Seasonal Variability of Water Vapour Dynamics in the Semi-Arid Region of Western India—Revelations From Isotopic Investigation at Mt. Abu, Rajasthan

Abstract

This study presents the first year-long continuous record of stable isotopes (δ¹⁸O, δD and d-excess) in atmospheric water vapour from semi-arid western India (except during periods of inoperable extreme weather conditions). Continuous in situ isotopic measurements of water vapour were made to monitor isotopic variations on daily to seasonal timescales and to discern the controlling factors. The observed temporal changes in isotopic composition are governed by variations in vapour sources and the contribution of recycled moisture. Seasonal mean values reflect a source-driven isotopic baseline: pre-monsoon [δ¹⁸O: −10.0‰; d-excess: −0.9‰], monsoon [δ¹⁸O: −10.6‰; d-excess: −5.3‰], post-monsoon [δ¹⁸O: −12.7‰; d-excess: −1.0‰] and winter [δ¹⁸O: −14.3‰; d-excess: +7.3‰]. Four distinct isotopic patterns, defined by coincident highs and lows in δ¹⁸O and d-excess, are linked to specific vapour sources: the northern Arabian Sea, evaporated terrestrial water bodies across the arid northwest, the wetter eastern interior of India and the Bay of Bengal. A dramatic isotopic shift at the end of April marks the seasonal transition from arid terrestrial to marine vapour dominance. Terrestrially recycled vapour dominates during non-monsoon months (64%–94%), while Arabian Sea contributions peak in June (~66%) and recede rapidly in post-monsoon. Vapour δ¹⁸O begins declining a median of ~14 h before rain onset (Wilcoxon p = 0.0005), demonstrating that vapour δ¹⁸O may serve as a reliable precursor to imminent rainfall. Atmospheric humidity and isotopic composition follow decoupled seasonal trajectories, confirming that vapour isotopes track moisture origin rather than amount, making them a uniquely powerful diagnostic tool for vapour dynamics in water-scarce regions.