OTT HydroMet

Water quality monitoring with hydrolab MS5


Courtesy of OTT HydroMet


The Škocjan caves are a unique natural phenomenon located in south western Slovenia and are preserved as the Škocjan Caves Park under UNESCO protection. One of the most famous network of underground caves in the world, they were formed by the flow of the Reka River as it bored tunnels through the region’s limestone geology. The river first disappears underground just outside the Škocjan village where it flows underground for thirty-four kilometres, resurfacing again close to the Adriatic coast.

A few hundred thousand years ago the ceiling of the cave collapsed approximately 200 metres from the original underground entrance, leaving two large sinkholes – the Velika dolina (up to 165 metres deep) and Mala dolina (120 metres) – leaving a natural bridge that separates the two.


The research and development institute of the Škocjan Caves Park Authority suggested a regular monitoring program of various water quality parameters to get a better understanding of the river conditions and possible influence of environmental changes in the region. Two Hydrolab multi-parameter probes were purchased (a MiniSonde 4a in 2003 and a MS5 in 2007) for monitoring primary water quality parameters in the Reka River running through the Škocjan caves.

The project required:

  • A device that can be programmed and used for unattended monitoring of water quality parameters
  • Measurement of conductivity, dissolved oxygen, pH, nitrate, temperature, and water level
  • Instrument installation directly in the river bed
  • Regular reports about the status of the Reka River

Monitoring Solution

The site where a Hydrolab probe is deployed for unattended monitoring is in the Svetina hall, which is about 200 meters downstream from where the Reka River disappears after the Velika dolina. A metal tube was attached to the wall of the river bed to hold the Hydrolab probe in a way that the lower end of the instrument is always below the waterline, even when the water level is extremely low. Using the metal tub as an installation mechanism makes it easy to remove the probe for maintenance and calibration.

The second Hydrolab multi-parameter probe is used for spot measurement of various portions of the Reka River through the park. Data is displayed on a laptop computer that is attached to the instrument.


  • Beside the standard parameters one sonde has also a nitrate sensor. We began with this monitoring recently. Results are good but vary on different locations and will certainly give a better picture of the water properties at different places. Over time, they will contribute to the acceptance of certain measures regarding the maintenance or improvement of the water quality in the Reka River and its afflux.
  • Maintenance and work with the sonde is not difficult, calibration is not complicated, too

Continuous monitoring of the Reka River in the Škocjan caves is a step to better quantify any potential watershed impacts on this sensitive and unique cave and river system. Data collected from these sites can be used for a better understanding of this ecological system and to review the influence of things like heavy rainfall and its entries into the water. The outcomes from the implementation and monitoring of these sites will help to improve the data gathering and the expansion of the area’s environmental preservation program.

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