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Determining DU (distribution uniformity)
for existing SDI systems


I was recently asked by a prospective client how to determine Distribution Uniformity for an installed SDI system. I know how to calculate the theoretical DU. The only way I know how to find out actual DU would be to dig up the tape in several places and measure the flow rates as the system is operating. Any other ideas?

by Randall Merriott


One of my favorite methods to estimate EU (or DU) for SDI is called the "Vandergulik Swag" technique: If half your field is brown and ugly then you have a 50% DU. You can also reverse it and say if half your field is a swamp........... you get the idea. Or if half your field is a swamp and the other half is brown and dry then you have a 0% DU

by Tim Wilson


Actual system DU can be estimated in the field as you mentioned, by digging and measuring. (The "Vandergulik Swag" technique mentioned by someone else is even less than a swag.) It's a lot of work which is why most people don't bother with it but sometimes you can be amazed by what you find (great to poor hydraulic design and sometimes plugging from all sorts of sources).

I recommend the Cal Poly method of evaluation with a few tips:

1. Do not take less measurements than the program calls for (there are very specific reasons for the number of measurements taken at which locations)

2. If digging up tape, be careful to not wipe the mud off it (you just might artificially plug the outlets)

3. I make little washers out of rubber hose to prevent water from running along the hose (and not into the measuring cups).

4. On drip tape systems I make one recommended change to the Cal Poly procedure and that is the first location of flow measurements. The Cal Poly procedure calls for the first flow measurements to be taken in the middle of a hose hydraulically closest to the pump. The reason for the middle of the hose is you need to be able to take all 16 flows at the same pressure (no significant friction loss in the middle of the hose if emitters are spaced far apart). To get a more accurate estimate of the actual application rate with tape systems, I take those measurements at the beginning of the hose where the pressure is the highest.

A NEW WINDOWS VERSION OF THE MICRO-IRRIGATION SYSTEM EVALUATION PROGRAM

CAN BE ORDERED FROM THE CAL POLY / ITRC (805)756-2434.

by Danyal Kasapligil


I may be missing something, (or are the Cal Poly "measurements", tensiometer measurements)?Granted it's nice to know the degree of distribution uniformity, but what really counts is uniformity of available water for the crop. And you monitor that quite directly with tensiometer readings of matric potential.

Now, for those readings to be uniform not only must the water-delivery be uniform, but so must the soil-profile. But if that's very variable, a uniform delivery of water will probably be inadequate anyway. So properly programmed tensiometer readings can really tell you what's important to your crop; warn you of the consequences of clogged emitters, breaks in tape, too large pressure-drops in long runs and/or non-uniform soil profiles.

by Len Ornstein


Just in case there are a lot of engineers out there without a sense of humor, the "Vandergulik Swag" technique was a joke, that's J-O-K-E. I hope I won't be kicked off the List for being so brash.

by Tim Wilson


Len, I think you're right. The uniformity of available water is what really counts. As long as the readings on the tensiometers are uniform, there's probably no need to dig up the tape and measure the DU. (I am curious how accurate tensiometers would be in estimating DU. Have any of the researchers out there planted a good number of tensiometers, taken readings, then dug up the tape at the same location, measured flows and then compared the two? It would be interesting to know what the DU was based on the tensiometers vs. that based on the flow measurements alone, and I guess the accuracy would depend a lot on the uniformity of the soil conditions.)

But what if the tensiometer readings aren't uniform? Is it because of emitter clogging? Is it because of poor design? Is it because of the soil or other factors? I would assume that there would be a certain amount of variability between the tensiometers themselves. The tensiometers are going to tell me I have a problem, but once I know I have a problem, I need to figure out what it is and how I can solve it.

I have calculated actual EU with microsprinkler systems using the ASAE method. Because of the nature of these systems you can also easily measure the pressure at each point, thus allowing you also to calculate the hydraulic uniformity. This helps make it easier to determine if the problem lies with the system design or with clogged emitters. I don't know how you can do this with tape without punching a hole in it. I just basically want to be able to let the customer "see" that his system is working properly, and if it's not, be able to quickly figure out why not.

by Randall Merriott


> Granted it's nice to know the degree of distribution uniformity, but what really counts is uniformity of available water for the crop.<

Yes, that is partially true. But you miss the point if you ignore the system aspects. After all, you may not be able to exchange your land and soil, but you may be able to buy a more uniform system. Conversely, the most uniform system may not be of much use on some land areas, so why bother with the best system. System, crop, soil, and climate, plus a host of other things need to be "matched" to make it all work. DU, EU, US and the other types of system uniformity terms are just some of the tools in the toolbox.

>It would be interesting to know what the DU was based on the tensiometers vs. that based on the flow measurements alone, and I guess the accuracy would depend a lot on the uniformity of the soil conditions.<

That method may give you a more "global" estimation of irrigation performance by integrating the system aspects with the soil aspects, but it may tell you less about what's causing what. If used properly, with other tools it might be a good tool for the tool chest. Sometimes knowing what performance a component aspect might be causing might then point you to look to the next possible problem, while if it is all integrated, you may have to split it back into components.

>I have calculated actual EU with microsprinkler systems using the ASAE method. I don't know how you can do this (pressure measurements) with tape without punching a hole in it.<

Depending on what information you want to obtain or what problems you want to isolate, you may be able to avoid measuring the pressures. ASAE EP-458 can be used to just measure US (the statistical uniformity of the emitter discharge rate) without measuring the pressures. Obviously, you can't isolate some of the problems without measuring the pressures. But if US turned out to be extremely high, the hydraulic uniformity, USH and the emitter performance variation may be moot points. I have also been told, you can repair the holes punched in the tape with goof plugs. A pressure gauge with a relatively large diameter hypodermic needle can be used for the pressure determinations.

>I just basically want to be able to let the customer "see" that his system is working properly, and if it's not, be able to quickly figure out why not.<

This may be too simplistic for your needs, But The pressure and flows along the system can OR should be determined by the system design. I know some of the manufacturers have design programs to calculate theoretical EUs or DUs, etc. The cumulative flows for the system and the pressures at the system inlet and the distal flushline could be measured on a regular basis (daily, weekly etc.). These values could be plotted with time and compared to the design values. If the deviations in flows or pressures change, the producer could then look for problems. The changes may be normal or maybe require immediate attention.

If the performance characteristics are significantly different than the design values immediately after installation, the producer and contractor should probably work together to find the cause, and that might mean much more rigorous procedures such as ASAE EP-458 or the Cal-Poly DU technique. The complexity of the procedure is sometimes altered by the difficulties of making the system measurements. IE, deep SDI systems with low flow tape products are more difficult than surfacesystems with higher flow emitters.

by Freddie Lamm


In using tensiometers to estimate the distribution uniformity of a drip irrigation system, Randall Merriott correctly mentions that there will be some error among the tensiometers. In studying optimal potato irrigation (not a series of drip irrigation trials) we dug up many paired tensiometers and paired Watermark soil moisture sensors and compared their readings with the soil water contents in the immediate proximity of the instruments. Much of the variability in instrument readings was related to spot to spot variability in soil water.

by Clinton C. Shock


>In studying optimal potato irrigation (not a series of drip irrigation trials) we dug up many paired tensiometers and paired >Watermark soil moisture sensors and compared their readings with the soil water contents in the immediate proximity of >the instruments. Much of the variability in instrument readings was related to spot to spot variability in soil water.<

Which MUST have been due to variability in local soil hydraulic conductivity.

With putatively uniform application of irrigation water, such variability (if it is of large enough magnitude) means that the crop DOES NOT uniform AVAILABILITY of water. In such soil profiles, what's needed in NON-UNIFORM application of water to yield uniform availability to the crop!

The only technology that can provide that, automatically, is the Irristat.

See: http://www.pipeline.com/~lenornst/index.html and go to the Irristat page.

by Len Ornstein