FRAC3DVS is an efficient and robust numerical model that solves the three-dimensional variably-saturated subsurface ow and solute transport equations in non-fractured or discretely-fractured media and which was developed at the University of Waterloo and at Universit e Laval. It has enjoyed widespread acceptance with both academics and groundwater professionals since its initial release in 1995 and many new features have been added to the code since then. However, FRAC3DVS did not have the ability to simulate fully-coupled subsurface/surface water systems in an e cient and straight-forward manner. Recharge spreading layer and MODFLOW-type river node schemes are simpli ed approaches that have been tried with limited success. In order to remedy this situation, a modi ed version of Hydrogeologic Inc.'s MODHMS surface water ow packages has been incorporated into the FRAC3DVS framework. These packages were originally developed to enhance the capabilities of MODFLOW so that it could handle more complex eld problems in a robust and e cient manner.

The resulting model, which we call HydroGeoSphere, is documented herein. Provided in the following chapters are detailed descriptions, formulations, veri cation and application examples, input instructions, output formats and sample data files for the various components of the model.

HydroGeoSphere is supported by a exible, user-friendly modeling interface that may be used to seamlessly prepare input data sets, or visualize and interpret simulation results.