What is acceptable distribution uniformity (DU)?

Application uniformity of microirrigation systems is usually measured using distribution uniformity (DU) or emission uniformity (EU). Emission uniformity is frequently used to describe the uniformity of the design of a microirrigation system while distribution uniformity is used to quantify the uniformity of a system field evaluation. The numeric value of DU and EU is the same for a given system.

The table below shows the recommended EU ranges for newly designed microirrigation systems. The DU of a system that has already been in use will at best remain constant over time. More likely, the DU will decrease with the age of the system. If the DU of the evaluated system is less than 70%, you should definitely take steps to determine why your irrigation uniformity is low and to improve the irrigation system's performance. The causes of DU values that are greater than 70% but less than the recommended design EU values in the table below are also well worth investigating. Remember the impact of poor irrigation uniformity: in order to provide adequate irrigation to the entire crop, you will have to give some of the irrigated area significantly more water than it actually needs, resulting in poor irrigation efficiency.

Recommended ranges of design emission uniformity (EU).

Emitter type	Spacing (m)	Topography	Slope, (%)	EU (%)
Point source on perennial crops	> 4	uniform steep or undulating	< 2 > 2	90 to 95 85 to 90
Point source on perennial or semi-permanent crops	< 4	uniform steep or undulating	< 2 > 2	85 to 90 80 to 90
Line source on annual or perennial crops	All	uniform steep or undulating	< 2 > 2	80 to 90 70 to 85

Source: ASABE EP405 Standard. February, 2003.

Ideally, you should know the EU of any new system (or the DU if a field evaluation is done of the new system) so that you can use that as a standard against which to compare DU's from future evaluations. Decrease in DU (increases in variability) over time would be a sign that emission device clogging was likely occurring. A close look at the field-collected data should yield evidence that the discharge from some emission devices is significantly less than that of others, and that is evidence of clogging.

If you were to start without an evaluation of a new microirrigation system to use as a reference, it will be much harder to tell whether lower-than-desired DU in future years is the result of poor original system design or whether it indicates clogging a well-designed system. It is hard to determine which, but there are two clues to look for:

1. If the system pressure is fairly constant (less than 10 to 20% difference) throughout the system but there is significant variability among emitter discharges resulting in a low DU, clogging is likely a problem.

2. If the variability in emission device discharge does not follow any pattern that can be explained by pressure changes attributable to elevation differences or friction losses, once again clogging is a strong suspect.

It should now be evident why it is extremely valuable to know where in the irrigation system measurements were taken during a system evaluation. Location referenced measurements provide valuable clues when you are trying to solve the mystery a low DU value.

If you believe you have a clogging problem and you want to solve it, click here.