Allan Fulton, UCCE Water Resources Advisor, Tehama, Shasta, Glenn and Colusa Counties; and Luke Milliron, UCCE Orchards Advisor, Butte, Tehama and Glenn Counties.
Advanced guidelines for walnut are provided in Table 1 (below). These guidelines consider the actual orchard measurements and normalized measurements after referencing baseline SWP (i.e. bars +/- Table 2 baseline, end of article). Research has shown the optimum crop water stress just before irrigation in walnuts is about -2 to -3 bars below baseline. This tends to correspond with field measurements ranging from -4 to -8. There are exceptions, especially for extraordinarily dry hot weather (e.g. >105 °F and <15 % relative humidity). Under very hot and dry conditions, orchard levels averaging -6 to -10 bars may be reasonable and correspond with -2 to -3 bars below baseline.
University experiments and production experience with intensive irrigation management that sustains orchards near baseline (no to minimal tree stress) throughout the season suggests this management strategy can increase tree loss and shorten the economic life of an orchard. Among the major tree crops grown in the Sacramento Valley, walnut may be especially sensitive to the negative consequences of maintaining a baseline or minimal water stress status. Concerns include higher incidence of root and foliar diseases, lower limb shading and dieback, and shortened orchard life. Orchards where SWP fluctuates more within these optimum ranges may both yield competitively and incur less tree loss.