How Farmers Irrigate
in California
Cooperative Extension University of California
Division of Agriculture and Natural Resources
LEAFLET 21414e
How Farmers Irrigate
in California
Most of California’s developed water supply-water that is
diverted from streams, stored in reservoirs, or pumped from
underground-goes to irrigate crops. Californians tend to
worry about this arrangement. They ask:
Why do farmers need so much irrigation water?
How does a farmer decide when to irrigate, and how much?
Why are there so many kinds of irrigation?
What are the roles of new technology and new scientific
methods of irrigation?
How much and where
About one-tenth of California, almost 10 million acres, is
irrigated farmland. About 85 percent of the state’s developed
water is used to grow crops on that land. Most of the irrigated
acreage is in central California: about 5 million acres in the
San Joaquin Valley, and 2 million in the Sacramento Valley
to the north. The rest is in valleys along the coast and in
southern desert areas.
The crop with the greatest irrigated acreage in California
is cotton. All the state’s orchards combined, including deci-
duous fruits, nuts, and citrus, occupy about as much irrigated
acreage as does cotton. Other crops occupying large amounts
of irrigated acreage are alfalfa, pasture, grapes, rice, corn,
tomatoes, and sugarbeets. Also, large acreages of wheat and
barley are grown in winter and spring with some irrigation
to supplement rainfall.
In an average year, about 36 million acre-feet of water go
to irrigate these crops, enough to fill a 1-acre reservoir to a
depth of 6,818 miles-almost to China. What happens to all
that water? In the process of meeting basic crop require-
ments, about two-thirds of it changes to water vapor and en-
ters the atmosphere. The other third either percolates down
to groundwater or flows out into drainage canals and streams.
Some of this return flow serves to flush salts from the soil.
Much of it is reused.
How water is applied
California farmers apply irrigation water in a dozen or
more different ways. All of these are either surface systems,
where water flows across the ground by force of gravity, or
pressure systems, where water is pumped through pipes or
hoses to sprinklers or emitters.
Surface systems, traditional in California, are used on
about 75 percent of the state’s irrigated farmland. There are
three types:
Furrow irrigation, commonly used for crops planted in rows,
such as corn, cotton, and tomatoes.
Flood (border) irrigation, used for crops such as alfalfa and
pasture, in which parallel strips of land are flooded.
Basin irrigation, often used in orchards, in which level areas
surrounded by dikes are filled with water.
During furrow and flood irrigation, the flow continues until
the desired amount of water has soaked into the soil along
the entire length of the sloping field. This means that some
tailwater generally runs off at the lower end. Also, more water
enters the soil at the upper end of the field, where it has been
flowing the longest.
CROP WATER NEEDS
Whether it’s a clump of grass or a pine tree, every growing plant
must have water for two vital functions: transpiration through its
leaves and evaporation from its surfaces and surroundings. To-
gether, these functions are called evapotranspiration (ET). Plants
can survive with less than their full ET needs, but only by slowing
growth. Without the growing season’s ET requirement, whether
from rainfall or irrigation, crop yield will fall.
The actual amount of water used by a crop for ET depends
largely on sunlight, temperature, humidity, and wind. On any day
at a given location, ET rates are similar for different crops. (Typ-
ical rate for a July day in the San Joaquin Valley: 0.3 inches of
water.) Season-long ET requirements are something else again;
they are low for crops that take less time to mature or that grow
during the cool months.
Wheat and barley (180 days of growth during winter and
spring) need about 15 to 20 inches of water for ET, amounting to
9 to 12 gallons per square foot for the growing season.
Corn (150 days of growth during summer) needs 25 to 30
inches for ET, or about 20 gallons per square foot.
• Alfalfa (spring, summer, and fall growth) needs almost 50
inches, or about 30 gallons per square foot.
Farmers generally apply more water than is needed for ET.
This additional irrigation allows for uneven distribution of water
in a field and leaching of salts from the soil. Irrigation is consid-
ered very efficient if 85 to 95 percent of the applied water is used
by the crop. More often, the figure is in the 70 to 80 percent range,
which may be the best possible under the circumstances. Below
50 or 60 percent is generally considered poor irrigation efficiency.
Because of this uneven water distribution, surface irriga-
tion systems are often thought to be less efficient than mod-
ern pressurized systems. In practice, they may be. However,
tests at the University of California and elsewhere have
shown that well-designed surface systems can be operated as
efficiently as sprinkler or drip systems. Particularly where
water is scarce and costly, surface systems may be among the
best, indicating it is not the system itselfbut how it is designed
and managed that makes the difference. Recycling tailwater,
for example, is one way to improve the efficiency of a surface
system.
Pressurized irrigation, applying water through sprinklers,
microsprinklers, and drip emitters, is now used on about 25
percent of California’s irrigated farmland. Sprinklers are the
most common, but use of drip systems is increasing rapidly.
Still, drip accounts for only a small fraction of the total: about
3 percent of all irrigated acreage in 1980, according to the
California Department of Water Resources (DWR).
THE SOIL RESERVOIR
After irrigation, the amount of water remaining in the root
zone and available to the crop depends on the soil’s capacity
to store water and the depth to which roots have grown.
Sandy soils hold less water than heavier soils. Coarse sands
have just enough space between particles to hold about ½ inch
of water per foot of soil depth. Fine clays have more, smaller
spaces, and commonly store at least 2 inches of available water
per foot of depth.
The depth of the root zone also varies: grass roots go down
only a few inches; tree roots, 8 to 12 feet or more. Even deep-
rooted crops may have their root zones restricted by under-
ground barriers (bedrock, hardpan, soil layers, high water
tables).
A 5-foot root zone in sandy loam can store about 6.5 inches
of available water, or 4 gallons per square foot of surface area
(1 acre = 43,560 square feet).
A 5-foot root zone in silty clay can store about 10 inches of
available water, or 6 gallons per square foot of surface area.
An important point: Plants cannot extract all of the mois-
ture in soil. The portion that can be removed by the roots is
called available water, but only part of this amount can ac-
tually be used in crop production. Some of the available water
must stay in the soil to avoid crop stress and reduced yields.
DRIP: THE MYTH AND THE FACTS
What type of irrigation can be used most easily to apply
water efficiently? Probably drip, i f the system is well designed
and operated.
But drip technology is not, as some have claimed, many
times better than other systems-a technological cure for Cal-
ifornia’s water problems. That is because two-thirds or more
of all water applied to crops in California goes to satisfy the
crop water requirement, the minimum without which plants
cannot grow properly. This is true regardless of the type of ir-
rigation system.
With all systems, water is applied in addition to this basic
requirement so all parts of the field will get enough water and
so salts will leach out. One objective of good irrigation is to
minimize this additional application. Drip irrigation makes
that easier.
Sprinkler and drip systems have several advantages:
They make it easier to control the amount of water applied,
and to distribute it evenly across the field.
They can apply water slowly, which is important on tight
soils where water infiltrates slowly.
They can be used on rolling land; in the case of drip, even
on steep hillsides.
In other words, applying the right amount of water where
and when it is needed is less complicated with sprinkler and
drip irrigation. Comparable efficiency is possible with surface
systems, but it takes more effort to apply good management.
Sprinkler and drip systems are easier to automate and may
save on some aspects of labor. To operate well, however, they
must be closely watched and maintained, and that may end
up requiring as much labor as surface irrigation.
Sprinkler and drip systems also have some obvious dis-
advantages. They are more expensive to buy and install, and
they require more energy. A particular problem with drip sys-
tems is the need for frequent close up inspection to find
plugged emitters.
The farmer’s decision
Regardless of which irrigation method is used, a grower
must make two decisions over and over again: When to ir-
rigate, and how much water to apply. Of course, anybody with
a garden, a lawn, or even a houseplant faces the same deci-
sions. But with so much more at stake, how does a farmer
decide? The cost of water and other such basic considerations
will have an influence. But more specifically, the grower can
look at three indicators: the plants, the soil, and the weather.
In practice, many growers go largely by the calendar, bas-
ing their irrigation plans on experience and, i f they use canal
water, on the irrigation district’s delivery schedule. Still, most
keep a wary eye on at least one of these indicators of crop
water need:
The plants. With some crops, such as beans and cotton,
experienced growers say they can see the first signs of stress
and irrigate in time to prevent yield loss. This is risky. Fur-
thermore, many crops show no visual symptoms until it is too
late. Scientific devices are used by researchers to measure a
plant’s water status, but these are still largely experimental
and only indicate when to irrigate, not how much. To deter-
mine the amount, it is necessary to look underground.
The soil. The amount of available water in the root zone
can be estimated with buried instruments that measure how
tightly water is held in the soil (tensiometers and gypsum
blocks) or with a device that directly measures water content
(a neutron probe). But there’s a problem: Each reading re-
flects conditions in only one spot, while actual conditions in
the field may vary widely. The farmer must either install
many expensive instruments-generally not practical-or
use information from a few to estimate overall conditions.
Since at least some guesswork seems to be unavoidable, why
not simply use a shovel, a soil tube, or even a steel rod to get
some idea of moisture in the soil reservoir? In fact, that old-
fashioned approach, properly used, often works well.
The weather. The actual amount of water used by a crop
is determined by still another factor-evaporative conditions,
or the evapotranspiration (ET) rate. Rates of ET are generally
the same throughout a uniformly planted field, and can be
measured fairly easily. This leads to the concept of water
budgeting, which many farmers, scientists, and state water
officials say is the best broad-based approach to improving
irrigation scheduling in California.
A farmer using water budgeting first determines the ca-
pacity of the soil reservoir. Second, the farmer decides how
much of that soil-stored water can be used safely without de-
creasing yields. This factor, the yield threshold depletion level,
.depends mainly on (1) the crop’s sensitivity to water stress
and (2) root distribution in the soil reservoir. It may be as high
as 90 percent or as low as 30 percent of the available water
in the root zone.
Third, the farmer keeps a record of the crop’s cumulative
ET losses since the last irrigation. Then, when the losses ap-
proach the yield threshold depletion level, it is time to turn
on the water again.
Water budgeting takes into account both soil moisture and
the crop’s changing water needs. With a reasonable idea of
how much soil-stored water can be used safely, and with
timely ET readings, a farmer can tell fairly accurately both
when to irrigate and how much water to apply.
CIMIS: A new approach
It would help to computerize the process. That is one goal
of a new state-sponsored program: California Irrigation Man-
agement Information System (CIMIS). Developed by the Uni-
versity of California, CIMIS links a network of more than 40
small, sophisticated weather-sensing stations in various
farm areas of the state to a central computer. Day by day, the
stations report weather conditions, and reference values for
ET are automatically computed. These values are then
matched with information on individual fields near each
weather station to recommend timing and amount for the
next irrigation.
The State Department of Water Resources financed devel-
opment of CIMIS, and now operates it as a public service. The
next step, involving DWR, UC Cooperative Extension, and
private irrigation consultants, is to show farmers how to use
CIMIS.
But which farmers, and why? Those are not easy questions.
Irrigation experts generally agree that:
Many California farmers already irrigate with reasonable
efficiency, so they have little to gain and much to risk from
changing their methods.
In many cases where irrigation is less efficient, the runoff
is used by other growers or contributes to streamflow.
But the experts also agree that, overall, there are gains to
be made in the efficient use of water on most farms, probably
through CIMIS or something like it. In studying the possi-
bilities, UC Cooperative Extension specialists have found
that:
Growers with irrigation problems are more likely to adopt
CIMIS.
Growers who can measure the amount of water they apply
are more likely to use CIMIS (most surface irrigators can-
not).
Use of CIMIS is more likely on trees and vines than on an-
nual crops.
Overall cost of water does not seem to be linked to use of
CIMIS, except in some local situations. High power costs for
pumping water often do make a difference.
This publication was prepared by Richard L. Snyder, Extension Bioclimatologist, Davis;
Blaine R. Hanson, Extension Irrigation and Drainage Specialist, Davis;
and Raymond Coppock, Extension Communications Specialist, Davis, with assistance
from others in the UC Cooperative Extension Soil, Water, and Engineering Program.
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Publication 21414e
Previous printing 1986. Digitally re-issued 2002; no content was updated from the 1986 printing.
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How Farmers Irrigate in California
Introduction
How much and where
How water is applied
Sidebar: CROP WATER NEEDS
Sidebar: DRIP: THE MYTH AND THE FACTS
The farmer’s decision
Sidebar: THE SOIL RESERVOIR
CIMIS: A new approach
For Information about This and Other Publications...
disclaimer: (Scan of Leaflet 21414. No content was updated from the 1986 printing.)
