The development of a better understanding and a predictive model for water influx is pivotal in scaling up the linked vertical well UCG process, since water influx strongly affects gas quality. A water-influx model is developed here which incorporates radial permeation of water through the coal seam, and steam generation by spalling enhanced drying of the coal and overburden. This random spalling process which enhances drying by exposing fresh wet surface is described by a surface renewal model. A cavity-growth model