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As in any living species,
water is crucial for long term growth and survival. In herbaceous
plants leaves can consist of more than 90% water. Fruits and vegetables
frequently have more than 95% water. Woody plants can consist of
more than 50% water (1). Water is essential in the process of photosynthesis
whereby light is trapped to produce carbohydrates from carbon dioxide
and water. A plant with sufficient water will have tugor pressure
for cellular expansion thereby enabling it to expand and grow. Plant
cell water helps moves sugars, gases, and salts throughout the entire
plant.
Water use terminology
Water is used in a cropped field in different ways. Leaves transpire
water through their stomata in the form of vapor in a process known
as transpiration (2). Water moves up from the soil through the roots,
stems, and finally out of the leaves in response to the much lower
vapor pressure in the atmosphere than in the soil. In a sense plants
can be considered "straws" sucking ground water out of
the soil. Transpiration helps to keep plant surfaces cool, thus
enabling photosynthesis and metabolism to continue. Transpiration
only occurs when stomata are open during the day thus explaining
why leaves appear wilted during the day but appear to have recovered
at night.
Water readily evaporates from the soil surface, as it does from
rivers and lakes, or any other wet surface. Evaporation is greater
after it has rained, at higher air temperatures, at higher light
levels, and during periods of higher wind speed. During cool and
cloudy weather water evaporation rates are considerably reduced,
thus explaining the reduced need for supplemental irrigation.
The combination of evaporation of soil water and transpiration from
leaves is known as evapotranspiration (ET). Essentially the crop
water requirement, or the amount of water needed to keep a plant
healthy, is represented by the term ET (3). Another term for ET
is consumptive use, which refers to amount of water used by plants
as they grow (4). With the general absence of summer rainfall in
all areas of the Pacific Northwest, ET has to be derived from residual
soil moisture from the winter rains or snows, or from supplemental
irrigation applied during the summer months.
Crop growth and ET
Nursery crop producers, whether they raise stock in the ground,
in containers, or in greenhouses, need to understand that total
plant growth is directly tied to ET. Maximum yield or growth will
not occur unless ET is met. In the wetter areas west of the Cascades,
where shade trees are grown on heavier sites, their ET can often
met by the residual soil moisture from winter rains. In the case
of Christmas trees, if seedlings are established on sites with enough
organic matter, during the winter months, they will survive and
grow for their entire life span without supplemental irrigation.
Conversely, nursery stock grown in containers always require supplemental
irrigation during the growing season (5). As containers have limited
volumes, the plants contained within will have smaller root systems,
and thus will need frequent, even daily irrigation. Container shape
also impacts ET. Wide, shallow containers will loose more water
due to evaporation, while tall narrow pots will not hold as much
water due to the greater gravitational force pushing the water towards
the bottom (6). Applying extra beyond a particular plant species'
ET will not improve yield. A plant can only transpire so much water.
Drought and ET
When a plant's ET is not met, due to either a lack of residual soil
moisture or inadequate supplemental irrigation, the plant will suffer
from the growth reducing effects of drought. Leaf stomata will close
in order to reduce transpiration, thus protecting the plant from
further water loss (7). However, stomatal closure will result in
less carbon dioxide uptake thus reducing photosynthetic rates. As
photosynthesis shuts down leaf expansion is curtailed, and buds
and shoots will not elongate. Eventually during severe drought the
plant will begin shedding leaves.
Beneath the ground drought will result in soil water shrinking
away from the interface with water-absorbing root hairs. If the
transpiration stream continuum from the soil-roots-stems-leaves-air
is broken the root hairs will start to die off further reducing
the ability of the plant to take up soil moisture. Newly transplanted
woody ornamentals that have been field grown and dug up bare-root
often suffer from a drought-like condition as the root hairs are
often lost during transplanting.
There are documented secondary effects of drought. A woody ornamental
suffering from the effects of drought is less able to ward off the
ravages of diseases. A weakened tree is predisposed to infection
by diseases (8). Conifers weakened by drought are more susceptible
to bark beetle infestations (9). Healthy trees have enough sap flow
to ward off the beetles are thus largely immune to attack. In areas
of the United States that are experiencing the long terms effect
of regional drought, entire forests are being lost due to the establishment
of large bark beetle infestations.
Estimating ET
In order to have a standardized method for measuring ET, thus relating
it to consumptive use, agricultural engineers studied the water
loss from a healthy stand of well managed turf (10). It was found
that the total of plant transpiration and soil evaporation was equal
to total evaporation from an open pan of water. The National Weather
Service measures evaporation using a standard evaporation pan called
a Class A pan (11). Pan evaporation is a measurement that combines
or integrates the effects of several climate elements: temperature,
humidity, solar radiation, and wind. Evaporation is greatest on
hot, windy, dry days; and is greatly reduced when air is cool, calm,
and humid.
A Class A evaporation pan is cylindrical with a diameter of 47-1/2
inches and a depth of 10 inches. Evaporation is measured daily as
the depth of water (in inches) evaporates from the pan. The measurement
day begins with the pan filled to exactly two inches from the pan
top. At the end of 24 hours, enough water is added, in measured
increments, to again fill the pan to exactly two inches from its
top. Evaporation values are collected over the evaporation season
(May through October).
By measuring the water use by plants at various stages of development,
and comparing this data to the ET data from the Class A pan, it
is possible to develop a relationship, referred to as the crop coefficient,
between the two sets of data. In this fashion it's possible to estimate
plant water use by simply measuring evaporation from the Class A
pan.
On-line access to evaporation data
Horticultural crop producers can access on-line pan evaporation
data from the U.S. Bureau of Reclamation's (12) Agricultural Weather
Network system referred to as AgriMet. Currently consisting of 70
automated weather stations situated in Oregon, Washington, Idaho,
and western Montana, this network provides daily access to evaporation
data, as well as other atmospheric data.
The most efficient method of supplying the water needs of nursery
stock is by using irrigation scheduling (13). Using this technique
means supplying only the amount of water that is needed: Multiply
daily pan evaporation by the crop coefficient. This type of work
has been conducted by Oregon State University in the North Willamette
Valley for a number of different woody ornamental container crops
(13). Irrigation scheduling works best when crops with similar crop
coefficients are grouped together in the same irrigation zone (14).
By using irrigation scheduling growers may save up to 25% of their
normal use (14). Irrigation scheduling works best under conditions
where over-head sprinklers are used to irrigate the crops.
Unfortunately AgriMet stations are more commonly situated in the
more arid regions of the Pacific Northwest. Currently there are
no stations in western Washington. There are three stations in the
principal ornamentals production area of the North Willamette Valley
of western Oregon.
References
1. Stress relief. 1998. Rita Hummel, nursery crop specialist, Washington
State University. American
Nurseryman, August 15, 1998.
2. Evapotranspiration
(ET) or crop water use. 1996. Norman Klocke, extension water
resources engineer, University of Nebraska.
3. Basic irrigation scheduling
in Florida. 1997. A. G. Smajstrla, water management specialist,
Agricultural and Biological Engineering Department, University of
Florida at Gainesville.
4. Basic irrigation terminology.
1995. Forrest Izuno, associate professor of water management, University
of Florida.
5. Irrigation 101. Dorota Haman, professor, Agricultural and Biological
Engineering Department, University of Florida at Gainesville. American
Nurseryman, October 1, 2000.
6. Irrigation of nursery crops, Chapter 14. 1994. Harold Davidson,
Curtis Peterson, and Roy Mecklenburg. Nursery Management Administration
and Culture, Prentice-Hall, Englewood Cliffs, New Jersey. Distributed
by American
Nursery Publishing Company.
7. Long-term
drought effects on trees and shrubs. 2000. Ronald Kujawski,
University of Massachusetts Amherst.
8. Plant
diseases development and management. 2001. Marcia McMullen and
Arthur Lamey, extension plant pathologists, North Dakota State University
Extension Service.
9. Ips bark
beetles in the south. 1983. Michael Conner, entomologist, U.S.
Department of Agriculture Forest Service.
10. Introduction
to evapotranspiration. 1998. Richard Allen, Utah State University.
In: Crop evapotranspiration - Guidelines for computing crop water
requirements - FAO Irrigation and Drainage Paper 56.
11. Evaporation,
evapotranspiration, and water use. 1982. Richard Farnsworth,
National Weather Service, NOAA.
12. AgriMet
evaporation data network. U.S. Bureau of Reclamation Agricultural
Weather Network. Headquarters are in Boise, Idaho.
13. Nursery
irrigation management. Part 1: Waste not, want not. 2002. Hannah
Mathers, state nursery crops specialist, The Ohio State University.
14. Waste no water. 2002. Hannah Mathers, state nursery crops specialist,
The Ohio State University, Columbus, Ohio. American Nurseryman,
November 15, 2002.
First posted:
December, 2004
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