METHODS
Samples of rainwater for stable isotopes (δ2H and
δ18O) analysis at the two sites were collected in a
5-L polyethylene bottle between January 2014 and April 2019 in the
Doñana National Park and in Seville city (Plaza de España). In total, 77
rainwater samples were collected with 40 rainwater samples from Plaza de
España (January 2014 to June 2018) and 37 rainwater samples in Doñana
(February 2016 to April 2019, see supplemental material). Sampling was
performed within intervals of at least 14 days basis if rain occurred
but frequently intervals were longer according to the rainfall incidence
especially during the summer season. To avoid contamination of the Laser
Spectrometer with Paraffin, only samples with a minimum of 1 L sampling
volume were analysed. In case of high intensity rainfall events leading
to complete filling of the rainwater collector, sampling was performed
after the respective event to avoid losing precipitation of subsequent
events. The amounts of rainfall which exceeded the collector volume and
therefore have not been collected are 114 mm of a total rainfall of 2135
mm between 24.01.2014 and 12.06.2018 at Plaza de España and 212 mm of a
total rainfall of 1373 mm between 5.02.2016 and 26.04.2019 at Doñana.
This corresponds to a loss of 5.3% and 15.5% at Plaza de España and
Doñana respectively whereby 67% of the not analyzed precipitation
volume at Doñana corresponds to only one meteorological event of 205 mm
in early December 2016, when 142 mm exceeded the rain collector volume.
Therefore, the individual samples are considered as representative
averaged precipitation weighted results of cumulated rainfall for the
respective sampling periods. Parallel sampling at the same dates was
performed for 6 intervalls to allow direct comparison between the two
sites.
The samples were collected by attaching a funnel to a high-density
polyethylene bottle (5L) containing paraffin oil to prevent evaporation
and the respective accumulated sample volume was registered for each
sampling event. Samples were collected without air bubbles in
polyethylene bottles and conserved immediately at 4°C in the
refrigerator until isotopic analysis in the laboratory. At both sampling
sites, some rainwater samples were taken twice on the same day for
double checking. All double checked samples concurred within the
measurement uncertainty and we used the average of both samples.
Meteorological data were taken from nearby official meteorological
stations, la Rinconada for the Plaza de España site whereas
precipitation and temperature data of the Doñana site were recorded on
site by a Meteorological station including a tipping bucket pluviometer
(Meter Group, Inc., USA).
The analyses of the stable isotopes of the rain (δ18O and δ2H) were
carried out at the laboratory of the Center of Hydrogeology of the
University of Malaga by using a PICARRO™ L2120‐i Cavity Ring-Down Laser
Spectrometer. The analytical method consisted of a continuous sequence
of three IAEA referred internal standards and seven samples to process
the instrumental drifting during the measurement cycles. From each
sample, a six injections running-mode was performed and the first three
injections were discarded to avoid memory effects. Blanks for the
isotope analyses were analyzed sporadically to confirm there was no
memory effect during combustion. The corrected isotope data for
δ18O and δ2H were reported as
”delta” (d) values in parts per thousand (denoted as ‰) enrichments or
depletions relative to the Vienna-Standard Mean Ocean Water (V-SMOW)
(ROZANSKI; ARAGUÁS-ARAGUÁS; GONFIANTINI, 1993). The accuracy of isotope
measurements was ± 0.1‰ for δ18O and ± 1‰ for
δ2H.
Statistical parameters of the regression lines were calculated by the lm
function of the R software (RCORETEAM, 2019) using ordinary least square
regression (OLSR) and precipitation weighted least square regression
(PWLSR). Calculation of local meteoric water lines was based on data
covering entire years ranging from February 2014-February 2018 for Plaza
de España and February 2016- February 2019 for Doñana. For the analysis
of the amount effect only samples without loss of precipitation with a
sample volume < 5L were considered. Data of precipitation and
temperature of the Doñana site was recorded on site by a Meteorological
station including a tipping bucket pluviometer (Meter Group, Inc., USA).
Data of the official meteorological station La Rinconada en Seville were
used for the Plaza de España site. Average temperatures assigned to
samples representing a certain time interval were calculated based on
daily temperatures at the respective site weighted by precipitation
amounts of the corresponding days.
Back trajectories for the Doñana site were computed with the Hysplit
Single Particle Lagrangian Integrated Trajectory (HYSPLIT) Model fromhttp://ready.arl.noaa.gov/HYSPLIT.
Considering an average residence time of 10 days, back trajectories of
240 h were produced for predominant rain events of samples with maximum
and minimum d-excess values starting at 500 m height which was
considered as representative cloud altitude. Start times were defined in
UTC times at the end of the respective major rain events. For each
simulation 24 back trajectories were started at 3 h intervals
representing a 3 day time period of the respective rain event.