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RESUMENEl sistema kárstico de Pico Frentes se ha desarrollado a favor de un conjunto calcáreo del Cretácico Superior cuya geometría plegada muy bien definida ha condicionado que los acuíferos se sitúen principalmente en tres sinclinales hidráulicamente conectados, con una capacidad de reservas subterráneas de entre 5 y 7 hm 3 . La recarga en este acuífero libre y en penillanura es autógena y difusa. El flujo subterráneo va dirigido a gran escala por el fondo de los sinclinales y a pequeña escala mediante corrientes subterráneas hacia los manantiales de Fuentetoba (210 l/s) y nacimiento del rio Mazos (50 l/s), surgiendo en aguas altas otras descargas menores. El análisis de los hidrogramas de estos manantiales indica un sistema de régimen muy variable y poco poder de regulación natural, característica de un acuífero típicamente kárstico, con gran capacidad de renovación y poco tiempo de residencia.Gracias a la simulación de los hidrogramas de estas surgencias mediante un modelo matemático de precipitación -escorrentía, se ha cuantificado de manera detallada el balance hidráulico medio para una serie de 20 años: aportación pluviométrica 16,86 hm 3 (100%), recarga natural 8,35 hm 3 (49,53%), EVT 8,50 hm 3 (50,41%), bombeo de agua subterránea 0,01hm 3 (0,06%), escorrentía superficial 0 hm 3 , transferencias subterráneas a otros acuíferos 0 hm 3 .Palabras clave: Hidrogeología kárstica; Hidrodinámica; Balance hidráulico; Cordillera Ibérica. ABSTRACTThe karst system of Pico Frentes has developed within an Upper Cretaceous calcareous series whose welldefined folded geometry determines that its aquifer reserves are held mainly in three hydraulically-connected synclines, with a groundwater capacity of between 5 and 7 hm 3 . The recharge to this unconfined peneplain aquifer is autogenous and diffuse. On a large scale, groundwater flow is directed by the base of the synclines, while on a small scale, it flows along groundwater conduits towards the Fuentetoba Spring (210 l/s) and source of the River Mazos (50 l/s), following a highly variable flow regime of low inertia, with other smaller discharges emanating during periods of high water. Analysis of hydrographs of these springs indicates a very variable rate system and little power regulating natural, characteristic of a typical karstic aquifer, with great capacity for renewal and low residence time.

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The Fuentetoba Spring, with a mean flow of 210 L s À1 , releases the discharge from an aquifer that comprises three hydraulically-connected synclines. The spring has a very irregular flow of between 8 L s À1 and 3,400 L s À1 due to the predominant water circulation that is non-Darcian turbulent flow as an underground torrent in the vadose zone, as well as through well-developed karstic conduits in the phreatic and epiphreatic zones, as attested to by speleological explorations. The long response times to recharge by the Fuentetoba karstic system, seen in the spring's hydrograph, are controlled by regional factors. Nevertheless, certain responses that have very long time lags under high water conditions might be governed in the final stretch of the flowpath by mechanisms of pressure or siphoning of the floodwave, and perhaps by constrictions or plugs of sediment in the conduits.