Spatial and Temporal Variability in Riverine Carbon and Nitrogen Export

Kelani River, Kalu River and Gin River, three largest typical perennial mountainous watersheds in the wet zone of Sri Lanka, analyzed monthly during the period of Southwest Monsoon (SWM) (May to September) in 2016, with objectives of providing baseline data for Organic Carbon and Nitrogen flow to the Indian Ocean and to understand the role of Sri Lankan carbon budget over global carbon cycle. Water samples were taken at the middle of the month using a Ruttner sampler at a mean tide level. Concentrations of TSM, DOC, TDN and DIN forms (NO2-, NO3- and NH4+) were estimated and DON and TON were calculated, as well as all their fluxes. Mean monthly rainfall over the river basin was calculated and used to understand the correlation between rainfall and fluxes. Exponential relationship (y = 34.371e0.0023x) was observed between average monthly stream discharge for each river and monthly mean rainfall over respective river basin, during 2016 SWM. Temporal variation of all 3 forms of DIN, DON and TDN was observed even within the monsoon period. Spatial variations were observed for NO2-, NH4+ and DIN were observed with a spatial variation within the three rivers.