Daily variability of isotopic composition

During the project three periods were selected for daily sampling of precipitation, 1 Oct. to 31 Dec.

2000, 1 Oct. to 31 Dec. 2001 and 1 Oct. 2002 to 31 March 2003. Data on climatic and isotopic FIG. 9. The ³H activity in monthly precipitation vs. air temperature relationship.

variations during selected periods are summarized in Table 3. To compare the same periods (October to December) the last sampling period was separated into two parts.

Table 3. Daily climatic and isotopic characteristics Location Sampling

Selected periods had very different characteristics. The mean air temperature was during the period from October to December in 2000 and 2002 much higher than for the 1961-90 normals that are 5.0 and 8.8°C for Ljubljana and Portorož Airport, respectively [2],[5]. In 2001 the mean air temperature was only slightly higher than in the reference period. From January to March 2003 was the mean air temperature slightly lower than for the 1961-90 normals that are 1.9 and 4.9°C for Ljubljana and Portorož Airport, respectively [2],[5].

Cumulative precipitation amount from October to December 2000 was much higher than in the reference period (i.e. 351 mm in Ljubljana, 286 mm at Portorož Airport and 419 mm in Kozina). In 2001 significant deficit of precipitation was observed [5] while in 2002 precipitation amount was close to normals [6]. In the first three months of 2003 precipitation amount was significantly lower than in the reference period (i.e. 259 mm in Ljubljana, 209 mm at Portorož Airport and 322 mm in Kozina).

Similar as for monthly precipitation we observed changes in isotopic composition in the southwest-northeast direction, with more negative δ¹⁸O and δ²H values in Ljubljana than in Kozina and at Portorož Airport (Table 3, Fig. 11-14).

Local meteoric water lines were calculated for each station during the individual sampling periods from daily data. The obtained slope values range from 6.1 to 8.7 while the d-excess values range from –2 to +19‰ (Table 4). The lowest d-excess was observed in Ljubljana (Table 4) during first three months in 2003 when precipitation events were rare and only 8 samples were collected (Table 1).

Furthermore, February was much colder and dryer while March was warmer and dryer than in the reference period. Consequently, evaporation of falling snowflakes and raindrops probably affected the isotopic composition of precipitation. On the contrary, d-excess values were frequently higher than 10‰ at Portorož Airport and in Kozina during October to December (Fig. 15, Table 4) and can be attributed to the influence of Mediterranean air masses. In Ljubljana this influence was less distinctive.

FIG.11. The δ²H vs. δ¹⁸O relationship of daily precipitation (1 Oct. to 31 Dec. 2000).

FIG. 12. The δ²H vs. δ¹⁸O relationship of daily precipitation (1 Oct. to 31 Dec. 2001).

FIG.14. The δ²H vs. δ¹⁸O relationship of daily precipitation (1 Jan.to 31 March 2003).

The comparison of obtained δ¹⁸O data in daily precipitation (October to December) in Ljubljana and at Portorož Airport with daily mean air temperature is shown in Fig. 16. At both locations positive correlation between stable isotope data and air temperature was observed:

Ljubljana: δ¹⁸O=0.5T−12.9 (r=0.58) (9) FIG. 13. The δ²H vs. δ¹⁸O relationship of daily precipitation (1 Oct. to 31 Dec. 2002).

The slopes of the linear fit, 0.5‰ per °C for Ljubljana and 0.2‰ per °C for Portorož Airport, are higher than those obtained for three-years monthly δ¹⁸O – air temperature relationship (Eq. 5 and 6, Fig. 8), and are typical for colder periods and regions [17].

The comparison of obtained δ¹⁸O data in daily precipitation (October to December) at all three stations with precipitation amount is shown in Fig. 17 but no significant correlation was observed, as it was also the case for monthly precipitation (Fig. 10). The scattering of the data demonstrates that irregularly distributed daily precipitation amounts, individual weather patterns, storm tracks as well as different air mass mixing influence the isotopic composition of precipitation in Slovenia considerably.

Table 4. Local meteoric water lines for daily sampling periods Location Sampling

period

Slope d-excess

(‰) Ljubljana 1.10. - 31.12.00

1.10. - 31.12.01

FIG. 15. Deuterium excess vs. daily precipitation amount.

FIG. 16. δ¹⁸O of daily precipitation vs. air temperature relationship.

FIG.17. δ¹⁸O of daily precipitation vs. precipitation amount.

4. Conclusions

Isotopic composition of oxygen, deuterium and tritium in precipitation was determined in monthly collected samples from three sampling stations in Slovenia from October 2000 to December 2003. In

The set of obtained data allows the following conclusions:

(a) Distinct spatial and temporal variations were observed in monthly and daily precipitation in SW-NE direction with the highest δ¹⁸O and δ²H values at the coastal station Portorož Airport and the lowest in Ljubljana.

(b) Different deviations from the Global Meteoric Water Line were observed for each station and d-excess values vary around 10‰, however much higher values were observed in autumn.

(c) Positive correlations between stable isotopic composition and air temperature were observed for monthly and daily data with higher slopes in Ljubljana than at Portorož Airport.

(d) The relationship between stable isotopic composition and precipitation amount is quite poor for daily as well as for monthly data.

(e) The estimated vertical δ¹⁸O gradient was the highest for extremely dry year 2003 (-0.33‰ per 100 m) and much lower during more rainy years 2001 and 2002.

(f) The lowest weighted mean tritium activities were observed at the coast and the highest in the central part of Slovenia. Observed tritium activity distribution is typical for the Northern Hemisphere.

ACKNOWLEDGEMENTS

The present study was financially supported by the IAEA (Research contract No. 11267) within the Co-ordinated Research Project on “Isotopic Composition of Precipitation in the Mediterranean Basin in Relation to Air Circulation Patterns and Climate”, by the Ministry of Education, Science and Sports of the Republic of Slovenia and by the Slovenian Nuclear Safety Administration.

The authors gratefully acknowledge the help of the co-workers of the Department of Environmental Sciences. We also thank to the staff of the Environmental Agency of Slovenia at the meteorological stations at Portorož Airport and in Kozina for their invaluable help in sampling. Many thanks are due to Dr. I. Krajcar-Bronić, Dr. N. Horvatinčić, Dr. B. Obelić, J. Barešić and B. Mustač from the Ruđer Bošković Institute for performed tritium analyses and valuable comments on the manuscript. We would also like to thank Dr. L. Gourcy from the IAEA for her assistance.

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ISOTOPE COMPOSITION OF PRECIPITATION AND WATER VAPOUR IN