This article presents a simple method for coupling the equations for fluid and heat flow in a porous medium with a rock mechanics model based on a simplified Mohr-Coulomb yield criterion. The aim is to investigate the effects of introducing a brittle-ductile transition zone (BDTZ) into models of the large scale hydrology of systems like the Taupo Volcanic Zone (TVZ) in New Zealand. The coupling between fluid/heat flow and rock mechanics is only partial in that it assumes that the local strain rate and lithostatic pressure are known a priori. A simple empirical relationship between the permeabilities of rock in its brittle and ductile states is also assumed, which mimics the effect of slow thermal creep in closing fluid pathways by reducing the effective permeability. This model is first applied to a number of simple situations where large scale convection of groundwater occurs above a depth of 20-km. These examples demonstrate the formation of a horizontal brittle-ductile transition zone under conductive conditions with uniform strain, and how this responds to local changes in pressure, temperature and strain rate. The presence of low permeability below the BDTZ effectively defines the greatest depth to which groundwater can steadily penetrate, providing a feedback to these models which influences the transport of heat and mass on all the length scales. The TVZ provides inspiration for the second model setting. Localised sources of water and heat, modelled on magmatic sill-like structures at a depth of 10-km, induce a BDTZ which shallows from 14-km to less than 10-km, consistent with geophysical estimates in the TVZ. A common feature of the models is that strong downflows of surface water occur in permeable regions adjacent to the heat sources, which depress the BDTZ by several kilometres between the geothermal areas. Water with near-surface temperatures can thus exist at great depths in these regions. Lastly, the models imply that the geothermal areas 123 336 W. Kissling et al. in the TVZ probably do not occur directly above any localised heat source, but are instead displaced towards the centre of the TVZ rift.