Conclusions

The study presents stable isotopic signatures of 77 cumulative rain water samples obtained between 2014 and 2019 at two sites in southwest Spain, collected with at least biweekly sampling intervals. Results show two additional Local Meteorical Water Lines (LMWL) representative for Southwest Spain which both are in agreement with the LMWL of Gibraltar and Morón de la Frontera.
Precipitation weighted average d-excess values of 11.8-13 ‰ point to an important influence of western Mediterranean semiarid vapor source beside the Atlantic influence.
Both δ2H and δ18O values do not show statistically significant correlations with temperature at both locations which is attributed to the variability of Atlantic and Mediterranean vapor source origin. Nonetheless, a seasonal fluctuation of δ2H, δ18O behavior is evident with more depleted values in winter and isotope enrichment during the warm season.
Results show a trend of more depleted values for higher sample volumes, which may be attributed to the rainfall amount effect leading to a decrease of heavy isotopes with the progress of precipitation. On the other hand, it may also reflect a temperature effect because higher rain intensity occurs more frequently during the colder season and correlates also negatively with temperature. Multiple regression analyses showed that only precipitation amount revealed statistical significance (p-value: 0.00347) whereas temperature has a negligible effect (p-value: 0.46).
Evaporation effects of single rainfall events during summer were identified by reduced d-excess values and enriched isotopic signatures below the GMWL. In contrast to typical Mediterranean sites, comparison of amount weighted (PWLSR) versus un-weighted (OLSR) regression leads to almost identical slopes due to the medium to high cumulative amounts of summer rainfall with low d-excess collected at both study sites.
Comparison of backward trajectories for samples with extreme d-excess values point to the importance of vapor source origin from the Mediterranean leading to more elevated d-excess values. Beside the different source origins as possible explanation of d-excess variability, results point to the variability rainfall intensity as an additional reason for the high d-excess variability.
The signatures of identical samples obtained at both sites show very similar trends and 4 of 6 samples of the site Plaza de España show lower d-excess values which may point to thermal influence but more samples of simultaneous rain events are necessary to confirm this hypothesis.