Rain-vapor interaction and vapor source identification using stable isotopes from semiarid Western India

Abstract

Results of a 4 year (2005-2008) study of stable isotopic composition of daily rain and ground-level vapor (GLV) at a semiarid station in western India are reported. The GLV samples were collected by complete cryogenic trapping. The sampling was mostly limited to the rainy season (June, July, August, and September) and about a month before and after. The maximum number of samples was collected during the year 2007. The GLV has a steady baseline δ¹⁸O and δD composition without distinguishable seasonal differences. The d-excess of GLV indicates that its isotopic composition has a significant contribution from kinetic evaporation of nonlocal water sources. During a rain event, GLV rapidly interacts with raindrops and tends to move toward isotopic equilibrium. On cessation of rain, the δ¹⁸O and δD of GLV quickly return to the typical baseline values. Therefore, use of isotopic composition of monthly rainfall for estimating average monthly isotopic composition of GLV can lead to erroneous results. Within a rainy season, certain large rain events have depleted δ¹⁸O and δD values compared to other equally large rain events with significantly enriched δ¹⁸O and δD. These isotopic differences are apparently not related to amount of rainfall. Variable magnitude of evaporation from falling raindrops and/or cloud liquid water fraction cannot explain the observed differences. Instead, it is shown that varying source regions (Arabian Sea or Bay of Bengal) and cloud top temperature may be responsible for observed differences.