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International audience; The spatial forest structure that drives the functioning of these ecosystems and their response to global change is closely linked to edaphic conditions. However, the latter properties are particularly difficult to characterize in forest areas developed on karst, where soil is highly rocky and heterogeneous. In this work, we investigated whether geophysics, and more specifically electromagnetic induction (EMI), can provide a better understanding of forest structure. We use EMI (EM31, Geonics Limited, Ontario, Canada) to study the spatial variability of ground properties in two different Mediterranean forests. A naturally post-fire regenerated forest composed of Aleppo pines and Holm oaks and a monospecific plantation of Altlas cedar. To better interpret EMI results, we used electrical resistivity tomography (ERT), soil depth surveys, and field observations. Vegetation was also characterized using hemispherical photographs that allowed to calculate plant area index (PAI). Our results show that the variability of ground properties contribute to explaining the variability in the vegetation cover development (plant area index). Vegetation density is higher in areas where the soil is deeper. We showed a significant correlation between edaphic conditions and tree development in the naturally regenerated forest, but this relationship is clearly weaker in the cedar plantation. We hypothesized that regular planting after subsoiling, as well as sylvicultural practices (thinning and pruning) influenced the expected relationship between vegetation structure and soil conditions measured by EMI. This work opens up new research avenues to better understand the interplay between soil and subsoil variability and forest response to climate change.

International audience; Mediterranean forests face an intensification of droughts caused by ongoing climate change. To improve our understanding of tree and forest responses to increasing drought, we explored over nine years, the effects of a 30% rainfall exclusion experiment on the water potential, sap velocity and primary and secondary growth of two co-occurring species (Quercus ilex and Pinus halepensis) in a French Mediterranean forest. In addition, native embolism was measured after six and nine years of exclusion onset. Water potentials decreased earlier during summer drought for both species in the rainfall exclusion plot, and to a higher extent during the drought peak for Q. ilex, involving earlier stomatal closure and reduced sap velocity. Sap velocity reduction persisted throughout the years in the exclusion plot. Outside summer the water potential difference between predawn and midday was similar between treatment which indicate that reduced water transport efficiency may be linked to decrease hydraulic conductance. Such differences were neither related to differences in xylem embolism, that remained similar between treatments, nor to change in secondary growth. In contrast primary growth measurements indicate that P. halepensis trees, and Q. ilex to a lesser extent, experienced reduction in total leaf areas in response to the rainfall exclusion. Globally, our results suggest that increase drought lead, for both species, to a decrease in sap velocity not mediated by increase embolism but rather by a reduction in primary growth. So far little conclusion can be drawn regarding the competitive advantage of one species over the other in the context of increasing drought related to climate change.

International audience; Aquifer recharge may depend mainly on the difference between precipitation and evapotranspiration. Hydrological models used to estimate groundwater reserves use evapotranspiration models that are mainly determined by climate demand. In particular, mechanisms of plant transpiration are neglected, although transpiration constitutes 70% of evapotranspiration. This is problematic when considering karst watershed, which are poorly documented at the interface between soil and atmosphere where vegetation and soil properties control water flows. To fill this gap, we propose an evapotranspiration model that integrates the processes of plant transpiration and soil evaporation. The dynamics of vegetation is evaluated using the Enhanced Vegetation Indexes from the Terra and Aqua Moderate Resolution Imaging Spectroradiometers. The soil evaporation calculation account for the impact of coarse elements at soil surface. The “Simple Crop coefficient for Evapotranspiration” (SimpKcET) model is tested at flux tower sites over forest of Font-Blanche, Puechabon and the agricultural area of Avignon. The simulated daily evapotranspirations are very close to the observations (RMSE ~0.5 mm.d-1), while the model is simple compared to other models proposed in the literature. The SimpKcET is implemented in a karst hydrological model to evaluate the impact of evapotranspiration estimation on the aquifer flow rate simulation. This approach is applied to the vast watershed of Fontaine de Vaucluse. In comparison to the water bucket model that is frequently used in karst models, SimpKcET provide ET simulations that are more in line with ET processes. A cross wavelet analysis highlighted the improvement of the simulated recharge and observed flow rate relationship brought by the consideration of evaporation and transpiration processes. The use of remote sensing data related to plant activity makes it possible to propose a parsimonious model that can be applied to all types of vegetation (agricultural, natural, mixed forest) and that can be transferred to other karst models.

Summary The unsaturated zone (UZ) of karst aquifers plays an important role in groundwater recharge processes. Comprehensive knowledge of UZ structure and hydrodynamic functioning is a key to better assess and manage groundwater resources in karst. In this extended abstract we present the results from a 2-year long Surface Nuclear Magnetic Resonance (SNMR) monitoring implemented in the Low-Noise Underground Laboratory (LSBB) site located within the Fontaine de Vaucluse karst hydrosystem in south-France. This exceptional 2-years SNMR monitoring demonstrated the efficiency of SNMR firstly to assess and validate the permanent presence of groundwater within karst hydrosystem UZ and secondly to monitor his temporal variations. Compared to boreholes, SNMR provides an integrated but accurate estimation of groundwater dynamics without disturbing the media.

International audience; Plant transpiration is a major component of water fluxes in the critical zone, which needs to be better characterized to improve our ability to understand and model the hydrological cycle. In water-limited ecosystems such as those encountered on karst environments, climate-induced changes in transpiration are expected to be strongly influenced by the ability of the vegetation cover to resist or adapt to drought. However, because of the high heterogeneity of karst environments, the amount of water available for trees can change within a stand, which may lead to significant differences in drought vulnerability resistance between trees of the same species. So far it is not known if soil and subsoil environment influence the magnitude of deep water extraction, at the intra-specific scale. Here, we investigate the variability in deep water extraction for six individual Quercus ilex trees growing on a karst substrate in a Mediterranean forest. We combined three approaches: (i) electrical resistivity tomography to determine the variability of soil/subsoil characteristics, (ii) isotope tracing to determine the origin of water transpired by plants, and (iii) predawn and midday leaf water potential (Ψ) to assess the trees’ water stress and transpiration regulation. Along the summer season, deep water extraction increased with drought intensity. Deep water use varies between individuals and according to drought intensity. At moderate water stress levels, we found no significant relationship between the origin of xylem water and soil/subsoil characteristics or individual stress level. However, at the peak of the drought (average predawn Ψ < −2 MPa), individuals that had the least total available water in soil/subsoil (0–2 m) relied more on deep water and were also subject to less water stress. These results suggest that trees with less favorable soil/subsoil conditions (i.e. low water retention capacity) in the near surface (0–2 m) adapt their root systems to exploit deep water reserves more intensively so as to enhance their drought tolerance, while trees with more favorable surface conditions exhibit greater water stress and may be more vulnerable to extreme droughts because of a lower root development in deeper horizons.

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&rsquo, 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.

International audience; Aquifer recharge assessment is a key factor for sustainable groundwater resource management. Although main factors of the spatial and temporal variability of recharge are known, taking them into account in a distributed or semi-distributed model is still a challenging task. This difficulty is increased in karst environments. Indeed, recharge of karst aquifers also depends on the organization of the karst network, which is both highly heterogeneous and difficult to characterize. We developed a reservoir model to simulate the spatial and temporal variability of recharge on karst watersheds. Special attention was paid to the link between model parameters and measureable or qualitative environmental factors of recharge. The spatial variability of soil reservoir capacity was estimated by multifactorial modelling (neural network). Intrinsic vulnerability indices were used to constrain the partitioning between slow and fast flows within the karst aquifer. Comparison of simulated and measured discharge at the outlet was used to calibrate and assess recharge model. The karst hydrosystem of the Fontaine de Vaucluse is renowned for its significant heterogeneity and anisotropy, which has so far limited the application of 2D or 3D modelling. The model developed was successfully applied to this system. Our results showed that the annual recharge is very heterogeneous on the test site. Spatialization of recharge improves discharge modelling as evidenced by increased KGE (from 0.8 to 0.9) and more realistic flows during drought periods. It is therefore essential to spatialize recharge in karst hydrogeological modelling to improve predictive capacity and better understand 1 functionning of the whole hydrosystem.

International audience; Southwestern Madagascar is a semi-arid region and a hot-spot of global change. On the Mahafaly plateau, people live with quasi-permanent water stress and groundwater, the only available resource, is difficult to exploit due to a complex hydrogeological environment. A methodology (suitable for humanitarian projects

International audience; To improve flood prediction in headwater catchments, hydrologists need to know initial soil moisture conditions that precede rain events. In torrential hydrology, soil moisture mapping provides a valuable tool for investigating surface runoff generation processes. In these mountainous environments, soil moisture prediction is challenging because of highly heterogeneous land cover and soil properties. This survey propose a methodology to study spatial soil moisture variations in the mountainous and torrential environment of the Draix Bléone experimental site—Laval 0.86 km2. This approach associates water content measurements at the plot scale with spatialized soil bulk electrical conductivity (ECa) measurements combined in a multivariate statistical analysis based on topographical parameters. Between the summer of 2015 and winter of 2016, four geophysical surveys were conducted under various moisture conditions and along the same pathway, using the Slingram electromagnetic induction (EMI) technique (EM31 device) in horizontal dipole to identify changes in soil properties to a depth of 3 m. These results were analyzed to determine water dynamics in this mountainous catchment. Temporal variations of ECa vary among land cover types (forest, grassland, and black marl). A significant relationship was observed between ECa and soil water content (SWC) measured with capacitive sensors in forest and grassland. A multiple linear regression produced using the spatial interpolation code LISDQS shows a significant correlation between ECa and landform units depicted on a high-resolution DEM. ECa variations decrease with distance to talwegs. Riparian zones appear as potential hydrological contributing areas with patterns varying according to moisture status. This study shows that multiple linear regression analysis and EMI make it possible to fill gaps between SWC plot measurements, over wide areas that are steep and that present numerous obstacles due to vegetation cover.

sous-presse; International audience; Karst environments are unusual because their dry, stony and shallow soils seem to be unfavorable to vegetation, and yet they are often covered with forests. How can trees survive in these environments ? Where do they find the water that allows them to survive? This study uses midday and predawn water potentials and xylem water isotopes of branches to assess tree water status and the origin of transpired water. Monitoring was conducted during the summers of 2014 and 2015 in two dissimilar plots of Mediterranean forest located in karst environments. The results show that the three monitored tree species (Abies alba Mill, Fagus sylvatica L, and Quercus ilex L.) use deep water resources present in the karst vadose zone (unsaturated zone) more intensively during drier years. Quercus ilex, a species welladapted to water stress, which grows at the drier site, uses the deep water resource very early in the summer season. Conversely, the two other species exploit the deep water resource only during severe drought. These results open up new perspectives to a better understanding of ecohydrological equilibrium and to improved water balance modeling in karst forest settings.

Inter-individual variability of tree drought responses within a stand has received little attention. Here we explore whether the spatial variations in soil/subsoil properties assessed through electrical resistivity tomography (ERT) could explain variations in drought response traits among trees. We used ERT to compute the percent variation in resistivity (PVR) between dry and wet conditions as an indicator of spatial variability in total available water content. PVR was computed in two different depth ranges (0-2 and 2-5 m) for eleven Quercus ilex stools in a Mediterranean forest stand. PVR values were compared to biological traits, including tree water status (predawn water potential (Ψ)), leaf traits (δ 13 C, leaf mass area (LMA)), and canopy defoliation measured after intense drought. We found significant correlations between PVR and biological variables. For Ψ , the nature and strength of the correlations vary according to the level of drought intensity. The correlation between Ψ and PVR was positive during well-watered conditions in the upper layer (0-2 m) and during water-limited conditions in the deeper layer (2-5 m). During most severe droughts, however, the Ψ was negatively correlated with PVR in the upper layer. Trees with lower PVR in the upper layer were also associated with water use efficiency (higher δ 13 C), higher LMA, and a lower level of defoliation after extreme drought. Overall, our results indicate that local differences in soil/subsoil properties affect tree response to drought and suggest that less favorable soil/subsoil conditions (lower PVR) can lead to lower water stress during the driest period and to lower defoliation after extreme drought. Plausible explanations for this better acclimation include higher stomatal regulation and improved deep soil and subsoil water exploration by trees located in more adverse conditions. We encourage the development of ERT in ecological studies to further explore the interrelated relationships between soil/subsoil, climate, and tree functioning.

Assessing underground hydrosystem recharge is crucial to characterizing their hydrogeological functioning. The common questions arising from a poor understanding of hydrogeological mechanism are about parts of the gross rain amount that evapotranspire and get temporarily stored within the soil. Evapotranspiration and soil water storage are largely influenced by the structure and the function of the aboveground vegetation, which is generally composed of heterogeneous forest layer in Mediterranean karstic systems. However, most models used to compute karst hydrosystem recharge rely on simplistic formulations of evapotranspiration (ET) that do not account for vegetation functioning. In this study, we used the vegetation process-based model CASTANEA to improve water transfer in the higher horizon of the karst system and recharge simulations. Effective infiltration was computed with CASTANEA or with a classical approach (based on precipitation minus ET) for a well-documented holm oak site in southern France. We then compared simulation results with outflow data measured at 33 m below ground. We found significant differences between the two calculation methods, up to 200 % of annual recharge in the case of a very dry year. The comparison of modelled effective infiltration with outflow data indicated that using CASTANEA improved assessment of the temporal dynamics of water recharge in this karst system compared to a more classical approach. Our approach constitutes a promising way to improve the simulation of karst hydrosystem recharge.

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.

Some portions of the porous rock matrix in the karst unsaturated zone (UZ) can contain large volumes of water and play a major role in water flow regulation. The essential results are presented of a local-scale study conducted in 2011 and 2012 above the Low Noise Underground Laboratory (LSBB – Laboratoire Souterrain à Bas Bruit) at Rustrel, southeastern France. Previous research revealed the geological structure and water-related features of the study site and illustrated the feasibility of specific hydrogeophysical measurements. In this study, the focus is on hydrodynamics at the seasonal and event timescales. Magnetic resonance sounding (MRS) measured a high water content (more than 10 %) in a large volume of rock. This large volume of water cannot be stored in fractures and conduits within the UZ. MRS was also used to measure the seasonal variation of water stored in the karst UZ. A process-based model was developed to simulate the effect of vegetation on groundwater recharge dynamics. In addition, electrical resistivity tomography (ERT) monitoring was used to assess preferential water pathways during a rain event. This study demonstrates the major influence of water flow within the porous rock matrix on the UZ hydrogeological functioning at both the local (LSBB) and regional (Fontaine de Vaucluse) scales. By taking into account the role of the porous matrix in water flow regulation, these findings may significantly improve karst groundwater hydrodynamic modelling, exploitation, and sustainable managementme portions of the porous rock matrix in the karst unsaturated zone (UZ) can contain large volumes of water and play a major role in water flow regulation. The essential results are presented of a local-scale study conducted in 2011 and 2012 above the Low Noise Underground Laboratory (LSBB – Laboratoire Souterrain à Bas Bruit) at Rustrel, southeastern France. Previous research revealed the geological structure and water-related features of the study site and illustrated the feasibility of specific hydrogeophysical measurements. In this study, the focus is on hydrodynamics at the seasonal and event timescales. Magnetic resonance sounding (MRS) measured a high water content (more than 10 %) in a large volume of rock. This large volume of water cannot be stored in fractures and conduits within the UZ. MRS was also used to measure the seasonal variation of water stored in the karst UZ. A process-based model was developed to simulate the effect of vegetation on groundwater recharge dynamics. In addition, electrical resistivity tomography (ERT) monitoring was used to assess preferential water pathways during a rain event. This study demonstrates the major influence of water flow within the porous rock matrix on the UZ hydrogeological functioning at both the local (LSBB) and regional (Fontaine de Vaucluse) scales. By taking into account the role of the porous matrix in water flow regulation, these findings may significantly improve karst groundwater hydrodynamic modelling, exploitation, and sustainable management.

The common hydrogeological concepts assume that water enters in karst media by preferential pathways. But it is difficult to identify these pathways, particularly if soil or scree covers the karst features. When and where does water enter in the hydrosystem? How fast? A unique large-scale Electrical Resistivity Tomography (ERT) surface-based time-lapse experiment was carried out during a typical Mediterranean autumn rainy episode (230 mm of rain over 17 days). A total of 120 ERT time-lapse sections were measured over the same profile during and after this event (30 days). The main goal was to evaluate efficiency and limits of the ERT to monitor water infiltration, under natural conditions. Apparent (directly measured) and inverted resistivity’s variation during the rainy event highlights some interesting zones. They could be interpreted as preferential pathways, where water dynamic seems quicker in term of moistening and drainage. Nevertheless, these results have to be interpreted reasonably because ERT does not provide enough precision to determine exact pathways geometry and functioning. In addition, forward modeling provided relevant data treatment limitations mainly for the deeper parts of the sections.

Assessing Electrical Resistivity Tomography (ERT) reliability is critical for interpretation step and decisions making. Scientists try to provide solutions with several quality indicators: Depth of Investigation (DOI) index, sensitivity, uncertainty and resolution matrix. Each indicator, presenting strengths and weaknesses, has to be cross-validated with others. Most of these indicators are already included in the commercialized inversion softwares. This paper deals with DOI index calculation. Two calculation methods appear/exist in the Literature: the one uses inverted resistivity and the other uses logarithm of inverted resistivity. Both solutions are compared in this paper with several synthetic models and field data set. In general, DOI index calculated using inverted resistivity point out systematically a less reliability in resistive zone of the model. DOI index map calculated using logarithm of inverted resistivity appears more robust in various configurations. This calculation method highlights quite accurately inversion artifacts.

International audience; Models are the only tools capable of predicting the evolution of groundwater systems at a regional scale, by taking into account a large amount of information. This study presents the association of a water balance model (WetSpass) with a groundwater flow and solute transport model (SUFT3D, saturated and unsaturated flow and transport in 3D) in order to simulate the present and future groundwater quality in terms of nitrate in the Upper Dyle basin (439 km(2)) Belgium. The HFEMC (hybrid finite element mixing cell) method implemented in the SUFT3D code is used to model groundwater flow and nitrate transport. Spatially distributed recharge, modelled with WetSpass, is considered for prescribing the recharge to the groundwater flow model. The feasibility of linking the WetSpass model with the finite-elements SUFT3D code is demonstrated. Time evolution and distribution of nitrate concentration are then simulated using the calibrated model. Nitrate inputs are spatially distributed according to land use. The spatial simulations and temporal trends are compared with previously published data on this aquifer and show good results.

International audience; Ground Penetrating Radar Electrical Resistivity Tomography Karst near surface features LSBB This paper highlights the efficiency and complementarity of a light package of geophysical techniques to study the structure of karst Unsaturated Zone (UZ) in typical Mediterranean environment where soil cover is thin or absent. Both selected techniques, 2D Ground Penetrating Radar (GPR) and Electrical Resistivity Tomography (ERT), are widely used in environmental studies and their application is accessible for a lot of scientists/engineers. However, GPR or ERT alone is not able to provide an enhanced characterization of geological features in karst media. In the present study, GPR results supply a near surface high resolution imaging and thus can provide relevant geological information such as stratifications and fractures. Despite the quality of the results GPR's investigation depth remains limited to around 12 m. Apparent and inverted resistivity provided by ERT surveys shows strong lateral and vertical variations. These variations can inform about general geological structuring and feature orientation. ERT is able to prospect down to 40 m but it's a low resolution integrative technique. In the study area the investigated limestone is a commonly electrical resistive formation (more than 2000 Ω.m). However deeper than 5–7 m, the ERT profiles reveal several zones of moderate resistivity (around 900 Ω.m). In these zones a stratification change corresponding to slanted bedding is clearly identified by GPR results. The combination of both GPR and ERT results can allow a well-established geological interpretation. These moderate resistivity zones with slanted beddings can explain the presence of a perennial water flow point 35 m below the surface of the studied site within the underground gallery of the Low-Noise Underground Laboratory (LSBB).

International audience; Deltaic aquifers are complex due to the important heterogeneity of their structure and their hydrogeological functioning. Auger drilling provides localized, but very robust geological and hydrogeological information, while geophysical surveys can provide integrated subsurface information. An effective, easy-to-use and low-cost methodology combining geological/hydrogeological information from Auger drillings and the results from three geophysical techniques (Electromagnetic mapping, Electrical Sounding and Electrical Resistivity Tomography) is being developed to characterize the structure of a typical Mediterranean deltaic aquifer. A first hypothesis about hydrodynamic properties of the aquifer is also obtained. The study area is located in the Rhone delta (Middle Camargue/southern France). Integration of geophysical and geological techniques allowed identifying the presence and lateral extension of the Saint-Ferreol paleochannel, the vertical contact between lagoonal-fluvial deposits and the marine clayed silt that separate the superficial aquifer from the deeper aquifer. Likewise, high north-south heterogeneity and east-west homogeneity were highlighted in the study area. Presence of clay in sandy deposits in the low areas implies changes in lateral hydraulic permeability. This fact, jointed to the low hydraulic gradient, suggests a slow groundwater flow in the local system. The Rhone delta presents a typical configuration of a Mediterranean deltaic aquifer, thus this methodological approach can be used for similar deltaic Mediterranean systems.

International audience; The structure of karst hydrosystems is complex and the related hydrodynamic functioning mechanism can be highly heterogeneous in both unsaturated and saturated zones. The Low-Noise Underground Laboratory of Rustrel (LSBB) is located in unsaturated zone (UZ) of Fontaine de Vaucluse (FdV) karst hydrosystem. In this way, the LSBB is a unique place to apply surface-based geophysical surveys due to numerous and various geological and hydrogeological knowledge under the investigated zone. We exhibit in this paper complementarity and efficiency of Electrical Resistivity Tomography (ERT) and Ground Penetrating Radar (GPR). Both GPR and ERT are well suited for the first phase of our study, in order to characterize the general structure of the investigated chosen zone. GPR results highlight the tectonic features which are also, but less accurately, seen by ERT apparent resistivity maps. ERT 2D inverted results have shown the presence of moderate resistivity zones within the limestone. The provided information allowed carrying out following step of our study to enhance understanding of local karst UZ e.i. apply Magnetic Resonance Sounding (MRS), Gravimetry and Low Frequency Electromagnetics (LF-EM).

This study aims to assess Magnetic Resonance Soundings (MRS) potential for the characterisation of the unsaturated zone of karst systems and thereby groundwater vulnerability assessment. The issues addressed are: (i) does water storage within the unsaturated zone of karst yield quantifiable MRS measurements ? if so, (ii) do different hydrogeological contexts yield different MRS signals ?, and (iii) what are the key factors influencing MRS signal ? The proposed methodology is based on a careful characterisation of the experimental sites, involving both hydrogeological and geomorphological analyses. MRS measurements have been performed on a total of 19 sites with varied hydrogeological and geomorphological setting. Analysis of the measured MRS signals shows that : (i) water storage within the unsaturated zone of karst does yield quantifiable MRS measurements, (ii) the first-order determinant of the MRS signals is the nature of the geological formation. These results confirm MRS applicability for the characterization of the unsaturated zone of karst aquifers and thereby vulnerability assessment. Future work will focus on the variability of the MRS response within a given geological formation.

(IF 3.63; Q1); International audience; Karstaquifers and watersheds represent a major source of drinking water around the world. They are also known as complex and often highly vulnerable hydrosystems due to strong surface-groundwater interactions. Improving the understanding of karst functioning is thus a major issue for the efficient management of karst groundwater resources. A comprehensive understanding of the various processes can be achieved only by studying karst systems across a wide range of spatiotemporal scales under different geological, geomorphological, climatic, and soil cover settings. The objective of the French Karst National Observatory Service (SNO KARST) is to supply the international scientific community with appropriate data and tools, with the ambition of (i) facilitating the collection of long-term observations of hydrogeochemical variables in karst, and (ii) promoting knowledge sharing and developing cross-disciplinary research on karst. This paper provides an overview of the monitoring sites and collective achievements, such as the KarstMod modular modeling platform and the PaPRIKa toolbox, of SNO KARST. It also presents the research questions addressed within the framework of this network, along with major research results regarding (i) the hydrological response of karst to climate and anthropogenic changes, (ii) the influence of karst on geochemical balance of watersheds in the critical zone, and (iii) the relationships between the structure and hydrological functioning of karst aquifers and watersheds.