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Physics‐based hydrologic‐response simulation: foundation for hydroecology and hydrogeomorphology

Two emerging and important disciplines within the large science of hydrology are hydroecology (Eagleson, 2002; Rodriguez-Iturbe and Porporato, 2004) and hydrogeomorphology (Sidle and Onda, 2004), each requiring an integrated understanding of hydrologic response at the surface and within the variably saturated subsurface. Obviously, the most useful tool for understanding ecological or geomorphic processes within a given hydrologically driven system is careful observation via detailed field measurements/experiments (e.g. Montgomery et al., 2002; Loheide and Gorelick, 2005). However, simulation of hydrologic response with comprehensive physics-based models can provide a strong foundation for concept development in both hydroecology and hydrogeomorphology. The simulation of hydrologic response has received considerable attention in the last half century (see Beven (2000, 2002) and Singh and Woolhiser (2002)). In an often cited paper, Freeze and Harlan (1969) proposed a blueprint for a distributed physically based hydrologic model, based upon numerical solution to the coupled partial differential equations that describe water movement on the surface and within the variably saturated subsurface. At least three hydrologic-response models have been developed in the true spirit of the Freeze and Harlan blueprint: (i) InHM (VanderKwaak, 1999), (ii) MODHMS (Panday and Huyakorn, 2004), and (iii) HydroGeoSphere (Sudicky et al., 2005). The Integrated Hydrology Model (InHM) was designed to estimate quantitatively, in a fully coupled first-order approach, three-dimensional (3D) variably saturated flow and solute transport in porous media, 3D variably saturated flow and solute transport in macropores, and two-dimensional (2D) flow and solute transport over the land surface and in open channels. Successful applications of InHM include those of VanderKwaak and Loague (2001) and Loague et al. (2005). Obviously, not all hydrologic-response simulations can (or should) be conducted with comprehensive physics-based models. Potentially the most effective use of physics-based simulation, related to hydroecology and hydrogeomorphology, is in the design of data collection strategies and identifying the next hypothesis-testing field experiment.