A fully integrated groundwater-surface-water model for southern Ontario: proof-of-concept and data release
Authors: Frey S K , Khader O , Taylor A , Erler A R , Lapen D
R , Sudicky E A , Berg S
J , Russell H A J 
Published: November 2019

A prototype groundwater-surface-water model for the southern Ontario Phanerozoic Basin Region has been developed with HydroGeoSphere (HGS), which provides a 3-dimensional, physics-based simulation of fully integrated groundwater - surface-water flow. To-date, the
model has been tested for its ability to reproduce average monthly surface water flow rates and groundwater levels, and its sensitivity to spatial and temporal resolution. The utility of the model has been demonstrated through an assessment of groundwater extraction influences on regional
groundwater levels, in order to showcase how it could be used to address water resources and hydrologic questions.
The full model domain encompasses 109,565 km2, with approximately 79,000 km2 being land area and the remainder being surface water area within the Great Lakes. To facilitate the assessment of sensitivity to spatial resolution, low- and high-resolution versions of the model have been constructed,
with surface water features resolved down to Strahler order 4 and Strahler order 3, respectively. The hydrostratigraphy in the model is constructed using recently released three-dimensional Paleozoic and Quaternary geological models, along with mapping that depicts depth to the base of high-sulphur
content groundwater. In total, the respective low- and high-resolution models consist of 21 and 16 layers, and 874,398 and 2,127,760 3-dimensional finite element mesh nodes. Both models incorporate spatially distributed soil and landcover, and spatially and temporally distributed evapotranspiration.
To assess model performance, 321 wells from the Provincial Groundwater Water Monitoring Network (PGMN) and 29 hydrometric stations from the Water Survey of Canada (WSC) were incorporated as validation targets. Simulation results show that both the low- and high-resolution versions of the model were
able to capture the magnitude and seasonal variation in both groundwater levels and surface water flow rates. Given that the model was subjected to minimal calibration, this is a meaningful validation of its performance. Differences in performance between the low- and high-resolution versions were
minimal for both the surface water flow rates and groundwater level results. However, with its higher spatial resolution and greater density of surface water features, the high-resolution version can be expected to provide a better representation of localized hydrologic processes. Conversely, with
its faster run time, the low-resolution version will better facilitate large ensemble simulations, such as those associated with uncertainty or climate change analyses.
Results demonstrate the feasibility and utility of a regional scale, fully-integrated hydrologic model for investigating important aspects of water resources. The model could evolve into a multi-objective groundwater/surface-water simulation tool for southern Ontario, wherein seasonal water balances
and general trends in groundwater and surface water availability under climate change and anthropogenic influences within the Great Lakes Region can be quantitatively assessed.