Water temperature records at the outlet of a fractured carbonate system during an annual cycle show two types of variation, slow variations over a small range, resulting from the thermal equilibrium between water and rock in the aquifier, and more rapid variations over a wider range, transmitted more quickly due to the inflow of water through fractures or karstic pipes. Starting at the outlet we present first a model of thermal exchange for flow in a conduit, then a model of thermal exchange in a porous medium between the surface and the reservoir. The comparison with the data recorded at Guichy (Ni¿vre, France) allows us to establish the fact that on this site the vertical flow between the surface and the reservoir is rapid through a fracture network as is the case for the flow at the outlet. ¿ American Geophysical Union 1993