THE HYDROGEOMORPHIC APPROACH TO FUNCTIONAL ASSESSMENT FOR PIEDMONT SLOPE WETLANDS

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THE HYDROGEOMORPHIC APPROACH TO FUNCTIONAL
ASSESSMENT FOR PIEDMONT SLOPE WETLANDS

• B. Vasilas, UD L. Vasilas, NRCS M. Wilson, NRCS

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Acknowledgements

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

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Outline

• Introduction to HGM
• Hydrology of slope wetlands
• Model variables

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HGM Approach

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

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HGM Approach

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

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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.

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Reference Wetlands

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

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Reference Standard Wetlands

• Subset of reference wetlands
• Exhibit the least anthropogenic disturbance
• Represent the highest functional capacity

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Model Development Variables

• Simple variables-presence of a surface flow
  outlet
• Complex variables-water chemistry
• Temporal variables-soil Eh

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User Friendly Variables

• Visual or easily measured
• No temporal restrictions
• Correlated to a quantitative measure of an
• attribute

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HGM Approach

• Function
• Process
• Attribute
• Variable

• Nutrient cycling
• Denitrification
• Organic carbon
• Leaf litter

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Hydrologic Characteristics

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

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Groundwater Driven

• High water quality
• Uniform inputs
• Buffered

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Hydroperiod Classification

• Seasonally saturated
• Permanently saturated
• Permanently inundated

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Retention Time

• Slope
• Surface roughness
• Connectivity

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Piedmont Slope Functions

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

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Hydrologic Source Variable

• Condition of catchment area
• Size
• Land use
• Disturbance

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Function Nutrient Cycling

• Process Microbial transformation
• Wetland attributes
• Hydrologic source condition
• Organic carbon (energy)
• Aerobic/anaerobic fluctuations

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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)

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Hydroperiod Variables

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

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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).

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Function Temporary Water Storage

• Processes Hydrologic inputs/outputs
• Attributes
• Hydrologic source condition
• Slope
• Surface area
• Microtopography
• Connectivity

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Function Removal of Pollutants

• Process Sequestration
• Attributes
• SOM accretion
• Plant biomass

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Function Removal of Pollutants

• Process Sorption to soil particles
• Attributes
• Hydrologic source condition
• Retention time
• Infiltration
• High cation exchange capacity

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Funtion Removal of Pollutants

• Variable
• Infiltration
• Slope
• Microtopography
• Herbaceous cover
• Soil porosity (texture)
• CEC
• Organic matter content
• Clay content (texture)

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HGM Model Development

• Reference domain
• Reference standard sites

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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.

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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

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Wetland Functions

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

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Limitations

• Model development is labor intensive.
• Maximum index value limited by pristine sites.

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Strengths

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

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Surrounding Land Use

• Connectivity to other wetlands-wildlife
• Agricultural-sediment and nutrient loading
• Development-hydrologic inputs

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HGM Functional Categories

• Hydrology
• Biogeochemical cycling
• Plant community
• Wildlife habitat

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Water Variables

• Quantity
• Quality
• Residence time

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Function Carbon Export

• Processes
• Organic carbon production
• Carbon transport (surface flow)
• Attributes
• Carbon production
• Carbon transport

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Function Carbon Export

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

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Function Particulate Retention

• Process Sedimentation
• Physical Attributes
• ? water
• Retention time ? water velocity
• Variables
• Slope
• Surface roughness
• Microtopography
• Herbaceous cover