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.