Understanding karst unsaturated zone (UZ) recharge dynamics is crucial for achieving sustainable management of karst hydrosystems. In this paper, we provide the first report of the application of surface nuclear magnetic resonance (SNMR) monitoring of a karst UZ during a typical Mediterranean rain event. This 79 days’ SNMR monitoring is a part of a more than 2 years of SNMR monitoring at the Low Noise Underground Laboratory (LSBB) experimental site located within the Fontaine de Vaucluse karst hydrosystem (southeastern France). We present eight SNMR soundings conducted before and after the rain event that accumulated 168 mm in 5 days. The obtained results demonstrate the applicability and the efficiency of SNMR for investigating infiltration dynamics in karst UZs at the time scale of a few days. We present the SNMR amplitudes that highlight strong signal variations related to water dynamics in the karst UZ. Infiltrated water cause increased SNMR signal during 5 days after the rain event. A significant draining process of the medium starts 15 days after the main event. Finally, after 42 days, the SNMR signal returns close to the initial state.

Estimating evapotranspiration (ET) is a primary challenge in modern hydrology. Hydrogravimetry is an integrative approach providing highly precise continuous measurement of gravity acceleration. However, large‐scale effects (e.g., tides, polar motion, atmospheric loading) limit the fine time‐scale interpretation of the gravity data and processing leads to residual signal noise. To circumvent this limitation, we exploited the difference between two superconducting gravimeters (SGs) vertically spaced by 512 m. The gravity difference allows to remove common large‐scale effects. Daily variation of the gravity difference is significantly correlated with daily evapotranspiration as estimated using the water balance model SimpKcET (p‐value = 4.10⁻¹⁰). However, this approach is effective only during rain‐free periods. In the future, comparison with direct ET measurements (e.g., eddy‐covariance, scintillometer) may confirm and strengthen our interpretation. Improved hydrogravimetric data processing could extend the proposed approach to other experimental sites equipped with a single SG.

The common hydrogeological concepts assume that water mostly enters and flows in fractured and karstified media through preferential pathways related to discontinuities. But it is difficult to locate discontinuities and even more to relate those to possible or effective water routes, particularly when soil or scree covers near surface features. When and where does water flow underground? How fast? Are we able to monitor the infiltration processes? A unique large scale Electrical Resistivity Tomography (ERT) surface based time-lapse experiment was carried out in fractured and karstified carbonate rock during a typical Mediterranean autumn rainy episode (230mm of rain over 17 days). 120 ERT time-lapse sections were measured over the same profile during and after this event (30 days). The gradient array was chosen for his robustness and rapidity. The site is covered by typical Mediterranean forest and is a good example of the surface conditions found in Mediterranean karst. There is no major karstification features (i.e. cave, sinkhole) or major tectonic accident (i.e. fault). In a previous paper, several commercial and research inversion software were tested on this dataset. This processing highlighted some limitations in inversion process. At the actual stage, apparent resistivity data provides insight about recharge/discharge processes that are almost valuable as the inverted resistivity results. Due to his quality, the availability of this unique dataset acquired under natural conditions will allow to the scientific and engineer community exploring advances and limits of ERT approach and to test new software or new data processing strategy.

We have conducted a comparative study to determine the most efficient and reliable way to calculate the depth of investigation (DOI) index to assess the quality of electrical resistivity tomography models. We compared the results of using resistivity and logarithm of resistivity after testing them on four synthetic models by direct modeling and a field case, in which the resistivity model was validated by auger drillings. We tested the two most commonly used acquisition arrays, dipole-dipole, and Wenner-Schlumberger. The index calculated with the logarithm of resistivity clearly appears to be more satisfactory than the resistivity-based index. The method based on resistivity systematically overestimates risk (high DOI) in areas of high resistivity, and it underestimates risk in conductive zones. As a result, we strongly recommend the use of the logarithm of inverted resistivity to calculate the DOI index.