Karst aquifers 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.Abbreviations: CADI, cellular automata-based deterministic inversion; Ex/Em, excitation/emission; NOM, natural organic matter; SLP, sea level pressure; SNO Karst, the French Karst National Observatory Service.Karstified carbonate formations contain 25% of the world's water resources. They cover a very large extent of the continental surface: 10% of the global continental surface, 30 to 70% of the Mediterranean area, 22% of the land in Europe, and 50% in France (Chen et al., 2017). In carbonate karst hydrosystems, the presence of fractures, conduits, and surface solution features leads to strong surface-subsurface interactions that result in significant water, mass, energy, and contaminant transport within the critical zone. Such heterogeneous systems are highly dynamic, with complex hydrologic, geochemical, and biological processes occurring across a wide range of spatiotemporal scales. As a result, they Core Ideas • SNO KARST is dedicated to the study of karst functioning. • Hydrodynamics and geochemistry are measured at springs and in karst compartments. • Process sampling was set up at nine sites in various climatic contexts.

A new multidisciplinary methodology is developed to determine the structural geometry of hard-rock aquifers. The Plancoët migmatitic basement, chosen as an example for the use of this methodology, is studied both at a regional (300 km 2 ) and a local (4 km 2 ) scale. The regional mapping, based on field mapping of the tectonic structures and the weathered layers, on geostatistics and on aerial photographs and Digital Elevation Model (DEM) analyses, demonstrates the influence of the tectonic structures on the weathering processes. Some geo-electric investigations, added to the first mapping at the local scale, lead to the building of a local geometrical model of the aquifer.Approche multidisciplinaire pour déterminer la géométrie structurale des aquifères de socle.Application à l'aquifère migmatitique de Plancoët (NE Bretagne, W France) Mots-clés. -Aquifère métamorphique, Analyse morphostructurale, Profils d'altération, Prospections électriques, Modèle géomé-trique Résumé. -Nous présentons ici une nouvelle méthode pluridisciplinaire pour déterminer la géométrie structurale des aquifères de socle. Le site migmatitique de Plancoët, choisi comme exemple dans cette étude, est étudié à l'échelle ré-gionale (300 km 2 ) et locale (4 km 2 ). La cartographie régionale, basée sur des observations sur le terrain des structures tectoniques et des horizons d'altération, sur une analyse géostatistique et sur des analyses de photos aériennes et du modèle numérique de terrain (MNT), met en évidence l'influence des structures tectoniques sur les processus d'altération.A l'échelle locale, des prospections géo-électriques ajoutées à la précédente cartographie permettent de construire un modèle géométrique local de l'aquifère.

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Using a statistical approach, Scheihing attempts to demonstrate the direct influence of recharge events in the Precordillera and the Andean Piedmont on water table variations in downstream alluvial fans. The author "unquestionably" discards the existence of ephemeral recharge processes in alluvial fans although they are frequently impacted by major floods. However, the author does not consider previous publications about recharge processes in arid alluvial fans, the hydraulic setting of the Pampa del Tamarugal Aquifer (PTA) and anthropogenic pressure changes in the PTA. Because the sustainable management of groundwater resources in drylands depends on a good understanding of hydrogeological processes, we propose a thorough reinterpretation of the short-and long-term PTA water table variations addressed by the author. In this comment, we illustrate the limits of the sole use of a statistical approach for characterizing both recharge processes and factors controlling the water table variations in the Atacama Desert.

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International audienceHard Rock Aquifers (HRA) have long been considered to be two-layer systems, with a mostly capacitive layer just below the surface, the saprolite layer and a mainly transmissive layer underneath, the fractured layer. Although this hydrogeological conceptual model is widely accepted today within the scientific community, it is difficult to quantify the respective storage properties of each layer with an equivalent porous medium model. Based on an HRA field site, this paper attempts to quantify in a distinct manner the respective values of the specific yield (Sy) in the saprolite and the fractured layer, with the help of a deterministic hydrogeological model. The study site is the Plancoët migmatitic aquifer located in north-western Brittany, France, with piezometric data from 36 observation wells surveyed every two weeks for eight years. Whereas most of the piezometers (26) are located where the water table lies within the saprolite, thus representing the specific yield of the unconfined layer (Sy1), 10 of them are representative of the unconfined fractured layer (Sy2), due to their position where the saprolite is eroded or unsaturated. The two-layer model, based on field observations of the layer geometry, runs with the MODFLOW code. 81 values of the Sy1/Sy2 parameter sets were tested manually, as an inverse calibration was not able to calibrate these parameters. In order to calibrate the storage properties, a new quality-of-fit criterion called “AdVar” was also developed, equal to the mean squared deviation of the seasonal piezometric amplitude variation. Contrary to the variance, AdVar is able to select the best values for the specific yield in each layer. It is demonstrated that the saprolite layer is about 2.5 times more capacitive than the fractured layer, with Sy1=10% (7%