## 1. Introduction

[2] The advantages and disadvantages of using physically based surface/variably saturated subsurface models to analyze flow systems have been the subject of a number of discussions in the literature [e.g., *Beven*, 1989, 1993, 1996a, 1996b, 2000, 2001a, 2001b, 2002a, 2002b, 2006; *Beven and Binley*, 1992; *Ebel and Loague*, 2006; *Grayson et al.*, 1992; *Loague and VanderKwaak*, 2004; *Loague et al.*, 2006; *Refsgaard et al.*, 1996; *Smith et al.*, 1994]. However, it is only in the last few years that models such as InHM [*VanderKwaak*, 1999], MODHMS [*Hydrogeologic*, 2000; *Panday and Huyakorn*, 2004] and HydroGeoSphere [*Therrien et al.*, 2005], which meet or exceed the original *Freeze and Harlan* [1969] blueprint, have been made available to the hydrological community for testing. Most of the initial applications of these ‘new generation’ surface-subsurface models have, to date, been limited to experimental plots or relatively small subcatchments [e.g., *Di Iorio*, 2003; *Heppner et al.*, 2006; *Jones et al.*, 2006; *Loague and VanderKwaak*, 2002; *Loague et al.*, 2005; *Pebesma et al.*, 2005; *VanderKwaak and Loague*, 2001]. In order for these data-intensive models to realize their full potential and to gain acceptance in the hydrological community, it needs to be demonstrated that they are capable of simulating hydrodynamic processes at larger scales.

[3] In this study, the physically based, surface/variably saturated subsurface flow model InHM [*VanderKwaak*, 1999] is applied to a hydrologically complex but reasonably well characterized 75 km^{2} watershed located in Southern Ontario, Canada. The primary objective of this study is to assess InHM's ability to simulate transient flow processes at a scale larger than has been previously attempted. This objective is accomplished by first calibrating the steady state subsurface of the system to hydraulic head data obtained from 50 observation wells while simultaneously matching the observed stream base flow discharge. The calibrated system is then subjected to two rainfall data series and the resulting discharge hydrographs are compared with the measured rainfall-runoff response. The computed hydrodynamic response of the system is next analyzed in terms of temporal variations in the contributing areas and surface-subsurface exchange fluxes occurring across the land surface during rainfall inundation and subsequent drainage phases. A secondary objective of this study is to demonstrate the advantages of using the fully integrated approach to investigate watershed-scale hydrodynamic processes. Traditionally, modeling studies at the watershed scale either treat the surface and subsurface systems as separate entities or employ a loosely coupled approach to link the two systems.

[4] InHM was originally developed at the University of Waterloo [*VanderKwaak*, 1999] and is capable of simulating water flow and solute transport over the two-dimensional land surface and in the three-dimensional dual continua subsurface (i.e., porous medium-macropore interactions) under variably saturated conditions. The two-dimensional form of the nonlinear diffusion-wave equation, together with Manning's equation to compute overland flow velocities, is employed on the surface while Richards' equation and Darcy's law are assumed to hold in the subsurface. Full coupling of the surface and subsurface flow regimes is accomplished by simultaneously solving one system of non-linear discrete equations arising from the control-volume finite element method to describe flow and solute transport in both flow regimes, as well as the water and solute fluxes between continua. Details concerning the theory, numerical solution techniques and example applications of InHM are given by *VanderKwaak* [1999] as well as *VanderKwaak and Loague* [2001].