on-irrigator moisture sensors for precision irrigation

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On-Irrigator Moisture Sensors for Precision Irrigation

Ian Woodhead, Adrian Tan, Ian Platt and Sean RichardsLincoln Agritech Ltd., Lincoln University, Christchurch, New Zealand

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PRESENTATION OVERVIEW:

1. Introduction to the moisture sensorHow does it enhance an existing variable rate irrigation system?

2. Operating principleHow does the sensor measure the microwave reflection of pasture land?

3. Soil moisture contentRelationship between sensor measurement and soil moisture

4. Sensor developmentSensor prototype and antenna

5. Measurement examplesShort lawn grass, medium and long pasture grass, at varying soil moisture

6. Conclusion

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Variable Rate Irrigation

•Pulsing sprinkler and varying system speed to achieve different application depth.

•Beneficial for different crops / non-crop areas; soil types; terrain; and obstacles.

•Need a computer database, knowledge of their effect on water intake, and algorithm.

•Irrigator needs to know the soil moisture content that are measured at a few locations.

Images sourced from ars.usda.gov

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Variable Rate Irrigation

Existing variable rate irrigation systems

• Systems of varying complexities, accounting for water budget, soil type, vegetation, growth stage, dynamics etc. Substantial modeling often required

• Systems with the ability to communicate and control sprinkler volume or irrigator speed

• Rely on indicative measurements of soil moisture at small sample size area

How does an on-irrigator sensor improve the system?

• Measures the soil moisture content of the ground area in front of the wetted pattern of an irrigator

• Measured soil moisture content at meter scale used to modulate the application rate

• Not entirely based on models/ prior measurements, but real time measurements

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Benefits to the environment and farmer

Benefits to the environment

• Improves soil water dynamics reduces nitrate leaching

Benefits to the farmer

• Correct application of irrigation improves nutrient availability and crop yield, and thus farm productivity.

• Improved feed quality and uniformity of dairy pasture enhances amount and consistency of milk production

• Lowers risk of water logging due to excessive water supply in areas of poor drainage

• More efficient utilization of water and energy lowers the cost of farming

Ref: K.C. Cameron et. al., Ann. Appl. Biol., vol. 163, pp. 145-173, 2013.

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How does the sensor work?

• Microwave sensor mounted on the boom

• Soil moisture in front is measured as the irrigator moves

• Sprinkler’s application rate is modulated accordingly

Soil moisture

Ground location

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Sensor’s Operating Principle

Transmitted signal:

Time

Time

Receivedsignal:

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Sensor’s Operating Principle

Transmitted signal:

Time

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Receivedsignal:

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Sensor’s Operating Principle

Transmitted signal:

Time

Time

Receivedsignal:

Ground backscatter

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Sensor’s Operating Principle

Transmitted signal:

Time

Time

Receivedsignal:

Ground backscatter

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Sensor’s Operating Principle

Transmitted signal:

Time

Time

Receivedsignal:

Ground backscatter

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Sensor’s Operating Principle

Transmitted signal:

Time

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Receivedsignal:

Ground backscatter

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Sensor’s Operating Principle

Transmitted signal:

Time

Time

Receivedsignal:

Ground backscatter

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Sensor’s Operating Principle

Transmitted signal:

Time

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Receivedsignal:

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Estimating soil moisture

• Extensive research has been conducted in empirical models of relating soil’s electrical properties with its moisture content.

• First order estimate of soil moisture content can be obtained directly due to dominating effects of water on electrical properties Ref: G.C. Topp, J.L. Davis and A.P. Annan, Water

Resources Research, vol. 16, no. 3, pages 574-582, June 1980

Accounting for pasture grass effect

•Method of calibrating the radar scattered signal to account for various crop types has been developed.

• Crops include corn, soybeans, milo and wheat at various stages of growth.

• Pasture grass is much shorter than crops, long grass might require minimal compensation.

Ref: F.T. Ulaby, A. Aslam, M.C. Dobson, IEEE Trans. AP, 22(2) March 1974, and IEEE Trans. GRS, 20(4), Oct 1982

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Sensor Prototype

• Sensor on mobile cart with supporting structure for operation in windy conditions

• Antenna height = 2.7m, pointing at 45° at ground in front

• Antenna is an array of log periodic dipoles:- Bandwidth = 400 – 1300 MHz- Gain = 11 dBi- 3-dB beamwidths = 45° ± 7°

• Agilent Fieldfox vector network analyzer

• PC for data collection, signal processing and display

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Sensor’s Antenna Design

• Array of 4 log-periodic dipole arrays and a wideband 1-to-4 power combiner.

• ~50 individual antennas in 0.4m x 0.4m x 0.5m volume

• Still a prototype, to be optimized in size.

Proposed method of small area illumination by the sensor’s antenna

Measured footprint of the antenna on ground.

Area = 0.62 m2

x (meter)

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Antenna height is 200 cm; Angle is 45

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BW1

(side-view)45°

2mθv

45cos2cr

(plan-view)θh

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Measurement #1: Lawn grass

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• Lawn grass, height = 2 cm to 5 cm

• Volumetric moisture content between 5% to 15%

• Validation with HS2 Hydrosense probe

Lawn grass behind the RFH building at Lincoln University

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Measurement #2: Dry pasture grass

• Pasture grass, height = 8 cm to 15 cm

• Volumetric moisture content of soil between 15% to 25%

• Validation with HS2 Hydrosense probe at 12 cm (blue line) and 20 cm (black line)

Pasture grass at Iverson Fields I9, Lincoln University

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Measurement #3: Wet pasture grass

• Pasture grass, height = 5 cm to 10 cm

• Volumetric moisture content of soil between 30% to 40%

• Validation with HS2 Hydrosense probe at 12 cm (blue line) and 20 cm (black line)

Pasture grass at Iverson Fields I9, Lincoln University

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CONCLUSIONExisting Variable Rate Irrigation System

ModellingSoftware

VRI Control System

Terrain:Digital elevation model

Soil Type:Available water holding capacity

Obstacles:User excluded areas

Crop intake:Crop type, growth stage

Soil Moisture:Measurements at few

selected locations

Soil Moisture:Real time, meter scale measurement in front

of irrigator

Proposed

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CONCLUSION

• On-irrigator moisture sensor can enhance precision irrigation by providing meter scale soil moisture mapping

• Development effort shows the feasibility, and limitations of such systems

• Measured soil moisture from a single sensor controlling the sprinkler water volume provides 1st level benefit

• There can be multiple sensors controlling the individual sprinklers to achieve the best water savings

• Cost savings can be achieved by multiplexing many (low cost) antennas with one single active element.

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