PS 18-170 - Environmental Change Onehealth Observatory (ECO²): Quantifying the role of groundwater in sustaining ecosystem services in a mixed-use humid continental climate watershed, Ontario, Canada

Background/Question/Methods

The role of groundwater in sustaining ecosystem services (ES) is often overlooked. Traditional approaches towards modelling water related ES focus only on the surface water system, with groundwater not included in the model framework. Such limitation poses a challenge when assessing ES in regions where groundwater plays a large role in governing hydrologic behavior. For instance, in Southern Ontario groundwater contributes significantly to surface water, with some areas attributing over 80% of stream flow to groundwater discharge. Groundwater also supports wetland function and plant water demands across the region, and under dry climatological conditions groundwater supports critical ecosystem function. In order to more fully capture the influence of groundwater when conducting ES assessments, it is imperative to develop modelling tools that incorporate groundwater processes. Fully-integrated groundwater – surface-water models intrinsically account for groundwater. In the study herein, the HydroGeoSphere (HGS) fully-integrated groundwater – surface-water model was used to evaluate groundwater’s role in supporting ES in the South Nation watershed (SNW) in Southeastern Ontario over the 2003-to-2017-time interval. The SNW encompasses ~4000 km² of mixed-use land cover, of which cropland, forest, grassland, urban, and wetland make up 38 %, 29 %, 20 %, 7 %, 5 %, respectively.

Results/Conclusions

The HGS model constructed for the SNW incorporated seven layers (soil and underlying aquifer and aquitard units). Model performance evaluation consisted of spatially distributed and transient surface water flow and groundwater level reproduction over 2003 -2017. Simulation output included transient total volume of water stored in the soil system (top 1 m), groundwater system, and overland/wetland system, as well as evapotranspiration. Of the three water storage compartments, groundwater exhibited the widest range of storage change over the simulation interval, with annual fluctuations on the order of 5x10⁹ m³, whereas soil and overland/wetland storage exhibited storage variations on the order of 5x10⁸ m³ and 1x10⁸ m³, respectively. Over the 15-year simulation interval, the largest variation in water storage occurred during a drought year, where groundwater, soil, and overland/wetland were all at their lowest, and where groundwater declined by almost 1x10¹⁰ m³. The excessive groundwater storage decline was due to groundwater discharging into the overland/wetland system as well as transmitting into the shallow soil system where it contributed to plant water needs. This is the first study of its kind to utilize a fully-integrated groundwater – surface-water model to quantify the dynamic role that groundwater plays in ES.