Infrared light measures water stress in grapevines

One morning at the Oakville experimental vineyard in Napa County, Chris Parry demonstrated a thermometer that can accurately detect vine leaf temperatures from yards away.

The "thermometer" is a camera mounted on a pole that shows promise as a new irrigation management tool that uses infrared light in order to measure directly a key piece of information about vine water stress.

"We have a little bit of experience using this on grapevines in Paso Robles and it looks pretty promising," said Parry, U.S. Department of Agriculture research plant physiologist.

Parry showed how to use infrared thermometers to measure grapevine water stress during Grape Day at the University of California research vineyard, as researchers discussed using a variety of modern sensors from the air or in the vineyard to manage irrigation.

The technology shows promise because it provides irrigation recommendations more precise than a combination of evapotranspiration (Et) and the crop coefficient, because it is based on water stress readings taken directly from the crop.

"The infrared thermometer tells you the temperature of the thing you point it at," Parry said. "From the temperature of the leaves, we can determine the plant stress. There is a plant water stress index based on the temperature. You can also convert that temperature to Et."

In previous research, Parry has already been able to correlate the relationship between the stress index and corn yield and hopes to develop a similar system for grapes.

One challenge will be bringing down the cost of the infrared thermometer (IRT) and other instruments needed to accurately assess crop water need.

"This IRT runs about $600; it is a research-grade piece of equipment," Parry said. "Using research-grade instrumentation, it would cost $5,000 to set it up in a vineyard so that you could tell remotely that it is time to irrigate. I already have some of those set up. We could work with cheaper radiometers to see if they work well with these models, so more growers can use it."

Another issue will be simplifying the calculations needed to translate IRT data into practical irrigation recommendations.

"The crop water stress index can be simple," Parry said. "With just the humidity, air temperature and IRT, you can come up with something. When you get more theoretical based by putting more inputs into it, it can get pretty labor intensive."

In order to do the most precise calculation on plant water need, you need to know, in addition to the leaf temperature, the leaf area, canopy height and width, solar radiation, wind, humidity and temperature.

Concord-based company Tule Technologies uses vine stress, Et and drip line pressure information to tell remotely the irrigation needs of vineyards.

"With those three pieces of information, we can tell a grower in an email how stressed their vines are, and how much water they need to apply to reach their target," said Tom Shapland, CEO of Tule.

The equipment is both closer to the vines than publicly accessible weather stations, and includes vine stress data in the calculations, he said.

Tule charges $1,500 a year per sensor for installation and maintenance of the system, and for regular updates on vineyard water status.

"The problem with imagery is you can't tell how much the wind is affecting the water stress," Shapland said. "Pressure bombs have to be done with utmost care. They have to be done with a magnifying glass. The trusty pressure bomb can be unreliable if it's not done with the utmost care."

Ceres Imaging of Oakland uses proprietary software to correct for the wind in interpreting the results of aerial photography that is timelier than satellite imagery.

"We contract with pilots out of Sacramento and Australia who do weekly flights," said Jenna Rodriguez, product manager at Ceres Imaging. "We started with drones, but they couldn't handle the equipment we wanted. We get very fine resolution. Aerial images also let us get coverage all over the state."

The equipment on the planes calculates water stress by photographing the canopy temperature, measures vigor with the normalized difference vegetation index, and uses the chlorophyll content to estimate the nutrient status of the crop.

Rodriguez first researched with Normalized Difference Vegetation Index photography while helping growers understand how earthquakes had impacted their water availability.

"I worked with an apple grower in Chile after the 2010 earthquake," she said. "Before the earthquake, he had a water table 10 feet down, but after the earthquake the water table came all the way up to the surface. He dug a 10-foot trench around the orchard to help it drain. I used the Normalized Difference Vegetation Index to monitor the progress."

More recently, Rodriguez used NDVI after the 2014 Napa earthquake, to see if vines were getting too much water in the root zone.

The water stress information, which can be more current than satellite images, costs $2 an acre per flight, but that price goes down with more flights or more acres.

"Water stress and nutrient status are the two most popular products," Rodriguez said. "We do have growers who do it weekly to monitor their irrigation."

(Bob Johnson is a reporter in Davis. He may be contacted at bjohn11135@aol.com.)