Nitrogen and Phosphorous Dynamics and Water Purification Ability of Vegetation and Soil in Uma Oya Catchment

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.