Let's talk about agricultural irrigation.......
by Freddie Lamm
The last time we talked, I said that one of the purposes of the IA Agricultural Irrigation Division is "to enhance the public image of irrigated agriculture." I also suggested that sometimes we shoot ourselves in the foot by the way we talk about irrigation. With that introduction, let me offer for discussion what I consider to be one of the most abused terms in irrigation, the infamous application efficiency, Ea. Now, before it appears that I start pointing fingers, let me confess, I too abuse the term application efficiency.
There are several definitions of application efficiency in the literature, but the one that I use was reported by Heermann et al. (1990). It is algebraically expressed as
Ea = Vsoil / Vfield
where Vsoil is equal to the volume of irrigation water needed for crop evapotranspiration to avoid undesirable water stress and Vfield is equal to the volume of water delivered to the field.
Before we delve into the workings of the equation, let's look at the history of the term, application efficiency. As best as I can tell, it is a product of this century. In my limited review of the earlier literature of this century, I couldn't find application efficiency, but I did find the terms "duty of water" and "permissible waste." "Duty of water" was defined by Brown (1920) as "the measure of the efficient irrigation work that water can perform, expressed in terms establishing the relation between the area of crop brought to maturity and the quantity of water used in its irrigation." Duty was usually expressed in terms of acre-ft/acre. In the language of the time (1920s), there were the terms "gross duty" and "net duty." "Gross duty" was the term applied to the irrigation amount diverted from the source to be used in irrigating an acre. It would include what we now call conveyance losses. "Net duty" represented the amount of water delivered to the field margin to irrigate an acre. It would include runoff, percolation, and evaporation losses during the irrigation event. Note from the definition, however, that the term "duty" was perceived by those of the time as efficient, even though there was still much inefficiency built into it. "Permissible waste," which is a term I am sure would not be politically correct today, was defined as the fraction of water that was lost in conveyance, unequal in-field distribution, deep percolation, and runoff. Fortier (1928) noted that even though "permissible waste" was still quite high, that it was "much less today than it was 20 years ago." He went on to say there would always be a limit to improvements that would be governed by economics. As you read these comments and thoughts of Fortier, does the word "deja vu" ring a bell.
Israelsen (1932) does include the term application efficiency, Ea, in his book. His definition at that point in time differs from the equation listed above and would be more closely associated with the term water storage efficiency. The water storage efficiency is the fraction of an irrigation amount stored in the remaining available crop root zone following an irrigation event. The use of water storage efficiency is discouraged by Heermann et al. (1990) because of the difficulty of determining the crop root zone and because the water storage efficiency can still be quite low while sufficient water is provided for crop production. Although application efficiency as defined by Israelsen does have a few technical differences with the Ea equation listed above, in many instances they can have the same value. Israelsen (1932) stated that Ea in some locales is probably as low as 20% and is seldom greater than 75%. He concludes there is "undoubtedly great opportunity to increase Ea at costs well within economical limits."
Now let's talk about the equation. It can be used for a single irrigation event or more as a term reflecting the seasonal performance. The difference in how it is used can be quite dramatic. For example, the first irrigation event using furrow irrigation can have a very low application efficiency if the length of run is long, if the furrows are freshly corrugated, if the stream size is wrong, or for several other reasons. However, subsequent irrigation events might have highly acceptable application efficiencies. Likewise, for sprinkler irrigation, one event might have a high application efficiency because the soil is extremely dry and is conducive to infiltration, whereas a similar event on a wet soil might result in excessive runoff and thus have a poor application efficiency. If you are an irrigator, you probably don't want to calculate your "standard practice" gross irrigation amount from the event with the worst application efficiency, but you could use that measured value of application efficiency if you were estimating what your net irrigation application for that single worst event was. Similarly, everyone in the business of selling irrigation systems would hate to sell on the basis of the application efficiency of that worst irrigation event, and would much rather pick the event where the application efficiency is high. A seasonal value of application efficiency has merit in some cases, but may be difficult to evaluate. The point I am trying to make is that we need to be particularly careful in how we use application efficiency with respect to time. In many of our attempts to model or predict irrigation performance, we assume the application efficiency is a fixed number for a system type when it is not.
Now let's look at the equation from a mathematical perspective. It is easy to manipulate Vfield so that Ea can be equal to 1 or 100%. Any irrigation system from the worst to the best can be operated in a fashion to achieve 100% Ea if Vfield is low. Increasing Ea in this manner totally ignores the need for irrigation uniformity. For Ea to have practical meaning, Vsoil needs to be considered to avoid undesirable water stress. What's the point of looking at the math? The point is that many people utilize the term application efficiency as a relatively static term without realizing that it can be easily manipulated to give any value that is desirable. There are efforts being made to mandate certain levels of application efficiency through water laws and regulations. I think we should all agree that irrigation should be used beneficially and wisely with minimal waste, but to raise the stature of such an easily manipulated term to such a high pedestal is wrong. If laws are needed, wouldn't some measure of the derived benefit of the irrigation be more appropriate? In addition, a high derived benefit would economically demand efficient application.
Now let's talk about those pesky application efficiency numbers for the particular systems. You've got a set you like and so do I. No, I am not going to tell you mine here in this column. Why should I? Chances are slim we would agree with each other. That's the point; there are lots of published numbers out there. If I get up on the wrong side of the bed and dislike sprinkler irrigation that day, am I being responsible if I find all the numbers that show evaporation losses as tremendously lowering application efficiency for sprinkler irrigation? Those numbers are out there if you want them. Oh, you don't like surface irrigation? No problem; those low number are out there too.
Well, what do I consider as the purpose of application efficiency? I would suggest that the purpose of application efficiency was not and should not be to compare irrigation system types even though I am still guilty of that abuse. The history implies to me the purpose of Ea was to help estimate the gross irrigation requirement once the net irrigation need was determined and vice versa. I think it was also to be used as a benchmark of the present condition to inspire us to make further improvements in irrigation.
I was unable to find a 12-step program to overcoming application efficiency abuse. To be honest, when I told one of our respected colleagues who knows a lot more about application efficiency than I do, that I was writing this essay on its abuse, his immediate response was that he had little hope it could ever be used correctly. Still, I think we can borrow from some of the those multi-step plans to set things right. We need to:
Recognize the abuse of Ea
Recognize the limitations of Ea as it pertains to system comparisons
Recognize that each measured or calculated Ea is different
Help educate the community about the proper use of Ea
Don't enable the abuse of Ea by others with your silence
Now before I close, let's put this all back in perspective. Yes, I am serious about my comments, but there is an element of light-heartedness in my approach. This is my opinion, it may not be your opinion. As professionals we should be able to agree to disagree. And finally, recognize that I am not equating the minor seriousness of misuse of a term with the seriousness of the important and unfortunate types of abuse that deal with our actions to ourselves or to our relationships.
In coming issues, I hope we can talk more about some of the positive aspects of agricultural irrigation. One of our colleagues saw the February 97 issue and suggested we talk about some of the agricultural irrigation success stories. I think that's a great idea. He also mentioned the importance of appropriate technology (i.e. why use the latest technology if the old technology meets all the needs?). This also would be a good topic to discuss. If you have some success stories you'd like to share, please let me know. If you have a story about how appropriate technology was the solution, let me know. Please recognize that I want you to be a part of the discussion, but that I may not be able to even respond to your submission because I bet there's a lot of stories out there. Please feel free to contact me by mail, Email, fax, or phone. In the mean time, let's talk about agricultural irrigation....
Brown, H. 1920. Irrigation, its principles and practice, as a branch of engineering. Third edition, London, England. P 32.
Heermann, D. F., W. W. Wallender, and M. G. Bos. 1990. Irrigation efficiency and uniformity. Chapter 6 in Management of Farm Irrigation Systems, pp. 125-149. Edited by G. J. Hoffman, T. A. Howell, and K. H. Solomon. ASAE Monograph, ASAE, St. Joseph, Michigan. 1040 pp.
Israelsen, O. W. 1932. Irrigation principles and practices. Wiley and Sons, New York, New York. 411 pp.
Fortier, S. 1928. Irrigation requirements of the arid and semi-arid lands of the Missouri and Arkansas river basins. USDA technical bulletin No. 36, Washington DC. 112 pp.
Freddie Lamm is an Associate Professor for Kansas State University conducting agricultural irrigation engineering research at the KSU Northwest Research-Extension Center, Colby, Kansas. He also serves as the Chair of The IA Agricultural Irrigation Division. Comments can be sent to postal address 105 Experiment Farm Road, Colby, Kansas 67701; Email: firstname.lastname@example.org; Fax: 913-462-2315; or Phone: 913-462-6281.