ANR Publication 8515 | February 2015http://anrcatalog.ucanr.edu
DAVID DOLL, University of
California Cooperative Extension
Farm Advisor, Merced County
KENNETH SHACKEL, Professor,
Department of Plant Science,
University of California, Davis
DROUGHT TIP
Drought Management for
California Almonds
Impacts of Stress on Almond Growth and Yield
Almond trees are tolerant to drought conditions and respond to water availability with increasing yields. Research has shown that trees are
able to survive on as little as 7.6 inches of water (Shackel et al. 2011), but
they produce maximally with 54 to 58 inches in many areas of California
(Sanden 2007). Minimizing water stress increases growth and yield due
to increased rates of photosynthesis and respiration.
Water and carbon dioxide are required by plants for photosynthesis. Water is provided
through the root system of the tree, while stomata, or “windows,” on the lower leaf surface
are responsible for allowing carbon dioxide to enter the leaf and oxygen to leave. As this
gas exchange occurs, substantial amounts of water vapor is also lost through the stoma via
transpiration. When the water loss potential from transpiration exceeds the amount of soil-
water the roots can easily absorb, the plant will begin to appear stressed. If water applications
through either irrigation or rainfall are not adequate to alleviate this stress, stomatal closure will be initiated, reducing
gas exchange, rate of photosynthesis, and production of carbohydrates. This limits the amount of energy available for the
many processes, negatively impacting vegetative growth and potentially fruit and kernel development.
The severity of stress determines its effect on the tree. Low to moderate levels of plant stress often occur within
orchards and may be beneficial. Research has shown that an application of moderate stress at the onset of hull split helps
to reduce the fungal disease hull rot and synchronize hull split (Teviotdale et al. 2001). Mild to moderate stress levels, if
monitored, are useful for irrigation scheduling, as plant stress levels indicate the current soil-water status (Fulton et al.
ANR Publication 8515 | Drought Management for California Almonds | February 2015 | 2
2014). Severe plant stress, however, should be avoided when possible,
as it impacts plant growth. Responses to severe water stress depend
on when the stress is imposed. Impacts on vegetative growth, fruit
and kernel development, and floral bud development are outlined
below.
Impacts on Vegetative Growth
The period after leaf out is a time of rapid vegetative growth that is
necessary to establish fruiting positions and carbohydrate reserves
for future yields. Water use in the spring is low at the beginning but
increases as leaves fully expand and the canopy develops. Typically,
the relatively cool temperatures, short day length, and high relative
humidity during this period mean that trees require less water, and
the water demand may be met by the soil-water stored in the root
zone from winter rains. In these cases, trees may grow relatively
stress-free with minimal irrigation until full leaf expansion around 4
to 5 weeks after bloom.
Vegetative growth is reduced by moderate to severe water stress
after full leaf expansion (or at any point in the growing season for
a long enough period). Research has shown that nut load is directly
related to canopy growth and size (Prichard et al. 1996; Lampinen et
al. 2007). Therefore, lack of canopy growth due to irrigation deficits
after full canopy expansion until harvest leads to a reduction of
fruiting spurs and future yield potential. One year of reduced spur
production will not necessarily lead to a dramatic decrease in next
year’s yield, but the effect can be cumulative if consecutive years of
deficit irrigation occur and the number of fruiting spurs decrease.
If the viable spur pool is already reduced due to a year of deficit
irrigation, future yields will decline more if deficit irrigation is
extended due to drought or other circumstances that may limit water
availability. This phenomenon has been observed in trials in Spain
and California, where fruit loads were unaffected by applied water
stress in the first 2 years of the 4-year trial but were reduced in the
final 2 years.
Impacts on In-Season Kernel Development
Fruit and kernel development follow a three-stage process (Kester et
al. 1996) (fig. 1). During stage I, rapid growth of the hull, shell, and
integuments occurs. The kernel begins to form as a white structure
with a translucent jelly. Stage I ends once the maximum external
dimensions of the hull, shell, and kernel have been reached, which
is about 2 months after bloom. Severe tree water stress rarely occurs
during stage I fruit growth (petal fall through late April and May)
due to stored soil moisture, shorter days, and cooler temperatures;
but if it does, it is thought that increased nut drop, smaller fruit,
and kernel size will be observed because of reduced photosynthate
directed toward cell division and expansion.
Stage II is a period of rapid fruit expansion. The hull and
shell reach maximum size about 2 months after pollination. This is
followed by shell hardening and kernel expansion (or hardening of
Figure 1: The three stages of almond fruit development and the typical
length and weight of the fruit at each stage.
Pericarp length (outside hull dimension)
Seed length
Embryo length
Endosperm length
Embryo dry weight
Le
ng
th
(c
m
)
W
ei
gh
t (
g)
STAGE 1
Growqth in size of fruit
STAGE 3
Increase in weight of seed
STAGE 2
Growth in
size of
embryo
4.0
3.0
2.0
1.0
0
2.0
1.0
Mar Apr May Jun Jul Aug Sep
ANR Publication 8515 | Drought Management for California Almonds | February 2015 | 3
the shell and expansion of the kernel), with a corresponding increase
in the dry weight of the kernel. This period, which is typically in late
May or early June in California, has a high seasonal irrigation demand
during which almonds are very sensitive to water deficits.
The final period of fruit growth is the preharvest period of
stage III. At this point, hull, shell, and kernel differentiation are
complete and the kernel begins to accumulate solids at a continuous
rate until harvest (maturity). Harvest is signaled by two events:
the onset of hull split and the formation of an abscission layer
between the peduncle and nut. Both of these events are impacted
by irrigation practices. Too much water can increase the duration of
the hull split period and thus delay the onset of harvest. In contrast,
too little water can decrease kernel weight and result in poor nut
removal due to an increase in the number of nuts with dried hulls
adhering tightly to the shell (i.e., “hull tights”).
Determining the timing and potential impact of water stress
during stage III of kernel development is complicated. A severe
stress imposed after kernel fill through harvest reduces kernel dry
weights and produces textured or shriveled almond kernels. If a
moderate, regulated stress level of 14 to 18 bars stem water potential
(SWP, discussed below) is imposed from kernel fill through hull
split, however, the impacts on kernel weight and size are minimized,
while the synchrony of hull split is improved. In a 4-year study, this
regulated deficit did not impact kernel yields over the experiment’s
duration, but it did reduce kernel weight by 2 to 3% compared with
the full irrigation treatment (Stewart et al. 2011). This reduction of
applied water saved 10 to 15% of the total season’s water budget.
It is critical to maintain irrigation through the post-hull-split/
preharvest period, as the too-high water stress level reduces kernel
weight and quality. A study conducted in Kern County determined
that there is an indirect relationship between the length of preharvest
irrigation cutoff and kernel quality (Goldhamer and Viveros 2000).
In this study, preharvest irrigation cutoffs occurred from as late as
8 days to as early as 57 days prior to shaking. Kernel quality was
negatively impacted when preharvest cutoffs extended beyond
15 days. This cutoff period, however, varies among soil types and
irrigation management strategies. Monitoring should be done on an
orchard basis. If needed, water should be applied to minimize the
stress during the preharvest timing (after hull split to harvest). This
prevents the negative impacts on kernel weight and quality.
Impacts on Development of Fruit Buds
Severe stress from deprivation of postharvest irrigation has been
found to decrease the next year’s crop yield more than does a
preharvest water deficit. Studies have shown that very severe
postharvest stress (–40 bars predawn leaf water potential) caused a
52% reduction in ‘Nonpareil’ bloom density and a 94% reduction
in fruit set, resulting in a 73.6% reduction in the following year’s
yield (Goldhamer and Viveros 2000). Furthermore, yield loss was
observed in all treatments across this trial that withheld postharvest
irrigations, regardless of the preharvest cutoff. This reduction was
attributed to stress impacts on floral bud development that occur
late in the season.
In contrast, a study near Manteca was successful in reducing
postharvest irrigations without any negative crop impacts (Prichard
et al. 1996). In this study, water was applied based on tree stress and
reduced based on low observed tree stress level. This reduction was
thought to be possible due to adequate soil-water reserves in the
root zone from irrigation and fall rainfall that may occur in some
almond-growing regions of California. Tree stress levels should be
monitored into the postharvest season to determine the need for
postharvest irrigation.
The timing of bud development varies by cultivar and
geographic region and can occur before and after harvest for
late- and early-harvested varieties, respectively (Lamp et al. 2001).
Since multiple varieties are typically planted within an orchard,
it is important to minimize stress during the period between
hull split and harvest in order to maximize the following year’s
production. If possible, water demands should be met through
the end of September. Care should be taken during this period
when using low-volume or micro-irrigation systems because the
lower application rates do not allow for quick recovery of severely
depleted soil moisture.
ANR Publication 8515 | Drought Management for California Almonds | February 2015 | 4
Understanding Deficit Irrigation (DI)
Water deficits occur when a tree’s water demand exceeds the
amount of water available in the soil. These deficits increase water
tension within the plant, and when this stress is high enough it will
negatively affect many plant processes. As described above, almond
trees have a varying tolerance of stress throughout the season.
Ideally, growers would achieve the most efficient use of irrigation
water (i.e., the most “crop per drop”) when they irrigate just before
water stress is low enough to cause a significant reduction in yield.
This method of applying water during critical almond development
periods and limiting water application during less-critical periods is
called strategic deficit irrigation (SDI).
Plant water stress is commonly evaluated by measuring
midday stem water potential (SWP) using a pressure chamber
or equivalent device. SWP is a direct measure of water tension
(negative pressure) within plant and is given in metric units of
pressure, such as bars (1 bar is about 1 atmosphere of pressure) or
megapascals (MPa; 1 MPa equals 10 bars or about 145 psi). Even
under fully irrigated conditions, the July SWP in almond trees
at midday can be as much as –1 MPa simply because this much
tension is required to pull water out of the soil and through the tree.
Technically, SWP should always be shown as a negative value (e.g.,
–1MPa), but in conversation we often omit mentioning “negative”
before the value. More information about the operation of the
pressure chamber can be found in Using the Pressure Chamber
for Irrigation Management in Walnut, Almond, and Prune (ANR
Publication 8503; Fulton et al. 2014).
Through the use of a pressure chamber, plant stress can be
readily monitored. Water applications can be made once specified
levels of stress are reached, reducing stress extremes and damage
to the current crop and future yields. Further, SDI may be used
to extend watering intervals and save water. The downside of this
approach, however, is the challenge of managing the minimal soil
moisture reserves required to achieve SDI while preventing too
much stress with micro-irrigation systems that are designed to “just
meet” peak crop water demand.
Putting It into Practice: Drought Management
Irrigation Strategies
As a consequence of the drought and diminishing ground water
supplies, water availability will be limited in many major production
areas. Growers will need to decide when to apply water to reduce
the impacts of stress on trees. The best place to start is to know
how much water has typically been applied annually to the orchard.
Once this estimate is known, it is possible to compare this amount
with what is considered a “fully irrigated” orchard. This distinction
is suggested because experience has shown that many commercial
almond orchards are under-irrigated. A mature, fully irrigated
almond orchard that shades about 80% of the ground area at noon
in midsummer can use approximately 49 to 58 acre inches of water,
depending on location (Goldhamer and Girona 2012; unpublished
data). This calculation is based on the 30-year averaged reference
crop evapotranspiration data for the respective area, as determined
by the statewide California Irrigation Management Information
System (CIMIS), and may vary slightly from year to year. This
point of comparison is needed since a further reduction of applied
water in a traditionally under-irrigated orchard can lead to severely
stressed trees and unintended outcomes. Keep in mind that
rainfall and soil moisture depletion are considered water applied
to an orchard and may reduce total required irrigation. For more
information on scheduling irrigation, see Scheduling Irrigations:
When and How Much Water to Apply, UC ANR Publication 3396
(Hanson et al. 1999).
Two irrigation strategies exist for drought management of
almond: hull split SDI and proportional deficit irrigation (e.g., 80%
of normal crop ETc.) The appropriate strategy depends on water
availability and use of a pressure chamber.
Hull Split SDI
Hull split SDI maintains full irrigation until the completion of
kernel fill. After kernel fill and until 90% hull split, irrigation
is applied only when trees reach SWP values of –14 to –18 bars
(Shackel et al. 2004). Field research has shown that this technique
decreases water use by as much as 34% during this period,
ANR Publication 8515 | Drought Management for California Almonds | February 2015 | 5
reducing total seasonal water use by about 15% while having
minimal impacts on the current and next season’s crop (Stewart et
al. 2011). In practice, it can be difficult to fine-tune the irrigation
schedule to this SWP threshold. Many growers initially reduce water
applications by 50% around mid-June and adjust the amount of
subsequent irrigations once stress levels increase and soil moisture
depletion occurs. Water should be applied prior to harvest to
improve hull split and reduce hull tights (Prichard et al. 1994).
This strategy is a particularly effective method for reducing hull rot
(Tetviotdale et al. 2001), if that is a problem, but it also improves
harvestability by reducing the force and time required for shaking,
which can benefit the long-term health of the orchard.
Proportional Deficit Irrigation
If a pressure chamber is unavailable or the anticipated seasonal
water deficit is greater than 15% for the seasonal ETc, reduced water
applications can be made by applying a fixed proportion of ETc. In
this method, the amount of water available for the season should
be calculated as a percentage of full ETc. This percentage should
be applied to spread the deficit evenly across the season. In other
words, if it is determined that enough water is available to supply
only 55% of ETc for the whole season, each irrigation would match
55% of the determined ETc for that irrigation period. An example
is given in table 2. Current-season and future yield loss should be
expected when using this strategy, but research has shown this
to be the most effective strategy in minimizing losses for large
irrigation deficits (Goldhamer et al. 2006).
Imposing whole-season SDI or applying water as a percentage
of ETc will help preserve kernel quality and future yields as much
as possible. Nevertheless, the current season’s yield will begin to
drop, and further declines in production can be anticipated in
subsequent years if a drought continues. By employing whole-
Table 1. Thirty-year average evapotranspiration rates for unstressed pasture (ETo)1 and almonds (ETc)2 in inches for several CIMIS
zones within almond-producing areas of California
Zone 124 Zone 145 Zone 156 Zone 167
Month Kc3 ETo ETc ETo ETc ETo ETc ETo ETc
Jan 0.40 1.24 0.50 1.55 0.62 1.24 0.50 1.55 0.62
Feb 0.41 1.96 0.81 2.24 0.92 2.24 0.92 2.52 1.04
Mar 0.62 3.41 2.11 3.72 2.30 3.72 2.30 4.03 2.49
Apr 0.80 5.10 4.09 5.10 4.09 5.70 4.57 5.70 4.57
May 0.94 6.82 6.44 6.82 6.44 7.44 7.02 7.75 7.31
Jun 1.05 7.80 8.20 7.80 8.20 8.10 8.51 8.70 9.14
Jul 1.11 8.06 8.93 8.68 9.61 8.68 9.61 9.30 10.30
Aug 1.11 7.13 7.90 7.75 8.59 7.75 8.59 8.37 9.28
Sep 1.06 5.40 5.73 5.70 6.05 5.70 6.05 6.30 6.68
Oct 0.92 3.72 3.41 4.03 3.69 4.03 3.69 4.34 3.97
Nov 0.69 1.80 1.23 2.10 1.44 2.10 1.44 2.40 1.64
Dec 0.43 0.93 0.40 1.55 0.66 1.24 0.53 1.55 0.66
Total (in) 49.73 52.61 53.73 57.72
Notes:
1Normal year evapotranspiration of unstressed grass (reference crop, ETo ) 30-year CIMIS average for the respective zone.
See cimis.water.ca.gov/App_Themes/images/etozonemap.jpg.
2Evapotranspiration rates for almonds were calculated by multiplying ETo by the crop coefficient (Kc).
3Referenced crop coefficient (Kc) (unpublished data)
4Zone 12 ETo rates from Chico, Fresno, Madera, Merced, Modesto, and Visalia.
5Zone 14 ETo rates from Newman, Red Bluff, and Woodland.
6Zone 15 ETo rates from Bakersfield and Los Banos.
7Zone 16 ETo rates from Coalinga and Hanford.
cimis.water.ca.gov/App_Themes/images/etozonemap.jpg
ANR Publication 8515 | Drought Management for California Almonds | February 2015 | 6
season SDI, stress imposed during stage I will reduce fruit load and
size. This leads to a reduced amount of photosynthate required to
fill the nuts during stage II, producing more complete kernel fill and
higher quality. This contrasts to the erroneous “feast then famine”
strategy of fully irrigating the almonds through stage I, then deficit-
irrigating in stage II. This latter strategy results in increased fruit set
or nut load but may reduce kernel fill and increase shriveling. Both
strategies lead to similar field kernel yield per acre, but quality and
marketability, and thus kernel price, will be reduced in the feast then
famine strategy. Future yield reduction depend on the severity of
stress applied during the postharvest period and the overall seasonal
impact on vegetative growth.
Managing Severe, Persistent Drought
Although a rarely used option, SDI can be implemented under
conditions of severe, persistent shortages of irrigation water
supplies. While not yet well researched or documented, reports
of past drought-stricken seasons suggest that trees can be kept
alive with as little as 6 to 8 inches of water (including stored soi
