Hydrologic Response Caused by Wetland Expansion at Huntley Meadows Park in Hybla Valley, Virginia
Creators: name: Stone, Stephen Fraser
Publisher: name: Old Dominion University Libraries
Year: 2017

description: The goal of this study was to understand the effects of wetland expansion across a watershed. The 2013 restoration and expansion of the wetlands at Huntley Meadows Park (Fairfax County, VA) performed by Wetland Studies and Solutions, Inc. provided the opportunity to study this process. The 630 ha park contains more than 364 ha of freshwater emergent and freshwater forested wetlands. The restoration and expansion project used a subsurface vinyl-piling dam that impedes groundwater flow leaving the wetland, thus expanding the existing pond and the surrounding wetland.

This study used a network of more than twenty monitoring instruments making observations of hydrologic and weather data, along with soils maps and soil borings, and observations of vegetation provided by scientists from Virginia Tech. Data from these sources were used to characterize the hydrologic drivers and responses throughout the area before and after wetland expansion for the purpose of developing wetland water budgets within Wetbud to model the effects of wetland expansion. Observations of water table elevations made throughout the park indicated the water levels in the pond at Huntley Meadows Park are not strongly influenced by regional groundwater flow. However, observations of diurnal fluctuations of the water table at monitoring wells located in emergent and forested/shrub wetlands revealed that spatial variations in actual evapotranspiration (AET) rates strongly influence the distribution of water throughout the park. The effects of AET are strong enough to induce a seasonal reversal in hydraulic gradients where water table elevations surrounding the pond are greater than the pond during the winter months and lower than the pond during the growing season.

Wetland expansion during the study initiated changes in vegetation and hydrology. To model the potential effects of AET that may change due to expansion, monthly crop coefficients (Kc) were developed with reference ET coming from the Reagan National Airport NOAA weather station and AET coming from diurnal fluctuations of the water table analyzed with a MATLAB-adapted version of White’s Method. Monthly predictions of head within the wetland and outflow through the weir, modeled using Wetbud’s Basic Scenario tools, were improved when area-weighted Kc values were applied to the model. Additionally, daily predictions of head, made using Wetbud’s Advanced Scenarios tools (a graphical user interface for USGS MODFLOW), were improved when spatially appropriate Kc values were applied to the model. However, when modeling differences in the distribution of plant communities from two consecutive years during the transitional period, there was little difference in predicted head values. Based on the differences in observed AET rates between emergent and forested/shrub wetlands, we suspect models of expanded wetlands that have had sufficient time to fully transition from the pre-construction distribution of plant communities to the design-intended distribution will require different distributions of crop coefficients and corresponding evapotranspiration rates in those models in order to accurately predict water levels for the design-intended distribution of plant communities.