 John Ossa |
Drip irrigation became more widely adopted in turf and landscape approximately 25 years ago. Now, more than two decades later, there is an established track record for the hardware in this application. So what have we learned about it, and what has changed? I posed these questions to a long-time proponent and expert on the subject, Tom Bressan of The Urban Farmer Store, an irrigation distributor based in San Francisco. “I’d say the biggest shift has been the move away from point source irrigation to creating a grid using pre-embedded emitter tubing,” Bressan says. Drip irrigation has an inherently high potential for efficiency that is better realized through establishing a grid of emitters over a root zone. Another less obvious but very important outcome of using a grid is it is possible through a simple formula to calculate the output of the system in terms of precipitation rate, as opposed to thinking about gallons per hour, per plant. Bressan made the point that “this allows people to understand drip in terms of what they already know about sprinklers, particularly when it comes to scheduling.” The formula that allows us to determine the drip system application rate in inches per hour, like sprinklers, is 231.1 (a constant) times the emitter output, divided by the product of emitter spacing in the tubing times the lateral spacing. Once precipitation rate is established, it should be matched to the soil type. What that means is the rate of application (precipitation rate), should not exceed the infiltration rate (the ability of the soil to “accept” water). Once the soil type and root zone depth are known, there is readily available information about how much water (measured in inches) can be held in the root zone. These are the key variables to understand for precise irrigation scheduling designed to replenish water in the root zone. Drip irrigation is not a solution for every crop and situation. It does, however, work best in situations where sprinklers don’t work. Obvious examples include containers, narrow landscaped areas, spaces that are irregular in shape and places where overspray is unacceptable. Another condition where drip irrigation works great is landscapes with mature shrubs that have developed to the point where the original spray system is now blocked by the mass of the plants. These are good opportunities for a retrofit to drip irrigation. In the past 25 years we have learned that a well-designed, installed and managed drip irrigation system delivers a very high level of irrigation efficiency. I asked Bressan what the greatest factors are that prevent a drip irrigation system’s success. He stressed that it’s not the hardware that’s the weak link, and that the CV rate (the manufacturer’s coefficient of variation, a measure of the variability of discharge) of the equipment is solid. The weak link, he said, is typically the lack of understanding of optimum emitter placement, and the lack of knowledgeable and diligent system maintenance. The way forward for any contractor that wants to assure his or her customer of a high efficiency irrigation installation is to learn the fundamentals and be diligent in the installation of the hardware. The mechanics of installing the tubing and emitters is relatively easy, but the fundamental principles of an even application of water through a given soil type require an understanding of soils and the basic irrigation principles at work. The contractor that makes the effort to do the math, and communicate the benefits to the customer, is the contractor that will be positioned for success. |
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