Assessing moisture origin as a potential driver of event-based precipitation isotope variability in a Western Mediterranean catchment (Apuan Alps, Italy)

Isotope hydrology of precipitation is crucial in environmental and climate research for investigating the complex interactions between the atmosphere, land surfaces, and the hydrological cycle across various time scales. Traditional approaches rely on monthly cumulative precipitation samples to quantify the isotopic effects due to climatic and geographical factors but cannot resolve the potential impact of moisture origin and atmospheric pathways. Daily or event-based sampling offers a more accurate representation of how moisture sources and transport affect isotope ratios in precipitation, enhancing the understanding of regional hydrological dynamics with implications for water resources and paleoclimate research. With the aim to evaluate the influence of moisture sources and pathways on the isotope composition of precipitation over the western Mediterranean, in this work we present the results of a study performed in the Apuan Alps (northwestern Tuscany), one of the rainiest areas in Italy and the Mediterranean. 53 event-based precipitation samples were collected over one year at two altitudes and analyzed for δ18O and δ2H. Lagrangian moisture uptake diagnostic was applied to trace the origin and pathways of moist air masses associated with precipitation events. Results highlighted the influence of moisture contributions from different source regions on the isotope values of precipitation. Depleted isotopic signals in winter and autumn were associated with increased moisture transport from distal sources, greater rainout during transit, and higher precipitation amounts. By contrast, enriched isotope values in spring and summer were driven by larger continental moisture recycling, sub-cloud processes and evaporation over the southeastern Mediterranean.