Soil Moisture Measurement for Irrigation Scheduling

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Soil Moisture Measurement for Irrigation
Scheduling

• Sanjay Shukla
• Agricultural and Biological Engineering
• UF-IFAS

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Irrigation Scheduling

• Appropriate water at appropriate time
• Crop need, soil properties, and weather condition
• Different methods
• Experience
• Calendar method (0.8 in every 4th day)
• Soil water measurement based irrigation
• Dynamic water balance water budget

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Soil Moisture Measurement

• Direct method
• Gravimetric method
• Indirect
• Tensiometric (energy status related to
  moisture)
• Tensiometers
• Resistance blocks
• Psycrometer
• Volumetric
• Nuclear method (Neutron probe)
• Dielectric methods
• Time Domain Reflectometry (TDR)
• Frequency Domain Reflectometry (FDR)
• Capacitance, TDT, ADR, Phase Transmission
• Other

After EDIS Bul. AE266
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Types of Devices

• Fixed
• measurements at a fixed location
• depths
• single (e.g. 8 in.)
• multiple (e.g. 4, 8, 12 in.)
• Portable
• fixed location access pipes with portable reader
• Hand held
• can measure moisture anywhere at farm/grove

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BREAK / DEMO
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Tensiometer

• Tensiometers
• Water potential
• Need to related tension to volumetric water
  content to know available water
• Soil water characteristics curve (Put a figure
  for a variety of soils)
• Advantages
• Relatively inexpensive (100) and easy to use
• Limitations
• Slow response, needs maintenance, manual reading,
  lack of contact in sandy soils

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Electrical Resistance

• Porous Blocks to measure electrical resistance as
  a function of water content
• Advantages
• Low cost and maintenance
• Ease of use
• Limitations
• Delayed response in sandy soils
• Dry conditions - reinstallation
• Errors in soils with high salinity

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Dielectric Method

• Based on mediums capacity (dielectric constant)
  to transmit high frequency electromagnetic
  wave/frequency
• D Const 1 for air, 2-5 for soil, and 80 for
  water
• Two approaches
• Time Domain Reflectrometry (TDR)
• Frequency Domain Reflectometry (FDR)
• Better devices than other types
• More costly than tensiometer and resistance types

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Capacitance Probe

• Capacitance probe
• 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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Examples of Types of Devices
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Capacitance Probe
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Portable Capacitance Probe
Cost 2000
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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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TDR Probes
Cost 600
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Use of Soil Moisture Measurement Devices

• Know your soils and crop root zone
• Contact USDA-NRCS or refer to county soils map to
  get the field capacity and wilting point
• Manage the soil moisture in the root zone between
  the field capacity and 50 of the plan available
  water (management allowable deficit (MAD) PAW
  FC PWP)
• example
• field capacity 12 wilting point 6, PAW 6
• point for turning the irrigation on 9

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Capacitance Probe Multiple sensors (4, 8,
12, and 20 in)
Irrigation
Stop - water below root zone
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Selection of Devices

• Factors for selection
• Size and management of the farm
• Soil-hydrologic factors
• How closely you want to fine tune irrigation
• Cost limitations
• 200 to 17,00
• Personnel available
• Be aware of the sensor accuracy
• Especially for sandy soils (FC 9 WP 4, PAW
  5)
• If possible, use multiple depth probes
• Using factory calibration does not always mean
  accurate results
• Most of the available probes are suitable for
  basic scheduling decisions

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Location of Soil Moisture Measurement

• Factors for selecting locations
• Soil series map (soil spatial variability)
• Known wet and dry areas
• Type of irrigation
• More the better
• Multiple soils within the irrigation zone
• Example 50 acre zone
• 49 acre, high WHC 2 acre, low WHC
• Soil moisture from 49 acre for managing
  irrigation
• 30 acre with high WHC and 20 acre with relatively
  low WHC
• Install multiple probes or use the moisture from
  20 acre

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Location of Soil Moisture Measurement

• Example
• Multiple sensor probes (cost and soil dependent)
  
• Fruit crops (orchards) Citrus - 10 probes for
  1000 acres
• Row crops Vegetable - 5 probes for 500 acres
• Manual/Portable
• Take measurements from different soil (and crop
  types)
• With experience, can do good irrigation
  scheduling
• Type of irrigation
• Install the probe within wetted area of the
  drip/microsprinkler

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Use of Telemetry

• Wireless transmission of data from field to
  office
• Requires less time (travel and personnel)
• Current data can be available on the internet
  anywhere, anytime
• Enviroscan Wireless logger
• C-Probe
• Lease the system
• Yearly contract