sNorth Dakota State University
NDSU Extension Service
No. 186 October 2000
http://www.ext.nodak.edu/extnews/snouts
Irrigation Workshops
Is Your Irrigation System Ready for Winter?
To Be or Not to Be -- Fall Irrigating, That Is!
Evaluating Operating Efficiency of Irrigation Pumping Plants
The North Dakota Water Users annual convention is
scheduled for Dec. 6 and 7 at the Radisson Inn, Bismarck. An
irrigation workshop for current irrigators will be held in conjunction with
the convention on Wednesday Dec. 6, and a workshop for new
or potential irrigators will be held on Dec. 7. A part of the
convention will be an irrigation exposition where irrigation suppliers will
demonstrate their products and services. On the afternoon of Dec.
6, the North Dakota Irrigation Caucus will hold its annual meeting.
If you have any suggestions for topics to cover at the
workshop, please give me a call, send an e-mail or a letter.
Is Your Irrigation System Ready for Winter?
Fall is always a busy time of the year and sometimes it is
easy to forget about getting the irrigation system ready for winter.
Here is a checklist to help you prepare your irrigation system for
the coming winter.
- Pump out or drain any buried pipelines.
- Chlorinate the well.
- Plug all holes in electrical control boxes to keep out rodents.
- Check all motor openings to see if they are
properly screened, again to keep out rodents.
- Drain water from pumps.
- Remove pump intake pipe from surface water sources.
- Check gearboxes on center pivot towers for water accumulation. Drain water and replace with oil.
- Remove and clean the end cap on the center pivot.
- Park the center pivot pointing in either a southeast or
a northwest direction.
- Check the condition of the gaskets in portable aluminum or PVC pipes.
- Loosen the belts on belt driven pumps.
Tom Scherer, (701) 231-7239
NDSU Extension Agricultural Engineer
tscherer@ndsuext.nodak.edu
To Be or Not to Be -- Fall Irrigating, That Is!
You know it's a hot topic and one not many people know a
lot about when I get six phone calls and three emails in one week
on the topic of fall irrigating. I really wasn't planning to write an
article on this topic, but it seems necessary to answer all the questions.
Let's say the questions are "Should irrigators be fall
irrigating?" and "Are there any documented benefits to fall irrigating?" Let
me answer the second question first. There are many good reasons
for irrigating in the fall after the heat of the summer season is
over. However, at the same time, irrigators need to make sure they
know why they are fall irrigating, the consequences, and what they
hope to accomplish. This leads us to the first question. Whether or not
to fall irrigate depends on the purpose.
Reasons for fall irrigating:
- Encourage germination of volunteer grain.
By fall
irrigating, a crop producer can establish a relatively
uniform flush of weeds and volunteer grain while at the same
time promoting vigorous, stress free weed growth. This
improves the efficacy of weed control by herbicides. If the objective
of tillage is to control fall weeds and volunteer grain, then
fall irrigation will enhance that operation.
- Facilitate fall tillage, whether for overwinter soil
conditioning or for organic matter
decomposition.
Tillage in the fall is performed for a variety of reasons
that include workload distribution, to facilitate spring
planting operations and residue management either to remove
the residue or facilitate its decomposition. Regardless of
the reason, added water to raise the soil moisture level prior
to tillage improves the tillage operation while reducing
the power requirements. Soil strength is inversely proportional
to the soil water content. What that means is the drier
the soil, the stronger the soil, and vice versa the wetter
the soil, the weaker the soil. By irrigating just to the point
of wetting the upper soil depth to field capacity, a producer
can facilitate the tillage operation and reduce the time
and energy required to complete the operation.
Another important aspect is the decomposition of
organic matter. Typically at the end of the cropping season, the soil
is very dry at the surface possibly even approaching
the wilting point. At this soil water content, organic
matter decomposition is slow. Microbes need moisture, and
added moisture up to a point facilitates the organic
matter composition.
- Water for fall plant growth. This applies primarily
to pasture, grasses and alfalfa. A lot of pasture and hay
ground in Montana is dominated by cool season grasses
plants which put on most of their growth during the cool months
of spring and fall. This is when they store sugars and
carbohydrates in the root system. Most folks can quickly
recognize the change of seasons by the way the grass in their
lawn grows. This also holds true for hay and pasture crops.
Begin irrigating in September when the days are
getting shorter and the nights cooler and you'll see good
grass growth. This growth is a potential valuable source of
winter feed after a good hard frost and a good shot for
spring growth. One thing that irrigators need to be aware of
when fall irrigating for plant water use is that alfalfa as tough
a crop as it is is pretty fragile when it comes to fall
water. Generally, a vigorously growing alfalfa crop needs about
six weeks of "conditioning," which is when growth is
slowing, water use is dropping off and before the soil begins to
freeze. Top growth might be killed by frost several times, but
root growth continues into October and even November.
This added moisture from fall irrigating facilitates root growth
and repeated top growth. Nevertheless, irrigators should not
fall irrigate alfalfa beyond late September. This will allow that six-week conditioning or hardening period.
This doesn't apply to grasses. The grasses are much hardier and are able to continue growing right up to
freezing. Moreover, because the grasses are predominantly
supported by a broad diffuse root system they are not subject
to damage from frost heave like alfalfa.
- Leaching of salts is reason not many irrigators think
about. Under a lot of irrigation conditions in Montana, the
net movement of water during the irrigation season is
upward, meaning soluble salts from the irrigation water and soil
are drawn upward toward the soil surface. Off-season
irrigation can be valuable at moving those salts down in the soil
and below the root zone. Off season irrigation water can also
fill the soil pores, enhancing the dissolving and diffusion
of those salts from the fine pores, so that when
moisture comes from rainfall or snowmelt, the salts are leached
further down into the soil below the root zone. (In fact,
in most cases in Montana, irrigators unknowingly rely on
this over-winter leaching of salts to satisfy the
necessary leaching requirement.)
- Soil moisture storage. Clearly this seems to be one that
is being argued a great deal these days. Some folks say
it doesn't do any good it all evaporates before
spring. Well, that's not true! There have been some studies
which have shown (on a very limited, random sampling) that
soil moisture that comes in the fall is evaporated from the soil
by the time planting season rolls around. The logical
argument then poses the questions: why fallow? An extensive
amount of research by scientists in both Montana and North
Dakota has shown that essentially all the moisture stored in the
soil, excluding that from irrigation, comes in the months
of September to December and March to June, with about
40% of the recharge moisture coming in the four fall months.
So, now the question becomes, can you outguess nature?
If you fall irrigate, what are the chances of getting
more moisture during the months of September-December
and March-June than you can hold in the soil? In other words,
are you wasting water by fall irrigating? Your guess is
as good as mine, but you can play the odds. On a sandy
soil, the available water storage capacity is about 1.5 inches
per foot. Therefore, if you have 4 feet of dry soil, let's say
at wilting point, about 6 inches of water can be
stored. Anything greater will be lost from the root zone but
will contribute to ground water and stream base flow during
the dry months. In this scenario, if you fall irrigated and
add 3 inches of water to a sandy soil, you'd still be able
to store another 3 inches of rainfall and snowmelt.
Compare that to a silt loam soil where you can store
2.2 inches of available water per foot of soil. That means you
can store between 8 and 10 inches of water in the soil, if it's
at wilting point. What's the likelihood of getting 8 inches
of effective rainfall in September-December? Effective rainfall
is any event that results in greater than 1/10th inch of
moisture infiltrating into the soil. We don't count the fact that 50%
or more of our snow cover sublimates (goes directly from
snow on the surface to a vapor, without infiltrating into the
soil). That's a good argument for adding some of that moisture in the fall.
- Effects of additional rain. The amount of research that
has been conducted on the benefits of "added rainfall effects" is mind boggling. Back in the days when we were
seriously looking at making it rain, and more recently as we
have looked at the effects of global warming on climatic
patterns, one of the issues that has been exhaustively studied is
that of added rainfall. Does it really make a difference if we
get one additional inch of moisture during the
non-growing season whether from rainfall or irrigation? You bet it
does. Researchers in Montana have shown that a single inch
of additional moisture can have a significant effect on grass
and forage production. Researchers from MSU have shown
that an additional inch of moisture is worth anywhere from 4 to
8 bushels of wheat per acre, from 400 to 500 pounds of
alfalfa per acre and significantly more than 4 to 8 bushels of
oats per acre.
There are some legitimate reasons for taking a serious
look at fall irrigation. The more important question is
why? What do you hope to accomplish by fall irrigating?
Clearly, we are not saving moisture, since eventually it all falls
into the hydrologic cycle and cycles through the oceans and
back to the atmosphere. But, by fall irrigating, we might be able
to create a little "added value" to a resource that most of
us take for granted, which tourists pay good money to get
in a bottle but pay better money to fish in, which often
begins its journey from the atmosphere in the
hydrologic cycle someplace here in Montana, and which always
seems to be in short supply here at the top of the watershed.
Dr. James W. Bauder, (406) 994-5685
Soil and Water Quality Specialist, Montana State University
jbauder@montana.edu
Evaluating Operating Efficiency of Irrigation Pumping Plants
Why efficiency?
Adding value to irrigated crop production requires that
the pumping plant and irrigation application system perform
well. Irrigators expect that an irrigation pumping plant should be at
its peak efficiency when it is installed. What is the peak efficiency
you ought to expect? Are you getting what you expected? How does
the pumping plant efficiency relate to your application system
design? Does the pumping plant flow rate match the application
system requirements? Does the application system meet the criteria
for crop water requirement? Through time, moving parts on
the irrigation pumping plant will wear causing a decrease in
pumping plant efficiency. How do you respond to such changes that
occur slowly over time and usually cannot be visually detected? Do
you then need to monitor pump performance on a regular
basis? Conducting a simple pumping plant efficiency test can provide
the answers to all of these questions and draw attention to
the importance of constructing quality irrigation systems at the
very beginning.
Testing pumping plant efficiency
An efficiency test will reveal how well the pumping plant
is performing and indicate its energy use. Efficiency is determined
by taking a ratio of output power and the input energy and
comparing against a standard. The output power is determined by
taking physical measurements of pump flow rate and also the
discharge pressure the pump generates. A portable ultrasonic meter
was used to measure the flow rate in gallons per minute (gpm).
The discharge pressure is measured in pounds per square inch
(psi) with a pressure gauge. The input energy is measured directly
with an AC multimeter capable to measure energy use in
kilowatt-hours per hour (KWH/HR). The standard energy use of an ideal
pumping plant with an electrical energy source and directly driven is
considered to be 0.885 KWH/HR. Typically, a pumping plant test at a
site will take about 20 minutes from beginning to finish.
Numerical efficiency test outputs (flow rate and energy use) are displayed
on the two instruments.
Evaluation of test data
Pumping plant efficiency tests were conducted at 37
irrigation sites this past summer. All the tested irrigation sites had
energy delivered from a three phase AC power source and the pumps
were driven directly by electric motors. The test performance results
are shown in Table 1. Comparative average performance data
is delineated for surface and groundwater source systems
and distinction highlighted between irrigation application systems.
There is significant difference in the efficiency ratings between
the sprinkler and gravity irrigation systems. Most gravity
systems could be considered inefficient. Even though the energy use
of groundwater based irrigation was significantly higher than
the surface water counterparts the respective extra energy use
was comparably similar.
Table 1. Pump plant efficiency test data and performance results.
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Water Source Application System
All ----------------- --------------------
Item Sites Surface Ground Sprinkler Gravity
----------------------------------------------------------------------------------
1 No. of irrigation sites 37 28 9 18 19
2 Irrigated area per site (acres) 101 66 172 133 66
3 Avg. flow rate (gpm) 1133 1215 888 740 1527
4 Avg. total head (ft) 106 73 204 178 34
5 Avg. motor horsepower 44 34 73 58 29
6 Avg. energy use (KWH/HR) 35.17 26.90 59.98 48.34 21.99
7 Avg. efficiency (%) 69 63 87 78 60
8 Avg. extra energy use (KWH/HR) 9.06 9.81 6.82 9.51 8.61
----------------------------------------------------------------------------------
----------------------------------------------------------------------------------
The overall pumping plant parameters relating flow rate
(gpm) with pressure head (ft) and electric motor horse power (HP) for
both the application systems (sprinkler and gravity) is illustrated
in Figure 1. There is clear demarcation between the gravity and
the sprinkler irrigation systems and their distinctive performance patterns. While sprinkler systems pumped less water the total
head and motor power used was much higher. The slope of
the motor HP used in sprinkler systems was much steeper than
their counterparts using gravity systems.
Figure 1. Pumping plant efficiency test parameters for sprinkler
and gravity irrigation system.
The performance of the two distinct application systems
was evaluated by relating energy use to their respective pumping
plant efficiency. Figure 2 depicts the even spread derived from
sprinkler systems versus the scattered patterns given by the gravity
system. The graph clearly delineates the potential sites where
corrective measures are warranted.
Figure 2. Efficiency relative
to energy use in sprinkler and gravity irrigation systems.
Conclusion
The construction of high efficient pumping plant is a key
to successful farming operation. For diagnosing problems
pumping plant efficiency test is essential to delineate energy use and
the flow rate. The test is most useful when conducted at new
irrigation sites where measures to ensure quality installation could be
taken. NDSU Extension Specialist in North Dakota routinely
conducts these tests on old and new irrigation systems. Farmers planning
to develop new irrigation facilities are encouraged to contact
County and Area Irrigation Specialist for arranging the test.
Aung Hla, (701) 652-3194
NDSU Area Irrigation Specialist
aung@daktel.com
Water Spouts, No. 186 October 2000
NDSU Extension Service, North Dakota State University of Agriculture and Applied Science, and U.S.
Department of Agriculture cooperating. Sharon D. Anderson, Director, Fargo, North Dakota. Distributed in furtherance of the
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upon request, 701/231-7881.
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