Properly done irrigation audits (in accordance with standards recommended by the Irrigation Association) collect a wide variety of data about an irrigation system. While all of this data provide insight on the status of an irrigation system, there are two measurements or calculations of especially high importance in an irrigation audit. Each of these calculations are determined at an irrigation valve or station level and are based on the amount of water collected in each catch can.
One of these measurements in an irrigation audit is distribution uniformity or DU. In simple terms, DU is just a value that indicates how evenly or unevenly water is applied to the landscape. Unevenly distributed water results in either wet spots or dry spots. However, this is not always visually obvious so the DU is the means by which this problem can be discovered.
The second of these irrigation audit measurements is precipitation rate. This value measures the amount of water applied per unit of time and it is usually calculated in inches per hour or centimeters per hour. As mentioned in an earlier article, it is crucial that the dimensions of the catch cans are the same. This will ensure that a good, solid precipitation rate can be determined. While a given station will have a few dozen catch cans, the precipitation rate calculated is essentially the average precipitation rate for the entire station.
The importance of the precipitation rate in an irrigation audit is in how the precipitation rate compares to the type of soil that water is absorbed into. How quickly water is absorbed into the ground depends on the type of soil. Some soils readily take in water while other types absorb water far more slowly.
Once the precipitation rate is calculated for a particular station and the type of soil is known for that same station, the irrigation audit will inform the interested parties how that particular station irrigation should be set up so that water is applied in a manner that allows it absorb into the soil. Not knowing this could result in a station running water too long for the soil to take in and water simply puddling or running off on the surface. That is wasted water.
Conversely, some soils require a longer sustained application of water in order to sufficiently penetrate the soil surface and reach the root zone of the grass or shrubs. Not doing so is another, but less obvious, way water is wasted. Water is applied but instead of running off it enters the soil but that is it. It does not reach the ultimate goal of the plant roots.
An irrigation audit tries to remove as much subjectivity and guesswork as possible in scheduling an irrigation system. Guessing is the common way that irrigation schedules are typically set up. However, a well done irrigation audit provides an opportunity for facts to dictate an irrigation schedule instead of vague rules of thumb.
Beighley Associates has provided expert landscape design services for over 30 years. With licenses in Oregon, Washington and Idaho, Beighley Associates serves residential, commercial and governmental clients throughout the Pacific Northwest. Find out how to contact BAI with your landscape design needs that a longtime Portland landscape architect like BAI can meet and exceed.
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