To Drip or Not to Drip, That Is the Question

Drip irrigation has been generally accepted as the most effective, water-efficient method of landscape irrigation. Research by Peter Waller, University of Arizona professor of agricultural and biosystems engineering, leads him to question this assumption. He argues for the use of bubbler irrigation. Patsy Waterfall, UA extension agent, water conservation, and Vicki Richards, project coordinator, Low 4, respond to Waller, claiming that the benefits of drip irrigation outweigh those of bubbler irrigation.

Bubbler, Not Drip by Peter Walker

In the past, irrigation companies as well as Arizona cities, state agencies, and university personnel have recommended drip irrigation of desert landscapes as the best method to conserve water in urban landscapes. However, I recommend bubbler irrigation over drip irrigation for most desert landscapes. Why? Our evaluation of 38 landscape drip irrigation systems revealed an average uniformity of less than 20% (> 70% is acceptable). Degradation of emitters and lack of adjustment of number of emitters as plants grew resulted in low uniformity. Site efficiencies ranged from 14% to 400% and revealed a lack of ability by most irrigation managers to calculate watering schedules. Of the 38 sites, only one received regular maintenance and adjustment of emitters to match tree canopy area — the only site with acceptable uniformity. During our drip system evaluations, we found that drip irrigation maintenance is hot, dirty, and thorny, and we received many ant bites. In a controlled experiment in Phoenix, desert plants (mesquite, acacia, encelia, and cassia) in bubbler irrigation treatments generally had significantly greater canopy growth than in most drip irrigation treatments, and rodents were continually chewing up one of our drip irrigation systems.

Drip irrigation, a technology that has varied success in a different environment — agriculture — is not suitable for most urban landscapes. Most city dwellers do not have the training, motivation, or time to manage drip irrigation systems. I recommend bubbler irrigation with water harvesting. Bubblers are inexpensive, easily adjustable with a screwdriver, and reliable. See the following web site for a description of the bubbler irrigation/water harvesting system — http://ag.arizona.edu/abe/urbbler/. The web site includes a calculator for flow and timer adjustment for drip and bubbler systems, details on installation of bubbler irrigation systems, and a summary of our drip irrigation research project.

Let us imagine that there were no irrigation systems in Tucson, and our job was to recommend an irrigation system for xeriscapes to the citizens of Tucson and Phoenix. What should we look for?

• Adjustable, visible, and measurable flow rate.

• Calculation of flow rate per plant/emitter and time between irrigation cycles must be simple.

• System should require infrequent maintenance, and maintenance must not be unpleasant.

Drip drawn by Jade Shepler, 6th grade, Apache School, Sierra Vista

• System must be immune to clogging, heat degradation, insect damage, and animal teeth.

• Attractive.

Compare bubbler and drip irrigation:

•Bubblers are visible and problems are noticeable — drip emitters and drip problems are invisible.

• Bubblers require minimal filtration — soil, calcification, and bacteria clog drip emitters.

• Bubblers do not degrade in heat — drip emitters degrade in heat and flow increases.

• Bubbler flow rate and wetted area are easy to adjust — drip flow rates are difficult to adjust.

• Bubblers are durable and require little maintenance — drip emitters are fragile and require far more maintenance.

• The bubbler irrigation/water harvesting system is attractive — buried drip irrigation systems are attractive.

However, I do recommend drip irrigation for vegetable gardens and steep slopes.

Drip, Not Bubbler by Patsy Waterfall/ Vicki Richards

Peter Waller has concluded drip irrigation is an unsuitable strategy because the systems evaluated in his study were not working properly and/or maintained adequately. He advo- cates the use of bubbler irrigation systems instead. Most drip irrigation problems cited in his study, however, re- sulted from improper maintenance or the use of older style or low-quality components. We believe that up-to- date, properly installed, well-maintained drip systems are still the most efficient, water conserving irrigation systems for urban landscapes.

Drip irrigation has many advantages over bubblers. The most important are application of water directly in the plant's root zone and ability to water many plants on one valve. Other benefits include multi-valving for mixed plantings with varying water requirements in the same physical area; reduced weed growth and surface evaporation; slow application rate which eliminates runoff and erosion; ease in installation and repair; and lower overall installation cost. In some situations bubblers can be effective, specifically with monocultures such as hedges, orchards, lines of trees and areas of groundcover. Even in these cases, however, uniformity and suffer if the grade is not level. Drip irrigation is much more adaptable to unusual situations, such as clayey soils, slopes, hilly areas, berms, and small, low density landscapes. Bubblers installed close to buildings and walls damage footings, stucco, adobe, and mortar.

Bubbler systems (bubblers, pipes, valves, trenches, and basins), like drip systems, require routine maintenance. Bubbler systems are not immune to vandalism and wear, particularly at commercial, institutional and multifamily sites. In addition, the basins and trenches need to be kept clean to prevent overflow. Because of higher flows, bubbler systems waste more water when leaks, breaks, or over-scheduling problems occur. Low flow drip irrigation is more forgiving, wasting less water because of the low flows.

The basins and trenches for directing and containing bubbler irrigation water are problematic. They are unattractive and are impractical for intensely used landscapes. Tree roots are generally 1½ to 4 times larger than the actual tree canopy, and most of the tree's water is absorbed in the area at the drip line and beyond. The basin for a mature 25-foot tree would be enormous. When basins are enlarged, surface roots, which take in water, will be destroyed. Drip irrigation maintenance was described as hot, dirty, thorny, with imminent danger from insects and rodents. Bubbler maintenance occurs outside (hot); if repairs are need, bubbler trenches are deeper than drip trenches and require more digging (dirty); maintaining and enlarging basins involves close encounters with trees and shrubs (thorny); and bubbler irrigation does not rule out the possibility of ants (bitey).

Peter Waller has destroyed the expectation that drip irrigation systems will last forever and be maintenance free. Additionally, he has brought to the forefront the desperate need for drip irrigation installation standards, certification of designers and installers, and on-site inspection of during system installation.We have no problem with the findings of this study, but to condemn drip irrigation entirely as an irrigation method, is throwing the baby out with the bath water.

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