Assessing the influence of climatic parameters and moisture sources on precipitation isotope composition from central Croatia: Insights from a two-year long daily-resolved dataset

Nova Grgosova Cave, central Croatia A continuous, high-resolution monitoring campaign was conducted to unravel the site-specific processes governing the behaviour of oxygen isotopes in daily precipitation (δ 18 O p ) over a two-year study period. A series of methods was applied including correlation analyses using meteorological variables, back-trajectory modelling and moisture source determination, as well as atmospheric reanalysis to link the δ 18 O p to precipitation types. The results reveal a strong positive correlation between δ 18 O p and 2-m air temperature throughout the whole year. A moderate negative correlation with precipitation amount is observed during the warmer part of the year, when mean daily temperatures exceed 12°C and convective precipitation dominates, resulting in more positive δ 18 O p values. Below the threshold of 12°C, the temperature effect prevails, resulting in more negative δ 18 O p values showing more extensive variability. Back-trajectory analyses reveal westerly-dominated air circulation, which is strengthened during winter and autumn, and weakened in the spring and summer. Moisture uptake calculations show that the majority of winter and autumn moisture is advected from large water bodies, i.e. the Western Mediterranean and North Atlantic, while continental sources are more pronounced in the summer and spring. The daily δ 18 O p data do not reveal clear dependence on the moisture source regions. However, longer time series would be needed to confirm this finding. • Stable isotope composition of daily precipitation (δ¹⁸Oₚ) in Croatia was explored. • Temperature effect is evident throughout the two-year period. • Rainfall amount effect is restricted to the warm season and driven by convection. • The Mediterranean Sea and North Atlantic Ocean are primary moisture sources. • δ¹⁸Oₚ shows no clear dependence on the origin of the moisture source.