EVALUATING WATER REQUIREMENTS OF DEVELOPING WALNUT ORCHARDS IN THE SACRAMENTO VALLEY

Transcription

1 EVALUATING WATER REQUIREMENTS OF DEVELOPING WALNUT ORCHARDS IN THE SACRAMENTO VALLEY Allan Fulton ABSTRACT Most of the research on irrigation of walnuts has primarily focused on plant water relations and crop responses in mature, bearing orchards. With the continued and rapid expansion of walnut throughout the Central Valley of California, questions are increasing about irrigation requirements of young, developing orchards. This applied research examined the water requirements of young developing Chandler walnuts from first through fourth leaf when irrigated at reasonable frequencies and durations to sustain desirable midday stem water potential levels and tree growth. The objective was to develop reasonable estimates of crop water requirements for developing trees with rapidly changing canopies. The intent was to provide guidelines to assist growers, particularly those without prior experience, with information to help them determine the appropriate irrigation frequency and duration to develop a newly planted orchard into a healthy, productive orchard within an optimal time frame. Results from the study suggested a reasonable seasonal water requirement for a first leaf tree planted in May (bare root Chandler trees ranging from ½ to ¾ inch diameter) is about 14 to 15 inches. When grown under a minimal pruned system on gravelly, sandy loam to silt loam soils, seasonal water use for second, third, and fourth leaf trees was estimated to be 21.2, 38.0, and 42.0 inches, respectively. OBJECTIVES 1) Evaluate water use by young, non-bearing Chandler walnut trees from planting through fourth leaf when irrigated at reasonable frequencies and durations to sustain desirable levels of midday stem water potential and tree growth. 2) Provide guidelines to assist growers, particularly those new to the walnut industry, with information to help them determine the appropriate irrigation frequency and duration to develop a newly planted orchard into a healthy, productive orchard within an optimal time frame. 3) Understand the relationship between real-time estimates of crop evapotranspiration (ETc) for mature, bearing walnut orchards and young, developing non-bearing orchards. SIGNIFICANT FINDINGS Water use was measured in young developing (first through fourth leaf) Chandler walnuts. Photosynthetically active radiation (PAR) in August was measured with a mobile light bar to characterize the extent of canopy development. PAR levels averaged between 7.2 and 49.0 percent as the orchards developed from a new planting through the fourth leaf. A table of semi-monthly estimates of crop evapotranspiration (ETc) has been prepared for the months of April through October. This table can be related to specific irrigation system designs to develop an approximate schedule of irrigation frequency and duration for developing orchards. Site specific conditions needs to be considered when applying these ETc estimates and adjusted accordingly. This table also includes estimates of seasonal ETc for first, second, third, and fourth leaf walnuts. California Walnut Board 113 Walnut Research Reports 2013

2 Similarly, a second semi-monthly table of approximate ETc is provided for first, second, third, and fourth leaf walnut trees. Except, these estimates are expressed as a percentage of real-time estimates for mature, bearing walnut orchards. These percentages are given because often real-time estimates of ETc for walnut are only available for mature, fully bearing walnut orchards and the estimates for developing orchards require adjustment for the smaller canopies. This table provides a first approximation of how to adjust. PROCEDURES A group of young walnut orchards ranging from first through fourth leaf in age was located on the west side of Tehama County in the northern Sacramento Valley. Each orchard consisted of the Chandler variety planted on Paradox hybrid rootstock at 120 trees per acre (26 x 14 spacing). A minimum pruning strategy was used to develop these orchards. Herbicide strip sprays were used to control weeds in the tree row. Orchard middles were mowed periodically. All of the orchards were under the same ranch management. The predominant soils ranged from gravelly, sandy loams to silt loams. During the first and second years, the young walnut trees were irrigated with drip irrigation. During the first year, each tree was irrigated with two, one gallon per hour drip emitters. An emitter was located eight inches to each side of the newly planted tree. In the second year, two additional one gallon per hour drip emitters were added for a total of four, one gallon per hour emitters. Two emitters were placed on each side of the tree approximately 8 and 30 inches to each side of the second leaf tree. Prior to emergence of the third leaf, the drip irrigation system was converted to full coverage, minisprinkler systems (Nelson R10). ETc was measured one or two times per week in two replications of first leaf trees and three replications of second, third, and fourth leaf trees. Applied irrigation water measured with inline flow meters, in-season rainfall that occurred after leaf-out was measured with an on-site rain gauge, and the soil storage contribution from winter rainfall was determined with a neutron probe by measuring the change in soil moisture for a six foot soil profile from leaf-out until the end of the season. The applied water, in-season rainfall (assuming 60 percent effective), and the change in soil storage from Spring to Fall were summed to provide an approximation of ETc. Irrigation frequency and duration was guided by taking midday stem water potential measurements one or two times per week. Figure 1 illustrates the typical seasonal pattern of midday stem water potential in these orchards. The management strategy was to maintain midday stem water potential levels between -4 to -8 bars during May through August. Beginning in September, more tree stress was allowed (-10 to -11 bars) to reduce late season shoot growth and promote the green shoot tissue to mature into woody tissue that is more tolerant of cold fall and winter temperatures. These levels indicated low to moderate tree stress that should promote sufficient vegetative growth but still guard against too much irrigation and associated root diseases. During years 1 and 2, drip irrigation was applied daily with the exception of Sundays. Daily run time ranged from about one hour when the trees were newly planted and increased to between two or three hours per day as the trees grew and the canopy size increased. Irrigation run times were sometimes split into two daily sets to encourage better lateral distribution and less deep percolation of the irrigation water California Walnut Board 114 Walnut Research Reports 2013

3 in the courser textured soils. Beginning in year 3 when the orchards were converted to minisprinkler irrigation, the orchards were generally irrigated one or two times per week. Canopy development and size was characterized by measuring photosynthetically active radiation (PAR) using a mobile light bar at solar noon during August. PAR levels averaged between 7.2 and 49.0 percent as the orchards developed from a new planting through the fourth leaf. The photos in Figure 2 show the progression of orchard canopy development from planting through fourth leaf. The first year of walnut harvest occurred during fourth leaf. Walnut yields were measured with portable weigh wagon in the same tree rows where PAR was determined. Yields typically averaged about 800 lbs per acre at fourth leaf. RESULTS AND DISCUSSIONS Table 1 provides semi-monthly estimates of crop evapotranspiration (ETc). Each season ETc was lowest in the spring at leafout, reached a maximum in mid to late July, and then declined in the fall as the trees approached dormancy. Estimates ranged from 0.1 to 2.1 inches per semimonthly interval for first leaf trees. Estimates of semi-monthly ETc increased to a range of 0.3 to 3.1 inches for second leaf trees and to 0.5 to 5.1 inches for third leaf trees. By fourth leaf, semi-monthly estimates of ETc ranged from 0.8 to 5.1 inches and were similar to levels published for mature orchards in the University of California Walnut Production Manual publication Seasonal water use by first, second, third, and fourth leaf Chandler walnuts averaged 14.6, 21.3, 38.5, and 40.1 inches, respectively, when irrigation frequency and duration was managed to sustain low to moderate crop stress (-4 to -8 bars midday stem water potential) from leaf out through early September. Moderate to high tree stress (up to -11 bars) was allowed in mid to late September and October to slow shoot growth and promote hardening and maturity of new shoot growth to guard against frost and cold injury. Table 2 provides semi-monthly estimates of ETc expressed as a percentage of ETc for mature, bearing walnut trees. These estimates are provided to assist growers who may want to use realtime estimates of ETc to account for current weather conditions instead of relying on the time and site specific estimates provided in Table 1. Usually, real-time estimates of ETc for walnut are given for mature, bearing orchards and rarely provided for young developing orchards with rapidly changing canopies. The semi-monthly estimates for first leaf walnuts ranged from 15 to 45 percent of ETc for mature, bearing orchards. Semi-monthly estimates of ETc for second leaf trees ranged from 35 to 60 percent of ETc for mature, bearing orchards. Semi-monthly estimates of ETc for third leaf trees ranged from 70 to 100 percent of ETc for mature, bearing orchards. Semi-monthly estimates of ETc for fourth leaf walnuts closely matched (100 percent) published estimates of ETc for mature, bearing orchards. These semi-monthly estimates of ETc for newly planted, developing orchards as outlined in tables 1 and 2 are only approximate guidelines based on limited data. Site specific conditions require that these guidelines be applied with caution and ideally they should be confirmed by either monitoring orchard water status with a pressure chamber or some form of soil moisture monitoring. These guidelines should provide a reasonable estimate of ETc for young developing orchards and assist growers, particularly those new to the industry, with determining reasonable California Walnut Board 115 Walnut Research Reports 2013

4 irrigation frequency and duration. However, they may very well require some adjustment for differences in weather, orchard design, cultivars and rootstocks, irrigation system design, pruning strategies, and management of orchard floor vegetation. Also, when applying these approximations of ETc, remember that these ETc estimates do not necessarily equate to the irrigation requirement. It is important to properly account for soil storage contributions from winter rainfall and effective rainfall that may occur after leaf-out. The irrigation requirement to meet these levels of ETc may be substantially less in the walnut growing regions of the Sacramento Valley than in the San Joaquin Valley because annual rainfall is usually higher in the Sacramento Valley. Figure 1. Example seasonal trend of midday stem water potential and the general water management strategy in these developing Chandler walnut orchards. California Walnut Board 116 Walnut Research Reports 2013

5 Figure 2. Illustration of orchard canopy development from first through fourth leaf. (A) Newly planted, June budded trees; (B) Growth of tree at end of first leaf; (C) Second leaf; (D) Third leaf; and (E) Fourth leaf. A B C D E California Walnut Board 117 Walnut Research Reports 2013

6 Table 1. Approximate ETc (inches) of developing walnut trees in semi-monthly intervals. Date First Leaf Second Leaf Third Leaf Fourth Leaf Apr Apr May May June June July July Aug Aug Sept Sept Oct Oct Season Total California Walnut Board 118 Walnut Research Reports 2013

7 Table 2. Approximate ETc of developing walnut trees (percentage of ETc by mature, bearing walnut trees) in semi-monthly intervals. Date First Leaf Second Leaf Third Leaf Fourth Leaf Apr Apr May May June June July July Aug Aug Sept Sept Oct Oct California Walnut Board 119 Walnut Research Reports 2013