By Larry E. Williams
Department of Viticulture & Enology
University of California-Davis, and Kearney Agricultural Center
SYNOPSIS: How much irrigation water is required to grow quality
winegrapes depends upon site, the stage of vine growth, row spacing,
size of the vines canopy, and amount of rainfall occurring
during the growing season. Below, growers are presented with means
to determine when irrigations should commence and to calculate full
vine water use based on the results of 10 years of field trials
in California winegrape growing regions. Implications for such information
to assist in vineyard irrigation management are included.
Coastal, winegrape production areas in California are characterized
by warm days and cool nights, although high temperatures (104† to
116†F) may occur for a few days each growing season. Some areas
may have fog lasting late into the morning.
Rainfall is greater in northern coastal valleys and diminishes as
one travels south. In coastal valleys, evaporative demand can range
from 35 to 50 inches of water throughout the growing season (between
budbreak and the end of October).
Many of the soils in the coastal production areas are clay loam
to clay-type soils, which at field capacity, generally hold more
water than sandy-type soils. Since the majority of rainfall occurs
during the dormant portion of the growing season in these areas
and vineyard water use can be greater than the soils water
reservoir after the winter rainfall, supplemental irrigation of
vineyards may be required at some point during summer months.
No matter where grapevines are grown, two major questions concerning
vineyard irrigation management must be answered:
1) When to start? and 2) How much water to apply?
When to start irrigating
Deciding when to begin irrigating can be determined several ways.
Soil-based tools such as a neutron probe and capacitance sensors
can determine the actual or relative amounts of water in the rooting
zone of grapevines. Plant-based tools, such as a pressure chamber,
can be used to measure vine water status.
Regardless of the method, a value is determined which
indicates that the vines may need water. Once this value is reached,
an irrigation event should occur.
Using a pressure chamber
Water has free energy, a capacity to do work. In plants, waters
free energy (or chemical potential) is usually referred to as water
potential. Pure water will have a water potential of 0 bars
(bar is the unit of measurement).
Any solute (such as sugars, mineral ions, and amino acids) added
to water will lower its water potential, i.e. the water potential
will become more negative. The same can be said of a plants
For example, when more water is lost from a leaf via transpiration
than moves into the leaf from the vascular tissue, its water potential
will become more negative due to a relative increase in its solute
concentration. This is important as water in plants and soils moves
from regions where water potential is relatively high to regions
where water potential is relatively low. Such differences in water
potential will result in movement of water from cell to cell within
a plant or from regions within the soil profile that contain more
moisture to those with less.
One way to measure the water potential of a plant organ in the field
(such as a leaf) is by using a pressure chamber.
The leafs petiole is cut and the leaf quickly placed into
the chamber with the cut end of the petiole protruding out of the
chamber. Once the leaf is removed from the plant, the tension in
the petioles xylem is released and the sap withdraws from
the cut surface and moves into the blade.
As the chamber is pressurized, the water potential of the leaf is
raised by the amount of pressure applied so that at the balance
pressure (the pressure required to force the sap to the surface
of the cut end of the petiole), the water potential is zero. The original
leaf water potential plus the balance pressure equals zero. Therefore,
the negative of the balance pressure equals the original leaf
A more complete explanation of the pressure chamber technique, theory,
possible errors, and problems can be found in Measurements of
plant water status, Hsiao.2
The water potential of a plant leaf will be greatest at pre-dawn,
then decline (become more negative) during the day to reach a daily
minimum, then increase as the sun sets. This type of pattern will
occur regardless of the availability of water in the soil profile.
However, pre-dawn and midday minimum values will be more negative
for plants experiencing soil moisture deficits than those given greater
amounts of water.7 Thus, leaf water potential
can be used to estimate the water status of a plant. Units of water
potential are expressed in bars, as mentioned above, or megapascals
(MPa) (1.0 MPa = 10 bars).
In all of my irrigation trials, I have measured leaf water potential
to assess vine water status. I usually measure midday leaf water potential
between 12:30 and 1:30 pm. I select mature, fully expanded leaves
exposed to direct sunlight (no shading on the leaf). I have found
that any fully expanded leaf on the outside of the canopy will be
appropriate as long as it is not senescent (starting to turn yellow),
diseased, or suffering from insect damage.
The leaf blade is enclosed inside a plastic bag (plastic sandwich
bags are satisfactory) and then the petiole is cut with a sharp
razor blade. The plastic bag enclosing the leaf blade is to minimize
transpiration between petiole excision and pressurization within the
chamber. I have found a difference in leaf water potential of 2 to
3 bars between bagged grape leaves and those that were not bagged
(the latter being more negative).
The bagged leaf is placed inside the chamber with the petiole sticking
out (see photo). The time from enclosing the leaf inside the plastic
bag to placing it inside the chamber should be 10 seconds or less.
The chamber is pressurized with compressed nitrogen until the sap
just exudes from the cut end of the petiole. If the sap forms
into a lens or hemisphere, then the sample has been over-pressurized.
The recommended rate of pressurization is less than 1 bar per second
initially, then slowed to less than 0.2 bar per second as the balancing
pressure is approached.2 The end point should
be observed with a magnifying lens and adequate light.
While the above description of the pressure chamber involves use of
compressed nitrogen, a new chamber has been developed that doesnt
require compressed gas cylinders. This chamber is pressurized via
a manual pump and is very portable.
Water potential values obtained by using this technique can be dependent
upon ambient vapor pressure deficit (VPD), which increases as relative
humidity decreases; temperature and light, because all of these contribute
to evaporative demand; time of day the measurement is made; and the
amount of water in the soil profile.
Since, time of day is very important, and the evaporative demand will
vary considerably throughout the day, I limit taking leaf water potential
measurements to one-half hour on either side of solar noon. That is
when a grapevine uses the greatest amount of water on a daily basis.6
I have found that midday leaf water potential values of fully irrigated
vines on a day of low evaporative demand (ambient temperature at the
time of measurement 85†F) will be approximately 1 bar higher (less
negative) than on a day when ambient temperature is 98†F at the time
of measurement. This is also true for vines that are deficit-irrigated.
One can also assess vine water status by taking water potential
readings prior to sunrise (pre-dawn leaf water potential) or by
measuring stem water potential at midday.
Stem water potential is determined by enclosing a leaf in a plastic
bag surrounded by aluminum foil, at least 90 minutes prior to when
readings are to be made. This procedure eliminates transpiration
and the leaf water potential will come into equilibrium with the
water potential of the stem (i.e. stem water potential).
A recent study on almond trees has shown that water potential values
measured on shaded leaves (covered with a damp cloth just before
leaf excision) are very similar to values of stem water potential.1
I have compared leaf water potential of leaves under naturally occurring
shade with stem water potential values of grapevines this past summer
and found the two are very similar in some instances. In other instances
they were not.
I believe the discrepancy is due to the fact that on some trellis
systems (such as the VSP), it is difficult to find leaves that are
completely shaded (no sunflecks present on individual leaves) or
in deep enough shade that transpiration truly is minimal.
Some researchers feel that stem water potential is a better measure
of vine water status than leaf water potential since it somewhat
minimizes the effects of the environment on an exposed leaf as outlined
However, Dr. Merilark Padgett-Johnson and I have demonstrated (unpublished
data) that pre-dawn leaf, midday leaf, and midday stem water potential
of Vitis vinifera cultivars and different Vitis spp. are all highly
correlated with one another and with other measures of vine water
status. Based on these findings, midday leaf water potential is
an appropriate and convenient means of estimating vine water status,
however, one must follow precisely the techniques outlined above.
It is interesting to note that D.A. Goldhamer and E. Fereres found
that a major source of variation in determining tree water status
(stem and shaded leaf water potential) is due to operator error.1
Thus, whoever is taking your vine water status measurements, whether
midday leaf or stem water potentials, should be cognizant of possible
errors associated with their technique.
My research indicates that the midday leaf water potential of vines
that are irrigated at 100% of water use is generally never more
negative than 10 bars (equivalent to a stem water potential
value of 7.5 bars).7 Measurements
are made one-half hour on either side of solar noon, that is 1 pm
In my current irrigation experiments, I generally do not initiate
the application of water until midday leaf water potential is at
or more negative than 10 bars.
At present, many growers and vineyard consultants do not begin irrigation
of white wine cultivars until a midday leaf water potential value
of 10 bars has been reached or a 12 bar value for red
wine cultivars. The date during the growing season these values
are obtained is dependent upon rooting depth of the vines, soil
texture, soil moisture content, vine canopy size, row spacing, trellis
type, and evaporative demand.
For example, in 2001, leaf water potential of Thompson Seedless
grapevines at the Kearney Ag Center did not reach 10 bars
until bloom (the first week of May), at which time irrigations began.
In a Cabernet Sauvignon trial near Oakville in 2001, irrigation
was not initiated in a trellis and irrigation study until a midday
leaf water potential value of 11 bars was measured. Accordingly,
irrigation was initiated on June 3, June 11, and July 10 for a VSP
trellis (1m X 1m planting), a Wye (or lyre) trellis with 9-ft. row
spacing, and a VSP trellis with 9-ft. row spacing, respectively.
Since soil type and rooting depth were the same for all trellises
in the trial near Oakville, the differences in the date irrigations
began (i.e. when leaf water potential reached 11.0 bars at
midday) were due to the differing amounts of water used by each.
Hence the rate water was depleted from the soil profile.
How much water to apply
I have spent the last 10 years determining irrigation requirements
for raisin, table grape, and wine grape vineyards in all major grape-growing
regions of California.
Regardless of grape type, in my opinion, once the decision to begin
irrigations is made, vine water requirements are dependent upon
evaporative demand at the location of the vineyard, stage of vine
development, and percent ground cover by the vines canopy.
This is because the amount of water depleted from the soil profile
has been significantly reduced by that time (especially if no water
had been applied from the time of budbreak to that point) and the
majority of the water a vine subsequently will use is dependent
upon what is applied.
The information needed to schedule irrigations at daily, weekly,
or other intervals throughout the growing season includes potential
evapotranspiration (ETo) and reliable crop coefficients
(kc). Potential ET (also known as reference ET) is the
water used per unit time by a short green crop completely shading
the ground. Ideally, the crop is of uniform height and never water-stressed.
Potential ET (ETo)
ETo is a measure of the evaporative demand of a particular
geographic region throughout the year. Current (or near-real time)
ETo data are available from the California Irrigation
Management Information System (CIMIS) which is operated by the California
Department of Water Resources.
There are more than 90 weather stations located around the state
where environmental data are collected to calculate ETo.
Environmental variables measured to calculate ETo are
mean, hourly solar radiation, air temperature, vapor pressure, and
wind speed. These variables are then used to calculate other variables
such as net radiation and vapor pressure deficit which are then
inserted into an equation to calculate ETo.4
Potential ET may also be obtained from weather stations operated
by other entities (such as stations operated by the Paso Robles
Vintners & Growers Association in the Paso Robles region).
Potential ET will vary seasonally and is low at the beginning of
the season, highest in mid-summer, and then decreasing thereafter.
Between budbreak and the end of October, ETo can range
from 35 to 50 inches of water in the coastal valleys of California.
For example, ETo from March through October in 1997 for
the Carneros region of Napa Valley, Greenfield in the Salinas Valley,
Paso Robles, and Fresno were 44, 44, 51, and 48 inches, respectively.
Historical ETo in the Santa Maria region for the above-mentioned
months is estimated to be 36 inches. Therefore, if identical vineyards
(same cultivar, trellis system, row spacing, canopy size, etc.)
were growing at all five locations, then seasonal vine water use
would be lowest in Santa Maria and highest in Paso Robles. The differences
would be due to varying evaporative demand at those locations.
The next piece of information needed to determine vineyard water
use is seasonal crop coefficients (kc). The kc
is the fraction of water a non- water-stressed crop uses in relation
to that of ETo:
kc = ETc ¸
where ETc is crop ET. The kc is dependent
upon the stage of vine growth, degree of ground cover (shading),
height, and canopy resistance (regulation by the vine or crop).
The kc will vary throughout the growing season; it is not a constant
fraction of ETo. It is low early on and then as the canopy
develops, it will increase (Table I).
In the past, seasonal crop coefficients have been developed for
vineyards in the San Joaquin Valley. Unfortunately, when these seasonal
crop coefficients were utilized in coastal valley vineyards, they
did not work very well.
Various means of adapting these crop coefficients to different trellises
and row spacings have included the use of another coefficient (canopy
coefficient) that is a function of canopy size.
In order to develop crop coefficients, one must be able to measure
or estimate grapevine water use throughout the growing season. With
the aid of a weighing lysimeter, I have determined seasonal crop
coefficients for Thompson Seedless grapevines grown at the University
of Californias Kearney Agricultural Center.3,
A weighing lysimeter is a sensitive piece of equipment that is able
to measure ET of plants on an hourly, daily, and seasonal basis.
The lysimeter at the Kearney Ag Center is comprised of a large soil
container (2m wide, 4m long, and 2m deep) that sits upon a scale.
The soil surface of the container is at the same level as the soil
level of the vineyard surrounding it. Therefore, the soil container
and scale are below ground.
Two grapevines were planted in the lysimeter in 1986. Vines were
also planted around the lysimeter with vine and row spacings of
approximately 7 1/2 and 11 1û4 ft, respectively. The trellis system
used for the vines is a two-foot crossarm approximately five feet
above the soil surface. The scale has a resolution of 0.2 mm of
water, which is less than 0.5 lbs.
As water is lost from the tank, either by evaporation from the soil
or transpiration from the vines, a datalogger records the loss in
weight (i.e. it measures ETc) hourly. The vines within
the lysimeter are automatically irrigated whenever they use 2 mm
of water (slightly more than two gallons) and are therefore assumed
not to be under water stress.
Water use of the vines in the lysimeter, between budbreak and the
end of October, from four years after planting until the present,
has ranged from 29 to 34 inches (approximately 1,400 to 1,700 gallons
per vine).6 Potential ET at the
same location over the same years has ranged from 42 to 47 inches.
Daily water use of vines growing within the lysimeter will average
10 to 12 gallons at maximum canopy, mid-summer.
During the 1998 and 1999 growing seasons, a study was conducted
to determine the relationship between leaf area of the vines in
the lysimeter, shaded area cast on the ground at solar noon, and
grapevine water use. Thus, leaf area was estimated and shaded area
on the ground was determined at various times throughout the growing
The study found that, at full canopy, shaded area on the ground
comprised approximately 50% of the total land area allocated per
vine within the vineyard.
In 1999, the shoots of the two vines growing within the lysimeter
were allowed to grow across the row middles on either side of the
lysimeter. The shaded ground area just prior to that time was about
60%. A support system was then constructed to raise the shoots (simulating
an overhead trellis system) and the percent ground cover increased
to approximately 75%.
Actual vine water use prior to raising the canopy was 12.7 gallons
per day, and after raising the canopy, it increased to 16.7 gallons
per day. This indicates that it was the orientation of the canopy
(determined by the trellis system) and not the total leaf area per
vine that dictated how much water the vine used, if the vine was
It was also found that vine water use ( ETc) and the
crop coefficient were linear functions of the amount of shade measured
beneath the vines at solar noon. The equation to describe the relationship
between the crop coefficient (kc) and percent shaded
area was: kc = 0.002 + 0.017x, where x is percent shaded
A linear relationship between the crop coefficient and light interception
(or shaded area), with a slope similar to that of the grapevines,
has also been found by Dr. Scott Johnson for peach trees growing
in a weighing lysimeter at the Kearney Ag Center.
There are several reasons why grapevine water use and the crop coefficients
may be related to the percentage of shaded area when measured at
1) The driving force of ET, net radiation, is greatest between
11am and 2pm.
2) Approximately 75% of the daily water use by vines growing
in the lysimeter occurs between 10am and 2pm.
3) The shade beneath a vine is an indirect measure of how much
solar radiation the vine is intercepting.
4) The shade beneath the vines varies only slightly between
9am and 3pm for east/west rows (row direction in the lysimeter vineyard).
5) As the season progresses, the vines canopy gets larger,
resulting in more light being intercepted (more shaded area on the
ground) and greater water use.
Impact of trellis and row spacing
There are numerous trellis systems used for winegrape production
in California today. There are systems in which little management
is used (sprawl systems) and those, which are highly manipulated.
The latter systems include the VSP (vertical shoot positioned trellis)
and vertically divided canopies such as the Scott Henry or Smart/Dyson
trellises. Horizontally divided canopy trellis systems include the
lyre, U and Wye trellises, and the GDC (Geneva Double Curtain).
Any of the above winegrape trellis systems that increase the percent
ground cover should also increase vineyard irrigation requirements
based upon observations using Thompson Seedless grapevines in the
In addition, as the tractor-row width decreases, the percent ground
cover or shaded area will increase. One would therefore assume that
vineyard water use would increase as the distance between rows decreased.
I have independently developed crop coefficients for two different
training/trellis systems: a VSP trellis with unilateral cordons
and a modified GDC trellis with quadrilateral cordons trained to
a four-foot crossarm. In order to develop kcs for both systems,
water was applied at various fractions of my initial estimate of
ETc for each trellis system. This was replicated in trials in two
The fractions of ETc used were 0.25 to 1.25 ETc in increments
of 0.25. The VSP-trellised vineyard was in Carneros and had a seven-foot
row spacing and the quad-cordon vineyard in Temecula had a 12-foot
A vine water balance was then determined for each irrigation treatment.
Water inputs from rainfall and irrigation and the soil water content
in each irrigation plot were measured.
Based upon my work at the Kearney Ag Center, if vines are irrigated
at full ET, soil water content will remain fairly constant. If vines
are being deficit-irrigated, then soil water content will decrease.
In addition, my previous work with Thompson Seedless indicated that
midday leaf water potential generally would not be more negative
than 10 bars if vines were irrigated at 100% of ETc.
These were the two main criteria used to determine if vines were
receiving applied water amounts that replaced water used by the
vines on a weekly basis. Once it was determined which irrigation
treatment (applied water amount) was equivalent to vine ETc,
I was able to calculate how much water the vines used at regular
intervals throughout the growing season and then to calculate seasonal
The maximum kc for the vines on a VSP trellis system,
in a vineyard with a seven-foot wide tractor row, was slightly greater
than 0.7 (Table I). The maximum kc for the modified GDC
trellis (12-foot wide tractor row) was approximately 0.75 (Table
II). Both of the above values are less than that found for Thompson
Seedless grapevines in the lysimeter where the maximum kc can be
close to 1.0.3
The VSP trellis seasonal crop coefficients initially developed on
the seven-foot wide row spacing have been tested in other commercial
vineyards with different row spacings, row directions, cultivars
and rootstocks, and at four different locations. At all locations,
irrigation treatments at various amounts of estimated full ETc
were included in a replicated trial.
The seasonal VSP trellis crop coefficients were adjusted either
upward or downward for narrower or wider row spacings, respectively,
from those developed in Carneros.
For example, the seasonal kcs for a six-foot row spacing
were increased by 1.17, relative to those developed in Carneros
on a seven-foot row spacing (7 ft ¸
6 ft = 1.17) while those for a 10-foot row spacing were decreased
by 0.7 (7 ft ¸ 10 ft = 0.7) relative
to those developed in Carneros (Table I). Therefore, the maximum
kc for VSP vineyards with tractor row widths of six feet,
eight feet, nine feet, and 10 feet, were calculated to be 0.82,
0.62, 0.55, and 0.49, respectively (Table I).
Crop coefficients developed for the VSP trellis will increase or
decrease water use per unit land area as row spacing decreases or
increases. However, if vine spacing within the row is the same among
those vineyards, then water use per vine will remain the same. An
example of this can be found in Table II. The water use per unit
land area (ETc in inches column), comparing a VSP trellis
at six- and nine-foot row spacings is greater for the six-foot row
vineyard but water use per vine (last column) is the same for both.
In order to validate my assumptions about row spacings and crop
coefficients used, the crop coefficient was considered appropriate
(i.e. applied water amounts replaced what the vines used) if midday
leaf water potential remained above 10 bars all season and
if berry weight was maximized at applied water amounts at 75% of
estimated full ETc (i.e., applied water amounts at greater
values did not result in a further significant increase in berry
size.) These parameters were based upon data collected on Thompson
Seedless in San Joaquin Valley.
At various points during the 1998 and 1999 growing seasons, shaded
areas under vines at all trial sites with VSP trellises, were measured
at solar noon.
This was done by placing a four-foot by four-foot piece of plywood,
on which a grid (six-inch by six-inch squares) had been drawn on
the ground beneath the vines and either visually estimating the
percent shade in each square of the grid or by recording an image
of the plywood with a digital camera. The image was downloaded to
a computer and the shaded area was determined with appropriate software.
Crop coefficients used to schedule irrigations that week in each
vineyard were compared with the crop coefficient calculated as a
function of shaded area measured that particular week. The equation
used was: kc = 0.017 x percent shaded area, where 0.017
is the slope of the equation describing the relationship between
the percent shaded area and the crop coefficient of Thompson Seedless
vines growing in the weighing lysimeter.
The data indicated that the kc calculated from percent shaded area
and the kc being used that week to schedule irrigations
were linearly correlated with one another (coefficient of determination
[r2] was 0.86). This also implies
that the use of the shaded area technique to calculate a crop coefficient
could be a viable tool in vineyard irrigation management to approximate
the value of potential vine water use at full ETc.
In the 2000 growing season, crop coefficients were developed for
vineyards using a lyre trellis and a high-density VSP-trellised
vineyard (the latter planted 1m x 1m; 4,049 vines per acre), using
the shaded area technique.
The calculated kc at maximum canopy development (i.e.
late in the summer) was 0.83 for the lyre (planted to nine-foot
wide tractor rows) and 0.91 for the 1m x 1m planting (Table II).
Results indicate that trellises such as the lyre, which spread the
canopy, will have higher per vine and per unit land area water requirements
than trellises that dont (such as the VSP), when both are
planted to with same tractor row width.
However, when there is a VSP trellis in a vineyard with a closer
row spacing (such as a six-foot vine row spacing or 1m x 1m planting),
water use per unit land area of those vineyards may be comparable
to the lyre planted on nine-foot or wider row spacing. This illustrates
that both trellis (or canopy type) and row spacing will determine
percent ground cover in the vineyard and ultimately potential vineyard
Also during the 2000 growing season, the shaded area was measured
beneath vines on vertically split canopies (such as the Scott Henry
and Smart/Dyson systems) to derive their crop coefficients. Vineyards
in which these systems were used included row directions east/west
Shaded area measurements taken at solar noon on a vertically-split
trellis planted to north/south rows were not similar to those planted
to east/west rows.
I subsequently determined that the shaded area of a vertically split
canopy planted to north/south rows measured an hour before or an
hour after solar noon was very similar to the shaded area of rows
oriented east/west when measured at solar noon.
I also have found that crop coefficients developed on east/west
rows were appropriate for north/south rows for both VSP trellises
and vertically-split canopies.
The water requirement of a vertically-split canopy is approximately
25% greater than that of a VSP canopy, when planted on the same
row spacing when the calculations are made after veraison (Data
was generated during the 2001 growing season).
use of measuring shaded area in your vineyard
As shown above, the shaded area beneath vines in your vineyard can
be determined by several methods. One can use a grid and then estimate
the amount of shade within each grid, then calculate the total shaded
area of the vine. The same grid can be used to produce images of the
shade on plywood with a digital camera.
There are software packages that allow calculation of areas of different
colors on the image. However, the digital camera cannot be used if
the vines canopy extends to the soil surface. Lastly, one can
use a tape measure to determine the average width of the shade beneath
the grapevine and then calculate the total shaded area. This can be
done where the canopy within the vineyard is highly uniform.
It should be noted that determining the shaded area of canopies early
in the growing season or in newly planted vineyards is difficult with
a measuring tape since such canopies are very discontinuous. It would
be hard to get an average width of the shaded area.
However, if one can obtain a good approximation of the amount of shade
under the vine, then estimation of a crop coefficient would prove
useful in determining the amount of water a vine would use at 100%
of vineyard ET.
Below is an example of how potential vineyard water use could be derived
using percent shaded area to estimate the crop coefficient:
A. Tractor row width = 10 feet
B. Vine spacing = 6 feet
C. Area per vine = 60 ft2
D. Average width of measured shaded area between two vines = 3 feet
E. Shaded area per vine = B x D = 6 ft. x 3 ft. = 18 ft2
F. Percent shaded area = E ¸ C =
18 ft2 ¸
60 ft2= 0.3 or 30%
G. Crop coefficient (kc) = F x 0.017 (slope of relationship
between kc and percent shaded area of Thompson Seedless)
= 30 x 0.017 = 0.51
H. Vine water use = ETo (obtained from CIMIS) x kc.
To determine a vineyards irrigation requirement, the equation
in H above can be used (ETc = ETo x kc).
Remember this equation is for calculating applied water volumes that
replace amounts of water the vineyard uses daily or at other intervals.
That is, the soils water reservoir would not be depleted. The
above equation will give full crop water use in mm or inches.
To determine water application amounts per vine or to calculate pump
run times, one must convert mm or inches into liters or gallons. The
metric conversion is one mm covering one hectare equals 10,000 liters.
The English conversion is one inch covering one acre equals approximately
Once the amounts have been determined, divide the liters or gallons
required per unit land area by number of vines per hectare or acre
(Tables II and III). The pump run time is also dependent upon the
number and sizes of emitters per vine and the irrigation systems
The seasonal progression of vine water use for a VSP trellis, planted
with 10 foot-wide tractor rows is shown in Table III. The data illustrate
how vine water use is lower early in the season due to a smaller canopy.
Once the canopy is established and fills the trellis (beginning the
middle of July), differences in ETo from week to week,
will result in differences in vine water requirements when this technique
to irrigate vines is used. Thus, the benefits of calculating vine
water use with this method is that both evaporative demand (ETo)
and the seasonal progression of a canopys development or trellis
type and tractor row spacing dictate how much water to apply.
The following illustrates how seasonal vineyard water use of one trellis
system may vary as functions of row spacing and of different trellis
systems at the same location.
The calculated vineyard water use between budbreak (April 3) and harvest
(September 21) in 2000 for a Cabernet Sauvignon vineyard, using a
VSP trellis and a six-foot tractor row near Oakville in Napa Valley,
was 17.7 inches and applied water was 89% of that value. Potential
ET for the same timeframe was 34.4 inches.
The calculated water use for a similar vineyard but with tractor row
widths of 7, 8, 9, and 10 feet would have been 15.2, 13.3, 11.8, and
10.6 inches, respectively. Calculated water use for a lyre trellis,
with 9-foot row spacing, and a 1m x 1m planting at the same location
would have been approximately 20 inches and 25 inches of water, respectively.
Effect of water amounts on growth and yield
Research conducted in my laboratory and elsewhere, indicates that
deficit-irrigation will have minimal effects on berry growth or yield
and with possible advancement in date of harvest and an increase in
In each of my irrigation trials, irrigation amounts that are fractions
of estimated full ET have been included. These amounts are applied
from the first irrigation of the season up until at least harvest
or beyond. This allows determination of the effects of both under-
and over-irrigation on vine growth, berry characteristics, and yield
of raisin and table grapes or wine quality of wine grape cultivars.
I have found that irrigation amounts, at approximately 80% of full
ET, maximize berry size for raisin and table grapes.6
Yield of Thompson Seedless vines grown at the Kearney Ag Center is
maximized at applied water amounts that are 80% of lysimeter water
use whether used for raisin5,6 or
table grape production (unpublished data).
|It has been found that the
yield of Thompson Seedless grapevines used for raisin production actually
decreases when applied water is greater than 100% of ET, determined
with the weighing lysimeter. This is due to reduced bud fruitfulness
and lower cluster numbers per vine.
Vegetative growth of Thompson Seedless vines generally increases as
applied water increases from no irrigation to 120% of ET.5,6
My studies in various table grape vineyards, in both the Coachella
and San Joaquin Valleys (over a four-year period), have demonstrated
that packable yields may be only slightly reduced at applied water
amounts equal to 50% of full ET.
It should be noted that the above results were obtained in vineyards
that were irrigated daily. Therefore, the irrigation frequency used
in those studies may have mitigated possible negative effects of deficit
My studies in Chardonnay and Cabernet Sauvignon vineyards in Napa
Valley and along the central coast of California have somewhat mimicked
results found with Thompson Seedless vines. In these studies, irrigation
frequency is only once or twice per week. Berry size is usually maximized
with applied water amounts at 75% of estimated full ET (Table IV).
Yields are slightly diminished at applied water amounts at 75% of
full ET (Table V).
Yields have been maximized, in some instances, at applied water amounts
equal to 50% of full ET, but this is dependent upon weather (years
in which rainfall may have occurred late into the spring), location,
soil type, and rooting depth. Pruning weights, a measure of vegetative
growth, generally increased as applied water amounts increased (Table
My wine grape studies have also addressed the effect of rootstock
on vine water use. At least three rootstocks were used at each site
(five in Paso Robles) and these included 5C, 110R, 3309C, Freedom,
140R, and 1103P.
I have found no differences in vine water use or vine water relations
among individual rootstocks at a specific location. This may be due
to the fact that all vineyards utilized the VSP trellis system. Shoot
hedging and positioning, as performed for the VSP trellis, does not
allow possible differences in vine vigor (increased canopy size) among
rootstocks to be expressed. In addition, drip irrigation may limit
exploration of the soil profile by each rootstock.
While, I have rarely found significant interactions between rootstocks
and irrigation amounts among the different fruit and yield parameters
measured at each site, I have found significant differences in those
parameters among rootstocks.
Small wine lots have been made of Cabernet Sauvignon grown at Oakville
and Paso Robles and Chardonnay from Carneros with sensory analysis
performed on each. While deficit irrigation will increase color of
wine made from Cabernet Sauvignon, there has been no consensus as
to preferences among the irrigation treatments.
One possible reason is that irrigation amounts that I calculate as
full ETc for the VSP trellis are much less than previously thought.
For example, calculated ETc for the Paso Robles Cabernet Sauvignon
vineyard (with 10-foot wide tractor rows) from budbreak to the end
of September has only been between 10 to 12 inches per growing season.
This is much less water than many growers presently apply.
Information regarding the use of a pressure chamber and estimation
of crop coefficients presented above can provide growers an objective
means in determining when irrigations should commence, while also
helping them estimate how much water a vine potentially may use at
I have used both the pressure chamber and measured soil moisture content
with a neutron probe in studies to determine when irrigations should
commence. Both have worked extremely well.
However, the measurement of a vines leaf (or stem) water potential
may be preferable, since it integrates the amount of water the vines
roots have access to throughout the soils profile. In addition,
measurements of leaf water potential can be made at more than one
site within the vineyard with relative ease.
Many times, determination of a vineyards soil water status with
a neutron probe in a commercial situation is based upon just one access
tube per vineyard. Placement of this single assess tube in the vine
row, including distance from an emitter (if drip irrigation is used),
may not accurately determine soil water status of an individual vine
nor account for variability within the vineyard due to different soil
types and/or rooting depths.
Calculation of irrigation requirements using crop coefficients and
ETo, provides an objective means to determine how much
water should be applied based upon the vines growth characteristics,
trellis, and row spacing. It is also reflective of changes in evaporative
demand from day to day during the growing season and from year to
For example, if evaporative demand is high one week, irrigation requirements
will be high, if it is lower the next week, then irrigation requirements
will be lower.
Initiating vineyard irrigation later in the growing season or the
use of deficit irrigation may restrict excess vegetative growth, whether
for grapevines grown in the interior valley of California or along
the coast. This would minimize the cost of canopy management for vines
that, in the past, became too vigorous due to excessive irrigation.
Grapevines can be deficit-irrigated at various fractions of estimated
full ETc with minimal impacts on yield but with a potential
to increase fruit quality. Thus, in most cases, one may not have to
apply water amounts that meet or exceed estimated vineyard water use
requirements presented in this article.
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management and irrigation scheduling. In: L.P. Christensen (ed.)
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Schaffer (eds.) Photoassimilate distribution in plants and crops:
Source-sink relationships. New York, Marcel Dekker, Inc. pp. 851881.
6. Williams, L.E. 2000. Grapevine water relations.
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