International audienceThe Galapagos Archipelago is nearly devoid of freshwater resources, but during six months of the year, a fog layer covers the windward slopes of the main islands. In order to investigate the hydrological importance of this phenomenon, a monitoring network was set up on Santa Cruz Island, at the center of the archipelago. Meteorological parameters were monitored together with throughfall and stemflow at two stations: a first in a secondary forest at the lowest fringe of the fog layer (400 m a.s.l.), and a second in shrub lands of the Galapagos National Park, at the center of the fog layer (650 m a.s.l.). Cloud water interception was quantified from the wet canopy water budget, based on a modified \emph{Rutter}-type canopy interception model. This methodology allowed the estimation of fog interception for short time intervals (15 min) and avoided the subjective separation into individual rainfall events. Fog was found to be a negligible water input at the lower site, but contributed up to 26 $\pm$ 16\% of incident rainfall at the higher site. Wind was shown to enhance fog interception, but this alone could not explain the difference in fog catch between the two sites. Higher liquid water content and more frequent fog occurrence contributed to the difference as well. This study highlights that the presence of fog may induce a marked increase of net precipitation, but this effect is restricted to the summit areas exposed to winds, located in the center of the cloud belt

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The hydrology of volcanic islands is poorly characterized due to their complex internal structure and challenging access. The Galápagos Islands, an isolated basaltic environment with unique ecosystems and growing anthropogenic pressure, suffer from scarcity of freshwater resources and from the lack of fundamental knowledge on their hydrology. To overtake these constraints and provide fresh water to the population, a geophysical survey and a hydroclimatologic network at watershed scale have been performed over Santa Cruz. An innovative helicopter borne transient electromagnetic method permitted a quasi-3D (an inversion scheme allowing for a 3D model description but with a local 1D forward algorithm) resistivity mapping of the massif and the identification of its hydrogeological potential. The latter is composed of a perched aquifer on the windward mountainside and a basal aquifer. Our hydroclimatologic network provided useful data records and allowed us to characterize the hydrodynamic properties of the basal aquifer. The geophysical data allow us to precise its extension. The conceptual hydrogeological model proposed for Santa Cruz confirms one of the models proposed for Le Piton de la Fournaise (Réunion Island). It has now to be confirmed with boreholes and a detailed geochemical study including noble gas analysis. To cite this article: E.