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Summary: The physically-based surface–subsurface HydroGeoSphere model is used to examine the hydrologic budget of the 286.6km2 Duffins Creek watershed, located 10km northeast of metropolitan Toronto. The primary objective of this study is to demonstrate the utility of the model to simulate the three-dimensional hydrologic response of the surface and subsurface flow systems in a large-scale watershed driven by multi-season precipitation events. The hydrologic model was calibrated to a consecutive sequence of hydrographs resulting from all precipitation events between the months of April and December 1986, while hydrographs from 1987 were used to test the model’s ability for prediction. Results demonstrate that a physically-based modeling approach which treats the surface and subsurface flow systems as an integrated continuum can capture the dynamic response of a large heterogeneous watershed as driven by transient precipitation events over a multi-season period. It was also revealed that full coupling of plant transpiration and surface–subsurface evaporation processes into the governing flow equations was critical for the successful calibration of the hydrographs. However, the ability of the model to predict the shape of individual hydrographs in the following year, during which the annual average precipitation rate decreased significantly, was diminished even though the annual average stream flows were closely replicated. We identify a number of data gaps that contribute to the decline in the predictive capacity of the model, including difficulty in establishing an appropriate initial condition, vertical mesh refinement issues within the root zone, and the reliance on a largely empirical relationship that distributes the net capacity for plant transpiration among various factors that may change from one year to the next. [Copyright &y& Elsevier]

Wastewater treatment plant (WWTP) discharge is often considered a principal source of surface water contamination. In this study, a three-dimensional fully-integrated groundwater–surface water model was used to simulate the transport characteristics and cumulative loading of an artificial sweetener (acesulfame) and fecal indicator bacteria (Escherichia coli) from WWTPs within a 6800 km2 mixed-use, highly impacted watershed in Ontario, Canada. The model, which employed 3.5 × 106 computational nodes and 15 layers, facilitated a comprehensive assessment of groundwater–surface water interactions under high and low flow conditions; processes typically not accounted for in WWTP cumulative effects models. Simulations demonstrate that the model had significant capacity in reproducing the average and transient multi-year groundwater and surface water flow conditions in the watershed. As a proxy human-specific conservative tracer, acesulfame was useful for model validation and to help inform the representation of watershed-scale transport processes. Using a uniform WWTP acesulfame loading rate of 7.14 mg person−1 day−1, the general spatial trends and magnitudes of the acesulfame concentration profile along the main river reach within the watershed were reproduced; however, model performance was improved by tuning individual WWTP loading rates. Although instream dilution and groundwater–surface water interactions were strongly dependent on flow conditions, the main reach primarily consisted of groundwater discharge zones. For this reason, hydrodynamic dispersion in the hyporheic zone is shown as the predominant mechanism driving acesulfame into near-stream shallow groundwater, while under high flow conditions, the simulations demonstrate the potential for advective flushing of the shallow groundwater. Regarding the cumulative impact of the WWTPs on E. coli concentrations in the surface flow system, simulated transient E. coli levels downstream of WWTPs in the watershed were significantly lower than observed values, thus highlighting the potential importance of other sources of E. coli in the watershed. Image 100146 • Integrated hydrologic models are effective for characterizing point source pollution impacts. • Acesulfame is an effective chemical tracer for validating watershed-scale models. • Groundwater – surface water interactions impart a dynamic influence on water quality. • With only point source WWTP emitters considered, instream FIB levels were underestimated. [ABSTRACT FROM AUTHOR]

Summary This work presents an iterative, water balance based approach to estimate actual evapotranspiration (ET) with integrated surface/subsurface flow models. Traditionally, groundwater level fluctuation methods have been widely accepted and used for estimating ET and net groundwater recharge; however, in watersheds where interactions between surface and subsurface flow regimes are highly dynamic, the traditional method may be overly simplistic. Here, an innovative methodology is derived and demonstrated for using the water balance equation in conjunction with a fully-integrated surface and subsurface hydrologic model (HydroGeoSphere) in order to estimate ET at watershed and sub-watershed scales. The method invokes a simple and robust iterative numerical solution. For the proof of concept demonstrations, the method is used to estimate ET for a simple synthetic watershed and then for a real, highly-characterized 7000 km 2 watershed in Southern Ontario, Canada (Grand River Watershed). The results for the Grand River Watershed show that with three to five iterations, the solution converges to a result where there is less than 1% relative error in stream flow calibration at 16 stream gauging stations. The spatially-averaged ET estimated using the iterative method shows a high level of agreement ( R 2 = 0.99) with that from a benchmark case simulated with an ET model embedded directly in HydroGeoSphere. The new approach presented here is applicable to any watershed that is suited for integrated surface water/groundwater flow modelling and where spatially-averaged ET estimates are useful for calibrating modelled stream discharge. [ABSTRACT FROM AUTHOR]