IRRIGATION SCHEDULING
I WANT TO STRONGLY EMPHASIZE THE NEED FOR STANDARDIZATION OF FORMULAS AND TERMINOLOGY. WITHOUT
STANDARDIZATION OF WHAT TO MEASURE, HOW TO MEASURE THINGS, HOW TO COMBINE DATA INTO FORMULAS, AND THE FORMULAS THEMSELVES, EVERYONE JUST TALKS PAST EACH OTHER.
The second widely used term is distribution uniformity:
“Minimum” amount of water received by a plant in the field
DU=
Average amount of water received by plants throughout the field
Where the “minimum” is defined such that 12.5% of the field values are less than the “minimum” value.
The idea of DU is this: Do all the plants in the field receive about the same amount of irrigation water? DU is not the same as “efficiency” because it does not measure if enough or too much water is applied. It is not expressed as a percentage partially because people would tend to confuse it with an efficiency term if it were.
Buckets are used to measure sprinkler overlap patterns as part of a sprinkler distribution uniformity evaluation at Cal Poly ITC. Photos: Charles Burt
The word “plants” is very important. An undertree sprinkler system may perform very similar to a drip/micro system in the sense that there are large, dry areas between the tree rows. It is not important to wet all the soil surface evenly, but a sprinkler system on row or grain crops should wet all the soil surface evenly because there are plants everywhere. The sprinklers may have the same nozzle size, but the evaluation procedure is completely different.
One part of a drip evaluation is measuring differences between SDI emitters all kept at the same pressure.
18 Irrigation TODAY | Summer 2023
The concept of DU is the same for all irrigation methods. We just measure different things with various irrigation methods. For example, we measure pressure differences between sprinklers on some sprinkler systems because we know that different pressures cause different flow rates. Similarly, we measure the time water sits on the ground at different points in a field with furrow irrigation because we know that different “opportunity times” result in different infiltration depths. Different soils in a furrow-irrigated field are somewhat comparable to mixed or worn nozzles in a sprinkler system.
The concept of DU is especially important for sprinkler and drip/micro designers because they can lock in a new system DU with the correct design of components. Everything is predictable for a new system. Surface irrigation design is different, because with surface irrigation the DU is impacted by numerous unpredictable factors related to soil uniformity and how the irrigators turn water into the field.
Because the DU with pressurized systems is predictable, the Irrigation Consumer Bill of Rights (available at
itrc.org/reports/ icbrgeneral.htm) states that a farmer should request, in writing, the new system DU. Note the word “system”; that means the DU must be measured across the field, not just down a single drip hose or sprinkler line, for example. It also means that whoever verifies the DU must follow standardized measurement and computation procedures — which is why Cal Poly ITRC on behalf of the California Dept. of Water Resources has spent so much time over the years standardizing the irrigation evaluations.
For those who are interested in drip/micro field DU evaluation results for systems of all ages, ITRC has detailed measurements and results from over 700 field evaluations at
itrc.org/irrevaldata. Those figures illustrate the type of picture that unfolds from evaluation program results.
Charles Burt, PhD, PE, CID, is the chairman of the Irrigation Training and Research Center at Cal Poly State University.
irrigationtoday.org
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