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.
Ultimately, it is the tree you are managing. Three important monitoring techniques to use as guides to scheduling irrigation are:
- Monitoring tree water status using a pressure chamber (pressure bomb)
- Checking soil moisture with an auger, tensiometer, etc.
- 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.
Fortunately, there are a number of resources for almond growers to use in planning and executing an irrigation management and scheduling program. In addition, research funded by the Almond Board continues to refine and improve upon this knowledge.
One resource is the California Almond Sustainability Program (CASP), which offers growers the opportunity to assess their operations and practices, benchmark them against others, and determine areas of improvement. Go to the Almond Board website (www.AlmondBoard.com/Growers) and click on the Sustainability tab. In addition to a self-assessment, the irrigation management module is rich with valuable references and tutorials (e.g., Irrigation Consumer Bill of Rights, Irrigation Scheduling 101) from UC, Cal Poly San Luis Obispo, and CSU Fresno. It is certainly worth reviewing. Even better, if you have not participated in a self-assessment workshop, you can do so by contacting Kendall Barton at (209) 343-3245 or firstname.lastname@example.org
Current Research Seeks Refinements
Recent research funded by the ABC and others, and led by UC Farm Advisors Blake Sanden, Dan Monk and Allan Fulton, is updating the almond crop coefficients ( Kc ) values used for tracking almond evapotranspiration. These updates will be available for use in the 2013 crop year.
Currently, the pressure chamber is the field production standard for measurement of tree water status, but alternatives are being investigated because use of the pressure chamber is labor intensive, time consuming, and not amenable to automation. To this end, Ken Shackel and UC farm Advisor David Doll are investigating a number of alternatives. The one which shows the most promise is a commercially available device called a psychrometer, which is sealed against a leaf or stem to measure water stress. This has the advantage of taking automated measurements, but the technology is still in the development phase.
Another alternative includes remote sensing (e.g., aerial overflights, satellites) combined with ground-level sensing. Sensing leaf temperature using infrared (IR) sensors is showing potential as a tool for irrigation management by determining plant water status. Mounting IR sensors along with other tools on a “mobile platform” to detect water status/stress could provide a more rapid and comprehensive orchard assessment. UC Davis ag engineers Vasu Udompetaikul, Shrini Upadhyaya, David Slaughter, Bruce Lampinen and Ken Shackel have mounted this suite of sensors to assess water status on a Kawasaki mule which is driven through the orchard.
Resources and links mentioned in this article can be found on the Almond Board website irrigation management page, www.AlmondBoard.com/Farmpress29.