There are a growing number of large-scale, complex hydrologic models that are capable of simulating integrated surface and subsurface flow. Many are coupled to land-surface energy balance models, biogeochemical and ecological process models, and atmospheric models. Although they are being increasingly applied for hydrologic prediction and environmental understanding, very little formal verification and/or benchmarking of these models has been performed. Here we present the results of an intercomparison study of seven coupled surface-subsurface models based on a series of benchmark problems. All the models simultaneously solve adapted forms of the Richards and shallow water equations, based on fully 3-D or mixed (1-D vadose zone and 2-D groundwater) formulations for subsurface flow and 1-D (rill flow) or 2-D (sheet flow) conceptualizations for surface routing. A range of approaches is used for the solution of the coupled equations, including global implicit, sequential iterative, and asynchronous linking, and various strategies are used to enforce flux and pressure continuity at the surface-subsurface interface. The simulation results show good agreement for the simpler test cases, while the more complicated test cases bring out some of the differences in physical process representations and numerical solution approaches between the models. Benchmarks with more traditional runoff generating mechanisms, such as excess infiltration and saturation, demonstrate more agreement between models, while benchmarks with heterogeneity and complex water table dynamics highlight differences in model formulation. In general, all the models demonstrate the same qualitative behavior, thus building confidence in their use for hydrologic applications.

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[1] We use an integrated, distributed groundwater-surface water-land surface model, ParFlow, to analyze integrated watershed response and groundwater-land surface feedbacks in the Little Washita River watershed, located within the southern Great Plains region of North America, under observed and perturbed climate conditions. Basin-scale hydrologic sensitivity to temperature and precipitation perturbations is shown to be greatest under energy-limited (direct runoff) conditions compared to moisture-limited (base flow) conditions. Feedbacks between groundwater depth and the land surface water and energy balance are shown to significantly influence surface fluxes under moisturelimited conditions, with differences in latent and sensible heat flux between areas of shallow and deep groundwater depth exceeding 75 W/m 2 under strongly energy-limited conditions. The influence of groundwater feedbacks on sensitivity of surface fluxes to changing climate conditions is shown to depend on changes in both moisture and energy availability over the watershed. Results demonstrate that hydrologic sensitivity to climate change depends on feedbacks between groundwater, overland flow, and the land surface water and energy balance. Results suggest not only that local and watershed response to global climate change depends on groundwater feedbacks but that the magnitude and seasonality of these feedbacks is sensitive to changes in climate.

Magnetotelluric studies of the Trans-Hudson orogen over the last two decades, prompted by the discovery of a significant conductivity anomaly beneath the North American Central Plains (NACP), from over 300 sites yield an extensive database for interrogation and enable three-dimensional information to be obtained about the geometry of the orogen from southern North Dakota to northern Saskatchewan. The NACP anomaly is remarkable in its continuity along strike, testimony to along-strike similarity of orogenic processes. Where bedrock is exposed, the anomaly can be associated with sulphides that were metamorphosed during subduction and compression and penetratively emplaced deep within the crust of the internides of the orogen to the boundary of the Hearne margin. A new result from this compilation is the discovery of an anomaly within the upper mantle beginning at depths of ~80–100 km. This lithospheric mantle conductor has electrical properties similar to those for the central Slave craton mantle conductor, which lies directly beneath the major diamond-producing Lac de Gras kimberlite field. While the Saskatchewan mantle conductor does not directly underlie the Fort à la Corne kimberlite, which is associated with the Sask craton, the spatial correspondence is close.