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Stable isotope composition of daily andmonthly precipitation in ZagrebJadranka Barešić a , Nada Horvat inčić a , Ines Kraj car Bronić a ,

Bogomil Obel ić a & Polona Vreča b

a Ruder Bošković Inst it ut e, Radiocarbon and Trit ium Laborat ory,Bij enička 54, 10002, Zagreb, Croat iab Jožef St efan Inst it ut e, Jamova 39, 1000, Lj ubl j ana, Slovenia

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Isotopes in Environmental and Health Studies

Vol. 42, No. 3, September 2006, 239–249

Stable isotope composition of daily and monthly

precipitation in Zagreb†

JADRANKA BAREŠIC‡, NADA HORVATINCIC‡, INES KRAJCAR BRONIC*‡,BOGOMIL OBELIC‡ and POLONA VRECA§

‡Ru --der Boškovic Institute, Radiocarbon and Tritium Laboratory, Bijenicka 54, 10002 Zagreb, Croatia§Jožef Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia

(Received 28 October 2005; in final form 7 February 2006)

Stable isotopic compositions (δ2H and δ18O) of daily precipitation collected in the period from October

2002 to March 2003 and monthly precipitation in the period from 2001 to 2003, as well as thecorresponding meteorological data (temperature, amount of precipitation), all collected in Zagreb,Croatia, are presented. δ

2H and δ18O values, both daily and monthly, show large variations due to

large temperature variations and the different origin of the air masses. Variations are larger for dailysamples than for composite monthly samples. Good correlation of δ

18O with temperature is obtainedfor both types of samples. On the basis of the correlation between δ

2H and δ18O, the local meteoric

water line is close to the global meteoric water line. Deuterium excess of both daily and monthlyprecipitation indicates that in the Zagreb area, the influence of air masses from the Mediterranean areaprevails in the autumn period.

Keywords: Atmospheric composition; Europe; Hydrogen-2; Mass spectrometry; Oxygen-18;Precipitation; Water cycles

1. Introduction

Isotopic composition of precipitation, including radioactive isotope 3H and ratios of stableisotopes 2H/1H and 18O/16O, has a wide application in the hydrological, meteorological andenvironmental studies, as well as in the studies of isotopic exchange and circulation processesin the atmosphere [1–3]. At the continental station in Zagreb, NW Croatia, the basic isotopicdata of monthly precipitation have been collected since 1976. The data have been includedin the Global Network of Isotopes in Precipitation [4–6]. In the frame of the InternationalAtomic Energy Agency (IAEA) Research Contract Tritium and stable isotope distribution in

the atmosphere at the coastal region of Croatia, we determined the isotopic compositionof monthly precipitation during the 3-year period at six stations (four maritime and two

†Revised version of a paper presented at the VIII Isotope Workshop of the European Society for Isotope Research(ESIR), 25–30 June 2005, Leipzig-Halle, Germany.

*Corresponding author. Tel.: +385-1-4571-271; Fax: +385-1-4680-239; Email: krajcar@irb.hr

Isotopes in Environmental and Health StudiesISSN 1025-6016 print/ISSN 1477-2639 online © 2006 Taylor & Francis

http://www.tandf.co.uk/journalsDOI: 10.1080/10256010600840226

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240 Jadranka Barešic et al.

continental) in Croatia, as well as that of individual daily precipitation during a 6-monthperiod in Zagreb [7]. The aim was to determine natural variations of radioactive isotope 3H,as well as the distribution of stable isotopes in precipitation over a relatively small area withdifferent climatic and geographical characteristics, and compare them with the long-term datafor the Zagreb station.

In this article, we present the δ2H and δ

18O values of monthly precipitation for the periodfrom 2001 to 2003 and daily precipitation for the period from 1 October 2002 to 1April 2003 atthe Zagreb station. Relations between stable isotopic composition and meteorological data, aswell as the relation between δ

2H and δ18O, and deuterium excess are presented and discussed

for both monthly and daily precipitation.

2. Sampling and measurements

Zagreb station, NW Croatia, (45◦49′N, 15◦59′E) has a typical continental climate charac-terised by large seasonal variations in temperature, whereas the seasonal variations in themonthly amount of precipitation are less pronounced, with minimum in winter and maximumin summer [8].

Daily precipitation samples were collected every morning in the period from 1 October

2002 to 1 April 2003 in a rain gauge at the Ru --der Boškovic Institute (RBI), Zagreb. Monthlycomposite precipitation samples were collected in the period from 2001 to 2003 in a rain gauge(emptied daily) situated at the Meteorological and Hydrological Service of Croatia, Zagreb-Gric. Meteorological data, i.e. daily air temperature and amount of precipitation, as well asthe monthly averaged air temperature and total amount of precipitation, have been obtainedfrom the Meteorological and Hydrological Service. Distance between the two sampling sitesis ∼700 m, the difference in altitude is 30 m (Zagreb-Gric 157 m; Zagreb-RBI 187 m), and thesites do not show particular differences in temperature and amount of precipitation.

Stable isotopic compositions of hydrogen, δ2H, and oxygen, δ

18O, were measured inmonthly samples at the Department of Environmental Sciences of the Jožef Stefan Insti-tute in Ljubljana on a Varian MAT 250 mass spectrometer and in daily precipitation at theIsotope Hydrology Section at the IAEA in Vienna. δ

18O was determined by equilibration ofCO2 with water samples, and δ

2H was measured on H2 generated by reduction of water overhot chromium [9].

3. Results and discussion

3.1 Meteorological data

The meteorological data for the periods of sampling of daily and monthly precipitation arepresented in figures 1 and 2, respectively.

Daily samples were collected between 1 October 2002 and 1 April 2003, i.e. in the colderperiod of year (autumn–winter) with a maximum temperature of 16.5 ◦C (23 October 2002)and a minimum of −6.1 ◦C (10 January 2003). Mean temperature of the whole 6-month periodwas 6 ◦C, whereas the mean temperatures for the first 3-month period (October–December2002) and the second 3-month period (January–March 2003) were 7.6 and 2.4 ◦C, respectively(table 1). The amount of precipitation in the first 3-month period was much higher than in thesecond one (table 1), and on 4 November 2002, the highest daily amount of precipitation wasrecorded (38.2 mm). Most of the precipitation in the sampling period was in the form of rain,

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Isotopes in precipitation in Zagreb 241

Figure 1. Daily air temperature and amount of daily precipitation for the sampling period, October 2002–March2003, in Zagreb.

Figure 2. Monthly mean air temperature and amount of monthly precipitation in Zagreb for the sampling periodof monthly precipitation (2001–2003).

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242 Jadranka Barešic et al.

Table 1. Temperatures (lowest and highest daily temperatures and the mean for theperiod indicated) and amount of precipitation (highest daily amount and total amount for

the period indicated) for sampling periods of daily precipitation samples.

Air temperature (◦C) Amount of precipitation (mm)

Sampling period Min Max Mean Max (daily) �

October–December 2002 −1.7 16.5 7.6 38.2 243.9January–March 2003 −6.1 10.8 2.4 27.0 106.2

October 2002–March 2003 −6.1 16.5 6.0 38.2 350.1

but in December, January, and February, snow was also collected (figure 1). March 2003 wasa very dry month with only two precipitation events.

Mean monthly air temperature for the period 2001–2003 shows typical seasonal variationswith winter minima and summer maxima (figure 2). The lowest mean monthly temperatureof −0.7 ◦C is observed in December 2001 (table 2) and the highest in August 2003 (25.8 ◦C).The mean annual temperature in the long-term period 1961–1990 [8] in Zagreb is 11.4 ◦C,with the lowest value of 0.5 ◦C in January and the highest of 21.3 ◦C in July. For the period1991–2003, all mean temperatures are ∼1 ◦C higher: the mean annual temperature is 12.5 ◦Cand the monthly means range from 2.1 to 22.6 ◦C. If we compare the mean annual temper-atures for period 2001–2003 with the long-term (both 1961–1990 and 1991–2003 periods)measurements, we see that the mean temperature for each year 2001–2003 was higher than thelong-term averages (table 2). In particular, 2002 was much warmer the whole year, whereassummer 2003 was much warmer (mean temperature 24.6 ◦C) than the average summer (meansummer temperatures are 20.4 ◦C for 1961–1990 and 21.9 ◦C for 1991–2003).

Maximum monthly amount of precipitation during the presented period is collected inSeptember 2001 (177.3 mm) and minimum in March 2003 (8 mm) (figure 2). These monthlyamounts are much higher or lower, respectively, than the long-term mean monthly amounts(maximum of 100.9 mm in June and minimum of 46.6 mm in February for the period 1961–1990). The long-term annual amount of precipitation was the same in the periods 1961–1990and 1991–2003 (table 2). The amount of precipitation for 2002 was ∼20 % higher and for2003 ∼30 % lower than the long-term mean annual amount. Summer 2003 was an especiallywarm and dry season (figure 2).

3.2 Stable isotopes in precipitation

δ2H and δ

18O of daily and monthly precipitation are shown in figures 3 and 4, respectively.The values δ

2H and δ18O, measured daily (figure 3) in the warmer period October–December

Table 2. Mean monthly temperatures (lowest, highest and mean monthly values) and monthlyamount of precipitation (lowest and highest), as well as total annual amount for period of

sampling of monthly precipitation (2001–2003), and two long-term periods.

Air temperature (◦C) Amount of precipitation (mm)

Sampling period Min Max Mean Min Max Annual �

2001 −0.7 23.6 12.7 9.2 177.3 7242002 2.2 22.5 13.2 7.0 156.9 10642003 −0.1 25.8 12.9 8.1 106.6 623

1961–1990 0.5 21.3 11.4 8831991–2003 2.1 22.6 12.5 884

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Isotopes in precipitation in Zagreb 243

Figure 3. Stable isotopic composition (δ18O and δ2H) of daily precipitation in Zagreb for period October

2002–March 2003.

2002 (mean values δ2H = −61.8 ‰, δ

18O = −9.0 ‰), were higher than in the colder periodJanuary–March 2003 (mean values δ

2H = −97.7 ‰, δ18O = −12.9 ‰). The highest daily

δ18O and δ

2H values were measured on 3 November 2002 (δ18O = −1.8 ‰) and 31 October2002 (δ2H = −3.9 ‰), respectively, whereas the lowest values were measured on 7 January2003 (δ18O = −20.0 ‰ and δ

2H = −150.8 ‰).

Figure 4. Stable isotopic composition (δ18O and δ2H) of monthly precipitation in Zagreb for period 2001–2003.

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244 Jadranka Barešic et al.

As a typical continental station, Zagreb shows a large seasonal variation in the stable isotopiccomposition (figure 4). For monthly samples, the highest δ

2H and δ18O values are measured

in summer of all 3 years (values −20 and −3.5 ‰, respectively). The lowest values aremeasured in January 2003 (δ2H = −119 ‰ and δ

18O = −16.5 ‰). Long-term (period 1976–1995) mean monthly δ

2H ranges from −91 (December) to −37 ‰ (July) and δ18O from

−12.6 (December) to −5.6 ‰ (July) [6]. For the whole 3-year period, the mean values areδ

2H = −57.6 ‰ and δ18O = −8.3 ‰, and they are slightly more positive than the long-termmean values −62.1 and −8.8 ‰, respectively. This difference can be explained by the highermean temperature in the 3-year period than the long-term mean temperature.

Comparison of seasonal variations in temperature and in stable isotopic composition,as well as the comparison of the corresponding daily values, indicates a good correla-tion between the air temperature and the isotopic composition of precipitation. Thus, δ

18Ovs. temperature correlations for daily and monthly samples are presented in figures 5and 6. The daily values (figure 5) showed that δ

18O vs. temperature slope for the 6-monthperiod was 0.65 ± 0.07 ‰/◦C for the mean temperature of 6.0 ◦C. The slope calculatedfor the first 3-month period (October–December 2002, mean temperature 7.6 ◦C) is lower(0.52 ± 0.09 ‰/◦C), and the slope for the second 3-month period (January–March 2003,mean temperature 2.4 ◦C) is higher (0.80 ± 0.12 ‰/◦C) than the slope for the whole 6 monthperiod. It can be noticed that the slope of the correlation is higher in periods of lower meantemperatures. On the basis of daily precipitation samples in Ljubljana and Portorož (Slovenia),the δ

18O vs. temperature slopes of 0.5 and 0.2 ‰/◦C for mean temperatures of 7.2 and 10.9 ◦C,respectively, were determined [9]. This is in accordance with the observed dependence ofthe slope δ

18O vs. temperature for the continental and maritime stations of Croatia for theperiod 2001–2003 [7]: the higher the mean temperature, the lower the slope. The slope variedbetween 0.37 and 0.15 ‰/◦C for the mean temperature range 4.1–17.5 ◦C at these stations inCroatia. Also, on a global scale, the slope of the δ

18O vs. temperature correlation is higher forcolder regions than for temperate regions [2].

Figure 5. Correlation between δ18O of daily precipitation and daily temperature for the sampling period October

2002–March 2003. Data for the two 3-month periods are shown separately.

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Isotopes in precipitation in Zagreb 245

Figure 6. Correlation between δ18O of monthly precipitation and mean monthly temperature for the sampling

period 2001–2003. Data are shown separately for each year. Regression lines are shown for each year and for thewhole period.

If we plot the same correlation for monthly precipitation (figure 6), for the whole 3-yearperiod (2001–2003), we obtain the slope of 0.33 ‰/◦C, which is the same as that for the long-term measurements (1976–1995) of monthly precipitation in Zagreb (0.33 ‰/◦C) [6]. Slopesfor individual years 2001, 2002 and 2003 are 0.28 ± 0.07, 0.29 ± 0.05 and 0.39 ± 0.07 ‰/◦C,respectively. Owing to a small number of data per year, the errors are large, and theseslopes agree within the given errors. However, the difference between the slope for monthlyprecipitation in a 3-year period (0.33 ‰/◦C) and that for the daily precipitation in thecolder 6-month period of a single year (0.65 ‰/◦C) is significant, which can be explainedby the difference in the mean temperatures for the respective periods, 11.4 and 6.0 ◦C,respectively.

We noticed that the mean δ18O in the period 2001–2003 (−8.3 ‰) is more positive than the

long-term mean δ18O (−8.8 ‰) [6]. Taking into account the long-term slope of 0.33 ‰/◦C,

we can determine that in the 3-year period, the mean temperature was about 1.5 ◦C higherthan the long-term mean temperature in Zagreb, which agrees well with the actual differencebetween the measured temperatures (long-term mean of 11.4 ◦C and the mean for the 3-yearperiod of 12.9 ◦C). Therefore, the mean δ

18O of precipitation followed the change in meantemperature.

The correlation between δ18O and δ

2H in monthly precipitation on a global scale is knownas the global meteoric water line (GMWL) δ

2H = 8 δ18O + 10 [10]. However, local meteoric

water lines (LMWL) based on the long-term monthly values can be significantly different fordifferent regions or locations, and their knowledge is of utmost importance for any hydro-geological application of stable isotopic data. The long-term (1976–2003) LMWL for Zagrebis δ

2H = (7.7 ± 0.1) δ18O + (4.5 ± 0.8), based on n = 270 monthly data with the correlation

coefficient r2 = 0.96 [7].The correlation between δ

18O and δ2H obtained for daily samples collected in the 6-month

period (figure 7) is δ2H = (7.9 ± 0.2) δ

18O + (7.5 ± 2.0), and it fits well to the GMWL aswell as to the long-term LMWL for Zagreb. MWL for monthly samples (2001–2003) isalso presented in figure 7, which is δ

2H = (7.3 ± 0.2) δ18O + (2.8 ± 1.8), n = 34, r2 = 0.98.

Slope and intercept are lower than those of the long-term LMWL, and this difference may be

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246 Jadranka Barešic et al.

Figure 7. Correlation between δ18O and δ

2H for daily (October 2002–March 2003) and monthly (2001–2003)precipitation samples.

due to exceptional meteorological conditions in the short observation period, especially warmsummer temperature with a low amount of precipitation (see Meteorological data).

Deuterium excess (d-excess), defined as d = δ2H-8 δ

18O [11] and determined from themeasured δ

2H and δ18O values, can be related to the meteorological conditions in the source

region from where the water vapour is obtained [3, 12]. Therefore, by comparing the d-excessfor different precipitation events, one may conclude the origin of water vapour. The air massesoriginating from the Atlantic ocean have a typical d-excess of 10 ‰, whereas the vapourgenerated over a closed basin, such as the Mediterranean Sea, is characterised by higherd-excess reaching up to 25 ‰ [2, 3].

Deuterium excess of daily precipitation (figure 8) shows that the average value of d-excessfor the first 3-month period (October–December 2002) is 10.2 ‰, and more than half of the

Figure 8. Deuterium excess of daily precipitation in Zagreb for period October 2002–March 2003. Mean d-excessvalues in the 3-month periods are also shown.

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Isotopes in precipitation in Zagreb 247

events have d-excess above 10 ‰, with the maximum of 18.3 ‰. These higher values indicatethe stronger influence of air masses from the Mediterranean area during autumn. The meand-excess for the winter months January–March 2003 is 5.5 ‰, and only one event in thisperiod has d-excess above 10 ‰.

Deuterium excess of monthly precipitation samples in a 3-year period is shown in figure 9.Generally, we see that higher d-excess values (above 10 ‰ and up to 16 ‰) prevail in theautumn period because of the influence of the Mediterranean air masses over the region inthis period. Lower values (close to or below 10 ‰) are noticed during winter–spring periodsunder the prevailing influence of the Atlantic air masses. Due to different seasonal influences,the d-excess in monthly samples decreases slightly with the increase of the mean monthly tem-peratures (d = (−0.30 ± 0.08) × t + (12.6 ± 1.2), n = 33, r2 = 0.30). The mean d-excessfor Zagreb is 7.8 ± 4.0 ‰, and it ranges from −5 to 15 ‰ [6]. There is no correlation betweend-excess and either the monthly amount of precipitation or the mean monthly vapour pres-sure [4]. Our data for precipitation along the Adriatic coast showed that the mean d-excessincreases along the coast from NW towards SE [7], showing the increasing influence of theMediterranean air masses.

Low values of d-excess (∼0 ‰) are recorded in some samples mostly in summer (Julyand August 2001, March 2002, June 2003; figure 9). Such a decrease of d-excess may beindicative of secondary evaporation processes (e.g. evaporation of falling raindrops in a warmand dry atmosphere) [13, 14] and is characteristic of small precipitation events at high tem-peratures. As mentioned earlier, summer 2001 and especially summer 2003 were warmerand drier than the average summers (figure 2, table 2). No exceptionally low d-excess valuewas observed during the rainy summer 2002 (figure 9). Negative d-excess values for threedaily winter samples (−9.06, −2.32 and −2.0 ‰; figure 8) can also be explained by a verylow amount of precipitation (<2 mm) in relatively warm and dry air (>7.0 ◦C). There isno correlation of daily d-excess values with either the amount of precipitation or the dailytemperature.

Figure 9. Deuterium excess of monthly precipitation in Zagreb for period 2001–2003.

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248 Jadranka Barešic et al.

4. Conclusions

The analysis of isotopic composition of precipitation in Zagreb, NW Croatia, both on monthlyand daily scales, and their correlation with the meteorological parameters, has shown thefollowing:

δ18O and δ

2H values of precipitation, both monthly and daily, show large variations dueto large temperature variations and different origin of the air masses. Variations are largerfor daily samples than for composite monthly precipitation. Good correlation of δ

18O withtemperature is obtained for both types of samples. Generally, the amount of precipitation hasno influence on the isotopic composition. The correlation between δ

2H and δ18O (LMWL) is

good (r2 = 0.98) and is close to the global MWL.Deuterium excess of both daily and monthly precipitation indicates that in the Zagreb

area, the Atlantic air-mass circulation prevails, whereas the influence of air masses from theMediterranean area is more significant in the autumn period (d-excess above 10 ‰). Severallow d-excess values have been determined for very small amounts of precipitation andrelatively high temperatures. The use of d-excess and its seasonal variations, in additionto δ

2H and δ18O data, can be useful in the identification of seasonal recharge of groundwater.

The presented data of isotopic composition of daily and monthly precipitation in the Zagrebarea, and their relations with the meteorological data, lead to the following conclusions aboutthe applicability of such data. Isotopic composition of individual precipitation can give us moreinformation on the particular air-mass trajectories, e.g. more information on the source of theatmospheric moisture for the particular event. Comparison of meteorological and isotopicdata obtained by the various stations along a single air-mass trajectory would allow us toimprove the understanding of evolution of isotopic composition of precipitation. Compositemonthly sampling is more appropriate for comparison with other (neighbouring) stations andfor climatological studies. The collected data of stable isotopic composition (δ2H, δ

18O) ofmonthly and daily precipitation in Zagreb during the 3-year period provide a database forstudying different influences on the isotopic composition of precipitation. These data can beapplied to study the air-mass movement (circulation pattern), climate variability/change, localand regional hydrological features and so on.

Acknowledgements

This study was financially supported by the IAEA (Research Contract no. 11265) within theCoordinated Research Project on ‘Isotopic Composition of Precipitation in the MediterraneanBasin in Relation to Air Circulation Patterns and Climate’ and by the Project 0098014 of theMinistry of Science, Education and Sport of the Republic of Croatia. We thank L. Gourcyand the staff of the IAEA Isotope Hydrology Section Laboratory, S. Žigon and Z. Trkovfrom the Jožef Stefan Institute, Ljubljana, for stable isotopic measurements and S. Vidic fromMeteorological and Hydrological Service of Croatia for meteorological data.

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