0. On-Irrigator Moisture Sensors for Precision Irrigation
Ian Woodhead, Adrian Tan, Ian Platt and Sean Richards Lincoln Agritech Ltd., Lincoln University, Christchurch, New Zealand

1. PRESENTATION OVERVIEW:
1. Introduction to the moisture sensor How does it enhance an existing variable rate irrigation system? 2. Operating principle How does the sensor measure the microwave reflection of pasture land? 3. Soil moisture content Relationship between sensor measurement and soil moisture 4. Sensor development Sensor prototype and antenna 5. Measurement examples Short lawn grass, medium and long pasture grass, at varying soil moisture 6. Conclusion

2. 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

3. 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

4. Benefits to the environment and farmer
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 , 2013.

5. 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

6. Sensor’s Operating Principle
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Ground backscatter

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14. Sensor’s Theory and Modelling

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26. Sensor’s Theory and Modelling

27. 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 , 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

28. 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

29. Sensor’s Antenna Design
Proposed method of small area illumination by the sensor’s antenna
Measured footprint of the antenna on ground.
Area = 0.62 m2
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.

30. Measurement #1: Lawn grass
Lawn grass, height = 2 cm to 5 cm
Volumetric moisture content between 5% to 15%
Validation with HS2 Hydrosense probe 20 15 10 5 2 4 6 8 12 14 16 18
Distance (m)
Volumetric Moisture Content (%)
Lawn grass behind the RFH building at Lincoln University

31. Measurement #2: Dry pasture grass20 10 5 6 7 8
Distance (m)
Volumetric Moisture Content (%) 40 30 50 60 9
Pasture grass at Iverson Fields I9, Lincoln University
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)

32. Measurement #3: Wet pasture grass20 10 5 6 7 8
Distance (m)
Volumetric Moisture Content (%) 40 30 50 60 9
Pasture grass at Iverson Fields I9, Lincoln University
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)

33. Real time, meter scale measurement in front of irrigator
CONCLUSION
Proposed
Existing Variable Rate Irrigation System
Terrain:
Digital elevation model
Obstacles:
User excluded areas
Crop intake:
Crop type, growth stage
Soil Type:
Available water
holding capacity
Modelling
Software
Soil Moisture:
Real time, meter scale measurement in front of irrigator
Soil Moisture:
Measurements at few
selected locations
VRI Control System

34. 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.