LIFE TRŠCA: First year of continuous water level observations of the »lake windows«

At the end of August 2024, the Karst Research Institute ZRC SAZU, within the framework of the LIFE TRŠCA project, upgraded the measuring network for continuous water level observations, with the main aim of determining local deviations in water levels at different locations of the polje. To the existing measuring points held by the Slovenian Environment Agency (ARSO) at Dolenje and Gorenje Jezero (Stržen watercourse) and in Cerknica (Cerkniščica tributary) and the Institute for Karst Research ZRC SAZU in Rešeto, Zadnji kraj, Narti and in two sumps of the Mala and Velika Karlovica caves, eight additional measuring points were added in the central part of the polje: confluence of Lipsenjščica and Stržen rivers, confluence of Žerovniščica and Stržen rivers, Zadnji kraj – Kotel, Levišča, Ajnce jame, Velika Ponikva, Trščenke and Osredki. Due to their morphology and direct connection to groundwater, most of these points are often called "lake windows".

At the end of July and the beginning of September 2025, the first and second visits to all measuring points were made, as they are only accessible at very low water levels. The primary reason for the visit was to transfer data about hydrological observations, whereas the secondary reason was precise geodetic measurements of the instrument locations. Due to the absence of a stable bedrock, some instruments were installed on concrete blocks, which can eventually settle into the muddy base. It turned out that after the first year of measurements, no changes were detected in the position of the blocks, which is indicating that the muddy base is sufficiently solid even during flooding.

The first year of measurements showed that the dynamics of flooding and drying out of lake windows is very diverse. It turned out that at almost every location the water level rises at a different time and also at a different speed rate. The same applies to drying out periods, as the water level at each location falls at a different period and at a different rate. Significant differences can be observed even at the closest locations, which is somewhat unexpected. The consistent fluctuation of the water level is thus present only at the highest water levels, when the water level is practically flat throughout the lake.

Due to the extremely diverse dynamics in the water level, it very often happens that the water level at one location is higher than elsewhere for a certain period, and lower during another phase. According to a preliminary analysis of the data, there may be several reasons for such a phenomenon:

• the isolation of individual locations (the locations are not well connected to each other by underground channels)
• spatial differences in the recharge area that supplies individual locations with water,
• local deviations in the distribution of precipitation (impacts during the rising water periods)
• and very likely something else.

For more precise interpretations, further measurements will definitely be needed, in which more high-water events will be monitored, but certainly new measurements within the framework of the LIFE TRŠCA project are already showing interesting results and, in general, the great importance of new measurements for further understanding of the Cerkniško polje hydrological dynamics.

Article was prepared by the Karst Research Institute ZRC SAZU.