Corn Irrigation in GA: Sensors, your eyes and hands in the field

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Corn Irrigation in GASensors, your eyes and
hands in the field

• Jim Hook
• C.M. Stripling Irrigation Research Park
• January 15, 2008
• GaACC supported Corn Research

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Merging Technologies to Answer Todays Difficult
Questions

• Soil Water Pressure
• Tensiometers

Images Allen G. Smajstrla and Dalton S.
Harrison, IFAS CIR487. http//edis.ifas.ufl.edu/A
E146. 1/15/2008
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Merging Technologies to Answer Todays Difficult
Questions

• Soil Water Pressure
• Tensiometers

Images Allen G. Smajstrla and Dalton S.
Harrison, IFAS CIR487. http//edis.ifas.ufl.edu/A
E146. 1/15/2008
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Images ICT International. http//www.ictchile.com
/2725jetfill.htm. 1/15/2008
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Merging Technologies to Answer Todays Difficult
Questions

• Soil Water Pressure
• Tensiometers
• Pros Highly accurate sensitive to soil in the
  same way a plant is.
• Cons Require frequent in-field service. More
  difficult to automate with remote sensing
• Costs 60-100/ unit 1.60/ac/y (3 dep, 3 loc, 5
  yr)

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Merging Technologies to Answer Todays Difficult
Questions

• Soil Water Pressure
• Gypsum Blocks

Images Gypsum blocks for measuring the dryness
of soilAG0294Ian Goodwin, TaturaSeptember, 2000.
http//www.dpi.vic.gov.au/dpi/nreninf.nsf.
1/15/2008
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Merging Technologies to Answer Todays Difficult
Questions

• Soil Water Pressure
• Gypsum Blocks

Images Gypsum blocks for measuring the dryness
of soilAG0294Ian Goodwin, TaturaSeptember, 2000.
http//www.dpi.vic.gov.au/dpi/nreninf.nsf.
1/15/2008
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Merging Technologies to Answer Todays Difficult
Questions

• Soil Water Pressure
• Gypsum Blocks
• Pros Very inexpensive. Easy to read and
  automate. Good in clayey soils.
• Cons Not very sensitive in sandy soils. Short
  life in acid soils (1 y)
• Costs 15-20/ unit 1.80/ac/y (3 dep, 3 loc, 1
  yr, 100 ac) or 0.60/ac/yr if you dig them up.

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Merging Technologies to Answer Todays Difficult
Questions

• Soil Water Pressure
• Watermark
• Granular Matrix Sensors

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Merging Technologies to Answer Todays Difficult
Questions

• Soil Water Pressure
• Watermark
• Pros Inexpensive. Easy to read and automate.
  Good in sandy soils. IrrigatorPro compatible.
• Cons Short life in acid soils (1-2 y).
• Costs 25-35/ unit 3.50/ac/y (3 dep, 3 loc, 1
  yr, 100 ac) or 0.85/ac/yr if you dig them up
  (4-yr).

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Merging Technologies to Answer Todays Difficult
Questions

• Soil Water Content
• Volume of water in soil (3 to 30)
• Typically 5 to 15 - Inches of water per foot of
  soil.
• Technology
• Capacitance/FDR (Frequency Domain Reflectrometry)
  
• Soil Dielectric Soil water content
• Cable tester/TDR (Time Domain Reflectrometry)
• Radio waves slowed by water Soil Water
  Content
• Neutron Probes - Neutron Attenuation
• Fast neutrons slowed by Hydrogen Soil Water
  Content

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Merging Technologies to Answer Todays Difficult
Questions

• Soil Water Content
• Capacitance Probes
• Dielectric properties of soil
• Modern probes can log data for every 5 minute and
  higher
• Single and multiple depth probes
• Manual measurements to continuous logging
• Manual Diviner, Echo Probe
• Automatic Enviroscan, EasyAG, C Probe
• Accurate measurement
• Data can be stored for 2-4 weeks and downloaded
  to computer

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Merging Technologies to Answer Todays Difficult
Questions

• Soil Water Content
• Capacitance Probes

Dynamax Delta-T PR2 Capacitance
Probe Portable, moved place to place Expense
spread over many locations Expensive tubes -
moderate to difficult installation
removal Tubes 100/site 1.5/ac/y probe (3
loc, 2y, 100 ac) High accuracy
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Merging Technologies to Answer Todays Difficult
Questions

• Soil Water Content
• Capacitance Probes

Adcon Sentec C-Probes Installed, into
moderately expensive tubes - moderate to
difficult installation removal Tubes 3
sensor probe 750/site 4.5/ac/y probe (3
loc, 5y, 100 ac) High accuracy used with local
or remote logging
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• C-Probe 750-1000

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Images ICT International. http//www.ictchile.com
/2725jetfill.htm. 1/15/2008
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• Portable Capacitance Probes 2000

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Aquaterr T-300 Temperature and Soil Moisture
Meter Aquaterr Instruments portable
capacitance-based probes provide instant direct
readings in a stainless steel and aluminum
housing that will last for years.
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• AquaSpy Formerly Agrilink

• AquaSpy Soil Moisture Probe Capacitance

• AquaSpy Soil Moisture Sensor Capacitance

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Theta probes Standing Wave Moisture Probe The
moisture measurement of the material is based
upon the fact that in a watersoilair matrix,
the dielectric constant is dominated by the
amount of water present. Then the soil water
content can be measured exactly because changes
in water content of the soil result in changes in
the dielectric constant of the soil. The MP406
has a high frequency moisture detector, which
uses the standing wave principle to indicate the
ratio of two or more substances forming a body of
material, each substance having a different
electric constant (Ka).
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Merging Technologies to Answer Todays Difficult
Questions

• Soil Water Content
• TDR Probes
• Estimate the dielectric constant by the travel
  time for electromagnetic wave to go through a
  transmission line (parallel rods)
• Measure average soil moisture content along the
  waveguide (soil cylinder of approx 1.5 times the
  spacing)
• Better accuracy
• Types of devices
• Portable e.g. Hydrosense (600)
• Automatic e.g. CS 616 (Campbell Sci.)
• Can be used to measure moisture at multiple
  depths

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• Campbell TDR CS605, 610, 615

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• Soil moisture equipment

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Merging Technologies to Answer Todays Difficult
Questions

• Soil Water Content
• Neutron Probes
• Pros Accurate (reference), Sensor moves place
  to place, Measure larger volume of soil than most
• Cons Homeland Security, Nuclear Regulatory
  Commission, Radioactive

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Sensors by application
PR2 Probe
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Sensors by Data Records

• Manual gauges
• Hand held meters (built in calculations)
• Data loggers left on-site (simple Hobo, Spectrum,
  WatchDog- to more expensive) keep a record
  until you read or down load it

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Sensors by Data Records

• Radio Communications
• Many communication pathways
• Licensed radio
• Broad spectrum radio
• Cell Phone
• Broadband wireless
• Satellite
• Two common data record presentation
• PC Based hardware and software
• Internet based data records
• Wireless internet download to portable devices

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Radio relayed soil water suction data from farm
fields to desktop computer or Internet
Early season Topsoil dries before subsoil
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Radio relayed soil water suction data from farm
fields to desktop computer or Internet
Peak growth Topsoil Subsoil dry out at the
same time
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Radio relayed soil water suction data from farm
fields to desktop computer or Internet
Irrigation needed at this level of soil suction
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Radio relayed soil water suction data from farm
fields to desktop computer or Internet
Rain/Irrigation wets topsoil but not subsoil.
Grain yield suffers.
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Merging Technologies to Answer Todays Difficult
Questions

• Sensor Placement

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basic principles For an excellent decscription
of capacitance sensors see this description on
Richard Mead's page. All the sensors within this
group use an oscillator to generate an AC field
which is applied to the soil in order to detect
changes in soil dielectric properties linked to
variations in soil water content. Like TDR
systems they are therefore also known as
dielectric sensors. Capacitance sensors consist
essentially of a pair of electrodes (either an
array of parallel spikes or circular metal rings)
which form a capacitor with the soil acting as
the dielectric in between. This capacitor works
with the oscillator to form a tuned circuit, and
changes in soil water content are detected by
changes in the operating frequency. Frequency
Domain sensors work similarly, but use a swept
frequency. The resonant frequency (at which the
amplitude is greatest) is a measure of soil
moisture, and the amplitude is a measure of soil
electrical conductivity. Like capacitance
sensors, their measurement is made at a single
frequency, but the exact frequency depends on
soil moisture content. Delta-T's ThetaProbes
also use an oscillator to generate an AC signal
but, as in TDR systems, apply this to a coaxial
transmission line that extends into the soil (via
an array of parallel metal spikes). Soil moisture
content is measured from the amplitude of the
standing wave which is set up when the reflected
AC signal interacts with the generated AC signal.
They operate at a single fixed frequency. Other
models of sensor (example the SENTEK) operate
from within access tubes and are not in contact
with the soil. This allows multiple sensors to be
lowered into an access tube and take measurements
at all depths.                                  
                           advantages Generally
capacitance sensors permit more freedom of choice
in the design of electrode geometry and operating
frequency than is the case with TDR systems. Most
of these sensors operate at lower frequencies
(100MHz or less) and can therefore detect "bound"
water in fine-particle soils. This bound water is
water that is strongly attracted to the surface
of soil particles and can constitute more than
10 soil moisture content. Much of it is
available to plants, but is not detected
effectively by TDR systems operating at
frequencies gt 250MHz. Less critical measurements
required compared to TDR, and no specific
knowledge of analysing wave-forms is required.
Most sensors can be connected to conventional
dataloggers.                                    
                         disadvantages Readings
are heavily influenced by moisture content and
air gaps in the soil volume nearest the
electrodes (this is also found with TDR systems).
With access-tube models, particularly, it is
extremely critical to have good sensor-tube-soil
contact for reliable estimation of soil moisture.
Systems operating at lower frequencies
(particularly at lt 20MHz) are more susceptible to
soil salinity errors. Less "control" and
detailed information is available compared to
purist TDR wave analysis.
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basic principles TDR depends on discontinuities
in the energy storage mechanisms which are
available. Combined with knowledge of the
propagation velocities of the waves in the medium
being used, these discontinuities can be located
by observing the change in energy levels at fixed
points in the media. Energy which does not become
dissapated returns to its source. The probe tips
of a TDR appliance present a discontinuity in the
wave propagation path of the energy initiated at
the signal source. The mathmetical description
of wave mechanics was developed by a fellow named
Schroedinger several years back. Basically, all
of the natural properties required for wave-based
appliances to operate come from tradeoffs between
two interlocked energy storage mechanisms. A
pendulum trades off potential and kenetic energy,
acoustic waves trade off transverse and
longitudinal compression and expansion and
electromagnetic waves trade off storage between
magnetic and electric fields. The presence of
water in the medium affects the speed of the
electromagnetic wave. (Slows it down sightly).
The accuracy of TDR measurements depends on
precise measurement of time and precise
calibration with the relative volumetric content
of water around the probe. Freedom from
interfering signals and capacitive disturbances
may also be a factor, i.e., if you are measuring
in a shallow metal tray, TDR may not work well.
The TDR probes tend to react in ways similar to
"rabbit ear" antennae used for television
reception. In addition, many signalling issues
must be resolved such as energy levels, pulse
widths and shapes, repetition rates, oscillator
stability, etc. The University of Utah's Soil
Science Dept illustrated html description, and
pdf version (which includes EC measurement
principles). Soilmoisture's description.
                                                  
          advantages Accurate, continuous, no
need for calibration, unnaffected by salts
(within limits - see disadvantages below).
                                                  
          disadvantages Complex electronics and
expensive equipment required for "pure" TDR.
Effects of salinity on TDR The TDR technique as
practised by many is indeed relatively
insensitive to salinity as long as the salinity
level is low enough that a useful wave form is
returned! The big problem is that, as salinity
levels increase, the signal reflection from the
ends of the rods in the TDR probe is lost. This
occurs because of conduction of the signal
through the saline soil between the rods. The
amount of conduction increases as the soil wets.
Thus, one can obtain quite good wave forms in a
very dry saline soil and useless wave forms in
the same soil when wet. To backtrack a little,
when people say that TDR is insensitive to
salinity they mean that the travel time is little
affected by salinity. So, as long as the wave
form can be interpreted, the water content
estimation will be good. That fact doesn't help
much if the wave form is not interpretable due to
signal loss. There have been many attempts to
solve this problem by coating the probe rods.
However, coating the rods introduces some other
problems (change in calibration, loss of
sensitivity, wear of the coating affecting
results, etc.). I think there are others out
there that could address the possibilities
associated with using coated rods. See Can.
Geotech Journal, 1985, No 22, pp 95-101, DE
Pattersson MW Smith. "Unfrozen water content in
saline soils results using time-domain
reflectometry" It is an old journal article but
delas with TDR, Saline soils and water content.
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ECH2O Soil Moisture Sensor Designed by Decagon,
a leading US instrumentation manufacturer, ECH2O
is a capacitance probe that measures dielectric
permittivity of the surrounding medium. In soil,
dielectric permittivity is directly related to
the water content. The ECH2O probe outputs a
voltage proportional to the dielectric
permittivity, and therefore the water content of
the soil
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