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Using Soil Attributes for HGM Wetland Classification

Richard Weber Wetland Hydraulic Engineer Wetland Team (Ret.) CNTSC, Fort Worth, TX

Data Sources:

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• Web Soil Survey • County Scale • Smaller than Reference Domain • Pre-built Interpretations

• Geospatial Data Gateway • Statewide Data • Raster and Vector • Larger than Reference Domain • User Selected Interpretations • MLRA Attribute Selection

“crunches” an entire State’s data

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Major Land Resource Areas

GIS Layer – Geospatial Data Gateway

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The Reference Domain

The area within which a defined Subclass Exists

Major Land Resource Area is a Good Default

MLRA Subdivisions may Exist for Further Refinement

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Map Unit Polygons – Also Occupy Landscape Positons in the Watershed

SSURGO Geomorphic Descriptions are Useful, i.e.: • Upland Drainageway • Terrace • Floodplain • Depression • Many, many others used

Hydrologic Soil Group - HSG

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Developed ONLY for use to determine Runoff Curve Number (RCN) – NRCS RCN Method • HSG – A, B, C, or D • A – Low runoff (think sponge) • Low Runoff infers high recharge • D – High Runoff (think Duck’s back) • High Runoff infers low recharge

• Important in assessing wetland watershed

• Q: Is a watershed of HSG A or B soils likely

to support Discharge Wetlands? • HSG is in both WSS and SSURGO data

Slope Class

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• Soil Map Units have Slope Class Data • Only Available in SSURGO • Directly Applicable to Mineral Flats (0-2%)

Willamette Valley, Oregon Wet Prairie (Tufted Hairgrass Provides Microtopography)

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Water Features •WSS Provides Flooding, Ponding, and Groundwater Frequency and Duration by Month – Quickly •SSURGO requires user to access multiple tables, BUT more detail is available •Flooding – Lotic – RIVERINE AND SLOPE Classes •Ponding – Lentic – DEPRESSION, and Depressions in RIVERINE

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Taxonomy: Use With Care: • Orders:

• Histosols – ORGANIC FLATS and discharge sites like SLOPE and Discharge DEPRESSION

• Require near continuous saturation to form and maintain

• Entisols – Young Soils like floodplains • Suborders:

• Aquic (wet), Fluvic (water formed) • Great Groups

• Includes Minerology – Calcic indicates discharge • Sub-Groups • Available in both WSS and SSURGO • Lack of evidence in the taxononomy does NOT mean

absence of the attribute • Presence of evidence in the name is more useful

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“Endo”saturated and “Epi”saturated Great Groups: Use With Care: • Endosaturated soils indicate Discharge • Episaturated soils indicate Recharge • Definition is depth of saturation down to 200

cm – Not meant to be a hydrodynamic interp.

• Typical Episaturated wetlands: Episaturated RIVERINE, Recharge DEPRESSION, MINERAL FLAT

• Typical Endosaturated wetlands: SLOPE, Discharge DEPRESSION, Endosaturated RIVERINE

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Minerals and Great Groups

• Groundwater Dissolved Minerals • At point of Discharge, ET leaves mineral

concentrations behind • Best Example is Calcium (calcic), ex:

“calciaquolls” • Horizonation - Bk

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Saturated Hydraulic Conductivity (Ksat) Function of: • Texture • Structure • Bulk Density • Others Increase in Ksat with Depth Indicates Vertical Downward Movement (Recharge) • Recharge Wetlands – MINERAL FLATS,

Recharge DEPRESSIONS • Episaturated RIVERINE • Ex. – Bt horizons

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Other Useful Soil Attributes from SSURGO:

• Geomorphic Description • Drainage Class • Slope Class • Taxonomy • Horizons • Water Features

• Flooding Freq. and Dur. • Ponding Freq. and Dur. • Groundwater Depths

Useful Database Tables: • Component • Map Unit • Map Unit Aggregated

Attributes (muaggat)

Digital Elevation Data Can be used also to Aggregate and Dis-Aggregate Map Units Beyond our Scope Today…

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Selection by Attributes •SSURGO Vector Polygons •Site or HUC-12 Scale •“Heads-up” Mapping

Red • Calcareous Wet Prairie • MINERAL FLAT Green • Calcareous Marsh • DEPRESSION

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HUC -12 Scale •Large Enough to Display all HGM subclasses •Small Enough to Visualize

Floodplain Landscape

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RIVERINE ATTRIBUTES

• Geomorphic Descrition – “floodplain” term • Water Features – “Flooding” means Lotic

inundation • Flood Frequency – higher than “Rarely” • RIVERINE Water Features can include Flooding,

Ponding, and Groundwater – all 3

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RIVERINE Soil System – Multiple HGM Sub-Classes • Look for more geomorphic description detail –

Ex. – “Backswamps, Natural Levees, Oxbows on Floodplains”

• Look for more Water Features. Ex. – “Ponding” means it’s a backswamp

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Problem! – Stream Functions Vary with Scale

2nd Order Kennebec

5th Order Kennebec

Map Unit Dis-Aggregation

SLOPE – Headwater Reaches

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• Look for: • High Water Table • Histosols or Histic

Epipedon Horizon • Non-Floodplain

geomorph (i.e. Upland Drainageways)

• No decrease in Ksat with Depth

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Landscape Functional Break: SLOPE to RIVERINE HGM Class

Blue – Ackmore Colo Complex, Endoaqolls, Upland Drainageways, WTD < 12” – SLOPE Class

Green – Floodplains

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SLOPE Wetlands – Watershed Reaches Above Floodplains

Attributes: • Taxonomy – “endosaturated” Great Group • Taxonony – Histosol or Histic • Geomorphic Description – Ex. “upland

drainageways” • Water Features – WTD < 12”, most months

Kansas Headwater

Idaho Headwater Fen

New York Headwater Fen

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Nebraska Rainwater Basin – Recharge DEPRESSION

Wyoming – Recharge DEPRESSION, Gillette

South Dakota Prairie Pothole – Recharge DEPRESSION South Carolina – Carolina Bay

Discharge DEPRESSION

DEPRESSION CLASS

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DEPRESSION - Recharge

• Horizonation – Perching Layer. Look for Bt horizon

• Drastic decrease in Ksat with depth

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DEPRESSION - Discharge

No perching layer

Taxonomy Ex. – Calcic subgroup

Taxonomy – Histosols or Histic

Recharge Go > GI Discharge Gi > Go Flow Through Gi = Go

High Infiltration Uplands Support Strong Discharge Wetlands

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MINERAL FLAT and DEPRESSION Brown – MINERAL FLATS • Interfluves (geomorphic desc.) • 0-2% Slope Class • Poorly Drained • Also – Decrease in Ksat with

Depth • Water Features – no ponding, no

flooding, only WTD < 12”

Yellow – DEPRESSION • Depressions (geomorphic desc.) • Ponding Frequency and

Duration • No Flooding

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Defining Landscape Ecological Function with the use of Hydrodynamic Vectors

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Wetland sub-classes: • Have a System Boundary • Same Water Budget within the boundary • Same Ecological Functions within the

boundary • Same Plant Community • Same Maintenance Processes • Same Response to Disturbance • Same Management Prescriptions • Same Soil Attributes

Hydrology Informed Wetland Classification

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Ecological Site Descriptions • Starts with Plant Community • States in Transition Driven by Disturbance Hydrogeomorphic (HGM) Landscape Classification • Landscape and Hydrology Based • Originally Developed for Wetland Functional

Assessment Cooperative Soil Survey • Mapping Driven by Landscape and Hydrologic

Factors

Existing Landscape Classification Concepts

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Tool for Wetland Definition Using Soils and Hydrology - Needs

• Simple • Qualitative to Quantitative • Translates to Spatial Mapping and

Modeling • Provides Visual Representation • Uses Existing Soils Database • Applies to all landscapes • One-dimensional to three-dimensional

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Nebraska Sandhills

Indiana Flatwoods

High Plains Colorado

Prairie Pothole Region

Uplands (Interfluves)

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Wet Interfluves- Mineral Flats

• < 1 acre Drainage Area • WT < 12” • Feeds Discharge Sites

Delmarva Peninsula

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Moist Upland Sites

Headwaters

Utah

Idaho Panhandle

Iowa Des Moines Lobe

Nebraska Sandhills

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Losing Watershed Reaches NOT A WETLAND

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Wet Headwaters

Boundary County, ID

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Groundwater Supply is increased with HSG A, B Soils

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Closed Recharge Depressions

Prairie Potholes

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Closed Recharge Depressions and Wet Headwaters

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Landscape Definitions Vector Parameters – Magnitude and Direction • Runoff • Runon • Recharge • Discharge Apply to Landforms: • Watershed Elements

– Interfluves – Headwaters – Stream Corridors

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Jasper County, IA • Brown –

Recharge/Discharge Mineral Flat

• Green – Runon/Discharge Depression

• Blue – Runon/Recharge/Discharge Headwater

• Yellow – Runon/Runoff/Discharge Floodplain

Questions?

Wet Headwater – Kansas Flint Hills photo: Jon Fripp

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Thanks!

NRCS is an Equal Opportunity Employer and Provider

Questions?

Wetland HydrologyTechnical Note: “Technical Note No. 3 - Soil Hydrodynamic Interpretations for Wetlands” http://www.nrcs.usda.gov/wps/portal/ nrcs/main/national/water/wetlands/restore/