Measurement
of Potential Tree Water Stress Effects on Almond Tree Growth and Yield
Larry
Schwankl
John
Edstrom
Ken
Shackel
Terry
Prichard
Jan
Hopmans
Introduction
The 20-acre Marine Avenue almond planting of the Nickelís Soils Laboratory
near Arbuckle, CA was established in 1990 to compare the response of 4 almond
varieties (Butte, Monterey, Nonpareil, and Carmel) to irrigation by various
microirrigation systems (surface drip, subsurface drip, and microsprinklers). A
key component of the comparison has been to apply the same amount of water since
planting with each of the different microirrigation systems. Thus, what is being
compared is the response of the trees to the various microirrigation methods;
not differences in the amount of applied water.
Previous years of investigation indicated that there was a strong trend
toward a yield and tree growth advantage associated with use of microsprinklers.
To investigate a possible cause of this difference - differential water stress among trees irrigated with different
microirrigation methods - a project was undertaken during the summer of 1998.
This project used the pressure bomb to measure the water status across the
season of Nonpareil almond trees irrigated by various microirrigation systems (microsprinklers,
surface drip, and subsurface drip).
During the 1999 growing season, the project was continued with some minor
changes in the trees/ microirrigation systems being monitored. In addition,
detailed soil moisture monitoring (using a neutron probe) was done around a
surface drip irrigated tree and around a microsprinkler-irrigated tree. The soil
moisture monitoring was undertaken to draw comparisons between soil moisture
information and leaf water potential measurements.
Objectives
| (1) |
Monitor, using the pressure bomb, leaf water stress of trees irrigated by
surface drip and microsprinklers to determine if there is differential water
stress occurring as a result of the type of microirrigation delivery system.
|
a.
|
Generally, none of the irrigation treatments exhibited elevated leaf water
potential levels until the end of August. Greater pressure bomb measurements
(more negative leaf water potential measurements) would indicate greater water
stress in the tree. At the end of August, irrigation applications were slightly
cut back to increase water stress to aid in hull split.
|
b.
|
There is a very definite reflection of irrigation practices in leaf water
potential measurements. Irrigating at less than tree ET estimates results in
increased pressure bomb measurements (greater water stress), and vice versa.
|
c.
|
Even though the irrigation intervals during the peak water use periods
differ for the drip-irrigated (daily irrigations) and the microsprinkler-irrigated
(3-day interval) trees, the leaf water potential levels remain nearly the same.
There was no difference across the season in tree water stress between the
microsprinkler- and surface drip-irrigated trees.
|
d.
|
Preliminary analysis of leaf water potential and neutron probe
measurements indicate that a 3-day irrigation interval between mid-summer
microsprinkler irrigations is appropriate. Extending the irrigation interval to
4 days or longer, under the low waterholding capacity soil conditions at Marine
Ave., resulted in increased pressure bomb levels (increased water stress
levels).
|
e.
|
Even though water stress was imposed beginning at the end of August, it
was not an extreme stress. The average pressure bomb measurements for both the
surface drip and microsprinkler treatments remained below 12 bars. It was not
until the beginning to mid-September when pressure bomb measurements rose to
nearly 15 bars.
|
f.
|
The pressure bomb measurements for the trees in the double-line surface
drip 200% ET treatment showed minimal stress throughout the season. The water
applications of this treatment were reduced beginning at the end of August by
shutting off one of the two lateral lines per tree row. It is likely that stored
soil moisture supplemented irrigations through harvest and the trees did not
exhibit increased (more negative) leaf water potential levels.
|
| (2) |
Monitor soil moisture in detail, using a neutron probe, within and
surrounding the soil volume wetted by the irrigation system. This will be done
for both a microsprinkler and a drip irrigated tree. Trees selected for soil
moisture monitoring will also be monitored for leaf water stress so that
correlations between the two measurements can be made.
|
a.
|
Soil moisture reserves decreased across the season for both the surface
drip and microsprinkler treatments. Much of this soil moisture depletion
occurred in the soil volumes not being refilled by the drippers or
microsprinkler.
|
b.
|
Irrigations exceeding almond ET during a week period resulted in an
increase in the stored soil moisture. The opposite was also true. Work is
on-going investigating where in the root zone (depth and position relative to
the tree) soil moisture is being depleted and refilled across the season.
|
c.
|
During the season, the amount of stored soil moisture relative to the
early season, fully recharged, soil moisture condition, decreased to a greater
extent around the surface drip irrigated trees than it did around the
microsprinkler-irrigated tree. The microsprinkler system has a larger wetted
area (and volume) than does the drip system. Soil moisture measurements indicate
that the drippers actually keep the soil wetter in the volume they recharge than
does a microsprinkler, but that wetted volume is smaller for a drip system.
|
| (3) |
Monitor tree trunk growth and almond yield, as a function of irrigation
method, to determine if there continues to be differences as a result of
microirrigation treatments.
|
Tables 1 and 2 summarize the almond tree growth and yields at the Marine Ave.
orchard. While the 1999 trunk crossectional area measurements have not been
statistically evaluated for significance, the microsprinkler-irrigated trees
continue to be somewhat larger than the drip-irrigated trees. The differences
seem to be decreasing as the orchard has reached full canopy development.
There does not appear to be any significant difference between the 1999
yields for the various microirrigation treatments in the Nonpareil almonds. In
fact, the Nonpareil microsprinkler treatment average is slightly (5%) less than
the average yield of the surface drip irrigation treatment. Now that the Marine
Ave. orchard canopy has reached full cover, it appears that there is less
difference in almond production between the microsprinkler-and drip-irrigated
trees.
