Precipitation isotopes to elucidate moisture sources in the Western Mediterranean: case of the Middle Atlas Mountains, Morocco

Morocco, being part of the Mediterranean basin, is characterized by a semi-arid climate heavily affected by climate change, spatial heterogeneity of the water resources along with its high demand. As the region heavily relies on precipitation to supply surface and groundwater, the restraints are a capital threat to its availability. Therefore, conducting studies on the spatio-temporal variations of precipitation ought to be a necessity. Herein, we present the results of our study conducted in the High Oum-Er Rbiaa catchment in the Moroccan Middle Atlas Mountains. Spatial and temporal monitoring of precipitation isotopic composition during a full hydrological year indicated a wide variation of isotopic values ranging from – 3.7‰ to – 13.1‰ and from – 17.1‰ to – 78.2‰ for δ 18 O and δ 2 H, between winter and summer respectively. Deuterium excess was marked by a strong seasonality, ranging from -9.2‰ to 27.8‰, with a mean value of 14.8‰. The preliminary local meteoric water line (LMWL: 2 H = 6.58 × 18 O + 8.3 ( R 2 = 0.92)) reflects the significant evaporation effect, whereas the high d-excess values might also suggest Mediterranean-sourced moisture. Global reanalysis data of the vertical integral of moisture flux combined with air masses trajectories simulations, confirm substantial Mediterranean-sourced moisture during the rainiest events in 2020–2021, as shown by the depleted isotope values (– 9.5 < δ 18 O (‰) < – 13.1; – 55 < δ 2 H (‰) < – 78.2) and high d-excess (18 < d-excess (‰) < 27). Contrariwise, summer precipitation displays weak and heterogenous moisture sources indicative of continental influence, as reflected by the enriched isotope values and high d-excess (0 < d-excess (‰) < 5) likely due to sub-cloud evaporation process. The findings allowed to emphasize the influence of large-scale water vapor transport of oceanic moisture contributing to winter precipitation within the region, whereas summer precipitation is affected by cloud microphysical processes generating local rainfall.