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Open AccessArticle

Long-Term Isotope Records of Precipitation in Zagreb, Croatia

1
Laboratory for Low-level Radioactivities, Division of Experimental Physics, Ruđer Bošković Institute, 10000 Zagreb, Croatia
2
Department of Environmental Sciences, Jožef Stefan Institute, 1000 Ljubljana, Slovenia
*
Author to whom correspondence should be addressed.
Water 2020, 12(1), 226; https://doi.org/10.3390/w12010226 (registering DOI)
Received: 29 November 2019 / Revised: 8 January 2020 / Accepted: 10 January 2020 / Published: 14 January 2020
(This article belongs to the Special Issue Use of Water Stable Isotopes in Hydrological Process)
The isotope composition of precipitation has been monitored in monthly precipitation at Zagreb, Croatia, since 1976. Here, we present a statistical analysis of available long-term isotope data (3H activity concentration, δ2H, δ18O, and deuterium excess) and compare them to basic meteorological data. The aim was to see whether isotope composition reflected observed climate changes in Zagreb: a significant increase in the annual air temperature and larger variations in the precipitation amount. Annual mean δ18O and δ2H values showed an increase of 0.017‰ and 0.14‰ per year, respectively, with larger differences in monthly mean values in the first half of the year than in the second half. Mean annual d-excess remained constant over the whole long-term period, with a tendency for monthly mean d-excess values to decrease in the first half of the year and increase in the second half due to the influence of air masses originating from the eastern Mediterranean. Changes in the stable isotope composition of precipitation thus resembled changes in the temperature, the circulation pattern of air masses, and the precipitation regime. A local meteoric water line was obtained using different regression methods, which did not result in significant differences between nonweighted and precipitation-weighted slope and intercept values. Deviations from the Global Meteoric Water Line GMWL (lower slopes and intercepts) were observed in two recent periods and could be explained by changes in climate parameters. The temperature gradient of δ18O was 0.33‰/°C. The tritium activity concentrations in precipitation showed slight decreases during the last two decades, and the mean A in the most recent period, 2012–2018, was 7.6 ± 0.8 Tritium Units (TU).
Keywords: precipitation; Zagreb; Croatia; stable isotope ratios; 2H/1H and 18O/16O; deuterium excess; local meteoric water line; δ18O–temperature relation; tritium precipitation; Zagreb; Croatia; stable isotope ratios; 2H/1H and 18O/16O; deuterium excess; local meteoric water line; δ18O–temperature relation; tritium
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Krajcar Bronić, I.; Barešić, J.; Borković, D.; Sironić, A.; Mikelić, I.L.; Vreča, P. Long-Term Isotope Records of Precipitation in Zagreb, Croatia. Water 2020, 12, 226.

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