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