INTRODUCTION
Precipitation is an essential component of the global water cycle,
constituting a crucial factor for wetlands, for irrigation as well as
for industrial and domestic uses in arid and semi-arid regions. The
study of the isotopic composition of stable isotopes as an environmental
tracer in atmospheric precipitation is an efficient means to improve the
current knowledge of the local and global water cycle (DANSGAARD, 1964).
δ2H and δ18O values of modern
precipitation are valuable tools for understanding regional climate
dynamics and moisture sources and for tracking changes in atmospheric
circulation on modern and quaternary time scales (CLARK; FRITZ, 1997).
The relationship between δ2H and
δ18O in precipitation at a global scale, is called
Global Meteoric Water Line (GMWL) (CRAIG, 1961) but due to the site
specific regional scale processes it varies in space. Therefore, the
understanding of the local water cycle benefits significantly from the
determination the Local Meteoric Water Line (LMWL) specific to a study
area which may show a clear deviation from the GMWL (GAMMONS; POULSON;
PELLICORI; REED et al. , 2006; SIMPKINS, 1995).
The local isotopic signatures of precipitation are mainly controlled by
regional scale processes, such as vapor origin and trajectory, rainout
history and meteorological conditions such as humidity and temperature
(ROZANSKI; SONNTAG; MÜNNICH, 1982). The regional environmental
background refers to the source of precipitation vapor. Local
geographical factors represent the various climatic factors during
precipitation, including rainfall amount, temperature, and relative
humidity. Analysis of δ2H and δ18O
is mostly used in a local area, so the local geographical factors should
be considered predominantly.
Among the local geographic factors, the temperature effect and the
rainfall amount effect may constitute important controls on the isotope
signatures. The positive correlation of temperature with heavy isotopes
enrichment in precipitation results from the fact that a higher energy
level is leading to the transfer of heavier isotopes into the vapor
phase. The δ18O temperature gradient is important
because it can be used to estimate paleo temperature changes and to
validate the accuracy of Global Climate Model outputs (ICHIYANAGI,
2009). (DANSGAARD, 1964) showed a strong correlation between the annual
mean δ18O and temperature on a global scale, with an
average gradient of 0.7‰/°C. (ROZANSKI; ARAGUÁS-ARAGUÁS; GONFIANTINI,
1992) showed that the long-term annual temperature effect averaged only
in European stations was around 0.60‰/°C. When sampled on an event or
daily basis, the correlation between precipitation isotopes and
temperature from the mid-latitudes is weaker compared with monthly or
annual timescales (BALDINI; MCDERMOTT; BALDINI; FISCHER et al. ,
2010). Seasonal variations are observed, where seasonal variations of
temperature are well pronounced. A remarkable temperature effect has
been discerned mainly in high and mid-high latitude continents
(DANSGAARD, 1964). In order to study the effect of temperature on
precipitation, δ18O is generally selected as an
indicator due to its higher temperature sensitivity of the fractionation
process as a consequence of the higher mass difference (ZHANG; WU,
2007).
The negative correlation of isotope content in precipitation with the
rainfall is called the rainfall amount effect and is related to the fact
that the content of heavy isotopes decreases with the progress of
precipitation. Nonetheless, the amount of this change at a specific site
is related to rainfall, temperature, relative humidity and other
conditions which explains the large variability of isotope signatures of
individual precipitation events and even during single precipitation
events. A monthly amount effect has been observed in low-mid latitude
oceans, islands, and monsoon areas (DANSGAARD, 1964; KRAJCAR BRONIĆ;
BAREŠIĆ; BORKOVIĆ; SIRONIĆ et al. , 2020; THARAMMAL; BALA; NOONE,
2017; YANG; MU; GUO; BAO et al. , 2019).
Also secondary evaporation below the clouds may play a significant role
in affecting isotopic signatures and d-excess. The preferential
evaporation of the light isotopes during the secondary evaporation
process leads to heavy isotopes enrichment and d-excess decrease. (YANG;
MU; GUO; BAO et al. , 2019) reported that rainfall is more
affected by secondary evaporation when rainfall is low leading to lower
d-excess values for low precipitation events. (HUGHES; CRAWFORD, 2012)
report that lower slopes calculated by non-weighing regression methods
tend to be found at sites with typical Mediterranean climates with hot
dry summers and mild wet winters, where low d-excess for small rain
events in summer have a dominant effect on the slope of the LMWL. To
correct this bias, they recommend to use a LMWL produced by a regression
method that is weighted towards those events.
(CORTECCI; DINELLI; MUSSI, 2008) compared isotopic precipitation
signatures of an urban and a rural site in Bologna (Italy). Their study
reveals that the LMWL of the urban site has a notably lower slope and
δ2H-intercept different from that of precipitation in
the peripheral non-urban area and the authors concluded that
precipitation in the urban center of Bologna undergoes appreciable
isotopic effects due to secondary evaporation during falling.
The study sites are mainly influenced by the North Atlantic Oscillation
(NAO), being the dominant mode of interannual atmospheric variability in
the Northern Hemisphere, and a clear influence on the isotopic
composition of rainfall was previously established (BALDINI; MCDERMOTT;
FOLEY; BALDINI, 2008). For the nearby Mediterranean region, the Western
Mediterranean Oscillation Index, an index measuring the difference
between the standardized atmospheric pressure recorded at Padua
(45.40°N, 11.48°E) in northern Italy, and San Fernando, Cádiz (36.28° N,
6.12°W) in Southwestern Spain (MARTIN-VIDE; LOPEZ-BUSTINS, 2006) has
being explored as another source of variability. (CELLE-JEANTON; TRAVI;
BLAVOUX, 2001) established a relationship between the isotopic content
of precipitation and the origins of air masses for the Western
Mediterranean basin defined as the Western Mediterranean Waterline
(WMMWL) with d-excess of 14‰ which falls between the Atlantic (10‰) and
the Eastern Mediterranean (22‰). Monthly weighted time series of
d-excess data between 1985 and 1991 of Western Mediterranean data from
Barcelona, Gibraltar, Tunis and Genoa revealed a much higher temporal
variability of δ18O signatures for the Barcelona
station. This was attributed to the almost equal influence of the
Atlantic and the Mediterranean vapor origin whereas Genoa and Gibraltar
are influenced mainly by only one vapor source, Mediterranean or
Atlantic, respectively (CELLE-JEANTON; TRAVI; BLAVOUX, 2001). A detailed
analysis (MORENO; SANCHO; BARTOLOMÉ; OLIVA-URCIA et al. , 2014)
based on single event sampling between 2010 and 2012 in north-east Spain
confirmed the high variability of δ18O signatures
compared to other Mediterranean locations documented by (CELLE-JEANTON;
TRAVI; BLAVOUX, 2001).
The Spanish network for isotopes in precipitation (Red Española de
Vigilancia de Isótopos en la Precipitación, REVIP) provides composite
monthly samples of precipitation collected since 2000 at 16
meteorological stations. The stations have a wide geographic
distribution, and are located in the main hydrographical basins, in
areas representative of the different climatic zones in Spain. The
nearest long term Global Network of Isotopes in Precipitation (GNIP)
site is located in the village Morón de la Frontera, representing the
only station of the Guadalquivir basin at a distance of about 130 km to
the Atlantic Ocean (TEIJEIRO; ARÉVALO; ZABALETA; CASTAÑO CASTAÑOet al. , 2007). This article therefore adds useful information
providing data directly measured at the coast line and in a mayor urban
area.
This study reports time series of δ2H,
δ18O and deuterium excess (d-excess or d) in
precipitation over a four to five-year period at two sites in
southwestern Spain with at least biweekly intervals. A local meteoric
water line and precipitation weighted average values are established
which can be used for hydrological studies in the region or at a
Mediterranean scale. Furthermore, the seasonal behavior of the isotopes
as well as secondary evaporation effects are described and the
importance of temperature and amount effects on the isotope composition
as well as possible thermal influence is investigated. Backward
trajectories were computed to investigate the vapor source impact on the
d-excess variability.