Micro Irrigation Systems For Almonds

John Edstrom UCCE
Farm Advisor

Larry Schwankl
UC Irrigation Specialist

Stan Cutter
Nickels Estate

Micro-irrigation systems are in widespread use throughout all central valley almond districts. Controversy continues as to the relative merits of the different types of systems-surface drip, microsprinkler/jet and subsurface drip, SDI. To evaluate these systems under commercial conditions a 22 acre replicated field trial was established in 1990 planted to Nonpareil (1/3), Butte (1/3), Carmel (1/6) and Monterey (1/6). Eight irrigation designs were initially under evaluation. Two types of SDI were abandoned this year (#7 & #8) leaving systems 1-6 :

Subsurface drip plots were established the first year using single hose surface systems and early in the second growing season converted to SDI. Hoses were shanked in to a depth of 15 inches 2 feet from trunks on the single hoses and 4 feet on either side of the row for double hose systems. Chlorine has been injected monthly during the growing season at 7 ppm to help emitter plugging.

The amount of applied water has been about equal between the different systems over the years. Micros generally received 2-3 irrigations per week mid season while all drip systems were run 5-6 days per week. Total seasonal water applied ranged from a low of 24 acre inches in 1998 to a high of 40 inches in 1997. These were based on evaporative demand as determined by Eto calculations from the Colusa CIMIS station.

Summary of Past Results:

Production in this planting has been quite good for the Butte, Monterey and Nonpareil varieties while Carmel performance, both in tree development and production have been consistently mediocre. Measurements show that trunk diameters of micro irrigated trees are also slightly larger than drip and SDI trees. On average over the years of this test the microjet irrigated trees have yielded more than trees on either type of drip system. This increase has been variable over the years and between the varieties. Nonpareil and Butte yields increased in some years with micros while Carmels have never shown a response. Monterey production has also been inconsistent between the three irrigation systems but with a slight advantage given to the micros. SDI yields have also been variable between the different configurations of SDI and between seasons and varieties. Generally yields have tended to be a little lower with SDI compared to drip and micro. Double hose SDI plots yielded more than single. Yields from the double surface drip hose plots have been very disappointing. This is probably due not to the dual hoses but rather the lower 0.5 gph output of these emitters and the resulting shallower water penetration. When yield differences are found between micros and drip, they are in the range of 10%. No differences in weight per kernel has ever been found for any irrigation system. All yield differences can then be attributed to more kernels per tree and not larger individual kernels.

Other Observations:

Early in the test several flexible hose risers were pinched closed (strangulation) by enlarging roots in the SDI plots. We suspect this is due mainly to the close proximity (within 2 feet) of the risers to the tree trunks. Of major concern has been the recent problem of root intrusion into the buried SDI hoses. Almond tree roots have grown in through the emitters plugging buried hoses and reducing flow of water. So far only the non herbicide impregnated SDI emitters have shown this problem. High concentrations of chlorine @ 240 ppm injected into the SDI hoses failed to solve this problem. Trifluralin herbicide injection is a preventative treatment. No currative treatments are available. Plugged lines will be replaced with dual surface hoses to revive these low performing trees.

Microjets have also exhibited problems, mainly due to insects (Fullers Rose Weevil) plugging the jets. Microjet irrigated plots also require 2-3 extra foliar herbicide applications per season and still weeds interfer somewhat with harvest. Weeds in the SDI plots are considerably less and the absence of surface hoses increases harvest efficiency greatly by eliminating hand raking and allowing harvest irrigations to proceed without wetting the crop. Surface drip systems allow harvest watering to some extent also but microjets have been problematic.

Latest Results

Production figures for 1999 (Table 1) show that all irrigation systems (that continue to function) yielded the same. Nor were differences in kernel size found between any of the irrigation systems, for any variety.

Flow meters measured 36" of applied water this season, equal to full 1.0 Et_c as calculated from Colusa CIMIS station data. The 1.2 treatment was added in 1997 to check the adequacy of our 1.0 ET_c calculated application rate. The 1998 data provided some indication that Butte responds to moisture levels higher than our ET_c amounts. Beginning in 1999 we readjusted our irrigation system efficiency factor which increased applied water by 10%. All systems continued to receive equal amounts of applied water, 5-6 days/week for drip/SDI and twice per week for Micros. Tree moisture stress levels as monitored by pressure bomb readings were minimal except for short periods around harvest. Leaf yellowing and drop was more evident on the Monterey variety again this season during harvest.

Due to the root intrusion problem discovered last year the SDI systems without trifluralin will be disconnected and converted to double hose Geoflow SDI. We plan to install this dual hose system at a depth of 6-8 inches (above the old 16 inch deep hoses ). Affected trees will be monitored for their recovery.

This year we experienced two insect related irrigation problems. First, earwigs invaded the irrigation system airvents resulting in clogged drip and microjet emitters. This problem was solved by attaching screens to all airvent openings. Second came the Fullers rose weevil depositing eggs in jet openings, again clogging the microjets and increasing maintenance expenses. Daily purging of all microjets may help avoid this problem in the future. If an insecticide is injected via the irrigation system for ant control then insect clogging will be avoided. Weeds were not as troublesome this year as reported last season do to more diligent herbicide "middles management" before harvest. But again, additional spot spraying of weeds was required mainly in the micro plots.

Subsurface drip continues to excel in relation to orchard floor management and harvest efficiency. After nine years of experience we are beginning to see more commercial potential for this management intensive technique. However, we have concerns regarding tree roots pinching buried hoses, the siphoning of silt into emitters upon system shutdown and damage by gophers. So far our experience shows that root intrusion can be avoided if the trifluralan impregnated emitters are used.

Table 1.
MICRO-IRRIGATION
ALMOND YIELDS
1999

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Copyright © 1999 Colusa County Cooperative Extension, Univ. of California
Last modified: September 01, 2000