USING GROUNDWATER MODELS TO FORECAST EFFECTS TO FENS IN HUMID TEMPERATE CLIMATES

Fen formation and persistence are by definition driven by characteristics of the groundwater system. Simulating fen-groundwater interactions, however, has not been well developed. In this work an existing regional groundwater model constructed for regional water supply was refined to relate changes in high-capacity groundwater pumping to changes in fen hydrology. The area of the study has multiple societal drivers that influence water use, including the Great Lakes Compact, a newly approved water diversion, and identified high-quality fen systems. The refinement included changes to the perimeter boundary conditions, stream and lake network, and groundwater recharge. To simulate fens, the Unsaturated Flow Package (UZF) for the USGS groundwater code MODFLOW was used. The UZF Package uses a computationally efficient kinematic wave formulation to simulate infiltration flow through the unsaturated zone. Less recognized, the UZF Package has the capability of handling the location and timing of groundwater discharge at the land surface.

Our work demonstrates that the intersection of the land surface and water table simulated with the UZF Package generally agreed with locations of identified fens. A UZF variable (SURFDEP) was used to tune this relation. Changes to the groundwater system from changes in pumping are easily handled by MODFLOW; how changes to the groundwater system translate to fen quality and persistence is less understood. Changes to hydroperiod were simulated using steady-state and transient approaches, which were evaluated for their ability to represent potential effects and suitability for communicating findings to stakeholders.