How to Calculate Distribution Uniformity and Adjust Irrigation Requirements
Distribution uniformity is expressed as a percentage, and is a relative index of the variability between emitters in an irrigation block. Distribution uniformity is defined as the average discharge of 25% of the sampled emitters with the least discharge, divided by the average discharge of all sampled emitters.
After emitter discharge has been measured in the field and recorded on the Drip System Evaluation Form you can calculate the distribution uniformity for the system and use that value to adjust the number of hours to irrigate the vineyard. After measuring discharge from sampled emitters, complete the calculations at the end of the blank form. See How to Sample Emitters for instructions on field data collection.
Because numerous calculations must be performed to convert sampled emitter discharge into gallons per hour, and emitters must be ranked in order of discharge rate, the process can become time consuming. There are many tools available online to help perform calculations, for example, the CA Sustainable Winegrowing Alliance has an interactive Excel worksheet available on their website.
Example Calculations
The following calculations are summarized on the SAMPLE Drip System Evaluation Form.
EXAMPLE: DETERMINE AVERAGE APPLICATION RATE
For each irrigation block, calculate the average of all your discharge rate measurements. If you measured the output of 36 drip emitters, find the average discharge rate (gph) of those 36 emitters.
Average discharge rate of all emitters = 0.48 gph
| Assumption: | ||||||
| Application rate per vine (gph) | = | 0.48 gph per dripper | x | 2 drippers per vine | = | 0.96 gph/vine |
| Average Application Rate is 0.96 gph/vine | ||||||
EXAMPLE: DETERMINE DISTRIBUTION UNIFORMITY
| Distribution Uniformity (%) | = | Avg. discharge of the 25% sampled emitters with the least discharge Avg. discharge rate of all the sampled emitters | x 100 |
To identify the 25% sampled emitters with the least discharge rate, the discharge rate of all sampled emitters should be ranked from lowest to highest. Then 25% of the emitters with the lowest discharge rate should be averaged together. For example, if 36 emitters were monitored, the average of the 9 emitters with the lowest discharge rates would be determined.
Assumptions:
Average discharge rate of all sampled emitters = 0.48 gph
Average discharge rate of the low 25% sampled emitters = 0.44 gph
| Distribution Uniformity (%) = | 0.44 gph 0.48 gph | x 100 = 92% |
Average Distribution Uniformity is 92% (This is quite good)
Using distribution uniformity to adjust irrigation requirements
The irrigation amount (gross irrigation amount) includes the water you have chosen to replace (net irrigation amount) plus some additional water to account for the inefficiencies of the irrigation system. The irrigation amount is determined in the following equations:
| Gross irrigation amount = | Net irrigation amount Irrigation efficiency (%) | x 100 |
"Irrigation efficiency" is difficult to quantify but if drainage (water that has moved below the vine's root system) and surface runoff is minimal, then irrigation efficiency can be approximated using the distribution uniformity. The above equation becomes:
| Gross irrigation amount = | Net irrigation amount Distribution uniformity (%) | x 100 |
DETERMINE NUMBER OF HOURS TO IRRIGATE
Assumptions:
Net irrigation amount = 12 gal per vine/wk (See When to Start Irrigating page)
Average application rate per vine = 0.96 gph
Distribution uniformity = 92%
| Gross irrigation amount = | 12 gal/wk 92% | x 100% = 13 gal/wk |
| Irrigation Time per week = | Gross irrigation amount (gal/wk) Avg. application rate per vine (gph) | = | 13 gal/wk 0.96 gph | = 13.5 hrs |
Number of hours to irrigate is 13.5 hours/week
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.