FACTORS CONTROLLING STABLE ISOTOPE COMPOSITION OF RAINFALL IN NEW-DELHI, INDIA

Temporal variations in the oxygen-18 and deuterium composition of New Delhi rainfall (1961-1983) have been analysed using statistical correlations. Although long-term values agree with the 'world meteoric line', short-term features can be seen following an evaporation line. Rainfall in the monsoon months (June-September) seems to be undergoing evaporation and subsequent isotopic enrichment during the course of its fall. The isotopic composition of rainfall events of less than 30 mm show deviation from the meteoric line. The isotopic composition and deuterium excess values appear to be governed by the intensity and distribution of rainfall and the trajectory of the moist air-mass movement. The long-term mean temperature and mean rainfall together account for about 80-95% of the long-term average variability of O-18 composition. Lack of correlation between monthly isotopic composition and rainfall reflects the possible impact of atmospheric circulation patterns on rainfall isotope contents. The necessity of isotope modelling of moist air-mass circulation is also discussed.