Impacts of soil erosion and forest quality on water quality in Samanalawewa watershed, Sri Lanka

Soil erosion is one of the serious environmental concerns in Sri Lanka which affects on water quality of watersheds. Forest quality is an important indicator of the healthiness of forests which often contribute to mitigate the adverse impacts resulting from watershed disturbances and to improve water quality in river watersheds. Thus, the prime aim of this study is to evaluate and map the rate of soil erosion and forest quality of the Samanalawewa watershed and to establish their relationship with water quality of the watershed. Soil erosion and forest quality of 12 sub-watersheds of Samanalawewa watershed were evaluated using the Integrated Valuation of Ecosystem Services and Trade-offs (InVEST), InVEST Sediment Delivery Ratio (SDR) and Carbon models developed by the NatCap-Project, Sanford University-USA and also using the Normalized Difference Vegetation Index (NDVI), respectively. Water quality parameters viz. temperature, pH, electrical conductivity, total dissolved solids, dissolved oxygen, biochemical oxygen demand, alkalinity, hardness, nitrate-N, phosphate-P were analyzed in monthly basis for water samples collected from 12 sampling locations (n = 12) in the sub-watersheds within month of May-July 2019. Pearson correlation was conducted to establish the relationships between water quality parameters and soil erosion and forest quality/NDVI of each sub-watershed. The results of the study indicated that the soil erosion of the watersheds ranges from 0 to 3555.7 t/ha/yr with an annual average of 139.9 t/ha/yr, which is similar to 28 times greater than the soil loss tolerance (T) in Sri Lanka. The average NDVI values (forest quality) of the sub-watersheds varied between - 0.2907 and 0.4628. All the water quality parameters in the study area were within the standards limits of SLS (614:2013). Estimation of vegetation cover of the study area using the InVEST Carbon model revealed that the above ground biomass is the dominant carbon storage among the other carbon pools. No significant correlation (p < 0.05) was found between the water quality parameters and the soil erosion rates in the watershed possibly attributed to the mitigation effects of the healthy forest cover within the study catchment. This was further confirmed by the significant negative correlation (r(2) = 0.521, p = 0.008) established between the average NDVI values and nitrate-N levels of the river water of the catchment. The findings of the study provided the empirical evidence of the contribution of the healthy forest cover on the improvement of watershed water quality. The outcome of the research is useful for formulating watershed management policies and planning for watershed management