engleski

Preliminary results from stable isotope investigations in the Kupa Drainage Basin (Western Croatia)

For the very first time stable isotope data of river waters from Western Croatia are presented. During a campaign lasting from October 27 to November 21, 2010 sixty-seven water samples from sites along the three hundred kilometres long course of the Kupa River, and along selected tributary rivers of its catchment of about ten thousand square kilometres, respectively. We want to emphasize that hydrological conditions during the sampling period were influenced by heavy rain falls midst of October. Our interpretation of isotope ratios in river water is mainly based on the relation between weighted mean δ18O and the altitude obtained from stations of the Global Network of Isotopes in Precipitation (GNIP), revealing an approximate vertical δ18O gradient of 0, 30‰ per 100 m, as reported by Vreča et al. (2006). In addition, the δ18O data from the GNIP station Zavižan predominantly indicate precipitation from the Adriatic coast, in contrary to the GNIP station Zagreb, the precipitation of which is influenced by more continental air masses. The catchment of upper Kupa River in the Gorski Kotar rises up to 1500 metres above sea level (a.s.l.). Upper Kupa is mainly charged by springs from big karst reservoirs, in which a residence time of groundwater of up to one year has to be considered. We interpret the δ18O values of Čabranka River (about 8, 07‰) as signals from maritime precipitation in this karstified catchment area. The δ18O value of upper Kupa River diminishes along its course from 8, 09‰ near Osilnica to 9, 06‰ west of Karlovac. After the inflow of tributaries south of Karlovac the oxygen isotope ratio of Kupa River water reveals a significant change because δ18O values of Dobra-, Korana- and Mrežnica River are lower ( 10, 45‰ to 9, 58‰). The catchment of Dobra River and Korana River rises between 400 and 880 metres a.s.l. We interpret the lower δ18O values of river waters from the drainage basins at those low mean altitudes as basically not caused by an altitude effect, but caused by precipitation out of more continental air masses.

Western Croatia; Kupa catchment; river water; stable isotopes

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