Vineyard Irrigation Distribution Uniformity

What is Irrigation Distribution Uniformity?

Distribution Uniformity Kit ContentsThe Lake County Winegrape Commission has assembled two Distribution Uniformity Test kits for growers to borrow and use in their vineyards. The DU Test kits have instruction in English and Spanish. Please stop by the LCWC office in Kelseyville to borrow one of the kits.

Please note: Distribution Uniformity testing is a prerequisite of the California Sustainable Winegrowing certification program.

Following vineyard establishment choices (location, variety, rootstock, spacing and trellis), vineyard water management is one of the most important annual cultural practices affecting winegrape quality.

There are two critical water management questions: 1) When to irrigate? 2) How much water to apply? To answer these questions, a grower must know: 1) the vine moisture status between irrigations, and 2) the irrigation system’s current performance characteristics to determine how long to operate the irrigation system.

How Evenly Is Water Applied To Your Vineyard?

Ideally, given uniform vine spacing, each vine in an irrigation block should receive the same amount of irrigation water over the same period of time. Growers should not rely on the manufacturer’s data specification sheets to estimate their drip system’s rate of water application, i.e. a half-gallon per hour emitter may not deliver 0.5 gallon per hour for a variety of reasons.

As the irrigation system ages, partial plugging of emitters by silt and clay, algae or chemical precipitates, is common and will cause the irrigation system to deliver different amounts of water throughout the irrigation block. As a result, growers may over-water some vines in order to satisfy the water needs of the vines in the areas where the system is partially plugged. This practice results in wasted water, energy and fertilizer.

What Is Distribution Uniformity?

Distribution Uniformity (DU) is a measure of how evenly irrigation water is applied across an irrigation block or field. The Application Rate (AR) is the average amount of water that is delivered to the vineyard block (acre-inches per hour) or individual vine (gallons per hour per vine) over a period of time. A DU field test will identify if there are problem areas in the irrigation system that need to be addressed to optimize overall irrigation efficiency. DU field tests should be performed every few years, just as winegrowers regularly calibrate sprayers, test fruit Brix levels, and conduct tissue, soil and water analyses. DU field tests are simple and can be accomplished by all vineyard workers that are provided an understanding of the objectives and the simple process for performing the field tests.

What Are The Risks Of A Low DU Value?

To illustrate the problem with a low DU value, for a vineyard that applies about four (4) acre-inches of irrigation water annually, and has a 70% DU, the amount of applied water at the end of season between the high and low quarter portions of the vineyard is alarming: 5.2 acre-inches applied to the high quarter and 2.8 acre-inches to the low quarter – a difference of about 46% in applied water, fertilizer and perhaps pesticides.

The goal is for a drip irrigation system to have a DU of 95% or better. Drip irrigation systems operating with DUs ranging from 85% to 95% are acceptable, 75% to 85% should be improved; and below 75% need to be improved

How To Conduct A Distribution Uniformity Field Test

Prior To Field Work

Obtain a Map of the Irrigation Block(s): Obtain a map or an irrigation plan that will be used in the field to record the locations of DU samples. The Google Maps satellite feature works well for this task and the LCWC staff can help you with this task. Select from 16 to 40 sample sites located throughout the block Choose sample sites as you would for fruit sampling and where you might expect to find the highest or lowest pressures; i.e. top and bottom of slopes.

Gather DU Test Equipment: The Lake County Winegrape Commission has two (2) test kits available for growers to borrow. The basic tools needed are: a pressure gauge (0-60 PSI, liquid filled) connected to a pitot tube, one 25, 50 or 100ml graduated cylinder, stop watch, hole punch, goof plugs, nylon paint strainer socks or bags, drip emitters to replace clogged emitters you come across, clip board, tape measure and field data sheets

In The Field

Measure Vine and Row Spacing: Measure and record the distance between vines and rows on the field data sheet. Note number of emitters per vine.

Measure and Record Line Pressures: Pressure testing of emitters is performed with a pressure gauge fitted with a pitot tube. Near each of your 16 to 40 sample sites, make a hole in the drip hose with the hole punch, insert the pitot tube/pressure gauge into the hole to get the PSI reading, remove the gauge, insert a goof plug, and record pressure on the data sheet.

Catch, Measure and Record Emitter Flows: At each of the sample sites, hold the graduated cylinder under the emitter, and record the volume of water captured during 30 seconds.

Check Lateral Lines / Hoses for Debris: Open the ends of a few lateral lines (~ 6 to 10 hoses) and place the nylon sock over the end of the hose to check for debris in the water. Note whether the debris is soil, algae or other material such as PVC shavings.

Back In The Office

Calculate the DU Value: The easiest and recommended method to calculate the DU value is to use the Excel calculation spreadsheet supplied by the LCWC. Simply enter your recordings into the Excel worksheet and your computer will automatically calculate the Application Rate (AR) of the drippers in Gallons per Hour (GPH) and Acre-Inches per Hour (Ac-In/Hr), the average of the lowest output emitters (called the Low Quarter – LQ), and the Distribution Uniformity (DU).

Determine if the DU Value is acceptable or if additional work is needed: If the DU value is less than 85% then refer to the maintenance information below to help determine what steps can be taken to improve the evenness of irrigation distribution.

Causes Of Poor Distribution Uniformity (≪85%) And How To Improve DU:

Dripper clogging: Partial & complete emitter clogging is one of the main causes of poor DU. Clogging maybe caused by: physical, biological or chemical factors.
Insufficient maintenance & repairs: flush, adjust and replace irrigation components: valves, filters, etc.

Pressure variations: Systems with non-compensating emitters, poorly designed or installed systems with elevation changes, and insufficient adjustment of pressure regulating valves may have pressure variations that negatively affect dripper discharge rates and need to be addressed.


DU testing over several years provides growers with an indication of their irrigation system’s “health”. A high DU value of 90%+ is essential for growers seeking to produce winegrapes of high quality while achieving good water, energy and chemical use efficiencies.

The results of a Distribution Uniformity test provide growers with the essential data necessary to implement precise irrigation scheduling practices that improve their financial bottom line, winegrape quality and yield and increase the efficiencies of water, fertilizer and energy uses.

Download a PDF document with forms and photos.


Prichard, T., L. S. P. Verdegaal, and R. Smith (2004). Deficit Irrigation of quality Winegrape Using Micro-Irrigation Techniques: University of California Cooperative Extension, Department of Land, Air and Resources, University of California Davis. Available at:

Schwankl, L. (2016). Maintenance of Microirrigation Systems – Emitter Evaluations: University of California Agriculture and Natural Resources.
Available at:

Schwankl, L., B. Hanson, and T. Prichard (2008). Maintaining Microirrigation Systems. University of California Agriculture and Natural Resources. Publication 21637.

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