Isotopic characterisation of precipitation and throughfall across forest, rural, and urban environments in Slovenia, during 2022-2024 sampling campaing

The isotope composition of oxygen and hydrogen in precipitation and throughfall (expressed as δ¹⁸O and δ²H) is used as tracers that provide insights into temporal climate variability relevant to hydrological and ecological research. This dataset includes the isotope composition of hydrogen and oxygen in monthly composite of precipitation from three stations: Ljubljana-Reaktor, Murska Sobota-Rakičan (Vreča and Malenšek 2016) and Drnovo and throughfall from two stations: Krakovski gozd and Murska šuma. The throughfall collectors used at Krakovski gozd and Murska šuma were built manually and adapted for forest conditions, with a larger opening than the standard Palmex design to prevent clogging by leaves. They were installed at ground level to limit solar exposure and reduce evaporation. At Drnovo, precipitation was collected using a standard Palmex RS-1 (Palmex, Zagreb, Croatia) in an open area. At Ljubljana–Reaktor, IJS staff collect precip itation immediately after each event using a modified collector (Vreča and Malenšek 2016), while at Murska Sobota–Rakičan, ARSO staff collect daily samples with a classical rain gauge. Samples from Murska šuma, Krakovski gozd, and Drnovo are collected by GeoZS staff, stored in 50 mL HDPE bottles, transported to the Geological Survey of Slovenia laboratory, refrigerated (5–8 °C), filtered (45 μm CA), and transferred to 2 mL vials for analysis. Monthly composite samples from Murska Sobota–Rakičan and Ljubljana–Reaktor are obtained by emptying event-based collectors into 5–10 L containers and stored at room temperature. In the Jožef Stefan Institute laboratory, these composites are filtered through 12–25 μm ashless filters to remove particles and stored in ≥30 mL glass bottles at room temperature until analysis. The samples collected from Ljubljana-Reaktor and Murska Sobota-Rakičan station were analysed at the Jožef Stefan Institute laboratory, with a Finnigan MAT DELTA plus dual inlet isotope ratio mass spectrometer, DI IRMS, Finnigan MAT DELTA plus, Finnigan MAT GmbH, Bremen, Germany) with an automated H2-H2O (Epstein and Mayeda 1953; Coplen et al. 1991; Avak and Brand 1995) equilibrator HDOeq48 Equilibration Unit (custom-built by M. Jaklitsch). The samples from Drnovo, Murska šuma and Krakovski gozd were analysed in the Geological Survey of Slovenia laboratory using a Picarro L2130-i laser isotope analyser. The reference materials USGS-45 and USGS-47, with known isotopic compositions, were used to normalise the data to the international VSMOW-SLAP scale. For independent control of measurement repeatability and verification of the memory effect, the internal laboratory reference material W-998 was analysed concurrently. Based on three replicate analyses of the laboratory reference materials, standard deviations for δ¹⁸O and δ²H were determined. Each sample was measured six times, with the first four injections discarded to eliminate the memory effect. The result for each sample is thus presented as the mean of the fifth and sixth measurements. Sampling of the monthly composite was performed between July 2022 and December 2024. Overall in this period, 149 samples were collected. Deuterium excess (d-excess) was calculated as: d-excess [‰] = δ²H – 8 x δ¹⁸O (Dansgaard 1964). The obtained data were included in the study of combined comparison of isotope composition in precipitation across rural, urban, and throughfall across forest environments in Slovenia. These data, supported by modern analytical and modelling tools, help reconstruct past climate and improve projections of future change in forested environments, and are key tracers for droughts, floods, and their impacts, and they clarify present hydrological processes.