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.
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. An interactive, bilingual Excel worksheet is available to automatically convert the volume of water collected in 60 seconds, or the number of seconds to fill a 35mm film canister into gallons per hour. The worksheet will also rank the emitter discharge and calculate emission uniformity.
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 Emission 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 as:
| Gross irrigation amount = | Net irrigation ammount 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 emission uniformity. The above equation becomes:
| Gross irrigation amount = | Net irrigation ammount Emission 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
Emission 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