During the 2009 season, presentations and recommendations by UC irrigation specialists continue to draw on past research to help almond growers cope with restricted water supplies – particularly those growers on the West Side of the San Joaquin Valley, south of the Delta where the State Water Project allocation is 30 percent and the Federal Central Valley Project allocation is even more severe at only 10 percent.
The past research focused on regulated deficit irrigation that employs moderate water stress from mid-June to harvest. This work, funded in part by the Almond Board, was done by a number of UC irrigation experts: David Goldhamer, Ken Shackel, Terry Prichard, Alan Fulton, and Larry Schwankl. As noted by Ken Shackel during a presentation, there has been a lot more research on irrigation and almond production than on irrigation and almond survival. Nevertheless, this past research has provided insights into maintaining trees at a survival level.
One very important example of this is the period of bud differentiation for next year’s crop from about mid-August to mid-September. Moderate stress during this period will have little effect on subsequent year’s nut numbers, but severe stress during bud differentiation has dramatically reduced fruit set the following season. Therefore, growers on a limited water budget should save some water for this period to protect next season’s crop.
Also as noted by UC irrigation experts, those who had to follow a severe water stress strategy – 50 percent or less of the seasonal tree-water use – should have applied water over the entire course of the irrigation season, including bud differentiation. The UC Drought Management Web site gives guidelines for almonds (http://ucmanagedrought.ucdavis.edu/Almonds.cfm). Use of plant-based monitoring with a pressure bomb to assess tree-water stress as measured by midday stem water potential has been a valuable tool and guidance is given on various scenarios.
The above past research indicates that almond trees can survive with as little as 6-12 inches of available water (combination of stored in the soil profile and applied) over the course of the season. At this very low level of available water, it appears best to spread the applied water over the season as much as possible, including bud differentiation, in proportion to almond evapo-transpiration (ETc).
As noted, system and application efficiency are critical to minimize losses. This includes a good, maintained irrigation system. Growers on a limited water budget should opt for fuller irrigations at critical stages rather than smaller doses more frequently, to minimize surface evaporation losses.
Current drought survival research
While the past research has provided guidance and insights, there are still many unanswered questions. A current Almond Board-funded research project led by Ken Shackel, “Drought Survival Strategies for Established Almond Orchards on Shallow Soil,” is addressing many of these. This work is being done at the Nickels Estate in Arbuckle. Key questions being addressed include:
• How much water do almond trees need to survive? Irrigation treatments include: none (only rain-fed), 5 inches applied in-season, 10 inches applied in-season, and a control of about 40 inches applied (100 percent ETc).
• What’s the impact of pruning out 50 percent of the canopy in the spring? This pruning was done once trees experienced mild to moderate stress to minimize any re-growth and none was observed in this instance.
• Do applications of reflective sprays such as kaolin (Surround R) help?
• How long does it take for trees to recover? Past research indicates normal production is not attained for at least two years once full irrigation is resumed.
The various treatments will only be imposed this season. In the next and subsequent seasons, all treatments will return to conventional management, including full irrigation.
Both the in-season impacts on yield as well as the carry-over effects on bloom and yield will be determined for an additional period of two to four years after this initial treatment year.
An update on this project will be given at the 2009 Annual Almond Industry Conference, Dec. 9-10, at the Modesto DoubleTree Hotel and Convention Center.
In addition, the Almond Board of California has developed a drought management portal on its Web site. The portal can be found under almond growers, Production Research, at www.almondboard.com.