Investigating the hydrological behaviour of the Upper Pivka Valley (Slovenia)

The Pivka River is a 20 km long stream located 40 km SW from Ljubljana (Slovenia), which disappears in the world famous Postojna Cave. While the river flows permanently on flysch rocks in the valley lower part, water is only present temporarily in the valley upper part due to the karstic nature of the aquifer located below the river. This aquifer is assumed to be linked to the larger Javorniki karst aquifer that belongs to the catchment of the Unica and Malenščica Springs, which drain water from the whole region. During high water period, the regional groundwater level rises up to 50 m, and 17 temporary lakes might appear on the valley surface. Because the hydrological situation in the Javorniki karst aquifer is assumed to affect flooding in the Pivka Valley, the interaction between both need to be understood. A network of nine automatic stations recording water level, specific electrical conductivity and water temperature at a 30 min interval has been progressively established in the valley since 2020. The three years dataset has been analysed with data collected in the water active caves of the Javorniki karst aquifer and at the Unica and Malenščica Springs. Results allowed elaborating a conceptual hydrological model of the region. They emphasized that the karst aquifer below the Upper Pivka Valley acts as an overflow of the Javorniki karst aquifer during high water periods, while it flows back into the Javorniki karst aquifer and further toward the Unica and Malenščica springs during the recession.