Irrigation management is not a simple task. The proper timing, frequency and amount of irrigation depend on a number of factors that must be taken into consideration. These include soil type(s) and water holding capacity; irrigation system type, efficiency and management; the amount of water stored in the soil from winter rains; weather conditions during the growing season; stage of development of the trees and the orchard; and the need for any additional water to leach salts out of the root zone (leaching fraction).

To ensure almond trees are neither over- nor under-watered requires diligent management and monitoring of these variables. To help almond growers refine their irrigation practices, the Almond Board of California has supported research on irrigation management and scheduling over many years.

Comprehensive Scheduling

Ultimately, it is the tree you are managing. Three important monitoring techniques to use as guides to scheduling irrigation are:

  1. Monitoring tree water status using a pressure chamber (pressure bomb)
  2. Checking soil moisture with an auger, tensiometer, etc.
  3. Tracking almond evapotranspiration (ET) using data provided by the California Department of Water Resources CIMIS program

A good overview of these tools is provided in a presentation by UC water management experts Ken Shackel, Bruce Lampinen, Terry Prichard and Blake Sanden  that was given at the 2008 Almond Conference and titled “Irrigation Scheduling — Putting It All Together for Efficiency and Production.”

These experts note the common pitfalls associated with failure to use all three tools. In high rainfall areas during normal years, there is a tendency to over-irrigate in the spring, which is implicated in lower limb dieback, and then have difficulty meeting the trees’ needs as the season progresses due to a shallow root system resulting from excessively wet conditions earlier. Conversely, in low rainfall areas, without these tools there is a tendency not to irrigate sufficiently in the winter to recharge the root zone before bloom.

Another critical time for irrigation management is the beginning of hullsplit, when regulated deficit irrigation is imposed. This can be monitored and quantified by using a pressure chamber to track midday stem water potential (SWP).  Research led by Ken Shackel and farm advisors shows that hull rot can be reduced 60 percent to 90 percent by imposing mild stress at initiation of hullsplit and maintaining stress for two weeks afterwards. Tracking tree water status with a pressure chamber, the goal is to reach mild stress with readings of -14 to -18 bars of pressure.

Pressure chamber readings should be done in concert with soil moisture monitoring to ensure deep soil moisture is not depleted and trees are not overstressed, making them susceptible to mite flaring, defoliation and reduced yield.

Finally, a critical time to for comprehensive and timely irrigation management is the period of bud differentiation in August and September. Studies show moisture deprivation during this period can reduce fruit set and yield the next season. As this is during the harvest to postharvest period — depending on variety —timely application of water can be challenging and requires careful monitoring and management using the three techniques mentioned above.