Modeling Hydrology

1
Modeling Hydrology

• Adapted from
• Dr. R. O. Evans, NCSU

NC STATE UNIVERSITY DEPARTMENT of SOIL SCIENCE
NC STATE UNIVERSITY DEPARTMENT of SOIL SCIENCE
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Example of Hydrograph
0
-10
-20
-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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NC STATE UNIVERSITY DEPARTMENT of SOIL SCIENCE
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Methods for Interpretation

• Modeling
• Model Calibration Method

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NC STATE UNIVERSITY DEPARTMENT of SOIL SCIENCE
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What is the hydrologic definition of a hydric
soil?
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NC STATE UNIVERSITY DEPARTMENT of SOIL SCIENCE
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Wetland hydrology occurs when

• the water table is at or near the surface (12
  inches or 30 cm) and,
• present for at least 5 percent (consecutive) of
  the growing season.
• in 50 out of 100 years

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NC STATE UNIVERSITY DEPARTMENT of SOIL SCIENCE
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Determining Wetland Hydrology
Month
1
2
3
4
5
6
7
8
9
10
11
12
0
6
12
18
Depth (in.)
24
30
36
42
48
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So based on this hydrograph is wetland hydrology
met?
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NC STATE UNIVERSITY DEPARTMENT of SOIL SCIENCE
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Hydrologic Models
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Hydrologic Models

• DRAINMOD
• Modflo
• Hydrus 2
• Colorado State

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DRAINMOD

• DRAINMOD (Skaggs, 1978)
• DRAINMOD Creams (Parsons, 1989)
• FLDNSTRM (Konyha and Skaggs, 1992)
• ADAPT (Chung et al., 1992)
• DRAINMOD-N (Breve et al., 1992)
• DRAINMOD-S (Kandil et al., 1992)

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NC STATE UNIVERSITY DEPARTMENT of SOIL SCIENCE
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Uses of DRAINMOD

• Hydrology of altered sites
• Is wetland hydrology being met?
• Use to test management methods for restoration
• Surface roughness
• Ditch spacing
• Use of berms
• Correlate indicators to hydrology
• Correlate morphology to hydrology
• Correlate redox to hydrology

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Rainfall
ET
Interception
Run-off
Run-On
Infiltration
Surface Storage
Water Table
Subsurface Flow
Seepage
Natural Forested Wetland or Pocosin
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DRAINMOD (Skaggs, 1978)

• Water balance on hour by hour - day by day
• as influenced by
• Weather
• Soil Properties
• Site and Landscape conditions
• Vegetation
• Predict/Simulate Hydrology

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Water Balance Analysis
Control Volume
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Rainfall
Evapotranspiration
Surface Runoff
Infiltration
Surface Storage
Water Table
Drainage
Ditch
Drain Tube
Seepage
Restrictive Layer
DRAINMOD Hydrology
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DRAINMOD was designed for this area
ET or Rainfall
Water Table
Lateral Seepage
Restrictive Layer
Vertical Seepage
Aqitard
Groundwater Aquifier with Constant Head
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NC STATE UNIVERSITY DEPARTMENT of SOIL SCIENCE
NC STATE UNIVERSITY DEPARTMENT of SOIL SCIENCE
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DRAINMOD can be used in this area to determine
wetland hydrology
ET or Rainfall
Water Table
Lateral Seepage
Restrictive Layer
Vertical Seepage
Aqitard
Groundwater Aquifier with Constant Head
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Detailed site specific inputs - overview

• Climate Data
• Drainage Data
• Soil Data
• Crop Data (if applicable)
• Not all parameters need to be measured, some can
  be estimated.

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Climate Data

• Long term data from nearest weather station
• PET supplied in DRAINMOD for NC
• In other locations PET can be estimated using
  temperature and the Thornwaite Method

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Soil Data

• Moisture release curve data for surface horizon
• Drainable porosity versus depth
• Upward flux
• Infiltration rate
• Hydraulic conductivity versus depth
• Depth to restrictive horizon

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Drainage Data

• Surface roughness
• Drain spacing
• Drain depth
• Effective drain radius
• Actual depth to impermeable layer
• Drainage coefficient
• Initial WT Depth

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Drainage Data

• Seepage
• Vertical
• Lateral
• Down slope
• Saturated hydraulic conductivity
• For each layer

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Basic run information
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Critical depth
Duration
Growing season
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Year by year data
Longest duration of saturation
Number of critical periods
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Number of years that meet the critical duration
and depth
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Model Calibration Method
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Model Calibration Method

• Designed for use in determining the appropriate
  DRAINMOD inputs in order to calibrate the model
  for a specific well
• Model calibration is considered successful when
  the standard error and average absolute deviation
  are lt 20 cm
• Reference
• Vepraskas, M. J., X. He, D. L. Lindbo, and
  Skaggs. 2002. Predicting long-term wetland
  hydrology from hydric soil field indicators.
  WRRI Report 342. 55p.

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Inputs

• On-site water table data
• On-site rainfall data
• Local rainfall data
• Site specific soil data for DRAINMOD input file

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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)

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Drain spacing too small or close together
34
Drain spacing too large or far apart
35
Drain spacing OK
36
Surface roughness too high
37
Surface roughness too low
38
Surface roughness OK
39
Drainable porosity adjusted
40
Final correlation
41
Calibration

• Run a 40 year DRAINMOD simulation using long-term
  rainfall data from closest site
• Query DRAINMOD how many years is the water table
  above 12 inches for for 14 consecutive days (5
  of growing season) or longer
• If 20 out of 40 years (50 of the years) have the
  water table above 12 inches for a period of 14
  consecutive days or longer then the site has
  wetland hydrology

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Notes about DRAINMOD

• DRAINMOD was developed for eastern North Carolina
• DRAINMOD was designed for flat topography
• DRAINMOD was designed for use in agricultural
  drainage systems
• DRAINMOD may not be appropriate to use in other
  areas with out extensive testing

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Notes about DRAINMOD

• In order to use DRAINMOD effectively training is
  required
• DRAINMOD is a valuable tool to evaluate wetland
  hydrology
• DRAINMOD has potential to help correlate
  indicators, redox, etc. to hydrology

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JUST LOOK FOR IT
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Threshold Simulation Method
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Threshold Simulation Method

• Adapted for use in onsite wastewater site
  evaluations
• The soil wetness condition is defined as
  saturation above 12 inches for 14 consecutive
  days (5 of growing season) in 50 of the years
• Reference
• Hunt, W. F., III, R. W. Skaggs, G. M. Chescheir,
  and D. M. Amatya. 2001. Examination of the
  wetland hydrologic criterion and its application
  in the determination of wetland hydrologic
  status. WRRI Report 333. 119p.

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Inputs

• On-site water table data
• On-site rainfall data
• Local rainfall data
• DRAINMOD soils input files

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Calibration

• 40 year simulation using long-term rainfall data
  from closest site
• Adjust DRAINMOD to find the parameters (ksat,
  drain spacing etc.) at which 20 years (50) have
  one 14 day period (or 5 of growing season) of
  saturation above 12 inches

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Calibration

• 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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Comparing Hydrographs

• Water Table Difference
• Compare TSH to MH when both exceed 12 inches
• If TSH is closer to surface than the MH then the
  site is an upland
• If TSH is further from the surface than the MH
  then the site is a wetland
• The indicator is only applied when both TSH and
  MH are above 12 inches

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Water Table Difference Example
0
-10
-20
-30
Threshold
Measured
-40
30 cm
-50
4/1
4/8
4/15
4/22
4/29
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Comparing Hydrographs

• Duration
• If TSH is within 12 inches of the surface for a
  longer duration than the MH then the site is an
  upland
• If TSH is within 12 inches of the surface for a
  shorter duration than the MH then the site is a
  wetland

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Duration Example
0
-10
-20
-30
Threshold
Measured
-40
30 cm
-50
4/1
4/8
4/15
4/22
4/29
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Comparing Hydrographs

• Volatility
• Compare TSH to MH when both exceed 12 inches
• The hypothesis is that water table will fluctuate
  less on wetter sites than on drier sites
• If TSH is more volatile than the MH then the site
  is WETTER
• If TSH is less volatile than the MH then the site
  is DRIER
• The indicator is only applied when both TSH and
  MH are above 12 inches

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Volatility Example
0
-10
-20
-30
Threshold
Measured
-40
30 cm
-50
4/1
4/8
4/15
4/22
4/29
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Example
0
-20
-30
-40
Threshold
-50
Measured
-60
1/1/03
2/1/03
3/1/03
4/1/03
5/31/03
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NC STATE UNIVERSITY DEPARTMENT of SOIL SCIENCE