Lyssette Elena Muñoz-Villers | Sofía González-Ríos | Guillermo Montero-Martínez

Abstract

Precipitation drives the terrestrial water cycle; determining its sources and variability is critical for advancing knowledge inatmospheric circulation, hydrology and climate impacts in urban montane environments. The present study analysed event-based rainfall isotopes to assess how moisture sources, influenced by synoptic-scale weather systems and local meteorologicalconditions, control precipitation during the wet season in Mexico City. We found that the isotope signature of rainfall exhibitedsubstantial event to-event variation (absolute range: 21.3‰ for δ18O and 171.7‰ for δ2H), reflecting the combined effects of bothlocal and remote atmospheric processes along moisture transport pathways. Trajectory analysis indicated that most air massestravelled over inland areas or followed combined land–ocean pathways. The former was associated with air parcels originatingover the Gulf of Mexico, the Pacific Ocean and continental regions, whilst the latter were mainly linked to air parcels travel-ling over the Caribbean Sea. Collectively, these air masses produced isotopically depleted rainfall with high deuterium-excessvalues compared to those that moved predominantly over oceanic regions. The moisture attribution analysis showed that 16%of the precipitation during the studied period was mainly sourced from recycled moisture over land surface regions within themixing layer, with the remaining fraction attributed to unidentified moisture uptake locations, highlighting the importance oflocal recycled moisture relative to externally advected sources. Consistently, high deuterium-excess values in rainfall were oftenobserved throughout the wet season (> 10‰; ~80% of the rain events sampled), providing further support for the re-evaporatedterrestrial moisture source contribution. Most rainfall events and accumulated precipitation at the study site were associatedwith air masses originating over continental sources and typically modulated by trough weather systems, with nearly 30% ofthe rainfall attributed to recycled moisture within the mixing layer. In contrast, air parcels transported from the Gulf of Mexicoand the Caribbean Sea – often influenced by troughs and tropical waves – were dominated by advected moisture sources (> 90%)from unidentified spatial locations. Under these conditions, rainfall exhibited the greatest variability in isotopic composition anddeuterium-excess. Although only a few rain events associated with frontal cold systems were sampled, these events exhibited thehighest levels of recycled moisture. These findings provide new insights into rainfall origin, moisture transport and sources forthe Mexico City Metropolitan Area, and foundation for future climate and hydrology studies in the region.This is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs License, which permits use and distribution in anymedium, provided the original work is properly cited, the use is non- commercial and no modifications or adaptations are made.© 2026 The Author(s). Hydrological Processes published by John Wiley & Sons Ltd.