We describe a model for regional initiation of shallow landslides based on an approximate analytic solution to Richards equation combined with an infinite-slope calculation. The model applied over digital topography computes pressure heads and factors of safety as functions of depth for geographic information system (GIS) grid cells at any time during and after rainfall events. An example is presented that simulates the progressive development of shallow landslides on steep slopes during a rainfall event. This example shows how this modeling provides insights into transient rainfall-caused processes that trigger shallow slope instability and consequent regionally distributed debris-flow events. Specifically, we infer that the spatial pattern of instability is primarily controlled by topography, while intensity and duration of rainfall, and the subsequent pore-pressure responses control the temporal pattern of instability. ?? 2003 Millpress.