Abstract
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).
Highlights
Water, especially groundwater, has become an invaluable natural resource, and the availability of freshwater is one of the greatest issues facing mankind today [1]
We describe the behavior of tritium activity concentration in the atmosphere and give a brief description of measurement techniques that have changed during the studied period
The isotopic composition of water constituents depends on isotope fractionation caused by phase transfers of water masses, which depend on the area of water origin and the precipitation amount [2,4,32,33,34]
Summary
Especially groundwater, has become an invaluable natural resource, and the availability of freshwater is one of the greatest issues facing mankind today [1]. Correlations between the precipitation isotope ratios recorded in the GNIP and meteorological quantities may provide additional evidence of recent climate change that appears to have manifested globally as well as evidence of the local weather situation. To find such evidence, one should have sufficiently long records of both climate data and the isotopic composition of precipitation. Monitoring of the isotope composition of monthly precipitation at a station in Zagreb (Croatia) has been performed since 1976 (tritium activity concentration, A) and since 1980 (stable isotope ratios of hydrogen (2 H/1 H) and oxygen (18 O/16 O)).
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