What Does the Professional Evaluation Information Mean?

Emitter discharge

The emitter discharge is the amount of water per unit time that the microirrigation emission devices discharge.  Usually this is expressed in gallons per hour (gph) or liters per hour (lph).  The discharge rate is measured in the field by collecting the discharge from the drippers or microsprinklers for a set period of time.  The evaluation team samples emission devices throughout the system to get a representative sample of the discharge rate (average discharge rate).  They will take into account pressure differences caused by elevation changes and frictional pressure losses.

How do you use the average emitter discharge value?

1. The average discharge rate value is very useful for irrigation scheduling.  By combining the average emitter discharge with evapotranspiration information, you can determine the required irrigation times.
2. If the average discharge rate, measured at the same pressure, decreases over time, clogging is the likely cause  An irrigation evaluation conducted when the system is new provides an excellent benchmark to compare future measurements.

Increases in emitter discharge rates are unusual. If this occurs, first check to make sure the most recent measurements were taken at the same operating pressure as previous measurements.  Another, infrequent, circumstance that causes an increase in average emitter discharge rate is when pressure-compensating (PC) emitters are beginning to clog.  During the early stages of PC emitter clogging, the emitter’s flexible orifice sticks open resulting in an increased emitter discharge.

Discharge uniformity measures

A perfect microirrigation system would have every emission device discharging at exactly the same rate.  This never occurs in the real world so you should expect to see discharge variability between emitters.

Some variations in emitter discharge follow patterns that are easily explained.  For example,

• Elevation changes may explain an increase or decrease in emitter discharge,
• Emitters farther from the head of the system often operate at a lower pressure due to friction losses, 
• Emitters farther along a lateral line may operate at a lower pressure than those at the head of the lateral line, again due to frictional pressure losses.

A significant decrease in emitter discharge at the tail end of lateral lines may indicate that emitters are clogged by silt and clay particles and that the system needs to be flushed more often.  These particles often settle at the tail end of lateral lines and unless they are flushed out of the system, they can clog emitters.

Emitter discharge variations throughout the system that appear to be random can be caused by:

• Manufacturing variability.  There are always slight variations in emitter discharge from one emission device to another due to manufacturing differences.  In quality equipment, these variations are small and are difficult even to measure in a field installation.
• Biological, particulate, and chemical precipitates. Biological, particulate, and chemical precipitate clogging can occur in a random pattern One emitter may show a reduced discharge due to clogging while an adjacent emitter will show no evidence of clogging.

Distribution uniformity (DU)

Emitter discharge uniformity measures variability in emitter discharge. A number of specialized terms are used to quantify the emitter discharge variability, but the most common is distribution uniformity (DU). Sometimes emission uniformity (EU) will be used to quantify discharge uniformity.  EU and DU give the same numeric value.  When used as part of a microirrigation system evaluation process, DU is defined like this:

To find the average of the low-quarter emitter discharge rates, rank all the measured emitter discharge rates, from lowest to highest, and then average the lower 25% of discharge rates.  For example, if 40 emitters were measured, you would rank them lowest to highest, and then average the discharge rates of the 10 emitters with the lowest discharge rates.  

For a new microirrigation system, the following table lists the recommended design EU ranges for new microirrigation systems of various types.  Emission uniformity is frequently used to quantify the predicted discharge variability of microirrigation system designs.

Table 1.  Recommended ranges of design emission uniformity (EU).

Source:  ASABE EP405 Standard.  February, 2003.

Due to wear on emitters, clogging, and leaks, the DU value for a microirrigation system often decreases over time.  Good maintenance will help to keep this decline to a minimum.

If you have one or more previous evaluation(s) that you can use as a reference for comparison, a DU decline of 5% or more is an indicator that clogging or a similar problem is occurring.  A corresponding decline in average emitter discharge would be further evidence of clogging.

If this is the first evaluation of the microirrigation system and the EU value is substantially below the recommended ranges given in Table 1, determination of the cause of variability should follow a 2-step process:

1. Examine the pressure measurements gathered during the evaluation.  Significant variability in pressures within the system will lead to emitter discharge variability unless you use pressure compensating (PC) emission devices.  When determining what to do to mitigate excessive pressure variability, you should discuss the matter with an irrigation professional, most properly a qualified microirrigation system designer.
2. If pressure measurements show relatively consistent operating pressure within the system, the emitter discharge variability is likely due to clogging of emitters.  For information on how to solve that problem, go to “I have a clogging problem and I want to solve it”.