Abstract:
Soil erosion and sedimentation are naturally inter-related processes occurring in the
landscape. Human interventions accelerate the soil erosion in catchments increases the
amount of sediment delivery to the rivers causing water quality deterioration. Uma Oya
watershed is an intensively cultivated landscape with high levels of human activities
contributing to land degradation. However, the empirical evidence on quantitative
assessment of soil erosion and their association with surface water quality is lacking in
the context of river catchments in Sri Lanka. Therefore, this study was aimed to
determine the relationship between soil erosion and its’ association with water quality
in Uma Oya watershed. The soil erosion rates of the ten selected micro-catchments
were evaluated using Integrated Valuation of Ecosystem Services and Trade off
(InVEST) Sediment Retention Model, which is based on Revised Universal Soil Loss
Equation (RUSLE). Stream water quality was assessed for water samples collected at the
stream sections at the base of each micro-catchment at monthly intervals.
Physicochemical water quality parameters and benthic macro-invertebrate indices of
EPT taxa and Chironomidae% were calculated to evaluate the overall ecological health
of the streams associated with micro-catchments. Pearson’s correlation test was
conducted for water quality and sediment delivery data to estimate their association.
The results indicated that average soil loss of selected micro-catchments varied
between 36.44 tons/ha/yr to 222.38 tons/ha/yr. Correlation analysis between average
soil loss and average water quality parameters at sub-catchment level indicated
significant (p<0.05) positive relationships with phosphate concentration,
%Chrinomidae, BOD, alkalinity and TDS concentrations. A significant (P<0.05) negative
correlation was detected with %EPT taxa. The findings of the present study suggest that
the stream ecological health and soil erosion in the catchment are closely related
indicating possible impacts of land degradation.
