sNorth Dakota State University
NDSU Extension Service

Water Spouts

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


Irrigation Workshops

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.

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:

  1. 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.


  2. 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.


  3. 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.


  4. 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.)


  5. 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.


  6. 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. 

----------------------------------------------------------------------------------
                                             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 Acts of Congress of May 8 and June 30, 1914. We offer our programs and facilities to all persons regardless of race, color, national origin, religion, sex, disability, age, Vietnam era veterans status, or sexual orientation; and are an equal opportunity employer. This publication will be made available in alternative formats for people withdisabilities upon request, 701/231-7881.

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