An Engineers Approach to Irrigation Management in Oregon Pinot
Noir .
John Selker, (PI), Emilie Baer (Student)
Department of Bioengineering, Oregon State University, Corvallis, Oregon.
Abstract
Vineyard managers must decide on the timing and amount of irrigation. In Oregon the
need for irrigation differs greatly year to year and field to field based on micro climate,
soil, root development, plant health, and root stock. We suggest the use of leafwater
potential as a useful management tool . We show that for this test to provide consistent
values, leaves should be wrapped prior to removal, should have consistent solar exposure,
but need not have precisely the same age. We illustrate the use of this measure with data
from the 2001 growing season taken in two Willamette valley fields . The data showed
that irrigation was not required throughout the season, and helps explain the unexpectedly
high yields noted by the vineyards.
Introduction
One goal of our study was to fmd tools which are able to evaluate soil moisture in
vineyards. It is implied that the tools have to be able to take accurate measurements in
dried soils and also that they can be used by the growers. We looked at the cost, the
difficulties of installing them and at the time it takes to obtain a reading .
The soil moisture sensors presently on the market make use of many different ways to
measure soil water content. Some are based on water pressure (tensiometric
measurements); others on electrical resistance in a block ofmaterial in contact with the
soil (watermark and gypsum block sensors) ; heat dissipation, psychorometric
measurement of water vapor pressure, and most recently the measurement of soil
dielectric. Of these technologies, only dielectric provides the range of measurement
required in Oregon s grape yards.
Among the sensors using dielectric coefficient of the soil, popular tools include the ESI
TDR Probe (http ://www.envsens .com/products/moisture/Mpoint/index.html) , the
Decagon Echo Probe (http ://www.decagon.com/echo/index .httnl), the Campbell
Scientific CS615/616 (http ://www.campbellsci .com/soilvol .html#cs6l5), the Vitel Probe
manufactured by Stevens instruments
(http ://www.vitelinc.com/products/hydraprobe logger.htm) and the Aquapro sensor
(http ://www.aquapro-sensors .com/). This list could be tripled in length given the rapid
expansion of tools based on dielectric measurement, many of which are very serviceable,
with equipment spanning prices from $50 to $50,000. For reasons of price, availability at
the time of the study, and flexibility, we included only the Aquapro tool in this study.
We found that the Aquapro is appropriate for the vineyards because it allows observation
ofthe entire soil profile, and can be employed with an investment of a few hundred
dollars. The Decagon Echo probe has come out in the last year and has some potential
advantages in it s radio telemetry options.
Contrary to other crops, such as tomatoes or corn, irrigation in vineyards cannot
be managed using soil moisture . There are several major reasons . First, with very deep
root systems, shallow soil moisture measurements may not reflect the real needs of the
plants . Further, unlike most agronomic crops which perform best with minimized water
stress, obtaining optimal wine quality requires significant stress at key periods. To
understand the changing water requirements, it is essential to use the plant water status
itself. The most practical tool presently available in this context is the measurement of
leaf water potential, which allows direct access to the water plant status. What does leaf
water potential mean? How can the measurements be taken, and decisions be made to
manage irrigation with them? Our study attempts to give some keys to the answers to
these questions .
Leaf Water Potential
Use of a pressure bomb and Application for irrigation
management in Pinot Noir
Definition and measurements
What does leaf water potential mean?
The leaf water potential measures the water status of a plant. The basic principle
is simple. As the plant is stressed the pressure required to extract liquid from a cut stem
ofthe leaf increases . Ifthe leaf is put under an increasing pressure Pert, with the cut stem
open to the atmosphere, at a certain pressure, liquid will be ejected from the stem. When
Pert=- Psap, the sap leaves the leaf, and we identify this as the water potential
How to measure leaf water potential
The leafwater potential is measured with a commercial pressure bomb using a tank of
compressed nitrogen gas (Figure 1).
Figure 1 : Schematic of a pressure bomb. After it is cut, the leafis placed in the pressure chamber (l .). The
pressure in the chamber is increased slowly (2.) while the cut surface ofthe sample is observed. When the
first drop of sap appears, chamber pressure is recorded (3.) (Photos from PMS web site) .
These techniques are employed in many crops (e.g., pine trees, mint, row crops) for
almost any kind of leaf. The next section will describe how to take the measurements in a
vineyard, and how they can help the growers in irrigation management .
Midday measurements in vineyards
When to take the measurements?
Two patterns of measurement could be followed in characterizing leafwater potential .
One is to take predawn measurements, and the other is to take midday measurements.
Through the predawn, the plant transpires almost nothing . Plant water potential is in
equilibrium with the soil. Moreover, leaf water potential measurements are more
consistent because microclimatic conditions are relatively homogeneous . Midday
measurements have to be taken on sunny days. They are less consistent, but they give an
idea of the maximum amount of plant stress and are more practical for the growers, both
for reasons of scheduling (taken between 12:00 and 2 :00 pm instead of 5 :00 am for
predawn measurements!) and because they provide a measure of the stress the plant feels
during its growth phase rather than telling the soil water conditions . For these reasons, we
explored the use ofmidday measurements. Our goals were first, to find out how to take
between a growing and an adult leaf). So, you can choose whatever adult leaf you want to
take the measurements .
Whatever the method you choose to take the measurements, you must be
consistent in your sampling choice. It means that you have to choose leaves which look
almost the same (color, size . . .) and which have almost the same solar exposure, without
being overly concerned about the age of the leaf.
How many measurements have to be taken?
Throughout the day, the leafwater potential changes. Here is an example of the
daily variability of leaf water potential with time.
Change in pressure during the day for a vineyard .
17 .5
17 .0
16 .5
16 .0
H
N
a
ME
'I
jf---~~Ioud->
BillI
15 .5
15 .0
14 .5
14 .0
13 .5
11 :00
11 :58
12 :56
13 :53
14 :51
Time (hrs)
th
Figure 2. Equilibrium chamber pressure as measured with a portable pressure bomb on
August 29 , 2000, two days after the final irrigation on this field (same date as
shown in figure 6). A small but clear separation between irrigated and unirrigated plants is apparent, averaging 0.67 bars (4%) .
This chart shows that clouds and irrigation influence the values. It shows also that
the choice of the leaf is very important . There is a large variability among the readings on
August 29 2000 . The leaf water potential is between -14.5 and -17.0 bars, i.e. 2.5 bars of
difference for the same day, in the same plot! In fact, each leaf has its own environment
and even though the samples seemed to be the same (color, size . . .), the stress level of
th
each is different because ofthis environment. To compensate for this variability and
obtain a relatively correct average, a sample of 3 to 6 leaves is suggested.
Summary of leaf Water Potential Measurement :
Questions
When
take
measurements?
Recommendations
Reasons
the Between 12 :00 pm and 2 :00 Maximum stress, consistent
pm
value.
Which leaf?
Adult, healthy,
exposed
Method
Plastic bag
How many samples?
Time required
3 to 6
Where?
fully
10 to 20 min
Take measurements in
different laces across lot
Limit leaf transpiration after
removal
Average closer to the real
leaf water potential of the
entire lot
Soil, plant vigor, solar
exposure all v
Irrigation management using leaf water-potential
When to begin irrigation?
Irrigation management can be based on leafwater potential as an indicator of the
stress level ofthe vineyard. In order to limit root extension and produce high quality
grapes, vines need water stress, within limits . Which target stress level should be
employed in Oregon? Unfortunately there is a paucity of literature on this subject, but we
employed levels suggested in several previous studies of irrigation ofred wine varietals
(see references l, 2, and 22). Based on our reading ofthe literature, we planned to start
the irrigation when the _ went below -1 .5 MPa (-15 bars) thereafter maintaining between
-12 and -15 bars. In the literature, -12 bars has been found to correspond to the initiation
of significant stomatal closure. At -15 bars the stress level is sufficient to inhibit
respiration, resulting in small fruits with low sugar content. Measurements were taken
twice per week. We planned to test the wine produced with the grapes coming from
irrigated vines and wine produced with grapes coming from unirrigated vines. As it
turned out, no irrigation was required in the 2001 season on our test fields, so no wine
comparison was undertaken.
As is apparent from Figure 3, potentials were far below target levels without any
irrigation. This may explain the unusually high fruit yields in the 2001 season. It would
65
appear that somewhat less rainfall or greater heat may have improved grape quality this
year. This also shows the importance of selecting sites with shallow soils so that the
grower may obtain high stress even in years ofrelatively high summer precipitation.
Date
8-Jun-01
0
28-Jun-01
18-Jut-01
-2
7-Aug-01
@ --Lemelson 2001
-+-Archery Summit 2001
"""' Expected
is
cd
o
-8
-10
Treatment 1
3
in our study
( without irrigation)
-18 -
LWP _,:15 bars `
Excess Stress
(less sugar, smaller fruit)
Treatment 2
in our study
(with irrigation)
-20
Figure 3 : Evolution of leaf water potential during the 2001 season . The stress of the plants
continuously increased, but irrigation was not recommended as the -15 bar stress level was not
achieved .
The choice ofthe -15 bar level cannot be taken to be universal, depending greatly
on root stock, climate, soil type and perhaps most of all the acclimatization ofthe field.
With proper conditioning and root stock, grape vines are able to adapt to extended
drought, and are able to support very high y without damaging fruit quality. On the
contrary, a leafwater potential measurement might not indicate stress, while the plants
show apparent distress . This was observed in our field trials where one sector had very
poor soil, and thus significant nutrient stress with smaller plants and yellow leaves but
concurrent low sap stress . Clearly overall plant stress can arise from a wide range of
sources, some of which will be apparent in the sap potential (e.g., lack of water, root
disease, poor root development) . Over-irrigation can cover up some ofthese stresses, but
may further impact grape quality. Hence we recommend combining leaf water potential
measurements and plant observations to determine the potential corresponding to a high
plant stress level. R.E. Smart (reference 24), for instance, found that leaf angle is an easy
and useful measurement.
Summary
Leaf water potential reflects plant water status, and therefore is a more robust and
informative measure than soil moisture in scheduling irrigation. Low water stress will
produce abundant watery low quality fruit, while excess stress produces low yield and
low sugar content. Leaf water potential measurements strongly depend on the
microenvironment of the leaves, so it is essential to employ a consistent and appropriate
method of sampling . Use ofplastic bags to reduce transpiration after cutting, consistent
solar exposure, and consistent time of day in leaf water monitoring is essential to obtain
useful data .
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