A two-dimensional model of fault-charged hydrothermal systems has been developed that considers the transient development of such systems including the effects of heat losses to the confining layers. The model can be used for theoretical studies of the development of fault-charged reservoirs. It can also be used to estimate the rate of recharge from the fault source and the time of evolution, using temperature data from wells. The model has been applied to the hydrothermal system at Susanville, California. A reasonable match with the areal temperature distribution in the primary aquifer and the temperature profiles of individual wells was obtained. This allowed an estimate of the recharge rate from the fault into the hydrothermal system to be obtained. As the calculated recharge rate (9¿10-6 m3/s m) into the Susanville hydrothermal system proved to be quite significant, a threefold increase in the potential of the Susanville hydrothermal anomaly for space heating purposes is predicted.