Micro-Irrigation Systems for Almonds- Nickels Evaluation

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

In 1997 we retrofitted some of the micro plots to apply 150% of Etc to check the adequacy of our water application rates.

This change resulted in higher yields for the Butte and Monterey varieties in 1999 but not for Nonpareil or Carmel when compared to micros applying 1.0 Etc. This provides some indication that Butte and Monterey respond to moisture levels higher than our ET_c amounts at least under high yield conditions.

The trial received about 32" of rainfall last season thoroughly wetting the soil profile to 4 feet. Abnormal weather resulted in 2.0" of rain falling in each of the months of March, April and May.

Detailed soil moisture measurements were taken in 1998 for trees irrigated with single drip hoses. Soil moisture levels declined predictably in the densely rooted soil beneath emitters. Soil moisture some nine feet out from the drip hose (beneath row middles) also declined from March to July. The pattern of decline indicates that tree roots extracted water far beyond the confined root zone irrigated in summer.

This supports the idea that even drip irrigated trees can widely access winter stored moisture. It also suggests that micro-irrigated trees may not have a big advantage in this regard. Typical rainfall for this location is 16-17" vs. 32" this last season. Apparently, spring root growth has extended rooting far beyond summer irrigated zones. Micro and drip tree roots accessed much of this moisture in 1999.

Leaf samples sent to the UC Davis lab found no differences in leaf % nitrogen between drip and microjet irrigated Nonpareils.

In summary, no system has been shown to be clearly superior. Each type of system has advantages and problems. Choice of a system should be based on the specific conditions at the orchard site ie: soil texture, depth, slope; water availability, quality, cost, pressure; frost danger; tree spacing, etc. We will continue to report on the progress of this long term trial.

Fisher’s LSD P=0.10

Almond dry nut yields (lbs./acre) by almond variety and irrigation treatment. Statistical comparison of yield was done by variety and by year. Numbers followed by the same letter are not significantly different at the 5% level.