Spatial variation of isotope composition in precipitation in a tropical environment: a case study from the Deduru Oya river basin, Sri Lanka

The stable isotope composition ( 18 O and 2 H) in the tropical precipitation collected from 18 locations throughout the Deduru Oya river basin in Sri Lanka, has been studied during August and September 2001, in order to characterize the isotopic composition of precipitation in the dry and intermediate climatic zones of Sri Lanka. The isotope compositions are described with respect to the distance from the coast and the altitude. The analyses show that δ 18 O vary from -5-1 to 1.39%c and SD vary from -35.71 to 12.55%o. The d-excess values range from -0-65 to 13.17 with an average value of ∼7. Regression for the δ 18 O - SD is y = 6-Sx + 4.9 (R 2 = 0-9) which is compatible with the precipitation in other tropical regions. The lower slope in the regression line and the lower d-excess value indicate high temperature events which were possibly aided by concentration through successive evaporation within the atmosphere. The spatial variation of isotope composition indicates two different cloud contributions for the rain events, of which one may be linked to the Indian Ocean contribution and the other to the high altitude condensation.