Do you know your irrigation DU?
19 Jan, 2009 By: Ron HallConservation advisor Chris Wright says that about 50% of the 7.8 billion gallons of water used outdoors in the United States every day is wasted. Most people, including grounds managers, don't know this because they don't know how efficient their irrigation systems really are.
In other words they haven't conducted what Wright describes as an action audit, which shows them just where the irrigation water is going in relation to the turfgrass water needs on their grounds or within their landscapes.
Action audit -- do it!
If you haven't conducted an irrigation audit on your property, do it, says Wright, Ewing Irrigation's Water Conservation Advisor, Western United States. Water waste translates into unnecessary expenses. Sometimes lots of expense. And this is beyond the actual cost of the extra water, itself, or the problems it's causing your turfgrass or the ornamental plants in your care.
Consider, for instance, the additional energy required to treat and move water that's ultimately wasted because of unnecessary irrigation. This in addition to the expense we all incur and share of building and maintaining water system infrastructure to meet relatively short mid-summer peak water demands.
Wright focuses on the importance of determining the efficiency of landscape irrigation systems that, he says, should apply just enough water to meet plant needs -- no more, no less. Anything more than that is waste; anything less affects the health and appearance of landscape plants.
Benefits of irrigation efficiency
Unfortunately, most landscape irrigation systems are not very efficient. Landscape irrigators must strive to improve irrigation efficiency, which
• reduces water use and lowers water costs,
• improves plant health and results in fewer wet or dry spots,
• reduces deep percolation water loss,
• reduces runoff,
• reduces fertilizer and chemical requirements and
• results in a more attractive landscape.
Wright said no irrigation system is 100% efficient, but most can be made more efficient, and many of them can reduce water use by 30% and still achieve the desired results.
Dry spots in turfgrass, which cause property owners to complain, are the main reason why irrigation water is wasted, he says. They can occur in turfgrass for several reasons, including the presence of hydrophobic soils, which can be improved by using soil wetting agents to increase soil water penetration. But homeowners (and a lot of grounds managers) aren't familiar with wetting agents and too often try to green-up dry spots by keeping sprinklers running longer. This, of course, puts too much water on surrounding areas of the landscape.
Start calculating
So, how do you know you're wasting water? And how do you determine how much you're wasting? You have to calculate the distribution uniformity (DU) of the irrigation system, how uniformly water is being applied, he says. DU is a measure of which areas of the turfgrass are getting too much water and which are getting too little. You can't correct system shortcomings until you identify and quantify how efficiently your system operates.
An irrigation audit is a 3-step process -- 1) site inspection 2) checking the system's performance and 3) irrigation scheduling. If you conduct the audit in this order you will find out what you need to know about your system, and what you will need to do to make it more efficient.
Start by walking the property, and looking for obvious problems, such as broken heads, misaligned heads, sunken heads, mismatched heads, incorrectly positioned heads, and high or low pressure. Do this for each zone, and report what you find on an Irrigation Audit DataSheet.
But let's get back to the concept of distribution uniformity (DU) and why it's important. The DU calculation, which you determine from an audit, reveals how evenly the water is being applied by the sprinkler system. At 100% DU the water is being applied perfectly even. This is not attainable in the real world. But achieving a higher percentage of DU is. And the higher the percentage of DU, the better the irrigation efficiency.
What's good and not so good
According to the IA, an irrigation system with rotary sprinklers that achieves 80% DU is considered excellent, 70% is good and anything lower than 55% is poor. The percentages are somewhat less for spray-type sprinklers with excellent DU pegged at 75%, good at 65% and poor being 50% of less.
In other words, at 50% DU the system will be applying twice as much water as the landscape needs, and at 70% DU about 30% more. Unfortunately, most irrigation systems have a DU on the lower percentage of the scale, said Wright
What does this mean to a property owner, in addition to the problems excess water may create in terms of and plant health? It means added expense.
But before we can add up the money savings a property can realize with a more efficient irrigation system, we have to learn the site's evapotranspiration (ET) rate. ET is a calculation of how much water is needed for optimum plant health based on the site's environmental and climatic factors, such as temperature, amount of sunlight, humidity and wind.
For his example, Wright usesan ET of .31 inches per day (April - September) for Las Vegas, meaning that to maintain a healthy turfgrass, an irrigation system would apply that much water each day, or 2.17 in. per week.
But if the DU of the system is 48% it would have to apply 4.52 in. (2.17 in./week X .48 = 4.52 in./week) of water to adequately irrigate the property.
By bumping up the DU to 70% through system repairs, upgrades or modifications to the system, the amount drops to 3.1 in. per week (2.17 in./week X .70 = 3.1 in./week).
It pays for itself
This represents a savings in water of 38,340 gal./week per acre of irrigated turfgrass. Assuming four acres are being irrigated, the water savings would be 153,360 gal./week. And, assuming that water cost $4 per 1,000 gal. (Wright's example), the property owner would save $613/week in water costs, or $14,722/year (irrigating six months).
While Wright cites a study showing this magnitude of water savings after 51 spray heads were retrofitted with MP Rotators, any upgrades or repairs to a system that boost DU will increase irrigation efficiency.
Of course, conditions (ET, the price and availability of water, customer expectations, etc.) vary from region to region, meaning that water and cost savings vary also. But the importance of understanding and seeking a higher percentage of DU is key to achieving greater irrigation efficiency.
As it often turns out, determining what needs to be done to improve a system and save water, based upon a site inspection and audit, is the easy part for a contractor or grounds manager.
Convincing an administrator, property manager or property owner, who has no idea what DU is or why it matters, that it's in the best interest of their property and their budgets or pocketbooks to do so -- that's the real challenge.
A sample datasheet and a point-by-point description of how to conduct an audit are available on the Web site of the Irrigation Association. Visit www.irrigation.org and download what you need.
(Editor's note: Wright made these observations at the first-ever WaterSmart Innovations '08 Conference this past October. The Conference, sponsored by the Southern Nevada Water Authority (SNWA), brought together more than 50 presenters representing a cross-section of irrigation, conservation, institutional and regulatory interests, all focusing on the fresh water issues.)
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