THE HYDROGEOMORPHIC APPROACH TO FUNCTIONAL

ASSESSMENT FOR PIEDMONT SLOPE WETLANDS

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

Acknowledgements

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

Outline

• Introduction to HGM

• Hydrology of slope wetlands

• Model variables

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

• Wetland classification

• Site selection

• Model development– Identification/selection of functions– Data collection– Generate variables– Calibrate

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

• Data collection sites for model development

• Represent a range in anthropogenic disturbance

Reference Standard Wetlands

• Subset of reference wetlands

• Exhibit the least anthropogenic disturbance

• Represent the highest functional capacity

Model Development Variables

• Simple variables-presence of a surface flow outlet

• Complex variables-water chemistry

• Temporal variables-soil Eh

“User Friendly” Variables

• Visual or easily measured

• No temporal restrictions

• Correlated to a quantitative measure of an

attribute

HGM Approach

• Function• Process• Attribute• Variable

• Nutrient cycling• Denitrification• Organic carbon• Leaf litter

Hydrologic Characteristics

• Hydrologic Source: Groundwater discharge– Toeslope seeps– Sideslope seeps

• Hydrodynamics– One directional (downslope)– Low-medium energy

Groundwater Driven

• High water quality

• Uniform inputs

• Buffered

Hydroperiod Classification

• Seasonally saturated

• Permanently saturated

• Permanently inundated

Retention Time

• Slope

• Surface roughness

• Connectivity

Piedmont Slope Functions

• Provide characteristic wildlife habitat

• Carbon export

• Temporary water storage

• Particulate retention

• Removal of pollutants

• Nutrient cycling

Hydrologic Source Variable

Condition of catchment area– Size– Land use– Disturbance

Function: Nutrient Cycling

• Process: Microbial transformation

• Wetland attributes:– Hydrologic source condition– Organic carbon (energy)– Aerobic/anaerobic fluctuations

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

• Soil– Presence/thickness of O horizons– Color/thickness of A horizons– Depth to redox features

• Plants– Species– Strata

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

• Processes: Hydrologic inputs/outputs

• Attributes:– Hydrologic source condition– Slope– Surface area– Microtopography– Connectivity

Function: Removal of Pollutants

• Process: Sequestration

• Attributes:– SOM accretion– Plant biomass

Function: Removal of Pollutants

• Process: Sorption to soil particles

• Attributes:– Hydrologic source condition– Retention time– Infiltration– High cation exchange capacity

Funtion: Removal of Pollutants

• Variable:– Infiltration

• Slope• Microtopography• Herbaceous cover• Soil porosity (texture)

– CEC• Organic matter content• Clay content (texture)

HGM Model Development

• Reference domain:

• Reference standard sites

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?

• 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

• Definition: biological, chemical, and physical processes that occur in wetlands

• Examples– N removal through denitrification– Surface water storage– Soil organic matter accretion

Limitations

• Model development is labor intensive.

• Maximum index value limited by “pristine sites”.

Strengths

• Regionalized

• Specific to a subclass

• Attributes easily and quickly measured

• Surrounding land use considered

Surrounding Land Use

• Connectivity to other wetlands-wildlife

• Agricultural-sediment and nutrient loading

• Development-hydrologic inputs

HGM Functional Categories

• Hydrology

• Biogeochemical cycling

• Plant community

• Wildlife habitat

Water Variables

• Quantity

• Quality

• Residence time

Function: Carbon Export

• Processes:– Organic carbon production– Carbon transport (surface flow)

• Attributes:– Carbon production– Carbon transport

Function: Carbon Export

• Variables:– Carbon production

• Woody debris• Leaf litter• Herbaceous cover• Soil organic matter

– Carbon transport• Slope• Channelization• Connectivity

Function: Particulate Retention

• Process: Sedimentation

• Physical Attributes: water– Retention time = ↓ water velocity

• Variables:– Slope– Surface roughness

• Microtopography• Herbaceous cover