Terms and definitions

Distribution Uniformity (DU)

DU in irrigation is a measure of how uniformly water is applied to the area being watered, expressed as a percentage. The distribution uniformity is either estimated during a normal site visit or we can be contracted to calculate it by performing an irrigation audit.

The most common measure of DU is the Low Quarter DU, which is a measure of the average of the lowest quarter of samples, divided by the average of all samples. The higher the DU, the better the performance of the system. If all samples are equal, the DU is 100%.

There is no universal value of DU for satisfactory system performance, but generally, a value >75% is considered acceptable.

Different DU

Distribution uniformity is used when determining the total watering requirement during irrigation scheduling.

For example, we may want to apply one inch of water to the area being watered. If the DU were 75%, then the total amount to be applied would be the desired amount of water, divided by the DU.

In this case, the required irrigation would be 1.33 inches of water, so that only a very small area received less than one inch. The lower the DU, the less efficient the distribution of water, and thus the more water that will be wasted when trying to achieve the minimum requirement.

Precipitation Rate (PR)

The speed at which an individual irrigation nozzle or irrigation system applies water. Measured in inches per hour (in/hr).

Matched Precipitation Rate (MPR)

A system or zone in which all the heads have proportional precipitation rates is said to have matched precipitation rates. A sprinkler by itself does not have a matched precipitation rate. Only when it is used with other sprinklers of similar precipitation rates would they be considered matched (matched implies two or more).

For example, if there were 3 sprinklers operating at the same time, but they all watered a different pattern, the precipitation rates should be different. A sprinkler that waters a 90-degree pattern should use 1/4 the amount of water than a sprinkler that waters a 360-degree pattern because it waters 1/4 of the area. When designed like this, the zone is said to have a MPR.

90-degree pattern180-degree pattern360-degree pattern
Example 12 GPM4 GPM8 GPM
Example 21 GPM2 GPM4 GPM

When designing sprinkler systems, matching precipitation rates can help to avoid wet and dry spots and excessive run times which leads to high water bills, increased pumping costs, or both.

A zone with a Matched Precipitation Rate (MPR) does not by itself have a high DU. In order to achieve the highest possible DU, proper sprinkler spacing is required as well as MPR.

Soil infiltration rate (IR)

Infiltration is the process by which water on the ground surface enters the soil. Infiltration rate in soil science is a measure of the rate at which soil is able to absorb rainfall or irrigation. It is measured in inches per hour or millimeters per hour. The rate decreases as the soil becomes saturated. If the precipitation rate exceeds the infiltration rate, runoff will usually occur.

Available water capacity (AWC)

Available water capacity or available water content (AWC) is the range of available water that can be stored in soil and be available for growing plants. This will not be more than the depth of the plant’s roots. The water readily available to plants is the difference between water content at field capacity (θfc) and the permanent wilting point (θpwp):

θa ≡ θfc − θpwp

Permanent wilting point (PWP)

Permanent wilting point (PWP) or wilting point (WP) is defined as the minimal point of soil moisture the plant requires to not wilt. If moisture decreases to this or any lower point the plant will wilt and can no longer recover its turgidity when placed in a saturated atmosphere for 12 hours.

Evapotranspiration (ET)

Evapotranspiration (ET) is the sum of evaporation and plant transpiration from the surface to the atmosphere. Evaporation accounts for the movement of water to the air from sources such as the soil, canopy interception, and waterbodies. Transpiration accounts for the movement of water within a plant and the subsequent loss of water as vapor through stomata in its leaves.

Allowable Moisture Stress (AMS)

Allowable moisture stress (AMS) which may be placed on the plant. For a lawn, some stress is allowed because you only need to keep the lawn green and healthy.

Effective Rooting Depth (ERD)

Effective rooting depth (ERD) of the plant to be watered, which affects how much water can be stored in the soil and made available to the plants.

Watering Window (WW)

Watering window (WW) amount of time in which water may be available for irrigation.


The slope of the land being irrigated as this affects how quickly runoff occurs, often expressed as a percentage, i.e. distance of fall divided by 100 units of horizontal distance (1 ft of fall per 100 ft (30 m) would be 1%).

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