With thermometers pushing past the 100-degree mark during the day in early June, along with dropping humidity readings, wind and trees in full leaf, evapotranspiration rates in the almond orchards of Merced County and surrounding areas of the San Joaquin Valley were rising. This marks the start of a period of peak water demand by the trees, which, typically, continues through August.

“Unless you irrigate to match that demand during this time, the trees will be stressed and kernel weights will suffer,” says David Doll, University of California Cooperative Extension farm advisor for Merced County.

He explains why: When trees start to lose more water through evapotranspiration than what is available in the soil, the stomata openings on the surfaces of leaves start closing. This reduces photosynthesis and production and movement of carbohydrates from the leaves into the nuts. With enough water stress, trees will also start dropping leaves. In fact, some growers have already begun seeing that this season, Doll reports.

“To maximize tree growth and yield, it is important to meet water demand through the year,” he says. “Embryo weight continues to increase until the abscission layer forms between the nut and peduncle, indicating the beginning of harvest. What catches many growers by surprise is that water stress after hull split can continue to reduce kernel weights.”

Meeting the water needs of trees this year could be a tough challenge for the many growers who must deal with significantly-restricted water supplies.

That’s why Doll has been advising growers to apply what water they do have this year proportionally throughout the season. For example, if only 20 percent of the usual amount of water is available, then he recommends irrigating at the appropriate times, as usual, but putting on only 20 percent as much water each time than normal.

“This strategy works well for growers facing a reduction greater than 15 percent of the total water budget,” Doll explains. “If the shortage is 15% or less, research shows that moderate water stress should be applied after kernel fill through hull split. This period of water reduction and stress will save about 15 percent of the season's water, but will only reduce kernel weight at harvest by 5 percent.”

Both of these strategies are recommended as better than applying water at higher-than-needed rates and letting the trees stress in between irrigations. “Trees respond more negatively to those big swings in water stress,” he says. “By providing a steady, but lower, water stress, trees will develop smaller nuts that require fewer carbohydrates to fill them. Over time, with a more moderate water stress throughout the season, the trees will produce leaves with fewer stomata openings that better tolerate drought conditions and remain on the tree.”

In addition to smaller nuts, many growers in his area are also facing increased pressure from spider mites, seeking to take advantage of the stressed condition of the water-short trees.

With warmer-than-usual weather this spring, California’s almond crop continues to develop at a faster-than-usual pace. Hull split in water-stressed orchards in his area could begin as early as June 20, Doll notes. That’s as much as 10 days sooner than normal. He expects hull split in well-irrigated blocks to start in the last week of June.

Doll estimates 70 percent to 80 percent of California’s almond growers have less-than-normal supplies of irrigation water this year.

“The only ones with enough water are those with good wells,” he says. “Even then, the quality of that groundwater is declining.

“Let’s just hope the weather forecasters are right about this being an El Nino year and we have a wet fall and winter,” he says.