Determine the difference

From an on-farm water management standpoint, it is desirable to achieve four things during an irrigation event. A professional wants to apply the correct depth of water at the correct time, giving the same amount of water to every plant in the field and avoid unrecoverable losses such as excessive evaporation.

The Cal Poly San Luis Obispo Irrigation Training and Research Center has developed the software, classes and procedures for on-farm irrigation evaluations (for drip, pivot, undertree sprinkler, linear move, wheel line and hand move, solid set, furrow, border strip, and basin systems) that have been used for more than 40 years in California. Each year we have a one-week short course for evaluation teams and others that combines lectures, laboratory and field exercises. The emphasis of the annual short course is on drip/micro irrigation simply because in California that is where the interest lies.

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. There is still a problem in our irrigation industry with people wanting to reinvent the wheel. There is no one perfect term/formula that describes all the aspects of irrigation performance. However, the 1997 landmark publication on the topic “Irrigation Performance Measures: Efficiency and Uniformity” (found at itrc.org/papers/efficiency.htm) is as valid today as it was more than 25 years ago.

Top formulas

The two most common terms and their formulas are irrigation efficiency, expressed as a percentage value between 0 and 100, and distribution uniformity, expressed as a value between 0.0 and 1.0. They are both defined in the Irrigation Association’s study materials for certified irrigation designers in drip/micro, as well as the CAIS certification.

The formula for irrigation efficiency is:

Irrigation efficiency recognizes that some of the applied water is not beneficially used; we can get close to 100% but it is impractical in the field. Typical uses that are considered nonbeneficial are excessive deep percolation beyond what is needed for salt leaching, uncollected field runoff and excessive evaporation. For example, a center pivot that rotates rapidly in a hot, dry environment may lose most of its applied water to nonbeneficial evaporation from plant and soil surfaces.

Hence, the recommendation with pivots is to rotate as slowly as possible without having runoff. Interestingly, one way to achieve a high irrigation efficiency is to underirrigate. With underirrigation there is typically little to no deep percolation or tailwater runoff from a field. But in that case, having a high irrigation efficiency does not lead to maximum crop production/quality per acre. This points out the inadequacy of any single term to describe irrigation performance.

The second widely used term is distribution uniformity:

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.

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.

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.