Stable isotopes of The River Sava as a tool for transit time investigations: a case study Ljubljansko polje, Slovenia

River water represents the spatial and temporal integrator of the isotopic composition of precipitation in a catchment area. Stable isotope measurements of oxygen and hydrogen (δ¹⁸O and δ²H) in stream waters and precipitation are widely applied to investigate hydrological pathways and transit times. In this study, we apply the stable isotope approach to improve knowledge on the hydrological characteristics of the River Sava, Slovenia, by performing monthly sampling of river water at two locations: Brod and Šentjakob and precipitation at one location (Ljubljana–Reactor), between 2020 and 2021. Gathered data was used for preliminary estimations of water transit times in streamflow. Moreover, different methods were used to determine the Local Meteoric Water Line and comparison with precipitation data for the period 1981–2021 to estimate temporal changes and transit times of the River Sava at selected locations.

The climatic characteristics of the investigated area are also reflected in δ¹⁸O and δ²H of precipitation that has been monitored since 1981. The δ¹⁸O and δ²H values of precipitation reveal strong seasonal variations, while the tracer output signal in the River Sava is dampened. Site-specific long-term (1981–2021) covariation of δ¹⁸O and δ²H is also in good agreement with Global Meteoric Water Line (GMWL), while short-period lines (2020–2021) differ in slope and intercept but lie close to the line GMWL. A longer time series is more suitable for the determination of the LMWL, as the error is much higher for shorter two-year periods. 

The exponential flow model produced mean stream water transit times of 4.2 and 3.1 years at Sava Brod and Sava Šentjakob, respectively, whereas estimated transit times were longer compared to the results of previous investigations. Although the identified results are hydrologically plausible, the limitation of this and previous studies are presented as uncertainties resulting from a short sampling period and low sampling frequencies.