Soil Moisture Measurement
for Irrigation Scheduling
Sanjay Shukla
Agricultural and Biological Engineering
UF-IFAS
Irrigation Scheduling
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Appropriate water at appropriate time
Crop need, soil properties, and weather
condition
Different methods
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Experience
Calendar method (0.8 in every 4th day)
Soil water measurement based irrigation
Dynamic water balance – water budget
Soil Moisture Measurement
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Direct method
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Gravimetric method
[After EDIS Bul. AE266]
Indirect
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Tensiometric (energy status – related to moisture)
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Tensiometers
Resistance blocks
Psycrometer…
Volumetric
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Nuclear method (Neutron probe)
Dielectric methods
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Time Domain Reflectometry (TDR)
Frequency Domain Reflectometry (FDR)
Capacitance, TDT, ADR, Phase Transmission
Other
Types of Devices
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Fixed
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measurements at a fixed location
depths
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Portable
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single (e.g. 8 in.)
multiple (e.g. 4, 8, 12 in.)
fixed location access pipes with portable reader
Hand held
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can measure moisture anywhere at farm/grove
BREAK / DEMO
Tensiometer
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Tensiometers
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Advantages
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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)
Relatively inexpensive ($100) and easy to use
Limitations
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Slow response, needs maintenance, manual
reading, lack of contact in sandy soils
Electrical Resistance
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Porous Blocks to measure electrical
resistance as a function of water content
Advantages
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Low cost and maintenance
Ease of use
Limitations
 Delayed response in sandy soils
 Dry conditions - reinstallation
 Errors in soils with high salinity
Dielectric Method
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Based on medium’s 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
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Time Domain Reflectrometry (TDR)
Frequency Domain Reflectometry (FDR)
Better devices than other types
More costly than tensiometer and resistance types
Capacitance Probe
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Capacitance probe
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Dielectric properties of soil
Modern probes can log data for every 5 minute and
higher
Single and multiple depth probes
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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
Examples of Types of Devices
Fixed
Portable
Hand-held
CS 616
C-Probe
EasyAg
ECH2O
Enviroscan
Grow-Point
Moisture-Point
Tensiometer
Theta Probe
Watermark
AquaPro
Diviner
AquaTerr
Hydrosense
Capacitance Probe
Portable Capacitance Probe
Cost = $2000
TDR Probes
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Estimate the dielectric constant by the travel time for
electromagnetic wave to go through a transmission line
(parallel rods)
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Measure average soil moisture content along the
waveguide (soil cylinder of approx 1.5 times the spacing)
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Better accuracy
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Types of devices
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Portable: e.g. Hydrosense ($600)
Automatic: e.g. CS 616 (Campbell Sci.)
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Can be used to measure moisture at multiple depths
TDR Probes
Cost = $600
Use of Soil Moisture
Measurement Devices
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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
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field capacity 12%; wilting point 6%, PAW = 6%
point for turning the irrigation on: 9%
Capacitance Probe – Multiple sensors
(4, 8, 12, and 20 in)
Irrigation
Stop - water
below root zone
Selection of Devices
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Factors for selection
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Size and management of the farm
Soil-hydrologic factors
How closely you want to fine tune irrigation
Cost limitations
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Personnel available
Be aware of the sensor accuracy
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$200 to 17,00
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
Location of Soil Moisture Measurement
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Factors for selecting locations
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Soil series map (soil spatial variability)
Known wet and dry areas
Type of irrigation
More the better
Multiple soils within the irrigation zone
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Example: 50 acre zone
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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
Location of Soil Moisture Measurement
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Example
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Multiple sensor probes (cost and soil dependent)
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Manual/Portable
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Fruit crops (orchards): Citrus - 10 probes for 1000 acres
Row crops: Vegetable - 5 probes for 500 acres
Take measurements from different soil (and crop types)
With experience, can do good irrigation scheduling
Type of irrigation
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Install the probe within wetted area of the
drip/microsprinkler
Use of Telemetry
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Wireless transmission of data from
field to office
Requires less time (travel and
personnel)
Current data can be available on
the internet anywhere, anytime
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Enviroscan – Wireless logger
C-Probe
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Lease the system
Yearly contract