Assessment of the water balance of a Dinaric karst polje (Planinsko Polje, Slovenia)

Poljes are flat closed depressions in karst terrains that are prone to regular flooding. The strongest floods can be several meters high and persist for months, making significant damages in infrastructures. To predict how climate change might affect the occurrence, amplitude and duration of the flood, a better understanding of the flooding dynamics is necessary. Among others, the computation of the water balance is a prerequisite. This method allows assessing when the polje will begin to flood, and gives information on the maximum water level potentially reached. However, this technique encounters the difficulty that a notable part of the inflow entering in poljes is generally ungauged, while it is challenging to quantify the outflow. This is because numerous springs and ponors activate only temporary with the rise of the water level. Moreover, many poljes are generally poorly monitored due to financial reasons. This work aims to see whether these drawbacks can be overcome. To this end, a typical Dinaric polje recharged by a combination of allogenic inflow and a rise of the regional groundwater level is equipped with a network of several measuring stations installed over its surface and in the nearby water-active caves. Combining an accurate Lidar of the surface with recorded water levels and inflow of the main two springs made possible to evaluate the polje flooding dynamics and to characterize its water balance. The method is able to quantify the polje total inflow, while an estimation of the outflow is presented. Then, the main ungauged signals affecting flooding are identified and separated. These values are used as input and calibration data in a numerical model aiming to reproduce the flood dynamics of the polje and its surrounding aquifer. Modelling results validate both water balance and conceptual hydrogeological model, and justify the significance of installing a network of several hydrological stations to monitor the poljes. The method can be applied to other poljes flooding in a complex way of superimposed input and output signals.