Soil Moisture Measurement for
Irrigation Scheduling
Sanjay ShuklaAgricultural and Biological
EngineeringUF-IFAS
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
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]
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
BREAK / DEMO
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
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
Dielectric Method 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
Time Domain Reflectrometry (TDR) Frequency Domain Reflectometry (FDR)
Better devices than other types More costly than tensiometer and resistance
types
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
Examples of Types of DevicesFixed Portable Hand-held CS 616 AquaPro AquaTerr C-Probe Diviner Hydrosense EasyAg ECH2O Enviroscan Grow-Point Moisture-Point Tensiometer Theta Probe Watermark
Capacitance Probe
Portable Capacitance Probe
Cost = $2000
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
TDR Probes Cost = $600
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%
Capacitance Probe – Multiple sensors (4, 8, 12, and 20 in)
Irrigation
Stop - water below root zone
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
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
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
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