Placement of Riparian Forest Buffers to Improve Water Quality

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Placement of Riparian Forest Buffers to Improve
Water Quality

• Mark Tomer1, Mike Dosskey2, Mike Burkart1, Dave
  James1, Matt Helmers3, and Dean Eisenhauer4

1USDA-ARS, 2USFS, Iowa St. Univ.3, Univ. of
Nebraska4
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Problem statement

• Riparian buffers offer opportunities to integrate
  agroforestry in agricultural watersheds.
• Riparian buffers can improve water quality.
• Mechanisms slowing water movement, encouraging
  infiltration, nutrient uptake/storage, water
  uptake, denitrification in shallow groundwater,
• However these benefits are difficult to establish
  at watershed scales.
• Buffers do not provide all these benefits at
  every riparian location.

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Objective

• If buffers function varies depending on location,
  then managers need techniques to identify
  locations where buffers will have the greatest
  water quality benefits.
• Our objective is to show several ways of
  identifying optimal locations for riparian
  buffers.

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Data sources / approaches

• Soil survey

• Terrain analysis

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Soil survey method

• Ranks soil map units for their capacity to trap
  sediment and allow water to infiltrate.
• Calculates a sediment factorSI D50 / R K L S
• Calculates an infiltration factorIF Ksat2 / R
  L S
• Relates these factors to VFSMOD output parameters
  that estimate buffer efficiency.

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Relationships of soil-survey factors with VFSMOD
output
Both efficiency factors assume a 24-hr, 2 yr
return event, and a 200 m slope length
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Interpretation is counter-intuitive!
Low STE indicates where sediment loads and load
reductions in buffer are greatest.
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Map of estimated parameters based on soil survey
(SSURGO) data
Sediment trapping efficiency (STE)
Water trapping efficiency (WTE)
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Maps based on soil survey (STATSGO) data
Sediment trapping efficiency (STE)
Water trapping efficiency (WTE)
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Map of STE at regional (MLRA) scale
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Mapped interpretations of a buffers capacity to
influence shallow groundwater or soil saturation.
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Note soil survey technique ranks soils for their
capacity to trap runoff and sediment, and
influence groundwater, as determined by soil
texture, hydric conditions, erodibility, slope,
and rainfall characteristics.
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Terrain analysis method

• Uses USGS national elevation data
• Similar in scale to soil survey
• Analysis provides images that reveal pathways of
  water movement, and areas of water accumulation

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Terrain Analysis method
Example map of contributing area
Wetness index Sediment transport
index As contributing area b slope
Moore et al. 1991, 1992
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Interpretation for riparian areas

• High Wetness Index
• Opportunity for runoff filtration
• Possibility of shallow groundwater
• Both benefits not assumed for all locations

• Sediment transport index
• Large values may indicate streambank
  stabilization needed
• Small values indicate probable areas of
  deposition

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Changes in NO3-N and total P concentrations
during a runoff event
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Discharge index
Riparian corridor analysis
dq Arc/Ac 1,000
Represents proportion of watershed area
contributed through an individual riparian
grid-cell. Related to capacity of riparian buffer
to measurably improve water quality in the stream.
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Iowas landform regions and study watersheds
Des Moines Lobe
Loess Hills
South Fork Iowa River
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Terrain indices differ along riparian corridors
according to stream order wetness index

Data from Keg and Silver Creeks in western Iowa
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Terrain indices differ along riparian corridors
according to stream order sediment transport
index

Data from Keg and Silver Creeks in western Iowa
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Buffers influence larger proportions of stream
water along low-order (headwater) streams
Data from Keg and Silver Creeks in western Iowa
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Result

• Opportunities for buffers to improve water
  quality are greatest along low-order (headwater)
  streams!

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Construction of stream-side maps (to identify
priority sites for buffers)

• Wetness and erosion indices extracted for cells
  adjacent to stream network.
• Series of large scale moving window maps
  constructed for field use.

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Conclusions

• Soil survey data can identifies map units most
  capable of trapping pollutants in surface runoff,
  or influencing groundwater.
• Terrain analyses identify where runoff and/or
  groundwater can most readily be intercepted by
  buffers.
• Two methods do not duplicate one another, and
  could be complimentary. Data sources have similar
  spatial scales.
• Detailed maps to assist conservation planning at
  the field scale can be constructed.
• Regional analyses are also possible using both
  methods. Data are broadly available.