Therefore, this study was aimed to determine the relationship between soil erosion and its’ association with water quality in Uma Oya watershed. The soil erosion rates of the ten selected micro-catchments were evaluated using Integrated Valuation of Ecosystem Services and Trade off (InVEST) Sediment Retention Model, which is based on Revised Universal Soil Loss Equation (RUSLE). Stream water quality was assessed for water samples collected at the stream sections at the base of each micro-catchment at monthly intervals. Physicochemical water quality parameters and benthic macro-invertebrate indices of EPT taxa and Chironomidae% were calculated to evaluate the overall ecological health of the streams associated with micro-catchments. Pearson’s correlation test was conducted for water quality and sediment delivery data to estimate their association. The results indicated that average soil loss of selected micro-catchments varied between 36.44 tons/ha/yr to 222.38 tons/ha/yr. Correlation analysis between average soil loss and average water quality parameters at sub-catchment level indicated significant (p<0.05) positive relationships with phosphate concentration, %Chrinomidae, BOD, alkalinity and TDS concentrations. A significant (P<0.05) negative correlation was detected with %EPT taxa. The findings of the present study suggest that the stream ecological health and soil erosion in the catchment are closely related indicating possible impacts of land degradation.