Is the deuterium excess in precipitation a reliable tracer of moisture sources and water resources fate in the western Mediterranean? New insights from Apuan Alps (Italy)

• Event-based and monthly precipitation were collected at five sites. • Local Meteoric Water Lines evidence sub-cloud evaporation of raindrops. • Positive relationship between d-excess and altitude is due to pseudo-altitude effect. • d-excess is locally altered by sub-cloud processes at lower altitudes. • Caution should be paid when using d-excess as tracer of moisture source. The deuterium excess is a function of the isotopic composition of oxygen and hydrogen in water. Traditionally, the d-excess is used worldwide to constrain the moisture sources of precipitation, as it mainly depends on the relative humidity and surface sea temperature at the evaporative sources, that control diffusion of the water molecules across the density gradient. A case in point is represented by precipitation deriving from vapor originating over the eastern Mediterranean Sea, characterized by the higher d-excess values (∼22‰) compared to the global average d-excess in precipitation of 10‰. Accordingly, d-excess values of ∼14‰ found in precipitation across the western Mediterranean are generally interpreted as the result of mixing of Atlantic and Mediterranean air masses. Nevertheless, several studies proved the d-excess values of precipitation evolves during transport of vapor and changes depending on local processes (e.g. sub-cloud evaporation of raindrops, continental moisture recycling, seeder-feeder mechanism), thus compromising the reliability of d-excess as a proxy of vapor source regions. With the aim to investigate the pattern of d-excess in precipitation at higher spatial and temporal resolution, and to evaluate its reliability for tracing the moisture source in the western Mediterranean, in this work we present the results of a study performed in the Apuan Alps (Tuscany, Italy), one of the rainiest areas in Italy. 19 single rain events were collected from 2020 to 2021 at two sites placed at different altitudes. Furthermore, monthly precipitations were collected at five sites at different elevations over different time intervals from 2019 to 2021. Event-based and monthly Local Meteoric Water Lines were computed at each site, indicating droplets evaporation beneath the clouds, especially at lower altitude sites. d-excess seasonally and spatially varied, with higher values registered at higher altitudes in winter and autumn, and lower values at lower altitudes in spring and summer. Large differences in d-excess were found among sites placed at different altitudes, and these differences ranged both at monthly and event scales. Mean and weighted mean d-excess vertical gradients of +0.73‰/100 m (r 2 = 0.92) and +0.51‰/100 m (r 2 = 0.95), respectively, were computed using 9 months in which monthly samples were collected at all five sites. This positive relationship between altitude and d-excess in precipitation is consistent with a process like sub-cloud evaporation that appears to be the most important process controlling the d-excess spatial variability in the mountainous areas. These findings evidence the importance of local processes acting on the d-excess and downsize the reliability of this parameter in tracing moisture sources in this area. Conversely, the d-excess vertical gradient, combined with the δ 18 O vertical gradient (i.e. altitude effect), could be used to identify the recharge areas of groundwater.