Quantitative Assessment of Soil Erosion and its Association with River Health of Uma Oya Watershed, Sri Lanka

Abstract

Soil erosion in river catchments increases sediment deliver to streams causing water quality deterioration. This study was aimed to determine the relationship between soil erosion and water quality of intensively cultivated Uma Oya watershed and to assess human-induced factors contributing to soil erosion in the watershed. The soil erosion rates of ten selected micro-catchments were evaluated using Integrated Valuation of Ecosystem Services and Tradeoffs (InVEST) Sediment Retention Model. The overall ecological health of the streams associated with micro-catchments was calculated by assessing physiochemical parameters and benthic macro-invertebrate indices in monthly intervals. A comprehensive questionnaire survey was conducted to obtain information on farmers' land use practices and their level of adoption of water and soil conservation practices (n=83). Person’s correlation test and multiple regression analysis were used to estimate association of water quality and soil erosion, and to identify human-induced factors that contribute to soil erosion. The estimated average soil loss of selected micro-catchments varied between 36.4 t/ha/yr to 222.4 t/ha/yr. Correlation analysis indicated average soil loss of the catchment is having significant (p<0.05) positive relationships with phosphate, Biological Oxygen Demand (BOD5), Total Dissolved Solids (TDS), alkalinity, %Chrinomidae and a negative correlation with Percentage Ephemeroptera: Plecoptera: Trichoptera (EPT) taxa in study streams. Results of the questionnaire survey indicated that the farm-level soil loss was attributed to family size, farmer's education and their soil conservation adoptions (p<0.05). The findings of the present study suggested close relations among the stream health and soil erosion, and need for human interventions in catchment management.