The question of the depth reached by groundwater in natural recharge to a geothermal field is of interest for geothermal development, since it can affect the nature of the recharge regime during withdrawal, and the volume of water within reach during exploitation. Also, useful inferences may be drawn about the large-scale permeability of the system if the groundwater flow regime is understood.

Evidence for the presence of thermal convection in the groundwater now appears to be well-established, although topographic effects may also be important (Studt and Thompson 1969, Healy and Hochstein 1973). regions which serve particularly well as illustration s are (1) the Imperial Valley of Southern California and (2) the Taupo Volcanic Zone of New Zealand. Both exhibit a number of quite well-defined zones of anomalously high heat flow (geothermal fields ), separated by distances of 10 to 15 Km, the intervening areas usually having very low heat flow. At Imperial Valley, the fairly permeable sands in which convection is likely to occur are overlain by sediments of low permeability, roughly 0.6 Km in thickness, and thermal conductivity alone without appreciable convection, commonly occurs in these upper layers (Palmer, Howard and Lande 1975). In the case of (2), the heat flow in areas surrounding geothermal fields is depressed practically to zero, and this has been interpreted by Studt and Thompson as being due to downflowing recharge water from precipitation. The water issuing naturally from geothermal field s is predominantly meteoric, but the residence times in the groundwater stage appear to be very long.

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