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Irrigation Offsets

This article was written by Jeremy Faludi in May 2007. We're republishing it here as part of our month-long editorial retrospective.

Irrigation%20offsets.jpg Green buildings do many things to save water, sometimes spending thousands of dollars to do so. But what if they could save double, or 10 times as much water for the same cost? And what if they could help bridge the rural-urban divide while they were at it? This could all happen with irrigation offsets.

As we've written before, cropland irrigation uses vastly more water than cities do. The amount of water wasted by inefficient irrigation is around four times the total amount of water used by commercial and residential buildings. So why spend thousands of dollars on greywater capture systems, low-flow toilets and showers, and other building improvements, to make an already tiny slice of the pie a few percent smaller?

Builders might have a bigger impact by financing water offsets--buying an efficient irrigation system for a farm or orchard somewhere. I was curious whether this would really make sense financially, so I ran the numbers on it, in a back-of-the-envelope way. It doesn't save as much as I'd initially thought [thanks for the correction, Nic], but the numbers are still impressive, especially for buildings in the dry southwestern US.

The cost to outfit a small office building with entirely low-flow fixtures (toilets, sinks, showers, etc.) is hard to calculate, because the prices not only vary a great deal, but are unavailable to the public. (Multiple places I called would not give price information to anyone but contractors, and the two architects I talked to had no idea of the numbers either.) So I took something easier to compare: the cost and water-saving of waterless urinals vs. normal ones. Waterless urinals cost about $500 - $600 apiece, while comparable status-quo urinals are $150 - $300. Let's say there's a $300 additional cost for being waterless. A small office building might have 20 urinals--this would be an added cost of $6000 for saving water in the building. The Presidio of Monterey did a study of their water savings after installing low-flow systems, and found that their 173 waterless urinals saved 0.755 acre-feet per month. That's just over 17,000 gallons per year per urinal. (An acre-foot is the amount of water required to fill an acre of land one foot deep. It's the standard unit of measure in large-scale water use in the US.) Your $6000 for 20 urinals will thus save you 340,000 gallons per year total. Sounds like a lot, right? Just wait.

The cost of a drip irrigation system for a small farm varies a great deal, and there are many sites that give estimates, but for good average numbers the Western Area Power Administration (getting data from Washington State University and the Idaho Department of Water Resources) says "Center pivot [sprinkler irrigation] systems have a total 'in-field' cost of $600 to $1,000 per acre... Drip or trickle systems cost $800 to $1,600 per acre". So let's conservatively say that it costs $600 more per acre to install drip irrigation. That means that when designing your small office building, instead of putting in 20 waterless urinals, you can take the same money and replace ten acres of sprinkler irrigation with drip irrigation.

How much water does ten acres of drip irrigation save? EnergyIdeas (a project of Washington State University) says "The 'water application' efficiency of a center pivot system ranges between 60% to 85% with an average of about 75%... The drip or trickle system has an efficiency of 80% to 95%, with an average of 90% assuming good water management practices are used." Those percentages must, of course, be applied to the total amount of water used. The USGS says that in the year 2000, "The average application rate [of irrigation water] was 2.48 acre-feet per acre for the United States". In many regions it is higher (California uses 3.37 and Arizona 6.21, for instance). That means that on average, a sprinkler system wastes .62 acre-feet per acre per year, and a drip irrigation system wastes .248 acre-feet per acre per year. So switching from sprinkler to drip irrigation saves .372 acre-feet per year per acre.

For our ten-acre farm, that saves over 1.2 million gallons per year--three and a half times the amount of water saved by spending the same money on waterless urinals in the building. And remember, this is a conservative estimate. If you were converting an Arizona farm, you might save 8.8 times as much water per dollar. The only way a building's plumbing even comes close is if it is a super-high-traffic building like an airport, where people are coming through and using the bathrooms constantly--in cases like this, one manufacturer claimed in Grist that a waterless urinal may save up to 40,000 gallons per year; even there, though, you would still save half-again the amount of water per dollar by having a farm irrigation offset.

Other Benefits

Drip irrigation also saves energy by requiring less water pumping. The EnergyIdeas article above estimates that "A 15% improvement in water application efficiency is thus expected to provide a 16.6% reduction in pumping system electrical energy use." Or, for absolute numbers, a study by the University of Florida said "Electric power units were estimated to consume 347 kw-hours per acre per year for crop maintenance drip irrigation and 432 kw-h/A for sprinkler irrigation." So our ten-acre farm would save 850 kilowatt-hours per year as well as saving water. This isn't a big amount of energy--roughly what you'd get by replacing nine incandescent lights with compact fluorescents--but it's a nice bonus. However, if you were building a single-family home instead of an office building and spent the same $6000 on water offsets, this bonus energy offset could be up to a quarter of the home's energy use.

A water offset like this might also help bridge the cultural and political divides between wealthy urbanites (who tend to create most environmental legislation) and rural farmers (who are often impacted by environmental legislation). Partnerships between buildings and farmers could be the perfect introduction to community-supported agriculture (the farmer could grow some produce used in an office cafeteria, or which employees sign up to buy), and perhaps other connections could bloom as well (using the farm for occasional retreats).


The hard part about setting up water offsets for buildings is the time spent finding good candidates to switch their irrigation systems. It might only work for very large buildings who can afford to hire a person to make the connections. Or it might require an intermediary firm to find the farmers and connect them to the architects, just like carbon offset firms act as intermediaries between consumers and the wind farms or forests being used as offsets. Problems like this have been solved before, however. William McDonough once told a building developer to plant ten square miles of forest to offset greenhouse emissions for the building being built, and there are half a dozen different carbon offset firms in North America alone. It may just be a matter of time before water offset companies exist alongside (or as part of) carbon offset companies.

Water Offsets for Green Buildings is part of our month long retrospective leading up to our anniversary on October 1. For the next four weeks, we'll celebrate five years of solutions-based, forward-thinking and innovative journalism by publishing the best of the Worldchanging archives.

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