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THE HYDROGEOMORPHIC APPROACH TO FUNCTIONAL ASSESSMENT FOR PIEDMONT SLOPE WETLANDS: 

THE HYDROGEOMORPHIC APPROACH TO FUNCTIONAL ASSESSMENT FOR PIEDMONT SLOPE WETLANDS B. Vasilas, UD; L. Vasilas, NRCS; M. Wilson, NRCS

Acknowledgements: 

Acknowledgements Funding provided by EPA, MDE, NRCS, ACOE, and FHA.

Outline: 

Outline Introduction to HGM Hydrology of slope wetlands Model variables

HGM Approach: 

HGM Approach Procedure designed to assess the capacity of a wetland to perform functions. Functions: biological, chemical, and physical processes (e.g. water storage)

HGM Approach: 

HGM Approach Wetland classification Site selection Model development Identification/selection of functions Data collection Generate variables Calibrate

Basic Assumption to HGM: 

Basic Assumption to HGM “…the highest, sustainable functional capacity is achieved in wetland ecosystems and landscapes that have not been subject to long-term anthropogenic disturbance.”

Reference Wetlands: 

Reference Wetlands Data collection sites for model development Represent a range in anthropogenic disturbance

Reference Standard Wetlands: 

Reference Standard Wetlands Subset of reference wetlands Exhibit the least anthropogenic disturbance Represent the highest functional capacity

Model Development Variables: 

Model Development Variables Simple variables-presence of a surface flow outlet Complex variables-water chemistry Temporal variables-soil Eh

“User Friendly” Variables: 

“User Friendly” Variables Visual or easily measured No temporal restrictions Correlated to a quantitative measure of an attribute

HGM Approach: 

HGM Approach Function Process Attribute Variable Nutrient cycling Denitrification Organic carbon Leaf litter

Hydrologic Characteristics: 

Hydrologic Characteristics Hydrologic Source: Groundwater discharge Toeslope seeps Sideslope seeps Hydrodynamics One directional (downslope) Low-medium energy

Groundwater Driven: 

Groundwater Driven High water quality Uniform inputs Buffered

Hydroperiod Classification: 

Hydroperiod Classification Seasonally saturated Permanently saturated Permanently inundated

Retention Time: 

Retention Time Slope Surface roughness Connectivity

Piedmont Slope Functions: 

Piedmont Slope Functions Provide characteristic wildlife habitat Carbon export Temporary water storage Particulate retention Removal of pollutants Nutrient cycling

Hydrologic Source Variable: 

Hydrologic Source Variable Condition of catchment area Size Land use Disturbance

Function: Nutrient Cycling: 

Function: Nutrient Cycling Process: Microbial transformation Wetland attributes: Hydrologic source condition Organic carbon (energy) Aerobic/anaerobic fluctuations

Function: Nutrient Cycling: 

Function: Nutrient Cycling Variables: Carbon (available vs. unavailable) Soil organic matter Woody debris Leaf litter Herbaceous groundcover (roots) Aerobic/Anaerobic fluctuations Hydroperiod (temporal) Microtopography (spatial)

Hydroperiod Variables: 

Hydroperiod Variables Soil Presence/thickness of O horizons Color/thickness of A horizons Depth to redox features Plants Species Strata

Summary: 

Summary Piedmont slope wetlands show sig. variability in hydroperiods. Variability due to position of groundwater discharge sites; as opposed to disturbance. Variability sig. impacts functional capacity (esp. nutrient cycling).

Function: Temporary Water Storage: 

Function: Temporary Water Storage Processes: Hydrologic inputs/outputs Attributes: Hydrologic source condition Slope Surface area Microtopography Connectivity

Function: Removal of Pollutants: 

Function: Removal of Pollutants Process: Sequestration Attributes: SOM accretion Plant biomass

Function: Removal of Pollutants: 

Function: Removal of Pollutants Process: Sorption to soil particles Attributes: Hydrologic source condition Retention time Infiltration High cation exchange capacity

Funtion: Removal of Pollutants: 

Funtion: Removal of Pollutants Variable: Infiltration Slope Microtopography Herbaceous cover Soil porosity (texture) CEC Organic matter content Clay content (texture)

HGM Model Development: 

HGM Model Development Reference domain: Reference standard sites

Functional Assessment: 

Functional Assessment Quantify the functional capacity of individual wetlands. Functional capacity: the degree to which a function is performed. Functional capacity is judged relative to a reference standard.

Functional Assessment-Why?: 

Functional Assessment-Why? Evaluation of wetland quality for Federal mandates Evaluation of anthropogenic impacts Evaluation for mitigation purposes (compensation “in kind”) Site selection for wetland enhancement Identification of environmentally-sensitive areas

Wetland Functions: 

Wetland Functions Definition: biological, chemical, and physical processes that occur in wetlands Examples N removal through denitrification Surface water storage Soil organic matter accretion

Limitations: 

Limitations Model development is labor intensive. Maximum index value limited by “pristine sites”.

Strengths: 

Strengths Regionalized Specific to a subclass Attributes easily and quickly measured Surrounding land use considered

Surrounding Land Use: 

Surrounding Land Use Connectivity to other wetlands-wildlife Agricultural-sediment and nutrient loading Development-hydrologic inputs

HGM Functional Categories: 

HGM Functional Categories Hydrology Biogeochemical cycling Plant community Wildlife habitat

Water Variables: 

Water Variables Quantity Quality Residence time

Function: Carbon Export: 

Function: Carbon Export Processes: Organic carbon production Carbon transport (surface flow) Attributes: Carbon production Carbon transport

Function: Carbon Export: 

Function: Carbon Export Variables: Carbon production Woody debris Leaf litter Herbaceous cover Soil organic matter Carbon transport Slope Channelization Connectivity

Function: Particulate Retention: 

Function: Particulate Retention Process: Sedimentation Physical Attributes:  water Retention time = ↓ water velocity Variables: Slope Surface roughness Microtopography Herbaceous cover