Perched in the limbs of almond trees at Fresno State are devices that monitor sap flow and tell growers how much water trees in the orchard are using and whether they are stressed.

“Sap flow collars or cuffs measure the life blood of the plant,” said Mike van Bavel, president of Dynamax, the company that placed the sensors into the trees.

Van Bavel spoke during a workshop on water use efficiency and the measurement of crop water stress in almonds at a program presented by the Fresno State Center for Irrigation Technology.

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Not far from the conference room where van Bavel spoke was the satellite office of Dynamax in a business incubator at Fresno State’s Water Energy and Technology Center. Dynamax is headquartered in Houston, Texas.

For the first time this year, the new technology is being used in the almond orchards. But it’s not the first time sap sensors have been used in California and elsewhere. Sap sensors have been used for seven years at 200 locations monitoring irrigation blocks in Napa and Sonoma vineyards.

Van Bavel says his company also has monitoring packages for other crops including pecans, peaches, cherries, plums, blueberries, strawberries, corn, and cotton.

He says the monitors “let the plant tell you how much water it is using and then you will know how much water you have to put back.”

Water savings

Goals include savings on water and energy use.

Vintners in Northern California say the monitors have saved them from using hundreds of thousands of gallons of water annually. With wine grapes, van Bavel said, deficit irrigation at the right time can enhance quality and other characteristics of the grape.

Van Bavel explains that the monitors are based on an idea his father had in the early 1980s.

The company has produced transpiration sensors since 1989. Some of the sap flow sensors included a system of needles – thermal dissipation probes – inserted into a limb.

But more recently, developed sensors, including those in the Fresno State orchard, are not invasive. They simply wrap around a portion of a limb.

Van Bavel says sap is “99.9 percent water” which courses through a plant “in a transpiration stream.” Solar heat is used to warm a strip around the limb, and measurements are taken to arrive at a “stem heat balance” that calibrates movement of sap upward.

“It’s done around the clock, provides real time data, and can be accessed remotely on a smart phone.”

In the Fresno orchard, Fresno-based PureSense, a real-time irrigation management company that also provides remote monitoring, has soil moisture sensors that help backstop findings of Dynamax.

Other forms of validation include information from CIMIS (California Irrigation Management Information System), pressure chambers, and weather stations.

How to do it

In the orchard, Dynamax field technician Ciro Ambriz demonstrated the steps in affixing a monitor to a branch. He first chose a healthy tree and a limb growing upwards, not horizontally.

Then Ambriz used sandpaper to smooth the surface of the branch and sprayed it with a thin layer of canola oil to allow the sensor to move with plant growth. He put a small amount of silicone grease on the inside of the sensor and installed the sensor around the branch, making sure the heater strip was overlapping to ensure proper contact and heating. 

He then wrapped the EXO Sensor with an elastic Velcro strap, being careful not to tighten it too much. Then a layer of Gore-Tex was added to keep the sensor dry, along with some zip ties.

Finally, a shiny layer of insulation was added, and the sensor was ready to be plugged into a cable and connected to a data logger.

Before put into use, van Bavel said, each sensor is run with and without heat “to test and characterize thermal gradients and check operations.”

In the test plot at Fresno State, there are two sensors each in two trees separated by about a quarter of a mile.

Periodically, sensors are moved to other trees to confirm consistency in operation, said Mike McClung, application scientist with Dynamax.

At one point, it was discovered sensors had been placed in a “crowed tree," he said - one that was overly shaded by limbs from neighboring trees.

More specifics

Interviewed after the field day, van Bavel said the cost for monitoring two sites with four sensors for a 7-9 month season amounts to about $7,000. That includes installation, service, and data access.

The orchard at Fresno State is part of the student farming operation at the university. It mirrors challenges faced by most California farmers this year in coping with drought conditions.

Bill Green, with the Center for Irrigation Technology, said the university has fallowed about 160 acres this year, mostly row and field crops that amount to about a sixth of the campus farm land.