The fact that major geothermal fields are commonly located above zones of major crustal structures suggests that thermal resources are large, but heat extraction depends on hydrologic conditions which constitute the limiting factor.

The heat is believed to originate by conduction in areas of anomalously high heat flow, and is transmitted and concentrated by deep circulating groundwater. The existence of horizontal flow of hot groundwater has been demonstrated in several fields, and large hot-water fields tend to be located in discharge basins with large intake areas.

In zones of horizontal flow, stored heat resources are probably minimal and, even in large systems such as Wairakei, the dominant resource is groundwater recharge. Consequently, preproduction estimates of resources based on stored heat and natural heat flow may be unreliable. Production from Wairakei has been characterized by decreases in pressure, temperature, and output, though recharge has increased threefold.

Vapor-dominated systems tend to be located in elevated areas of groundwater recharge, where an important factor appears to be a potent heat source beneath the reservoir. The most important resource is probably recharge, and extraction of stored heat from rock depends on reservoir permeability.

Conservation of heat and disposal of saline waters by reinjection may be impracticable where recharge is an important contribution to water resources.