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
