A Monsoon-Season Precipitation δ18O Isoscape for India: Insights Into Moisture Recycling and Atmospheric Transport

Abstract

The spatial distribution of precipitation stable isotopes (“isoscapes”) provides important insights into atmospheric moisture transport, land–atmosphere interactions, and regional hydrological processes. In this study, we construct a monsoon-season precipitation oxygen isotope (δ¹⁸O) isoscape for India and investigate the mechanisms governing its spatial variability. Despite the limitations imposed by the sparse observational network, the reconstructed isoscape reveals a pronounced southwest-to-northeast isotopic gradient, with relatively enriched values over southwestern India and progressively depleted values toward northeastern India. While an inverse relationship between rainfall amount and δ¹⁸O is evident over central India, the large-scale spatial pattern cannot be explained solely by the classical amount effect. Instead, the observed isotopic distribution appears to be controlled by the combined influences of large-scale advection of moisture through monsoon circulation and moisture recycling. A phase-space analysis framework is used to examine the interactions between advected atmospheric moisture and evapotranspired moisture and to identify their characteristic timescales. The analysis reveals a strong spatial correspondence between precipitation isotope variability and recycled rainfall, demonstrating that terrestrial evapotranspiration exerts a first-order control on the isotopic composition of precipitation at the continental scale. This finding provides a mechanistic explanation for how moisture recycling shapes precipitation isotope variability across the Indian monsoon region. The study further shows that isotopically depleted rainfall associated with Bay of Bengal low-pressure systems is partly offset by the continuous advection of relatively enriched moisture from the Arabian Sea, contributing to the observed isotopic patterns over central India. In addition, an updated Indian Meteoric Water Line (IMWL) is derived using an expanded precipitation isotope dataset spanning mainland India and the island territories. Together, these results highlight the importance of land-atmosphere coupling and moisture recycling in controlling precipitation isotope variability and provide a new framework for interpreting isotope-based hydrological and paleoclimatic records from the Indian monsoon region.