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.