Stable water isotope signatures of dual monsoon precipitation: A case study of Greater Cochin region, south-west coast of India

Abstract

Precipitation samples of various spatio-temporal scales were collected from coastal, midland and urban regions of Greater Cochin, Ernakulam district, Kerala for a period of 1 yr (2015–2016). The collected samples were analysed for stable water isotopes (SWI) (δD and δ18O), to understand these variations in the precipitation source and the factors governing its isotopic characteristics during precipitation. The δ18O in rainwater varies from −8.73‰ to 0.29‰ in urban, −12.21‰ to 2.59‰ in midland and −9.99% to 0.97‰ in lowland regions. Spatio-temporal variations in SWI were observed in various regions, suggesting altitude and continental effect followed by the establishment of a regional overall local meteoric water line (LMWL) δD = 8.06 (±0.15) δ18O + 12.5 (±0.68). Among the coastal, midland and urban regions, the highest slope (~8.3) and intercept (~13.0) were observed in the urban region, which designates the variations in temperature along spatial and different layers of the atmosphere in the urban region, resulting in the deviation of isotopic characteristics. The overall deuterium excess (d-excess) value is ~10‰ during the south-west monsoon (June–September), suggesting a moisture source of marine origin. A d-excess of ~13‰ is observed during the north-east monsoon, indicating a moisture source from the continental contribution (October–December). The results of the moisture source obtained from the d-excess value are also supported by back-trajectory analysis. Thus, the present study on isotopic characterisation of precipitation and its controlling factor may enhance our understanding of the Indian monsoon and its dynamics in the west coast region of India.