Motorists driving by the West Central Research and Extension Center at North Platte may have noticed a cluster of different sensors in one of the center's research fields. It's what Daran Rudnick, Nebraska Extension agriculture water management specialist at WCREC calls a "sensor cluster."

There are numerous ways to measure soil water availability. With access to irrigation tools to measure volumetric water content, electric conductivity, canopy temperature, plant stem diameter, precipitation, as well as evapotranspiration, it can be a challenge to determine which tool or combination of tools is the right one in a specific situation to use to make an irrigation scheduling decision.

"A lot of questions we get from irrigators are in regards to, 'What sensors should be used?' since there are so many on the market, and 'How relevant are they to our soils?'" says Daran Rudnick.

So, starting last year, Rudnick and other researchers at WCREC began testing different sensors to measure soil water availability. The goal is to showcase different technologies for producers, conduct statistical analyses to see how each sensor performs, compare them to one another, and provide that information back to producers and manufacturers.

Putting sensors to test
Now in the project's second year, Rudnick is testing out 10 different sensors replicated four times at multiple depths on the plot — from 4 to 48 inches deep, depending on the sensor.

The cluster contains several single soil water sensors, including an EC5 and 5TE from Decagon Devices (now known as Meter Group), a TDR315 time domain reflectometer from Acclima, a CS616, CS655 from Campbell Scientific and a Steven HydraProbe II. Multi-sensor probes from John Deere Field Connect, AquaSpy and AquaCheck are also being tested.

STALK-MOUNTED SENSOR: The cluster includes plant sensors from Phytech mounted directly on corn and soybean stalks. These sensors measure micro-variations in plant stem diameter — that is, shrinking and swelling, which may help monitor water uptake, water stress and transpiration within a plant.

Capacitance probes, a form of electromagnetic sensors, measure volumetric water content based on dielectric properties of the soil. Time domain reflectometry sensors estimate soil water content by sending an electromagnetic wave along a cable attached to a sensor with rods inserted in the soil and by measuring the travel time of the electric pulse after it's reflected back. The probe correlates the travel time to dielectric constant to estimate soil water content.

It also includes Irrometer's model R tensiometer, which measures water tension or matric potential. Tensiometers consist of a water-filled tube with a hollow ceramic tip placed in the soil, and equilibrate with the soil by pulling water out of the tube as the soil dries, or pulling water into the tube as the soil wets. This creates or releases tension in the tube, which can be measured.

It includes a neutron moisture meter by CPN, which emits "fast neutrons" into the soil where they are thermalized (slowed down) by colliding with hydrogen atoms, which provides an accurate estimate of volumetric water content.

The site also includes an Arable Mark — a sensor that features an acoustic rain gauge, as well as a spectrometer, barometer, radiometer and infrared thermometers.

The research plot also contains plant sensors from Phytech mounted directly on corn and soybean stalks. These sensors measure micro variations in plant stem diameter – that is, shrinking and swelling, which may help monitor water uptake, water stress, and transpiration within a plant.

The previous year, Rudnick also investigated MPS-2 and MPS-6 sensors from Decagon Devices, Watermark sensors from Irrometer, and tensiometers from Hortau.

Searching for answers
A key goal for the project is to help improve grower comfort and familiarity with sensor technology, and Rudnick notes one challenge for adoption is sensor placement. When installing a single capacitance probe on 120 acres, for instance, it can be challenging to determine the optimum location for a good representation of the field.

Another key goal is helping the producer understand the differences in sensors, and how they perform in different regions. An example is differences in thresholds where irrigation is needed.

With such a wide range of sensor technologies, field conditions and environments across the state, one of the recurring questions is how to calibrate different sensors for different conditions.

"When we evaluate sensors, one conclusion might be that they need to be site-specifically calibrated. The producer's response is: How do you propose I calibrate this in my field?" Rudnick says. "This year we're looking at different approaches to calibrate them, so we can share that information with our producers but also industry partners for them to share with their clients."

"For example, there might be a difference in internal calibration techniques," Rudnick adds. "If the sensors overestimate volumetric water content, we can work with those companies, show them our data analysis, and they can work with their producers."

Sensors were donated for this project by 21st Century Equipment, AquaSpy, AquaCheck, Phytech, Frenchman Valley Co-op, Hortau, and Arable.

To learn more, contact Rudnick at [email protected].