RELATIONSHIP OF WATER QUALITY WITH SOIL EROSION AND FOREST HEALTHINESS IN SELECTED SUB-CATCHMENTS OF SAMANALAWEWA WATERSHED

Abstract

Soil erosion and forest healthiness are important parameters which affect the water quality of watersheds. Thus, the prime objective of this study is to evaluate and map the rate of soil erosion and forest quality of the Samanalawewa watershed (SWW) and to establish their relationship with water quality of the watershed. Integrated Valuation of Ecosystem Services and Trade-offs (InVEST) Sediment Delivery Ratio (SDR) model, and Normalized Difference Vegetation Index (NDVI) were mainly used to calculate soil erosion, and forest healthiness of SWW with the support of Remote Sensing and Geographic Information System. Water quality parameters: temperature, pH, electrical conductivity, total dissolved solids, dissolved oxygen, biochemical oxygen demand, alkalinity, total hardness, nitrate, phosphate were analyzed. The study revealed that the present human-induced soil erosion of the catchment varies from 0 to 139.9 t/ ha/ yr with an annual average of 14.5 t/ ha/ yr, which is ~3 times greater than the soil loss tolerance (T) in Sri Lanka. The Average NDVI values/ forest quality varied between 0.329and 0.462 among 12sub- catchments indicating that SWW is having moderate vegetation healthiness. Further, Pearson correlation test was conducted in order to establish a relationship between water quality parameters, soil erosion, and forest healthiness / NDVI of each sub-watershed. However, there was no direct relationship between water quality parameters and soil erosion rates. Nonetheless, a negative correlation between NDVI and nitrate (r2= 0.521, p=0.008) has been found. The findings of the research suggest that catchment water quality is not significantly affected by soil erosion of the catchment possibly due to the presence of fairly healthy forest cover in the catchment