| Irrigation is by far the biggest consumer of water in Colorado, and irrigated
agriculture has been identified as a potential non-point source of pollutants
to our State’s water supplies. Virtually every irrigator is aware of this
potential problem, and most are keen to do the best job they can to maintain
quality as well as quantity of our water supplies. USDA-ARS in Fort Collins
has worked with farmers for many years to provide best irrigation management
practices to minimize the adverse impact of irrigated agriculture on public
waters. To us, precision irrigation means two things: precision with respect
to area and precision with respect to time. Because crop nutrients, such
as nitrogen (and many pesticides) are very readily dissolved in water, thus
move through the soil if excess water moves through the soil, precision
irrigation in either context means less opportunity for contaminating our
water supply because more ag chemicals are kept in or on the soil where
they belong.
The time aspect of precision irrigation is called “irrigation scheduling,”
and many farmers as well as farm advisors are aware of this terminology.
Irrigation scheduling can mean using field devices, such as gypsum blocks
or expensive instruments to measure when the soil can hold more water,
or it can mean computation of how much water is required by the crop at
its current stage and by weather conditions, to meet crop needs in the
near future. Either approach can provide guidance as to when to irrigate
and how much water to apply, and may provide the opportunity to reduce
irrigation application significantly — thus reducing costs of pumping,
nitrogen fertilizer, and other chemicals, as well as protecting water
quality.
Today, the Colorado Climate Center (CCC) at Colorado State University
CSU, in cooperation with CSU Cooperative Extension, CSU Ag Experiment
Station, USDA, and several commodity groups, maintains a network of 36
weather stations across the state. These stations report their weather
to CCC daily, and these data are used to compute the water used by a variety
of crops. The weather data as well as crop water use (evapotranspiration
or ET) are posted on the Internet.
In years past, the concept of irrigation scheduling has been promoted
on the basis that we can calculate the ET quite accurately if we know
the appropriate weather data from the past few days, and that we can determine
how much irrigation water is applied if we assume that the irrigation
system applies the same amount of water everywhere in the field. The concept
of precision farming now allows us to take a different approach to the
concept of precision irrigation — one of applying just the right amount
of water needed in each area of the field. Seldom do we have enough information
to say that the crop in one square foot of a field requires more water
than that in another area. However, we do know how differences in soils
affect how much of the water applied is held for later crop use. We also
know that irrigation does not result in the same amount of water being
stored in the soil in one part of the field as in another. With gravity
irrigation systems, changes in slope, soil type, tillage, wheel traffic,
irrigation stream, set time, and distance from the head ditch all effect
the amount of water stored in the soil. Even with center pivots, irrigation
is not the same from one point of the field to another, but depends on
topography, sprinkler package, and end gun operation.
What is appropriate irrigation management and the outcome of that management,
also depends on the climate. For example, in the eastern US, where irrigation
may account for 6 to 8 inches of water annually to supplement rain, it
may be feasible to irrigate sandy spots only, hoping that heavier soils
will hold enough water to last until a rain in the near future refills
the soil profile. In these cases, we may well find that some soils have
a lower ability to produce a crop, and that precision farming best dictates
that inputs be reduced in those areas that have lower production potential.
Thus, a system with the ability to put different amounts of water in different
areas may be worthwhile.
In Colorado, where corn requires 24 to 27 inches of well-timed water
to produce at its potential, rainfall is not as large a factor (typically
providing 7 to 10 inches during the growing season). In this case, we
have the potential to control the total water picture much better than
irrigators in areas with more rainfall. Where water is the controlling
factor of production, and particularly where nutrients can be applied
with the water, there is little reason to believe that one soil has a
lower potential production than another soil. We see evidence in the fact
that many of the very sandy soils, considered not irrigable prior to center
pivots, produce as much or more than the “good” or heavier soils of the
major river valleys.
We believe that precision irrigation implies that we need to design the
irrigation system to apply water as uniformly as practical, then to operate
the system to apply just the amount of water the crop needs. We are presently
conducting studies on the same center pivot irrigated fields as our colleagues
at Colorado State, and I will use information from those studies to illustrate
the importance of uniformity. In the past, irrigation engineers believed
that a system capable of applying water with a “coefficient of uniformity”
(CU) equal to 80 was a good system. However, we now know that we pay dearly
in terms of water required, and consequently in terms of both energy and
lost chemicals, if we have an irrigation system with low uniformity and
we try to keep most of the field “well irrigated.” Figure 1. shows how
much water we must pump, as a percentage of what the crop requires, for
different values of uniformity if we keep 85-90% of the field “well irrigated”.
Thus, if we increase uniformity from 85 (132%) to 95 (108%) we can reduce
the total water required by 24%! There are several reasons that uniformity
of a center pivot may not be up to par — nozzles worn from pumping sand,
improper nozzles installed (either poor design, or nozzles not installed
where they were intended), large changes in elevation on systems without
pressure regulators (or regulators that no longer work). One of the big
factors in nonuniformity of irrigation is use of an end gun. When the
field is planted to near the limit of end gun reach, it is obvious from
plant growth that water is not applied uniformly. Less obvious is the
variation of water application along the pipeline as pressure changes
when the end gun turns on or off.
The range of total seasonal irrigation application over one of our study
fields (CU = 86) in 1999 is shown in Figure 2. As a result of topographic
changes, incorrect nozzle sizing along the pipeline, turning the end gun
on and off, and stopping the system when it rained, then moving back to
the pivot road before starting the next irrigation, the water application
is less than uniform, even on this “well designed” system. Less than half
the field receives within 5% of the average irrigation application, and
significant areas receive either a deficit or a surplus exceeding 10 percent.
Overall, if the irrigator manages to keep 85-90% of the field well watered,
he will need to pump 28 percent more water than the crop requires. The
concept of precision timing is equally important, as studies in several
areas show that the average irrigation is frequently on the order of 30
percent more than the crop actually needs.
Our current work is concentrated on determining the effect of this “imprecise
irrigation,” combined with fertigation, soil differences, and estimated
leaching, on the availability of nitrogen fertilizer to the crop, and
ultimately on crop yield.
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Click here to view a Word file containing Figure
1.
Click here to view a Wordperfect file containing
Figure 2.
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This
is similar to saying there are three people, “Big & Strong”, “Medium-built”
and “Short & Puny” and that if we feed a lot (a huge meal) to the
“Short and Puny” person, we will be able to get the same amount of work
done out of the “Short and Puny” person as compared to the “Big &
Strong”. We all know that’s not going to happen. There may be some exceptions,
but generally speaking, if we feed the “Short & Puny” guy the same
huge meal as the “Big & Strong” person, the “short guy” most likely
will be running back and forth to the bathroom.
