This paper presents a conjunctive 2-D surface and 3-D subsurface flow model, which deals with the interaction between surface and subsurface flows. The model uses the non-inertia approximation of the Saint-Venant equations for two-dimensional unsteady surface flow and a modified version of the Richards equation for three-dimensional unsteady unsaturated and saturated subsurface flows. The equations are written in the form of 2-D and 3-D heat diffusion equations respectively. In the model the surface and subsurface flow components are coupled interactively using the common boundary condition of infiltration through the ground surface. Overland flow is generally classified into Hortonian and saturation overland flows in watershed hydrology. The conjunctive model was, after verification with experimental data, applied to a hypothetical watershed to simulate examples of 2-D saturation overland flow and 3-D saturated and unsaturated subsurface flow. The model demonstrates that overland flow is initiated when rainwater supply exceeds infiltrability, not only in Hortonian but even in saturation overland flow. The model also simulates overland flow, infiltration, seepage, subsurface flow, and the interaction between subsurface and channel flows. The conjunctive model, as a general model to deal with the interaction between surface and subsurface flows, contributes as a tool for improved detailed simulation of 2-D surface and 3-D subsurface flows and their interaction.