Abstract:
Uma Oya catchment is one of the sub catchments of the Mahaweli watershed which is highly disturbed by
the intensive crop cultivation practices. Nitrogen (N) and Phosphorous (P) exported from the catchments
end up in the streams and reservoirs causing water quality problems. However, empherical evidence on
the levels of nutrient runoff and their association with surface water quality is not available for river
catchments in Sri Lanka. Therefore, this study aimed at determining the levels of nutrients (N, P) exported
from the Uma Oya catchment and value the ecosystem service provided by the natural vegetation by
retaining nutrients. Integrated Valuation of Ecosystem Services and Trade off (In VEST) Nutrient
Retention Water Purification Model was used to estimate the amount of N and P exported from 10 sub
catchments within the main catchment. The highest N and P export rates predicted by the model were
63.2 kg/ha and 20.3 kg/ha, respectively. Higher nutrient export loads (kg/year) were recorded for sub
catchments Boralanda, Kottegoda, Bandarawela, Pahala Ambegoda, Nawela, Erabadda and Horadoruwawa
while lower values were recorded for Ohiya, Boragas, and Gangasiri. Calculations showed the ecosystem
service for nitrogen retention as LKR million 2.77 and LKR million 1.39 for phosphorous retention per
year for all 10 sub catchments.
