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Monitoring the water movements in karstic areas is a fundamental but challenging task due to the complexity of the drainage system and the difficulty in deploying a network of observations. Gravimetry offers a valid complement to classical hydrologic measurements in order to characterize such systems in which the recharge process causes temporarily accumulation of large water volumes in the voids of the epi-phreatic system. We show an innovative integration of gravimetric and hydrologic observations that constrains a hydrodynamic model of the Škocjan cave system (Slovenia). We demonstrate how the inclusion of gravity observations improves water mass budget estimates for the Škocjan area based on hydrological observations only. Finally, the detectability of water storage variations in other karstic contexts is discussed with respect to the noise performances of spring and super-conducting gravimeters.

Water supply from karst sources is a worldwide natural resource and the exploitation is tied to the knowledge of the positions of the hydrologic channels. We show that surface deformation induced by flood events in karst conduits is observable, and consists in uplift and outward movement from the hydraulic channel. Precipitation events produce the natural occurrence of subsurface hydraulic overpressure up to 1 MPa. Numerical modeling shows that the stresses are so strong to uplift and dislocate the surface by several mm and induce tilts in the order of microradians. The naturally induced deformation is compatible with a transient internal pressure loading of a channel. The results can be used to find new channels with dense GNSS networks. Sea water incursion and channels accessed for tourism could be monitored. Seismicity has been shown to have a seasonal variation in some areas, which could be explained by the subsurface stresses induced by the natural subsurface overpressure. The pressure induced deformation is expected to be observed in all karstic systems worldwide.

Depicting the water dynamics in a Karst environment frequently represents a challenge due to the more complex drainage system compared to porous media and to logistic difficulties in deploying an efficient monitoring network. The observations of key physical parameters such as discharge are usually carried out sparsely and only if a direct access to the drainage channels is possible. For this reason, indirect geophysical methods, such as gravimetry, could be a valid complement to hydrological prospections to study the hydrodynamics of such systems. A typical karst environment is the “Carso/Kars” region, which is a 600km2 limestone aquifer, shared between Italy and Slovenia. The aquifer is constituted by an extended network of caves, shafts, and conduits fed by autogenic water and the allogenic contribution of the Reka river. The Reka river sinks in the Škocjan caves and continues its underground flow for over 30 km, finally outflowing in the Adriatic sea at the Timavo Springs. The river shows high discharge variability between dry and wet periods (0.3- 350 m3/s) and as the karst conduit system cannot efficiently drain large discharge, huge water masses are temporarily stored in the epiphreatic voids during flood events. In July 2018 we installed a gravimeter near the Škocjan caves with the aim to monitor continuously the water variations. In this contribution, we present the gravity data and the processing in order to remove tidal and other non-hydrologic components and, thus, to decipher the gravity variations linked to the Karst water circulation. Some challenges in the processing arise since the study area is close to the Adriatic sea, where marine tidal and non-tidal contributions are unneglectable. Our gravity measurements revealed many effects that are linked to the local hydrology: one important event flooded the caves in February 2019 causing a gravity change of 40 microGal. The event is compatible with the gravity estimate derived from a hydraulic model of the cave which reported an accumulation of about 10 106 m3 of water in the whole cave system in 1.5 days. The case discussed in this contribution represents a first promising application of gravimetry for tracking the water paths in the Classical Karst, easily applicable to other sectors of the Karst for which only little information about the hydrological system exist.