With California in an enduring drought and water allocations significantly reduced or reduced to zero in many areas, many growers and managers are stuck with much less water than walnuts use for optimal production. In certain crops (e.g. wine grapes, oil olives, almonds), water stress in certain developmental timeframes (“regulated deficit irrigation” or RDI) is not harmful, or may even be beneficial.
Unfortunately, an effective RDI strategy has not yet been identified for walnuts and sustained moderate to high water stress (stem water potential below -8 bars) at any growth stage has been shown to affect walnut crop productivity and quality.
Different factors of walnut productivity are vulnerable to water stress depending on the timing and extent of that stress. Figure 1 shows the generalized water use from walnut as measured by evapotranspiration throughout a growing season and the different stages of nut and shoot growth occurring in the same timeframe.
With kernels developing in July, August, and September, water stress during this time can impact kernel size and quality. Perhaps more important, this is also the time that buds are developing, and severe stress in this period will typically reduce bloom counts and yield potential next season.
With less water available, and deficit irrigation infeasible, good system maintenance and precise irrigation application are the best approaches for walnut. For system maintenance tips, check out UC ANR’s maintenance of micro-irrigation systems site, and our article on irrigation system maintenance for guidance on how to check system pressure uniformity, flush irrigation lines, and manage emitter clogging.
For more precise irrigations, a good place to start is with weekly crop-water loss estimates, or evapotranspiration (ET). We supply weekly ET reports which can be found on the Sac Valley Orchards ET page, and where you can subscribe for weekly emails to be sent directly to your inbox.
While ET replacement irrigation scheduling is a great start, using a pressure chamber to directly measure tree stress via stem water potential is the most precise way to gauge the stress orchard trees are experiencing.
Waiting to irrigate until pressure chamber readings show that trees are 2 to 3 bars drier than the fully watered baseline (e.g. fully watered baseline is -4.5 bars, irrigating when trees near -6.5 to -7.5 bars). Sac Valley Orchards has a series of how-to guides on measuring and interpreting stem water potential for everyone from beginners to long-time users.
Even with a highly uniform system and precise irrigation accounting for the climate, soil, and tree measurements, reducing applied water to stressful levels may be unavoidable. During the last drought Allan Fulton and Rick Buchner, Farm Advisors Emeriti, created a drought strategies guidance document to explain different strategies and expected outcomes depending on the level of irrigation reduction.
For more detailed information on drought mitigation irrigation strategies for walnut, please refer to that publication. More recently, Ken Shackel, professor of plant sciences at UC Davis, Allan Fulton, and others reported new start-of-irrigation guidance for walnuts not yet incorporated into the drought strategies document, but which offer additional opportunities for water savings heading into next season.