Modeling Hydrology

1
Modeling Hydrology

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

2
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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Methods for Interpretation

• Modeling
• Model Calibration Method

4
What is the hydrologic definition of a hydric
soil?
5
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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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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Modeling

• A Soil Scientists introduction to DRAINMOD

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Modeling Approaches

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

• Water balance on hour by hour - day by day
• as influenced by
• Weather (rainfall and ET)
• Soil Properties (permeability, layering,
  restrictivehorizons, soil-water characteristic,
  infiltration, etc)
• Site and Landscape conditions(topography,
  drainage system, seepage)
• Vegetation
• Predict/Simulate Hydrology

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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 input data

• Hydraulic conductivity versus depth
• Soil-water release curve
• Upward flux, infiltration rate,
  drainableporosity versus depth
• Depth to restrictive horizon
• Drainage parameters - surface roughness, drain
  spacing, drain depth, outlet condition
• Climate data

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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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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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Model Method
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Detailed site specific inputs

• Climate Data
• Drainage Data
• Soil Data
• Crop Data (if applicable)

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Climate Data

• Long term data from nearest weather station
• PET supplied in DRAINMOD for NC. Others need to
  be measured.

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Soil Data

• Moisture release curve data for surface horizon
• Drain volume
• Upward flux
• Infiltration

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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
Critical depth, Duration, Growing season
Year by year data Number of critical
periods, Longest duration of saturation
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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Model Calibration Example
0
Measured
Simulated
-20
-40
-60
4/1
6/1
8/1
10/1
12/1
2/1
4/1
6/1
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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