How Uniform Is Your Micro-Irrigation System?

(Reprinted with permission of Citrus & Vegetable Magazine - May 2000 issue)
By Thomas Obreza

As of 1996, almost 90 percent of Florida's 800,000 citrus acres were micro-irrigated using either micro-sprinklers or drippers. In addition, about 10 percent of Florida's vegetable acreage was micro-irrigated, and a significant portion of the ornamental industry uses this method as well. The small orifices in micro-irrigation emitters are easily clogged by particulate matter, chemical precipitates and biological growths. If a micro-irrigation system is not maintained properly, the chemical and biological nature of Florida's irrigation water can eventually cause emitter plugging, even if the water is highly filtered.

The water distribution uniformity of even the most well-designed microirrigation systems will suffer as the degree of emitter plugging increases. As irrigation uniformity declines, nitrogen (N) fertilizer distribution uniformity will also decline, especially if the irrigation system is used to apply N in solution (fertigation). If the operator of a partially plugged system operates it long enough to sufficiently irrigate the plants watered by the lowest flow rate emitters, plants watered by non-plugged emitters will be overirrigated, and N leaching might occur.

The USDA-Natural Resources Conservation Service operates several Mobile Irrigation Laboratories (MILs) throughout Florida that provide irrigation system evaluation to producers, landowners and the urban community. In 1999, the Lower West Coast MIL based near Naples evaluated 90 citrus micro-sprinkler irrigation systems representing almost 3,000 acres in Southwest Florida. In these evaluations, a series of single emitter flow rate measurements were taken in representative grove blocks so that system emission uniformity (EU) could be calculated and potential problems in the system identified. Ideally, system EU should be 90 percent or greater. Table 1 summarizes the results.

Table 1. Emission uniformity of 90 micro-irrigation systems measured in 1999 by the Lower West Coast Mobile Irrigation Lab
Emission uniformity	Rating	Number of systems and percentage of total
> 90	Excellent	26 (29%)
80 - 89	Good	26 (29%)
70 - 79	Fair	22 (24%)
60 - 69	Poor	16 (18%)

The weighted average EU (with respect to acreage represented) was 78.5 percent (in the "fair" range). While 42 percent of the systems were rated "fair" or "poor," 79 of the 90 systems (88 percent) were identified as having some degree of emitter plugging. Therefore, emitter plugging is a major cause of low EU in Southwest Florida citrus micro-irrigation systems.

EU is evaluated by measuring the flow rates of 16 individual micro-sprinklers from a single irrigation zone. EU is defined as the average flow rate of the four lowest-flow emitters divided by the average flow rate of all 16 emitters. If a Southwest Florida citrus irrigation system was tested, and an EU of 78.5 percent was found, how would it relate to potential nitrate leaching? If we assume that the irrigation manager runs the system long enough that the lowest flow rate emitters apply the desired volume of water per tree during each irrigation while fertigating with N, then up to one-third of the applied N could be lost from areas of non-plugged emitters that were over-irrigated. If dry N fertilizer had just been applied, the portion of it that was placed within the irrigated zone would be subject to leaching due to overirrigation in these areas.

Therefore, one of the problems with the non-uniform micro-irrigation system is unintended leaching of N if the system is operated for the lowest flowrate emitters. (If the system is not run for these emitters, then some trees will be water-stressed.) This example stresses the importance of irrigation system maintenance to prevent emitter plugging. Methodology to accomplish this objective will be the subject of future "Immokalee Reports." CVM

Thomas A. Obreza is an associate professor of soil and water science at the University of Florida's Southwest Florida Research and Education Center, Immokalee, 941-658-3400.