Central Valley trials ask walnut trees: When do you need to be irrigated?

You’ve seen them alongside country roads all over California’s Central Valley: Walnut trees – entire orchards – knocked over, their dirt-covered roots up in the air, shriveling in the sun.

As the state’s $474-million walnut industry struggles to regain its footing against a host of environmental and economic challenges, scientists at UC Davis are testing a new tool that could help farmers with one of those problems.

Water.

Ken Shackel and team, in the Department of Plant Sciences, have a new tool to help farmers decide when to irrigate their walnut orchards. Their method saves precious water and keeps the trees, and their roots, healthier.  It uses a device that is inserted into a tree branch to directly measure how strongly the tree needs to pull to get the water up from the soil. The innovative method measures stem water potential, or SWP. It’s being tested in nine working orchards in the Central Valley, with the help of colleagues in UC Cooperative Extension.

Early indications suggest the SWP method also could be used profitably in other tree crops and woody perennials, and potentially, in other irrigated crops, Shackel said . By shifting away from the current system of estimating trees’ water needs based on the weather or soil moisture, to a system of measuring the trees directly, farmers could use their increasingly scanty water more efficiently and improve the long-term sustainability and profitability of their operations.

“In every case, farmers will be able to reduce their risks and maximize conservation of their irrigation water when they can make decisions based directly on the trees’ level of pull on the water,” said Shackel, a professor in the department.

Despite the challenges faced by California walnut growers, they still produced nearly all of the nation’s walnuts in 2022, according to the California Department of Food and Agriculture. With nearly 376,000 acres of orchards still growing in 2023, and requiring about 1.1 million acre-feet of water yearly, according to the California Walnut Board/Commission, farmers need a better way of knowing how much soil moisture they can use before they start to irrigate their orchards.

The Walnut Board/Commission supported the early stages of this research.

The current project by Shackel and team is in its first year of work. It’s funded by the United States Department of Agriculture, through the Natural Resources Conservation Service. The $1.3-million grant supporting the five-year project comes through the NRCS’s On-Farm Trials program to evaluate innovative water management systems. The goal is to help growers be more efficient and effective with dwindling water supplies.

A little early-season stress makes trees stronger

Farmers typically start irrigating early in the season, especially walnuts.  The trees stay dormant until early- to mid-April, so when they awake, the weather already can be quite warm. The thinking is that early irrigation helps maintain soil moisture for later use and avoids stressing the trees, said Tyler Dowd, assistant project scientist.

But previous studies have shown that  irrigating too early can harm tree health. Scientists think early irrigation could impact the roots, leading to symptoms of nutritional deficiencies seen in the leaves later in the season, particularly at harvest, Shackle explained.

Rather than saving water, Shackel said, “research suggests that delaying irrigation in the spring allows for better root development. Delayed trees are actually less water-stressed later in the season, when irrigation is temporarily discontinued for harvest.”

To help them decide when to irrigate, some walnut farmers already use devices called pressure chambers, or pressure bombs, to measure when trees are stressed for water. By taking the bomb out to their orchard and putting a leaf into it, a farmer can measure how strongly a tree is pulling on water, which is a way of understanding water stress, Dowd explained. This pull results from the process of evapotranspiration -- the combined loss of water from the soil by evaporation and from the plants themselves through the leaves (transpiration). Other methods farmers commonly use to schedule irrigation include following evapotranspiration charts that estimate water loss over time for broad geographic areas.

For their project, Shackel and team want to delay irrigation early in the season and use the pressure bombs to watch for a little bit of “good” stress before turning on the water. “This lets the trees tell us when they need water,” Dowd explained.

SWP: Using precision sensors in a new way

In contrast to most commonly used ways of timing irrigation, Shackel’s previous research also has shown that monitoring stem water potential – a technique that gives information in real time – can reduce irrigation by as much as half in walnuts, while maintaining or even improving tree health.

To measure SWP, the team is coupling information from the pressure bombs with readings from a little electronic device called a microtensiometer. They insert the device permanently into a tree branch, and it directly measures the tree’s pull on water. Throughout the day, each microtensiometer  transmits readings electronically to a receiver, which Dowd monitors from his desk.

“It’s much more convenient than pressure bombs, which require farmers to head out into each orchard and take measurements,” Dowd explained. “We can see what the trees are experiencing in real time, several times a day, rather than the ‘snapshot’ method of using a pressure bomb.”

Shackel has been working with FloraPulse, a company based in Davis, to develop the microtensiometers being used in these trials. The company already has been using the sensors in crops including grapevines, almonds, apples and other fruit trees, inserting the devices into the main stem. But that doesn’t work in walnuts, because the tree trunks ooze fluid that prevents measurement, FloraPulse CEO Michael Santiago explained.

“Ken came up with the idea of installing the sensor into the 'dead' pith in the center of a branch to avoid the wounding response and measure stem water potential,” Santiago said. “Preliminary testing and improvements during 2024 showed that this strategy is viable, though tricky.”

The scientists expect that, when farmers use this highly accurate and localized method for measuring SWP, they can base their irrigation decisions on actual plant stress rather than estimated water use, Dowd added.

Overcoming barriers to adoption

Using the microtensiometers to measure stem water potential is a more practical approach to using irrigation water more efficiently and improving the sustainability of a walnut operation. But, new practices always face some skepticism.

The Natural Resources Conservation Service plans to use the results of On-Farm Trials evaluations such as this one to refine guidance documents, technical tools, conservation practice standards and business practices for growers across the nation.

That knowledge could save growers enormous costs if they can keep their already established walnut orchards in the ground. In California, from October 2022 to August 2023, nearly 29,000 acres of walnut trees were bulldozed, according to a study by the California Walnut Board.