Hydrometeorological processes and evaporation from falling rain in Indian sub-continent: Insights from stable isotopes and meteorological parameters

Rigorous analysis of measured isotopic composition (δ¹⁸O and δD) of 556 daily rainwater samples collected at four Indian stations viz., Jammu, Jorhat, Hyderabad and Ahmedabad, is done in conjunction with satellite and model derived meteorological and isotopic parameters to discern prominent hydrometeorological processes and factors in four different climatic zones in the Indian subcontinent. A new Indian Meteoric Water Line (IMWL), better representing the different climatic zones, including the semi-arid western India, has been defined: [δD = (7.6 ± 0.1) δ¹⁸O + (8 ± 1); R² = 0.96; P < 0.05; N = 556]. The lower slope of IMWL compared to the Global Meteoric Water Line signifies the role of evaporation from falling rain throughout the Indian subcontinent, though it is surrounded by large marine water body, and bordered by lofty Himalayan mountains in the north obstructing the monsoonal winds. This study provides new quantitative insights about various hydrometeorological processes across the Indian subcontinent. Some of the important inferences drawn from this study are: (i) The estimated evaporative losses from falling raindrops for the four stations are [Jammu: Maximum 52% and Minimum 8%; Jorhat: Max 15% and Min 4%; Ahmedabad Min 8% and Hyderabad Max 29% and Min 15%]. (ii) Increased availability of surface waters due to flood in Brahmaputra River and high value (88%) of column averaged RH results in the lowest evaporative loss from falling raindrops at Jorhat in Northeast. (iii) The high Cloud Liquid Water Content (CLWC) over a longer span of altitude facilitates the interaction of falling raindrop with isotopically depleted ambient vapor and results in isotopically depleted rain. (iv) The observed inverse relationship between the evaporative loss from falling raindrop and the column averaged RH confirms that the post condensation kinetic processes are realistically accounted in this study. (v) Validation with measured isotope data of rain shows that the LMDZ-iso modelled rain fails to incorporate evaporative isotopic enrichment and generates negative bias.