Given the critical water situation for central San Joaquin Valley farmers, the University of California’s annual Statewide Pistachio Day focused on helping pistachio growers make the most of a limited water supply.

UC Davis Irrigation Specialist David Goldhamer shared strategies for budgeting irrigations for what he calls “maximum water use productivity”.

If poorly timed, he said, water stress in pistachios will have significant yield impacts for many seasons to come. But if timed well, trees can recover from water stress quickly with little impact on yield for current and future crop years.

“Pistachio trees are very deep rooted and can survive in extremely dry conditions. But that drought tolerance says nothing about productivity,” Goldhamer said. “The question is, in a drought situation, where we know we are going to be managing a limited water supply and we know we are going to be stressing our trees, how can we stress them, or specifically, when can we stress them and how much, with minimal impact on current and future years’ production.”

Goldhamer found that while overall water use for pistachios is high relative to other tree crops, there are definite periods in the development of the crop in which full water applications can be withheld with minimum impact on yield or grade sheets.

For years, Goldhamer has been researching regulated deficit irrigation in pistachios under normal hydrology years as a strategy to solve horticultural problems and ultimately get higher farm profits through reduced water costs. He is now looking at ways to apply that knowledge to years where reduced water availability is a foregone conclusion.

Unlike other tree crops, such as almonds, where growth phases of the tree and nut development seem to overlap, Goldhamer said pistachios actually have a distinct period from mid-May through June when nut development slows and trees are able to withstand water stress.

In California pistachios, stress should be avoided early in the season, in April and May, when shell diameter grows rapidly. Goldhamer calls this period of development Stage 1. Likewise, stress should also be avoided during Stage 3, when kernel development begins in early July through harvest. Pistachio yields can best withstand stress, he said, during Stage 2 — after shell diameter is fully achieved in mid-May, but before kernel development starts in early July.

“If you are on a limited water supply, save your water for Stage 3,” Goldhamer concluded.

In addition to timing stress to the optimum stage of tree and nut development, Goldhamer said growers can also irrigate for maximum water use productivity by using available tools to establish a water budget and irrigation schedule for their orchards.

Over the last three decades Goldhamer has established evapotranspiration crop co-efficients for pistachio trees, which can be multiplied by ET reference values from CIMIS to determine water use in pistachios for a particular location and time of year. This information, along with soil moisture and leaf water potential data from the orchard and information on the irrigation system application efficiency, provide valuable tools for establishing a water budget and irrigation schedule for pistachios.

“The water budget is an extremely easy thing to do, it’s free and it’s a good starting point to tell us how we should manage our water,” he said.

Variables such as crop location, tree spacing, irrigation method and application rate and efficiency will help a grower calculate the orchard water use rate for a two-week time period at a certain time and location.

Growers can convert that potential water use from inches per day to applications in gallons per tree by multiplying inches per day by tree spacing and a .622 equation factor, and then dividing that by the application efficiency rate of the irrigation system.

UC Davis Irrigation Specialist Larry Schwankl said that once the water budget and schedule is established, the next step is to fine-tune the irrigation system to be sure it is running at maximum efficiency.

Schwankl said growers should determine their application rate before making any attempts to limit applications to the orchard.

“Information on drought strategies requires that the micro-irrigation system is running well and you know how much you are applying,” he said. “If you’re talking about applying only a certain amount during different growth stages, you really have to have a handle on how much you are applying with your irrigation system.”

It is also important to address issues such as pressure differences and clogging that can impact application rate of the irrigation system. Pressure compensating emitters can help avoid over-irrigating. Managing the system to prevent clogging from sand and particulates, mineral or chemical precipitates or organic content in the irrigation water is also critical, he said.

Clogging from sand and particulates can be addressed with a good filtration system that should be cleaned regularly. Flush the system every three to four weeks to rid the system of particulates that escape the filtration system.

Adjust the water pH to address chemical precipitates, and address biological clogging through filtration and the use of a biocide such as chlorine or copper.

“It’s important to keep on top of your maintenance,” Schwankl said. “You want to be sure, especially in these drought situations, that you are putting water on to the best of your ability.”