What is emission uniformity?
Emission uniformity (EU) measures how evenly a drip zone's emitters put out water. Two emitters rated the same by the manufacturer rarely discharge the exact same amount once they're installed, because pressure drops along a lateral, emitters partially clog, and manufacturing tolerances vary from one unit to the next. EU turns a set of measured catch volumes into a single number that says how close the worst-watered quarter of your emitters comes to the average.
The University of California's Cooperative Extension defines EU as the average discharge of the lowest-performing 25% of tested emitters, divided by the average discharge of all the emitters tested. A field study of 112 low-volume irrigation systems in the southern San Joaquin Valley, published in California Agriculture, rated the results on a fixed scale for systems already in service.
| Emission uniformity | Rating | What it suggests |
|---|---|---|
| 90% or higher | Excellent | Pressure and emitters are performing about as evenly as a real installed system does. |
| 80% to 90% | Good | Typical of a well-designed, reasonably maintained system. |
| 70% to 80% | Fair | Worth investigating; often pressure variation or early clogging. |
| Below 70% | Poor | The 1983 field study traced about 60% of cases this low to pressure problems and much of the rest mainly to plugged emitters. |
That scale is for systems that have already been running for at least a season. Standards written for new-system design, such as the ASABE EP405 microirrigation standard referenced by USDA NRCS, set separate target ranges for a newly designed system, which are typically tighter than what an older system in the field will show. This calculator applies the field-evaluation scale above, since it's built around a catch test on an existing zone.
When this isn't the right tool. This calculator is for point-source drip and microirrigation emitters, evaluated with a catch test. For a sprinkler zone's catch-can test, use the distribution uniformity calculator instead; the math is the same, but sprinkler spacing and precipitation rates need different inputs.
How this emission uniformity calculator works
The calculator ranks your catch volumes from lowest to highest, averages the lowest 25% of them, and compares that to the average of every catch. If you add a current run time, it also works out how much longer to run the zone so the driest-tested quarter still gets your target amount.
1. Low-quarter average
Low-quarter count
Emitters in the low quarter = round(Number tested ÷ 4), minimum 1
The lowest-discharging emitters, by this count, are averaged together. This matches the worked examples published by University of California Cooperative Extension.
2. Emission uniformity
EU
EU (%) = (Average of low-quarter catches ÷ Average of all catches) × 100
Source: Handley, Vaux and Pickering (1983); University of California Cooperative Extension, Maintenance of Microirrigation Systems.
3. Flow rate (if you enter a catch time)
Average flow rate
Flow rate = Catch volume ÷ Catch time × 60
Converted from fl oz per minute to gallons per hour (÷128), or from mL per minute to liters per hour (÷1,000).
4. Run-time adjustment (if you enter a current run time)
Adjusted run time
Adjusted run time = Current run time ÷ (EU ÷ 100)
This is the same low-quarter logic irrigation auditors use to convert a target depth into a gross application: dividing by the uniformity fraction lengthens the run so the worst-watered quarter still reaches the target, at the cost of over-watering the better-watered areas.
Rounding policy
EU is shown as a whole percent, calculated directly from the numbers you enter. Catch volumes and flow rates are shown to 2 decimal places in US units and 1 decimal place in metric units, matching the precision used in the calculation. Run time uses whole minutes once it's under an hour, and hours and minutes above that.
When this calculator isn't the right tool
It's built for a point-source emitter catch test on one drip zone. Use a different approach for:
- Sprinkler and spray zones, evaluated with a catch-can test. See the distribution uniformity calculator.
- Drip tape or tubing with closely spaced perforations, which is treated as a line source. See the drip tape calculator.
- Sizing a new system before it's installed, where you need a manufacturer's coefficient of variation rather than a field catch test. See the drip emitter calculator.
- Diagnosing why pressure is uneven along a lateral or between manifolds. See the multiple-outlet friction loss calculator.
Emission uniformity calculation example
A grower catch-tests 12 emitters on a drip zone for 2 minutes each, collecting (in fl oz): 30, 29, 31, 28, 30, 32, 27, 29, 31, 28, 30, 26. The zone currently runs 30 minutes.
- Sort and find the low quarter: lowest to highest gives 26, 27, 28, 28, 29, 29, 30, 30, 30, 31, 31, 32. With 12 emitters, the low quarter is round(12 ÷ 4) = 3 emitters: 26, 27, 28 oz.
- Low-quarter average: (26 + 27 + 28) ÷ 3 = 27.00 oz.
- Average of all catches: the 12 values sum to 351 oz, so 351 ÷ 12 = 29.25 oz.
- Emission uniformity: (27.00 ÷ 29.25) × 100 = about 92%, rated Excellent (90% or higher).
- Average flow rate: 29.25 oz ÷ 2 min × 60 ÷ 128 = about 6.86 gph average; the low quarter runs about 6.33 gph.
- Adjusted run time: 30 min ÷ (92% ÷ 100) = about 32.5 minutes, roughly 2.5 minutes more than the current 30-minute run.
The same calculation in metric
A second zone is tested with 12 catches (in mL), each timed for 1 minute: 70, 75, 78, 80, 82, 84, 86, 88, 90, 92, 94, 96. The zone currently runs 45 minutes.
- Low-quarter average: lowest 3 values (70, 75, 78) average to (70 + 75 + 78) ÷ 3 = 74.3 mL.
- Average of all catches: the 12 values sum to 1,015 mL, so 1,015 ÷ 12 = 84.6 mL.
- Emission uniformity: (74.3 ÷ 84.6) × 100 = about 88%, rated Good (80% to 90%).
- Average flow rate: 84.6 mL ÷ 1 min × 60 ÷ 1,000 = about 5.1 L/hr average; the low quarter runs about 4.5 L/hr.
- Adjusted run time: 45 min ÷ (88% ÷ 100) = about 51.2 minutes, roughly 6 minutes more than the current 45-minute run.
Both examples show why a "good" or "excellent" rating still calls for extra run time: EU below 100% always means the driest-tested emitters are getting less than the average, so reaching your target everywhere means giving the better-watered majority a bit more too.
Running a drip emitter catch test
You need small containers (paper cups work), a way to time the test consistently, and a measuring cup or graduated cylinder. Pick one zone and one operating condition to test at a time; mixing zones or pressures in the same batch of catches will distort the result.
- Choose sampling points across the whole zone, not just near the valve. Include emitters at the start and end of laterals, and on laterals farthest from the supply line, since that's where pressure loss shows up first.
- Place a cup under each emitter you're sampling, without moving or bumping the emitter.
- Run the zone for a fixed time, such as 2 to 5 minutes for typical point-source emitters, or longer for very low-flow emitters so you have enough volume to measure accurately.
- Measure and record each cup's volume right after the test, before evaporation changes the reading.
- Enter every value into the calculator above, along with the catch time if you want a flow rate too.
Common mistakes
Testing too few emitters, or emitters clustered in one spot, is the most common error; it can make a genuinely uneven zone look fine, or the reverse. Changing the catch time partway through, letting a cup overflow, or testing while a nearby zone is also running (which changes system pressure) will all skew the result. If any single reading looks far out of line with the rest, check that emitter for a clog or a kink in the tubing before assuming the whole zone is at fault.
Using the result
A rating of good or excellent generally means the system doesn't need design changes, just the run-time adjustment above and normal maintenance. A fair or poor rating is worth investigating before you simply run the zone longer, since a longer run time treats the symptom, not the cause. Check the filter and flush the lines with the drip filter sizing calculator, confirm regulated pressure with the drip pressure regulator calculator, and re-test after any fix.
Factors that change emission uniformity
EU is a snapshot of one test, and several things can pull it down between tests or explain why one zone differs from another.
| Factor | Effect on EU | What to check |
|---|---|---|
| Pressure loss along laterals | Emitters farthest from the supply run at lower pressure and discharge less. | Friction loss calculator; shorten laterals or upsize tubing. |
| Pressure differences between manifolds | Manifolds farther from the source or pump run lower pressure. | Pressure-regulate each manifold; check valve settings. |
| Clogged or plugged emitters | Reduces discharge at affected emitters, often unevenly. | Flush lines; check filter condition with the filter sizing calculator. |
| Manufacturing variation | Even new emitters of the same model vary slightly in discharge. | Check the manufacturer's coefficient of variation when selecting emitters. |
| Emitter age and wear | Uniformity typically declines gradually as a system ages. | Compare against a previous test; a drop of 5 points or more suggests investigating. |
| Elevation change | Uphill runs lose pressure; downhill runs can gain it. | Use pressure-compensating emitters on sloped ground. |
| Water temperature | Can shift discharge slightly, especially in pressure-compensating emitters. | Test at a typical operating temperature. |
Frequently asked questions
What is a good emission uniformity for a drip system?
A field study of low-volume systems by Handley, Vaux and Pickering (California Agriculture, 1983) rates emission uniformity above 90% as excellent, 80% to 90% as good, 70% to 80% as fair, and below 70% as poor for systems that have been running for a season or more. Most well-designed, well-maintained drip systems land in the good-to-excellent range. A new system that tests below 70% usually has a fixable cause, such as pressure variation or clogged emitters, rather than a design flaw.
How many emitters should I test for an EU calculation?
Test more than the calculator's 4-emitter minimum. University of California Cooperative Extension's published worked examples sample 36 to 40 emitters spread across the field, catching each one for the same timed interval. A handful of emitters from one spot won't show pressure loss along a lateral or between manifolds, so spread your sampling points across the full length and width of each zone.
What causes low emission uniformity?
The usual causes are pressure differences along laterals or between manifolds, partially clogged or plugged emitters, manufacturing variation between emitters of the same model, and worn or damaged emitters on an aging system. A 1983 California Agriculture field study of 112 low-volume systems attributed about 60% of low-uniformity cases to pressure problems and about 50% of the remainder mainly to plugged emitters.
How do I run a drip emitter catch test?
Turn on one zone, place a small container under each emitter you're sampling, and run the system for a set time, such as 2 to 5 minutes for point-source emitters. Measure the volume each container collected and enter every value into this calculator. Pick sampling points spread across the zone, including the first and last emitters on a lateral, rather than testing several emitters next to each other.
Does emission uniformity change over time?
Yes. University of California Cooperative Extension notes that emission uniformity on an in-service system tends to decline over time from clogging, wear and mineral or biological buildup, rather than staying at its as-installed value. A drop of 5 percentage points or more since a previous test is a sign of a developing problem worth investigating, such as a clogged filter or degraded emitters.
How is emission uniformity different from distribution uniformity?
They're the same underlying calculation, the low-quarter average divided by the overall average, applied to different equipment. Emission uniformity (EU) is the term used for drip and microirrigation emitters; distribution uniformity (DU) is the term used for sprinkler catch-can tests. University of California Cooperative Extension notes the numeric result is calculated the same way for both. Use the sprinkler distribution uniformity calculator instead of this tool for spray or rotor zones.
See the distribution uniformity calculator.
Can I use emission uniformity to fertigate safely?
Uniformity affects how evenly fertilizer or other chemicals get applied, since they travel with the water. USDA NRCS's microirrigation design standard requires a design emission uniformity of 85% or higher for systems that inject fertilizer, pesticides or other treatment chemicals through the irrigation system. That's a design requirement for new NRCS-assisted systems, not a general rule for every existing system, so treat a field-tested EU below that as a reason to improve uniformity before relying on the system for chemigation.
Does EU apply to drip tape the same way as point-source emitters?
The low-quarter calculation works the same way, but drip tape's closely spaced perforations are treated as a line source rather than individual point-source emitters, and published guidance sets different manufacturing-variation targets for the two. This calculator's job is a point-source emitter catch test; the drip tape calculator covers tape-specific sizing instead.
See the drip tape calculator.
Who built and checked this calculator
Licensed Professional Engineer and Irrigation Association Certified Irrigation Designer with more than 20 years in irrigation design, system auditing and landscape water management. Full profile
How this page was checked
- The formula and both worked examples (US and metric) were verified by hand against the University of California Cooperative Extension method.
- The EU rating bands were checked against the cited 1983 California Agriculture field study, which states them for systems already in service.
- The calculator was checked against a zero-variation test case (all catches equal) to confirm it shows 100% EU with no run-time adjustment needed.
Sources
- Handley, D., Vaux, H.J. Jr., and Pickering, N. (1983). Evaluating Low-Volume Irrigation Systems for Emission Uniformity. California Agriculture, Jan–Feb 1983.
- University of California Agriculture and Natural Resources. Drip Emitter Evaluation Example. Maintenance of Microirrigation Systems.
- University of California Agriculture and Natural Resources. What Is Acceptable Distribution Uniformity (DU)? Maintenance of Microirrigation Systems.
- USDA Natural Resources Conservation Service. Conservation Practice Standard 441: Irrigation System, Microirrigation.