# Drip uniformity test: how to measure emitter flow and calculate CUD in the field

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Drip irrigation

Similar flow rates help to distribute water evenly, but the depth applied also depends on the actual flow rate and irrigation time…

By Team Perin Irrigação

Published in October 23, 2026 (2026-10-23)

9 min read

![Drip system in the Perin Irrigação collection](https://perinirrigacao.com.br/assets/perin/responsive/service-drip-640-055633cbf116.webp)

Photograph from the Perin Irrigação collection.

## Before you start

How to perform the drip uniformity test: collection points, flow rate, pressure, calculation of the uniformity coefficient (CUD) and reading the result.

> **Summary:** similar flow rates help to distribute water evenly, but the depth applied also depends on the actual flow rate and irrigation time. The uniformity test evaluates the distribution of flows in the field with simple material: containers, beaker, stopwatch and manometer. This guide shows how to choose collection points, measure flow and pressure, calculate the coefficient of uniformity of distribution (CUD) and interpret the number to decide whether to adjust management, clean the system or review the design.

## Why measure uniformity

When the irrigation time has been defined with the nominal flow, field measurement allows checking whether this reference corresponds to the actual flow. If part of the drippers delivers less water, due to clogging or insufficient pressure, the plants in that region receive less than calculated. At points where application exceeds need, water, energy and fertilizer may be wasted.

An evaluation of watermelon lots irrigated by drip in the Tabuleiros Litorâneos Irrigation District of Piauí, published by researchers with the participation of Embrapa Meio-Norte, found lots with an average flow rate of 15% to 29% below nominal. The authors note that flow rates lower than nominal lead to incorrectly applied water depths and that, in these cases, the irrigation time needs to be recalculated. In the same study, a batch with a CUD of 67% became 81.6% after maintenance, which shows the value of measuring before and after intervention.

The University of Florida Extension (UF/IFAS) publication on microirrigation evaluation recommends testing new systems to confirm design and installation and creating a baseline, and repeating the evaluation each year to track the effect of clogging and component changes. More frequent assessments may be necessary when there is already an obstruction problem.

## What will you need

- Containers that fit under the emitter.
- Graduated cylinder, preferably with milliliter resolution.
- Stopwatch (can be the one on your cell phone).
- Portable pressure gauge with polyethylene tube socket, if possible.
- Worksheet and ribbons or stakes to mark the points.
- Preferably, two people: one positions the containers, another controls the time and takes notes.

## Field test step by step

### 1. Prepare the system

Carry out the test with the system in normal operating conditions, at the same working pressure used on a daily basis. Clean the filters before starting, as the researchers in the Piauí study did, so that the assessment reflects the state of the lines and emitters, and not a saturated filter at the time of collection. If you want to diagnose the system exactly as it operates, also measure it with the filter in its current state, before cleaning.

Evaluate one sector at a time, so as not to mix pumping, main network and lateral line problems.

### 2. Choose and mark collection points

The sample needs to represent the entire sector. For the CUD calculation in this guide, use a **32 points**, described in the document *Irrigation Efficiency for Integrated Melon Production*, from Embrapa Agroindústria Tropical:

- Select four side lines: the first, the next 1/3 and 2/3 of the lead line length, and the last.
- On each side, measure eight emitters: the first, last and next at 1/7, 2/7, 3/7, 4/7, 5/7 and 6/7 the length of the side.

Identify and record points before collection, without choosing emitters based on appearance. UF/IFAS suggests marking them with the system turned off to avoid this influence.

The result represents the sample and operating condition at the time of testing. In sectors with irregular geometry or unevenness that are not well represented by this route, the distribution and number of points need to be defined in a technical assessment.

### 3. Measure the flow rate of each emitter

With the system turned on and the pressure stabilized, place the container under the dripper, collect for a fixed time (for example, one or two minutes, as suggested by UF/IFAS) and measure the volume in the beaker. Another way is to time how long the emitter takes to fill a known volume.

Convert to liters per hour:

**flow rate (L/h) = volume collected (mL) × 60 ÷ collection time (min) ÷ 1,000**

A completely clogged dripper enters the account with zero flow; deleting it hides the problem. In the Piauí study, the collection was repeated three times at each point and the average was used, which reduces reading error.

### 4. Measure the pressure

Measuring pressure at the beginning and end of the lines and at high and low elevations helps to separate hydraulic problems (pressure) from emitter problems (clogging or wear). UF/IFAS describes the use of a portable pressure gauge with a needle suitable for insertion into the lateral line or, in compatible emitters, a flexible tube attached to the emitter, keeping it installed. Use connections and procedures appropriate to the model evaluated.

With pressure-compensating drippers, the flow only remains within the pressure range stated by the manufacturer; below it, it falls even with a clean emitter.

## How to calculate coefficient of uniformity (CUD)

An index widely used in drip irrigation is Keller and Karmeli's distribution uniformity coefficient (CUD), adopted in the Embrapa Meio-Norte study. It compares the average of the room of emitters with the lowest flow with the average of all sampled flows:

**CUD (%) = 100 × (average of the 25% lowest flow values) ÷ (average of all flows)**

If the overall average is zero, the CUD cannot be calculated; First identify why there is no discharge at the measured points.

To calculate:

1. List all measured flow rates, including zeros.
2. Calculate the overall average.
3. Order the 32 flows from smallest to largest and separate the eight smallest, which form the bottom quarter of the sample.
4. Calculate the average of this bottom quarter.
5. Divide by the overall average and multiply by 100.

### Hypothetical example

*Illustrative premises, do not represent a real system:* sector with 32 sampled points, general average of 1.90 L/h and average of the 8 lowest values of 1.60 L/h.

CUD = 100 × 1.60 ÷ 1.90 ≈ 84%

Also compare the general average with the nominal flow of the emitter. If the dripper is specified for 2.0 L/h and the measured average is 1.90 L/h, the sector applies about 5% less water than the irrigation time assumes. Uniformity and average flow answer different questions: the first tells whether the water arrives evenly, the second tells whether it arrives in the planned quantity.

## How to interpret the result

The Embrapa Meio-Norte study uses Merriam and Keller's (1978) classification for CUD:

| CUD | Classification |
| --- | --- |
| CUD \> 90% | Excellent |
| 80% ≤ CUD ≤ 90% | Good |
| 70% ≤ CUD \< 80% | Regular |
| CUD \< 70% | Inefficient |

This classification is the historical reference adopted in the aforementioned study. The UF/IFAS publication works with statistical uniformity (Us), calculated in another way. Your results and ratings should not be treated as equivalent to the CUD in this guide. Compare assessments made with the same method and under recorded operating conditions.

In addition to the classification, note where the lowest flows are:

- **Concentrated at the end of lines or at high elevations:** indication of insufficient pressure, due to loss of pressure (long lines, insufficient diameter), uneven terrain, poorly adjusted regulator or leaks. UF/IFAS points out that low hydraulic uniformity usually originates from the project.
- **Scattered without a pattern, with adequate pressure:** indication of partial clogging or manufacturing variation and wear between emitters. Attention goes to water quality, filtration, washing the lines and, when water analysis indicates, chemical treatment.
- **Average flow well above nominal:** may indicate excessive pressure in the sector; in the Piauí study, average flows of 3.58 and 3.15 L/h, in emitters of nominal 2.3 L/h, were attributed to excess pressure.

## What to do after the test

1. **Save the spreadsheet with date, sector and points** to repeat the test in the same locations and compare.
2. **Adjust irrigation time** if the average flow differs from the nominal and the uniformity is acceptable; with low CUD, lengthening the time increases the excess in the highest flow emitters, so prioritize the cause.
3. **Attack the cause.** For obstruction, review filtration and maintenance; the contents about [irrigation filters](https://perinirrigacao.com.br/en/conteudos/filtros-de-irrigacao/) and about [clogging of drippers](https://perinirrigacao.com.br/en/blog/entupimento-de-gotejadores/) detail this path. For pressure problems, the review involves sectorization, diameters and regulators.
4. **Test again after intervention** to confirm the effect.

## FAQ

### Can I test with the newly installed system?

Yes, and it is recommended. UF/IFAS recommends evaluating new systems to confirm design and installation and create a reference for future comparisons.

### How many drippers do I need to measure?

In this roadmap, there are 32 emitters per sector: four lateral lines and eight positions in each, according to the Embrapa Agroindústria Tropical document. It is not a universal number for any sector geometry. When the distribution of these points does not represent the area well, sampling must be defined in a technical assessment; do not freely round off the bottom quarter of another number of points.

### Does high uniformity guarantee that the irrigation is correct?

No. It indicates that the water is distributed equally; The amount applied depends on the irrigation time, the actual flow rate and the needs of the crop.

## Conclusion

The uniformity test uses simple material and allows estimating the average flow rate and discharge uniformity of the sampled emitters. It does not directly measure the water available to the roots nor does it replace management according to the crop's needs. Measuring flow and pressure at well-distributed points, calculating the CUD and observing where the lowest flows are helps investigate management, maintenance and design problems. To better understand the system, see the about page [drip irrigation](https://perinirrigacao.com.br/en/gotejamento/).

If your test results raise questions, or if you want to evaluate a system before expanding it, the Perin Irrigação team can help interpret the data. Inform the municipality, crop, approximate area and water source via WhatsApp [(14) 99824-5219](https://wa.me/5514998245219) or by email contato@perinirrigacao.com.br.

## Sources consulted

Research carried out on 10/21/2026.

- [BARROS, W. S. de; DANIEL, R.; MADEIRA, B. B.; SANTOS, F. J. S.; RODRIGUES, B. H. N.; SILVA, L.C. *Evaluation of drip irrigation systems in the Tabuleiros Litorâneos Irrigation District of Piauí*. XXXIII Brazilian Congress of Agricultural Engineering, São Pedro-SP, 2004. Alice/Embrapa Repository:](https://www.alice.cnptia.embrapa.br/alice/bitstream/doc/1185645/1/AvaliacaoSistemasIrrigacaoGotejamento.pdf)
- [SMAJSTRLA, A. G.; BOMAN, B.J.; HAMAN, D. Z.; Pitts, D.J.; ZAZUETA, F.S. *Field Evaluation of Microirrigation Water Application Uniformity* (BUL265/AE094). UF/IFAS Extension, University of Florida:](https://edis.ifas.ufl.edu/publication/AE094)
- [GONDIM, R. S.; FREITAS, J. de A. D. de; MIRANDA, F. R. de. *Irrigation Efficiency for Integrated Production of Melon (Cucumis melo L.)*. Fortaleza: Embrapa Agroindústria Tropical, 2003. (Documents, 70). CUD sampling and calculation protocol, p. 13–15:](https://www.infoteca.cnptia.embrapa.br/infoteca/bitstream/doc/425183/1/doc70.pdf)

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