The use of conservative geochemical and isotopic tracers along with mass balance equations to determine the pre‐event groundwater contributions to streamflow during a rainfall event is widely used for hydrograph separation; however, aspects related to the influence of surface and subsurface mixing processes on the estimates of the pre‐event contribution remain poorly understood. Moreover, the lack of a precise definition of “pre‐event” versus “event” contributions on the one hand and “old” versus “new” water components on the other hand has seemingly led to confusion within the hydrologic community about the role of Darcian‐based groundwater flow during a storm event. In this work, a fully integrated surface and subsurface flow and solute transport model is used to analyze flow system dynamics during a storm event, concomitantly with advective‐dispersive tracer transport, and to investigate the role of hydrodynamic mixing processes on the estimates of the pre‐event component. A number of numerical experiments are presented, including an analysis of a controlled rainfall‐runoff experiment, that compare the computed Darcian‐based groundwater fluxes contributing to streamflow during a rainfall event with estimates of these contributions based on a tracer‐based separation. It is shown that hydrodynamic mixing processes can dramatically influence estimates of the pre‐event water contribution estimated by a tracer‐based separation. Specifically, it is demonstrated that the actual amount of bulk flowing groundwater contributing to streamflow may be much smaller than the quantity indirectly estimated from a separation based on tracer mass balances, even if the mixing processes are weak.

[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,

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[1] Surface ground-penetrating radar (GPR) techniques have been used by a number of previous researchers to characterize soil moisture content in the vadose zone. However, limited temporal sampling and low resolution near the surface in these studies greatly impedes the quantitative analysis of vertical soil moisture distribution and its associated dynamics within the shallow subsurface. To further examine the capacity of surface GPR, we have undertaken an extensive 26 month field study using concurrent high-frequency (i.e., 900 MHz) reflection profiling and common-midpoint (CMP) soundings to quantitatively monitor soil moisture distribution and dynamics within the shallow vadose zone. This unprecedented data set allowed us to assess the concurrent use of these techniques over two contrasting annual cycles of soil conditions. Reflection profiles provided high-resolution traveltime data between four stratigraphic reflection events while cumulative results of the CMP sounding data set produced precise depth estimates for those reflecting interfaces, which were used to convert interval-traveltime data into soil moisture. The downward propagation of major infiltration episodes associated with seasonal and transient events are well resolved by the GPR data. The use of CMP soundings permitted the determination of direct ground wave velocities, which provided high-resolution information along the air-soil interface. This improved resolution enabled better characterization of short-duration wetting/drying and freezing/thawing processes, and permitted better evaluation of the nature of the coupling between shallow and deep moisture conditions. The nature of transient infiltration pulses, evapotranspiration episodes, and deep drainage patterns observed in the GPR data series were further examined by comparing them with a vertical soil moisture flow simulation based on the variably saturated model, HYDRUS-1D. Using laboratory-derived soil hydraulic property information from soil samples and a number of simplifying assumptions about the upper and lower-boundary condition, we were able to achieve very good agreement between measured and simulated soil moisture profiles without model calibration; this is a strong indication of the overall quality of the GPR-derived soil moisture estimates. The only notable difference between simulated values and GPR water content estimates occurred during extended dry soil conditions near the surface.