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
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
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
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
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
and go to the Irristat page.
by Len Ornstein