Site Instrumentation Methods

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Site Instrumentation Methods by Jim Richardson
and Mike Vepraskas
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Overview

• Criteria for picking monitoring sites
• Selection of equipment and installation
• Data collection interpretation
• Rainfall measurements

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Selecting Sites to Monitor

• Pick sites that are representative of a large
  area--both in terms of landscape position and
  microtopography
• Pick at least one site in an area known to be
  hydric, and one site in the upland (transect).
• Replicate sites at the same landscape position
  (traverse).

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Boundaries

• In many cases you want to monitor across
  boundaries between
• Wetland vs. Non-Wetland
• Hydric vs. Non-Hydric Soils

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Think Shallow

• Hydric soils are saturated either on the surface
  or within 12 in. of the surface.
• Do not focus too deeply
• Be sure to monitor within 12 in. of
• surface

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Upland (Not hydric)
Hydric Boundary?
Hydric
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Microtopography should be similar at an
installation
10 ft
Piezometers or wells will give different readings
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Where do you place your Instruments?
Potential hydric soil zone
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Where do you place your Instruments?
Monitoring Sites
Hydric soil boundary
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Where do you place your instruments?
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Flooding Depth and Duration are Important too
Flooded and ponded For long duration Hydric soils
Flooded for Short duration (not hydric?)
Alluvium
Till
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Use Transects Across Hydric Boundaries
Wells and piezometers
Possible Boundaries
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Soils Considerations

• Complete a profile description for each plot
  where equipment will be installed.
• Estimate the depth to any layers that may perch
  water for long periods, and any sand deposits.
• From the description, estimate the depths that
  wells and piezometers will be installed.

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Piezometers
Well
Sand
Clay
Sand
Shallow piezometer contains water, but
lower piezometer does not. Perched water table
detected.
Water drains down well, Saturation undetected
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Soil Descriptions Suggest Boundaries
A
Bw
Bk
Btg
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Overview

• Criteria for picking monitoring sites
• Selection of equipment and installation
• Data collection interpretation
• Rainfall measurements

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Finding the Water Table

• Wells should be used to identify water tables.
• Piezometers measure pressure, and not the free
  water surface.
• Wells work best when they dont penetrate a layer
  that is perching water or intersect large cracks

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Wells and Piezometers
Wells are tubes that contain many holes to let
water in.
Piezometers contain few holes, and let water in
mainly at their bottom.
Wells show the depth to the water table
Piezometers show if soil around holes is saturated
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Piezometers vs. Wells

• We suggest placing one well at each site whenever
  you need to know where the water table is.
• Piezometers should be used to conform to the
  technical standard (one in the upper 10 in.)
• Do not use wells in Vertisols or any other clayey
  soil where bypass (crack) flow can occur

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Vertisols have large, continuous cracks that
carry water deep into the soil when soil is dry
Water flow
Wells have many holes, and some holes will be
next to cracks
Dry Ped
Water in well is from Crack Flow. Soil is not
saturated
Dry PED
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AQUIFERSCONFINED UNCONFINED
Unconfined Aquifer- in soil
SATURATED ZONE
Confining aquitard (clay)
Confined aquifer- sand
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Organic Domes in Natural Bays
Dome
Organic soil
Sand
Clay
Sand
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Domed Organic Soil
0
Mucky Mineral
-75
Sand
-169
-170
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Domed Organic Soil forms over point of
upwelling water?
0
Surface
Organic
Mucky Mineral
-75
Sand
-169
-170
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Suffolk Scarp
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Dismal Swamp
Scarp
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Suffolk Scarp
Stream
Sandy loam
Clay confining layer
Saturated Sand (confined aquifer)
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Stream has cut through Confining layer. Water
from confined sand flows into channel.
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If stream is dammed, water flows onto plain.
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Thick organic deposits form on plain. Dismal
Swamp
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Stratified Soil with Confined Aquifer
Where do you place your piezometers?
A-horizon (Loam)
SUGGESTION Place wells in surface aquifers down
to confining layer. Place piezometers in
confined aquifers.
E-horizon (Loam)
Bt-horizon (CLAY)
C- LOESS (silt)
Btb-PALEOSOL (CLAY)
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Ideal Piezometers In permeable layers
Avoid Piez. Will not fill in clays
Well
A-(Loam
E-(Loam)
Bt (CLAY)
C- LOESS
Btb-PALEOSOL (CLAY)
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Recharge - EpiSATURATED or PERCHED
A-(Loam
E-(Loam)
Bt (CLAY)
Water levels
C- LOESS
Btb-PALEOSOL (CLAY)
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Discharge - EndoSATURATED
A-(Loam
Water levels
E-(Loam)
Water in loess Under pressure Artesion Condition
Bt (CLAY)
C- LOESS
Btb-PALEOSOL (CLAY)
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Loose Cap use Pop (soda) can
Basic Piezometer Installation Method
Drill air hole In tube if cap Is tight
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Loose Cap use Pop (soda) can
Basic Well Installation Method
Drill air hole In tube if cap Is tight
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Fabric Covers or Socks for Wells and Piezometers
Cover holes With porous fabric.
Tape
Sand and Soil may fall into holes in Wells
and Piezometers.

• Can use
• Geotextile,
• Drain Sock,
• Womens nylons

Knot at bottom
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For Flooded Sites Use a Surface Marker Along with
wells and piezometers
Gauge showing water height above surface
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High Water Table Indicator Stolt, Univ. Rhode
Island
Well pipe cut in half To see inside
Magnet
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Another Type of Well Recorder
This is Circular 1409, http//edis.ifas.ufl.edu/C
H151. B. J. Boman, and T. A. Obreza, Cooperative
Extension Service, University of Florida
Gainesville, FL 32611.
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Overview

• Criteria for picking monitoring sites
• Selection of equipment and installation
• Data collection interpretation
• Rainfall measurements

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Time to Make Readings

• Collect data weekly during a critical time of
  year when water levels are high (usually
  winter, spring, and fall).
• Monthly readings may be adequate when water
  levels are low (e.g. in summer).

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Typical Wetland Hydrograph
Winter Peak
Summer ET
ponded
Surface
Saturated lt 30cm
Water Table Depth (cm)
-30cm
Spring Falling Limb
Fall Rising Limb
Time (Months)
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How often do you make measurements?

• Measure at least weekly
• Daily measurements are needed for some modeling
  work
• If you use automated systems, set for daily
  measurements, and visit monthly.

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Water levels in piezometers differ with depth in
recharge and discharge areas
Recharge
Discharge
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Piezometer Readings

• Data must be plotted separately for each
  piezometer.
• Note when free water present at depth of slotted
  portion of tube.
• Can plot depth of water or plot saturatedor
  not saturated as a bar over time.

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Finding the Water Table with Piezometers
Where is the water table in this soil?
2 ft.
Piezometers
6 ft.
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If you must use a piezometer to identify a water
table,then use the water level in the shallowest
piezometer for your estimate
Water table
2 ft.
6 ft.
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Water Table Fluctuations during rain events

• In some soils, water tables move up and down
  quickly during and after a rain.
• In other soils, there is much less fluctuation.
• The amount the water table rises during a rain is
  related to the soils drainable porosity.

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Influence of Drainable Porosity on watertable
fluctuation
High drainable porosity (little fluctuation)
Low drainable porosity (much fluctuation)
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Drainable Porosity
Is roughly the Volume of Air-Filled Porosity When
soil is wet to Field Capacity. It is related to
the volume of macropores, or large cracks, root
channels, and pores between sands and gravels.
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Low Drainable Porosity
1 inch of rain raises water table by 10
inches because of low amount of Pore Space
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High Drainable Porosity
1 inch of rain raises water table by 2
inches because of high amount of Pore Space
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Very porous layer of organic soil
1 inch of rain must fill much pore space to
saturate soil
Little water table Fluctuation in this zone
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Soils with Low Drainable Porosity

• Water table fluctuation is large
• Water tables may rise 2 ft. or more after a
  large rain event, and also fall within several
  days.
• Water tables need to be read weekly or more often
  in these soils.
• Unless water tables are read daily, you arent
  recording maximum level.

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This device is best for soils with Low Drainable
Porosity, where weekly readings are made
Magnet
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Water Table Fluctuations in Discharge Areas
RECHARGE
FLOWTHROUGH
DISCHARGE
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Carolina Bays
1.6 km (1 mile)
Carolina Bays Oval-shaped depressions oriented In
a NW-SE direction
N
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This Bay has ridges on western and eastern sides
Does Ground Water flow Into Bay here?
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Water Balance for Juniper Bay Today (2002-03)
PET 845 mm
Precip. 1115 mm
Surface Outflow 707 mm

• Storage
• 6 mm

Wetland
Groundwater Inflow-Outflow 543 mm
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Well on East Side of BayHourly water table levels
Days
Depth (in.)
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Sawtooth PatternCaused by ground water inflow
Days
Depth (in.)
Fall in water table during day due to Et
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Wetland is Discharge Area
Days
Depth (in.)
Rise in water table at night due to groundwater
inflow
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Well at Center of Bay
8/1/02
8/2/02
8/3/02
8/4/02
8/5/02
8/6/02
8/7/02
8/8/02
8/9/02
8/10/02
8/11/02
8/12/02
8/13/02
8/14/02
-20
No daily fluctuation no ground- water inflow
-25
-30
Water Table Depth (in.)
-35
-40
-45
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Recharge
Today
Flowthrough
Discharge
GiGo
GiltGo
GigtGo
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Overview

• Criteria for picking monitoring sites
• Selection of equipment and installation
• Data interpretation
• Rainfall measurements

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Rain gauge
Solar Panel
Net Radiometer
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Rainfall

• Daily rainfall measurements are critical for
  identifying years or normal rainfall
• Each site should be instrumented with an
  automatic recorder
• A manual recordering gauge is needed (essential)
  for backup

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Rainfall

• Locate the gauge on level ground, about 30 in. or
  more above the surface.
• The gauge needs to be in the clear, at a distance
  of twice the height of the nearest obstruction

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Rain gauge located in clearing at a distance from
the trees that Is equal to twice the height of
the trees
Gauge 30 in. above ground
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Rainfall contd.

• Nearest available weather stations usually arent
  near enough for daily data
• Be sure to protect gauges against bird
  droppings--install wires that cut feet.

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Rainfall Variability for a 750 acre Site
How Many Gauges Do You Need?
Gauges not working
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Points on Rainfall

• Rainfall data are just as important as water
  table data.
• One gauge per 750 acres is adequateif the gauge
  is working
• More rain gauges are fine, but they increase the
  chance that one or more gauges will fail.
• Use only the amount of equipment you can keep in
  working order.

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Summary

• Select sites that represent large areas
• Pay attention to microtopography
• Use wells for water table measuremens, and
  piezometers to find saturation depths

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Summary

• Install wells and piezometers so they dont leak
  by by-pass flow from surface.
• Read instruments weekly, and be sure to visit
  site monthly if using automated equipment
• Measure rainfall whenever possible

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Appendix
How do you know if your rainfall is
normal? Sprecher, S.W. and A.G. Warne. 2000.
Accessing and using meteorological data to
Evaluate wetland hydrology. ERDC/EL
TR-WRAP-00-1, U.S. Army Engineer Research and
Develop. Center, Vicksburg, MS.
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Normal Rainfall

• Some people use average rainfall for normal.
• This means rainfall is almost always
• above or below normal.

• USDA uses a Range of Normal Precipitation
• Ranges allow you to classify rainfall as being
• Wetter than normal
• Normal, or
• Drier than Normal

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WETS Data Tables

• Ranges of normal precipitation are given on WETS
  data tables,
• Available for virtually every weather station in
  the U.S.

WETS Tables are found at www.wcc.nrcs.usda.gov/cl
imate/wetlands.html
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WETS Station GREENVILLE 2, NC3638
Creation Date 10/23/2002 Latitude 3537
Longitude 07723 Elevation 00030 State
FIPS/County(FIPS) 37147 County Name Pitt
Start yr. - 1971 End yr. - 2000 ---------------
--------------------------------------------------
-------- Temperature
Precipitation
(Degrees F.) (Inches)
----------------------------
---------------------------------
30 chance
avg
will have of avg
---------------------
-----------------days total Month avg
avg avg avg less more w/.1
snow daily daily
than than or fall max
min more
-------------------------------------------
------------------------------ January 52.0
31.0 41.5 4.42 3.53 5.33 8
0.9 February 55.8 33.2 44.5 3.45
2.34 4.16 6 1.4 March 63.9
40.2 52.0 4.07 3.19 4.83 7
0.6 April 73.0 47.9 60.5 3.19
2.18 4.31 5 0.0 May 79.9
56.8 68.3 4.04 2.79 5.01 7
0.0 June 86.2 64.7 75.5 4.46
3.00 5.25 7 0.0 July 89.9
69.4 79.7 5.24 3.75 6.45 7
0.0 August 88.1 67.8 77.9 5.89
3.65 7.03 7 0.0 September 82.9
61.9 72.4 5.50 2.78 7.13 6
0.0 October 73.5 48.9 61.2 3.27
2.00 4.34 4 0.0 November 64.7
40.4 52.6 2.85 2.03 3.46 5
0.0 December 55.6 33.6 44.6 3.23
2.10 4.02 6 0.4 -----------------
-------------------------------------------
----- ---------------------------------------
-------------------------- Annual -----
----- ----- ------ 44.45 52.35 --
---- ---------------------------------------
-------------------------- Average 72.1
49.7 60.9 ------ ------ ------ --
---- ---------------------------------------
-------------------------- Total -----
----- ----- 49.61 ------ ------ 75
3.4 ---------------------------------------
-------------------------- ------------------
--------------------------------------------------
-----  
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Rainfall WETS TABLEDefines Normal Rainfall
Normal rainfall for August is 3.65 to 7.03 inches
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Corps Rainfall Analysis
In order to interpret single season water table
data
Measurements made during normal rainfall may be
unacceptable if prior 1-3 months were wetter than
normal.
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Rainfall Analysis contd.

• Three ways are available to evaluate antecedent
  rainfall
• Direct Antecedent Rainfall Method
• Moving Total Antecedent Rainfall Method
• Palmer Drought Index

We feel the first method is reliable. It is
illustrated next See Sprecher and Warne (2000)
for info on others
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Direct Antecedent Rainfall Method
Rainfall during previous 3 months is examined
to determine if the current month has a wet, dry
or normal condition Calculations are made
using a relative weighting scheme
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Direct Antecedent Rainfall Method

Most recent month is weighted more heavily than
the 3rd month
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Applications
Hydric Soil Technical Standard and Wetland
Hydrology Technical Standard Rainfall from the
month of interest is ignored and previous 3
months are used in the calculation