California is in its third year of drought, with 2013 the driest year on record. Late winter rainfall contributed little to seasonal totals, and there is not enough time left in the rainy season to make up for shortages. Under these conditions, growers with reduced levels of water should use all the technology available to optimize irrigations.

How best to manage irrigation in a short water year is a complex question for almond growers. A workshop at The Almond Conference last December looked at irrigation strategies for drought management to help provide almond growers with some answers for season-long water management under different availability scenarios.

Season-long Planning

Applications of available water should be spread out as much as possible over the season, typically mid-March to mid-November, in proportion to almond evapotranspiration (ETc). For season-long planning purposes, historical values can be found on UC’s drought management website at http://bit.ly/almondET. Current Almond Board–funded research is updating these ETc values; therefore, the values5 for the date periods given in the table should be used as the relative proportional ETc over the course of the season, and should be converted to the percent of total season-long ETc, which is the sum of the inches in each column (location).

In scheduling irrigation, the pressure chamber should be used to determine the stem water potential of the trees. Orchard irrigations should not be initiated until the trees reach about -4 bars off their baseline, or about -12 to -13 bars, say Ken Shackel (UC Davis Plant Sciences) and Merced County farm advisor David Doll. Irrigations should be at the percentage of almond evapotranspiration (ETc) that can be afforded — for example, if 15% of water is available for the season, typically mid-March to mid-November, apply 15% ET at each irrigation.

Growers interested in obtaining a pressure chamber can contact their UC farm advisor for options. Information on how to use the chamber is provided on the UC Fruit and Nut Research and Information Center website at http://bit.ly/PressureChamber. Reference baseline values of mid-day stem water potential for your area can be obtained at http://bit.ly/stemwaterpotential.

Almond Board–funded research by Shackel demonstrates that almond trees can survive through the year on as little as 6–8 inches of water (5–10% ET). This included the 2–4 inches of water that were available within the soil profile.

Bud Differentiation

An important consideration in planning is addressing the period of bud differentiation for next year’s crop, which starts about mid-August and continues through the month of September. Moderate stress during this period will have little effect on subsequent years’ nut numbers, but severe stress during bud differentiation has dramatically reduced fruit set the following season, according to research.

Once the season commences, starting at bloom, real-time estimates of ETc (=Kc*ETo) are available using the California Irrigation Management System (CIMIS) online at http://bit.ly/irrigationmanagement. Starting at bloom, keep a running total, and also account for any meaningful rain and irrigation/fertigation.

When water supplies are severely restricted, the impact on almond trees and crops is unavoidable. Yields are reduced in the drought year as well as the subsequent year, with reduction proportional to the degree of stress. Fortunately, production recovers by the second to third season after drought with sufficient water supplies.

UC researchers have urged growers not to take aggressive actions in reducing tree size or crop load in response to water shortages this year. Severe pruning will increase new growth, which would increase the leaf surface and ETc of the tree. Crop thinning has a similar effect and is also not recommended. By reducing crop load, the source/sink ratio of the tree is disturbed, causing the tree to put nutrients into vegetative growth instead of the nuts. Furthermore, in-season nitrogen applications should also be reduced in order to reduce vigorous shoot growth.

Irrigation System Accuracy

Other important actions under these conditions are to control weeds that compete for water, and to be sure your irrigation system is performing optimally. According to Jim Anschutz with Ag/H2O, a member of the Water, Energy and Technology Center at Fresno State University, it takes 1.8 acre-feet of water to compensate for a distribution uniformity (DU) of 75 percent.

Speaking at the annual conference of the California Irrigation Institute in Sacramento in January, Anschutz added that a 10–15-year-old system may have a DU as low as 45 percent to 65 percent, and at a DU of 75 percent in almonds, the potential loss of revenue at $3 per pound would be $570 per acre. UC Davis has developed guidelines for DU testing, which are published at http://bit.ly/DUtesting.

At a Jan. 28 almond drought management meeting, David Doll and Ken Shackel also noted the following very important issues:

• Under deficit irrigation, expect to see differences in tree water stress according to soils. Use the pressure chamber to identify areas of stress and adjust your irrigation approach before these areas become a severe problem.

• There is no evidence that heavy pruning, kaolin/whitewash sprays or reducing bud/crop load do any economic good to mitigate drought conditions.

• Fertility programs need to be throttled back, but not lacking. For instance, in-season adjustments for nitrogen should be based on early-season leaf sampling (see http://bit.ly/leafsampling) coupled with crop estimation.

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