Effect of Soil Erosion on Water Quality of Streams in Badulu Oya Watershed in Sri Lanka

Abstract

Badulu Oya catchment is an intensively cultivated landscape in Sri Lanka. Soil erosion due to agricultural practices can reduce soil fertility and deteriorates of surface water quality. However, empirical evidence on soil erosion rates of the catchment and their association with surface water quality is less documented in Sri Lanka. The soil erosion rates of the 10 selected micro catchments within Badulu Oya catchment were evaluated using, Integrated Valuation of Ecosystem Services and Tradeoffs (InVEST) Sediment Retention Model. Physical, chemical water quality parameters and benthic macro invertebrate indices EPT percentage taxa and Chironomidae percentage were calculated to evaluate the overall ecological health of the streams associated with micro- catchments. Pearson's correlation test was conducted to estimate the association between water quality and sediment delivery data of the micro catchments. A comprehensive household (HH) questionnaire survey was conducted to obtain the information from the farming community (n=71), on their land use, knowledge on soil and water conservation practices and the degree of interventions in catchment management. The average soil loss of ten micro catchments varied between 66.0t/ha/yr to 165.9t/ha/yr. Correlation analysis of average soil loss and average water quality parameters at micro-catchments indicated significant (p<0.05) positive relationships suggesting soil erosion in the catchment is affecting the stream water quality. Results of the questionnaire survey indicated that the soil erosion rates of the catchment are related to farmer's education level and their level of soil conservation adaptations (p<=0.05). The finding of the present study emphasized the need of human interventions in catchment management.