Isotopic composition of hydrogen and oxygen in precipitation at the station Ljubljana (Reaktor), Slovenia: period 2011–2024

Abstract

Long-term monitoring of the isotopic composition of precipitation is essential for obtaining insights into hydrological processes. This study presents an updated and expanded dataset of stable isotopic composition of hydrogen (δ²H) and oxygen (δ¹⁸O), deuterium excess (d-excess), and tritium activity concentration (A(³H)) in monthly composite precipitation samples collected at the SLONIP station Ljubljana (Reaktor), Slovenia for the period 2011–2024. The dataset is evaluated together with meteorological observations for Ljubljana (Bežigrad) and Ljubljana (Hrastje) national stations to assess temporal variability and is compared with previously published data for the 2007–2010 period.

The mean precipitation-weighted values for 2011–2024 were −8.41 ‰ for δ¹⁸O, −56.1 ‰ for δ²H, 11.1 ‰ for d-excess, and 6.7 TU for A(³H), with all parameters except A(³H) showing an increase compared to 2007–2010 period. The increase of δ¹⁸O and δ²H is consistent with rising air temperatures, whereas the decrease in A(³H) reflects the radioactive decay of ³H originating from atmospheric nuclear weapons testing. Temporal trends reveal a seasonal cycle of δ²H and δ¹⁸O, with summer maxima and winter minima, strongly correlated with air temperature (r = 0.75 for δ²H and r = 0.78 for δ¹⁸O). The local meteoric water line (LMWL), δ²H = (7.76±0.07) δ¹⁸O + (9.11±0.65), has a lower slope and intercept for the period 2011–2024 than for the period 2007–2010. Precipitation amount does not substantially influence mean isotopic values, although isotopic extremes were typically associated with low precipitation events. No statistically significant differences were found between precipitation-weighted means of δ²H, δ¹⁸O, d-excess, and A(³H), calculated using precipitation amounts from the Ljubljana (Bežigrad) and Ljubljana (Reaktor) stations. Continued long-term monitoring within the SLONIP programme is essential for statistically robust climate and hydrological assessments in Slovenia and the wider geographical region.