Stable isotope characteristics of precipitation of Pamba River basin, Kerala, India

Abstract

Stable isotope composition of precipitation from Pamba River basin, Kerala, India, is evaluated to understand the role of spatial and temporal variations on rainwater isotope characteristics. Physiographically different locations in the basin showed strong spatial and temporal variations. δ18O varied from −7.63 to −1.75‰ in the lowlands; from −9.32 to −1.94‰ in the midlands and from −11.6 to −4.00‰ in the highlands. Local Meteoric Water Lines (LMWL) for the three regions were determined separately and an overall LMWL for the whole of the basin was found to be δ2H = 6.6 (±0.4) δ18O+10.4 (±2.0). Altitude effect was evident for the basin (0.1‰ for δ18O and 0.8‰ for δ2H per 100 m elevation), while the amount effect was weak. The precipitation formed from the marine moisture supplied at a steady rate, without much isotopic evolution in this period may have masked the possible depletion of heavier isotopes with increasing rainfall. Consistently high d-excess values showed the influence of recycled vapour, despite the prevailing high relative humidity. The oceanic and continental vapour source origins for the south-west and north-east monsoons were clearly noted in the precipitation in the basin. Rayleigh distillation model showed about 30% rainout of the monsoon vapour mass in the basin.