Flee From Parsimony: Escaping the Boundary Condition Paradox in Groundwater Modelling

A recording of Aquanty's July 2026 webinar "Flee From Parsimony: Escaping the Boundary Condition Paradox in Groundwater Modelling" Abstract: Questions being posed of hydrologic models by clients, governments and regulators are becoming broader ranging and more complex, for example, how will the model results be affected by climate change; what are the cumulative regional effects on groundwater resources; how will water quality be affected; and, what are the effects on surface water-groundwater (SW-GW) interaction? Groundwater models typically have recharge zones prescribed by the modeller, with surface water features delineated with specified boundary conditions. The location and timing of recharge is highly variable, and specifying the location and magnitude of recharge is to some degree assuming a model input that should be a model output, and that is the boundary condition paradox. In a climate change assessment, if recharge is prescribed, how do you propagate changing precipitation patterns, shifting evapotranspiration, and altered snowmelt timing through the model? Cumulative regional effects require spatial scales and process interactions that simple recharge zones sub-divide somewhat arbitrarily. Contaminant transport is sensitive to where and how fast recharge actually occurs, not where it might be assumed to occur. Dynamic SW-GW interaction zones are precisely where prescribed boundary conditions are most likely to be wrong and most consequential. Integrated surface-subsurface hydrological modelling with HydroGeoSphere (HGS) is essentially the antithesis of the parsimonious recharge-zone approach. By fully coupling surface water and groundwater in a physically based, variably saturated model, HGS avoids the boundary condition paradox by construction. Recharge is an output from the simulation of precipitation, evapotranspiration, overland flow, and unsaturated zone flow dynamics. In a world where municipalities, mining companies, oil and gas operators, agricultural operations, and regulators are asking questions that a simple recharge zone model structurally cannot answer, perhaps the classical tools are now mismatched to the questions being asked. Presenter Bio: Dr. Michael Callaghan is a Senior Applications Engineer at Aquanty Inc. with over 25 years experience in the geoenvironmental sector, including over 20 years of numerical modelling experience ranging from soil column scale to large river basin scale, spanning diverse applications such as river sedimentation modelling, salt and pesticide leaching to environmental receptors, hydrologic risk assessment for agriculture, regulatory permitting for mine development, and more recently, integrated hydrologic modelling for groundwater supply. To learn more about HydroGeoSphere visit the Aquanty website: https://www.aquanty.com/hydrogeosphere