A Comparison of Soil Wetness by Morphological and Modeling Methods

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A Comparison of Soil Wetness by Morphological
and Modeling Methods

• D. L. Lindbo, E. Severson,
• M. J. Vepraskas, and X. He
• Soil Science Dept.
• North Carolina State University

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Acknowledgements

• USEPA
• USDA-NRCS
• Water Resources Research Institute
• USACOE
• EPA 319(h) program
• NCDENR, DEH-OSWW Section
• The Nature Conservancy
• Land Owners

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How to determine the water table?

• Soil morphology
• Mottles
• lt 2 Chroma
• Redoximorphic features
• Monitoring

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An underlying question

• Should the water table (soil wetness) determined
  by morphology be at the same depth as the water
  table (soil wetness) determined by
  monitoring/modeling?

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Methods of Determining Wetness
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Redox Concentration (Pore lining)
1. Plant root grows into soil
5. Reduced Fe moves away from decomposing
root Reduced Fe oxidizes, soil turns red
2. Root dies and starts to decompose
3. Water table rises
4. Bacteria continue to decompose root Oxygen
reduced Nitrate reduced Fe reduced and removed,
soil turns gray
6. Water drains from root channel
7. Root completely decomposed
8. Water table drops
Redox depletion
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Redox concentrations
WT 4
4 chroma depletion
WT 3
3 chroma depletion
WT 2
WT 1
lt 2 chroma depletion
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Site locations
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Typical Transect
Redox and Temperature Probes
Recording Wells
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Morphology
Mottles
Redoximorphic features
3 4 chroma depletions
lt 2 chroma
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Morphology
For this discussion only Redoximorphic features
Depletions (all) lt2 chroma will be used for
comparison to monitoring methods
Redoximorphic features
Depletions (all)
lt 2 chroma
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10 36 days 15 54 days 20 72 days
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Water table by morphologyGeneralizations

• WT by mottles is shallowest
• WT by lt 2 chroma depletions is deepest
• WT by any redoximorphic depletions is shallower
  than WT by lt 2 chroma depletions
• WT by any redoximorphic feature is shallower
  than WT by lt 2 chroma depletions or any redox
  depletions

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Monitoring Methodologies

• Maximum
• Average of wet season
• 14-day Method
• Weighted Rainfall Index (WRI)
• Threshold
• Calibration

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Maximum and Average
Maximum
Average
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The 14-day Continuous Saturation Method

• Daily water table readings
• On-site rainfall
• Full soil and site evaluation
• WETTS Data for nearest weather station
• Monitor during wet season (January April)

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Interpretation of Rainfall Data
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30-day cummulative
Acceptable zone
Acceptable zone
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14
12
30th percentile
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Rainfall (cm)
8
6
4
2
0
1/15/03
1/29/03
2/12/03
2/26/03
3/11/03
3/25/03
4/22/03
5/20/03
1/1/03
4/8/03
5/6/03
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14 days saturation
0
-10
-20
Water table
-30
Depth
-40
-50
-60
-70
-80
1/1
4/8
5/6
1/15
1/29
2/12
2/26
3/11
3/25
4/22
5/20
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Weighted Rainfall Index

• Determine the 5 month (Dec. April) cumulative
  rainfall (from closest 30 year record) for
  location at the 30th, 50th, 70th, 80th
  percentile. (Maps provided by OSWW)
• Record daily water table depth from January to
  May
• Record daily rainfall from December to May

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Weighted Rainfall Index

• Calculate 5 month cumulative rainfall for site
• 0.5Pdec Pjan Pfeb Pmar Papr 0.5Pmay
• Determine what percentile current rainfall is in
  for the given site using maps generated by OSWW
• Determine number of day (hours) of saturation
  that will be equivalent to WT

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Rainfall Index Maps for the 80th 100th
percentile
18.5
Rainfall at site 20
Therefore the rainfall at site was between the
80th and 100th percentile.
If the rainfall was lower than 19 then the site
would be in a lower rainfall percentile.
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19.5
21
20
20.5
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Weighted Rainfall Index
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Interpretation of Weighted Rainfall Index Method

• In practice, only 1 monitored season is required
  for this method.
• For this study
• Required duration is attained for a given year
  based on its rainfall (source nearest weather
  station),
• then historic water levels were determined for
  each usable year based on the calibrated models.

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Threshold

• 40 year simulation using long-term rainfall data
  from closest site
• Adjust DRAINMOD to find the parameters (ksat,
  drain spacing etc.) at which 12 years (30) have
  one 14 day period of saturation above XX
  centimeters

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Threshold

• Compare the DRAINMOD simulation, using current
  on-site rainfall data, to the on-site measured
  hydrograph
• If the threshold simulated hydrograph (TSH) is
  lower than the measured hydrograph (MH) then the
  site is wetter than that threshold level

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Threshold Simulation
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Calibration

• Use local rainfall and wet season (Jan.- April)
  measured hydrograph
• Adjust DRAINMOD parameters (primarily drain
  spacing) until the standard error between the
  measured hydrograph (MH) and simulated hydrograph
  (SH) is minimized (should be lt 20 cm)
• Run a 40 year DRAINMOD simulation using long-term
  rainfall data from closest site

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Model Calibration
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Calibration

• Query DRAINMOD for the depth that is exceeded for
  14 consecutive days or longer in 12 out of 40
  years
• 12 out of 40 years indicates that 30 of the
  years the water table rises above that depth for
  a period of 14 consecutive days or longer

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Lenoir Series
Maximum
WRI-High
6
All Redox
Threshold
14-day
12
Calibration
Depletions
WRI-Low
lt 2 chroma
Ave. of wet
18
24
30
36
105
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Noboco Series
6
Maximum
12
Threshold
18
14-day
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Calibration
Ave. of wet
30
All Redox
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Depletions
42
lt 2 chroma
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Goldsboro Series
Maximum
6
Ave. of wet
Threshold
12
Calibration
18
14-day
All Redox
Depletions
24
lt 2 chroma
30
36
42
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Foreston Series
Maximum
6
WRI-Low
12
Threshold
All Redox
18
14-day
Calibration
Depletions
WRI-Low
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Ave. of wet
lt 2 chroma
30
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Ortega Series
6
12
18
Maximum
All Redox
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Calibration
14-day
Depletions
30
Threshold
Ave. of wet
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lt 2 chroma
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Summary

• Redoximorphic features indicate a water table
  shallower than lt2 chroma features
• Monitoring only and monitoring and modeling are
  more conservative than morphology
• Monitoring/modeling methods have greater
  variability than morphologic methods
• The 14-day and calibration methods agree best
  with redoximorphic features

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Conclusions

• Whatever method or evaluation protocol is used it
  MUST correlate to the use of morphology on
  unaltered sites. In other words it must be
  scientifically valid.
• Determining the water table ignores the question
  How long can a system remain saturated and still
  treat the wastewater? Only when this question
  is answered can we truly begin to determine the
  water table for system design
• Whatever the method and evaluation protocol it
  should be relatively simple and inexpensive to
  facilitate its use. (OPINION)

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WE JUST NEED MORE DATA