[1] Vegetation zonation and tidal hydrology are basic attributes of intertidal salt marshes, but specific links among vegetation zonation, plant water use, and spatiotemporally dynamic hydrology have eluded thorough characterization. We developed a quantitative model of an intensively studied salt marsh field site, integrating coupled 2-D surface water and 3-D groundwater flow and zonal plant water use. Comparison of model scenarios with and without heterogeneity in (1) evapotranspiration rates and rooting depths, according to mapped vegetation zonation, and (2) sediment hydraulic properties from inferred geological heterogeneity revealed the coupled importance of both sources of ecohydrological variability at the site. Complex spatial variations in root zone pressure heads, saturations, and vertical groundwater velocities emerged in the model but only when both sources of ecohydrological variability were represented together and with tidal dynamics. These regions of distinctive root zone hydraulic conditions, caused by the intersection of vegetation and sediment spatial patterns, were termed ''ecohydrological zones'' (EHZ). Five EHZ emerged from different combinations of sediment hydraulic properties and evapotranspiration rates, and two EHZ emerged from local topography. Simulated pressure heads and groundwater dynamics among the EHZ were validated with field data. The model and data showed that hydraulic differences between EHZ were masked shortly after a flooding tide but again became prominent during prolonged marsh exposure. We suggest that ecohydrological zones, which reflect the combined influences of topographic, sediment, and vegetation heterogeneity and do not emphasize one influence over the others, are the fundamental spatial habitat units comprising the salt marsh ecosystem.

[1] Results are presented in which a physically-based, three-dimensional model that fully integrates surface and variably-saturated subsurface flow processes is applied to the 75 km 2 Laurel Creek Watershed within the Grand River basin in Southern Ontario, Canada. The primary objective of this study is to gauge the model's ability to reproduce surface and subsurface hydrodynamic processes at the watershed scale. Our objective was first accomplished by calibrating the steady-state subsurface portion of the system to 50 observation wells where hydraulic head data were available, while simultaneously matching the stream baseflow discharge. The level of agreement between the observed and computed subsurface hydraulic head values, baseflow discharge and the spatial pattern of the surface drainage network indicates that the model captures the essence of the surfacesubsurface hydraulic characteristics of the watershed. The calibrated model is then subjected to two time series of input rainfall data and the calculated discharge hydrographs are compared to the observed rainfall-runoff responses. The calculated and observed rainfall-runoff responses were shown to agree moderately well for both rainfall data series that were utilized. Additionally, the spatial and temporal responses of the watershed with respect to the overland flow areas contributing to streamflow and the surface-subsurface exchange fluxes across the land surface during rainfall inundation and subsequent drainage phases demonstrate the dynamic nature of the interaction occurring between the surface and subsurface hydrologic regimes. Overall, it is concluded that it is feasible to apply a fully-integrated, surface/variably-saturated subsurface flow model at the watershed scale and possibly larger scales.Citation: Jones, J. P., E. A. Sudicky, and R. G. McLaren (2008), Application of a fully-integrated surface-subsurface flow model at the watershed-scale: A case study, Water Resour. Res., 44, W03407,

The ability to simulate contaminant migration in large‐scale porous formations containing a complex network of discrete fractures is limited by traditional modeling approaches. One primary reason is because of vastly different transport time scales in different regions due to rapid advection along the discrete fractures and slow but persistent diffusion in the porous matrix. In addition to time‐related complexities, standard numerical methods require a fine spatial discretization in the porous matrix to represent sharp concentration gradients at the interface between the fractures and the matrix. In order to circumvent these difficulties, the Laplace transform Galerkin method is extended for application to discretely fractured media with emphasis on large‐scale modeling capabilities. The technique avoids time stepping and permits the use of a relatively coarse grid without compromising accuracy because the Laplace domain solution is relatively smooth and devoid of discontinuities even in advection‐dominated problems. Further computational efficiency for large‐grid problems is achieved by employing a preconditioned, ORTHOMIN‐accelerated iterative solver. A unique feature of the method is that each of the several needed p space solutions are independent, thus making the scheme highly parallel. Other features include the accommodation of advective‐dispersive transport in the porous matrix and the straightforward inclusion of dual‐porosity theory to represent matrix diffusion in regions where microfractures exist below the modeling scale. An example problem involving contaminant transport through an aquitard into an underlying aquifer leads to the conclusion that deep, essentially undetectable fractures in clayey aquitards can greatly compromise the quality of groundwater in the impacted aquifer.

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[1] The spatial variability of hydraulic conductivity in a shallow unconfined aquifer located at North Bay, Ontario, composed of glacial-lacustrine and glacial-fluvial sands, is examined in exceptional detail and characterized geostatistically. A total of 1878 permeameter measurements were performed at 0.05 m vertical intervals along cores taken from 20 boreholes along two intersecting transect lines. Simultaneous three-dimensional (3-D) fitting of Ln(K) variogram data to an exponential model yielded geostatistical parameters for the estimation of bulk hydraulic conductivity and solute dispersion parameters. The analysis revealed a Ln(K) variance equal to about 2.0 and 3-D anisotropy of the correlation structure of the heterogeneity (l 1 , l 2 , and l 3 equal to 17.19, 7.39, and 1.0 m, respectively). Effective values of the hydraulic conductivity tensor and the value of the longitudinal macrodispersivity were calculated using the theoretical expressions of Gelhar and Axness (1983). The magnitude of the longitudinal macrodispersivity is reasonably consistent with the observed degree of longitudinal dispersion of the landfill plume along the principal path of migration. Variably saturated 3-D flow modeling using the statistically derived effective hydraulic conductivity tensor allowed a reasonably close prediction of the measured water table and the observed heads at various depths in an array of piezometers. Concomitant transport modeling using the calculated longitudinal macrodispersivity reasonably predicted the extent and migration rates of the observed contaminant plume that was monitored using a network of multilevel samplers over a period of about 5 years. It was further demonstrated that the length of the plume is relatively insensitive to the value of the longitudinal macrodispersivity under the conditions of a steady flow in 3-D and constant source strength. This study demonstrates that the use of statistically derived parameters based on stochastic theories results in reliable large-scale 3-D flow and transport models for complex hydrogeological systems. This is in agreement with the conclusions reached by Sudicky (1986) at the site of an elaborate tracer test conducted in the aquifer at the Canadian Forces Base Borden.Citation: Sudicky, E. A., W. A. Illman, I. K. Goltz, J. J. Adams, and R. G. McLaren (2010), Heterogeneity in hydraulic conductivity and its role on the macroscale transport of a solute plume: From measurements to a practical application of stochastic flow and transport theory, Water Resour. Res., 46, W01508,