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Irrigation Scheduling with Tight Water Supplies
An older irrigation system may have a distribution uniformity
as low as 45–65%.

California is in its third year of drought with no relief in sight, and 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.

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 -8 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 ET that can be afforded — for example, if 15% of water is available for the season, water at 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 here and here.

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 includes the 2-4 inches of water available within the soil profile.

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%. 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 45–65%, and at a DU of 75% in almonds, the potential loss of revenue at $3 per pound would be $570 per acre. UC Davis has developed guidelines for DU testing.

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 status and stress according to soils. Use the pressure chamber to identify areas of severe stress and adjust your irrigation approach before these areas become a 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 coupled with crop estimation.
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