THE VINE NOVEMBER 24, 2008
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Switching to drip irrigation seems like a pretty commonsense way for farmers to
save water. Instead of using giant sprinkler systems or ditches to get water to
crops—losing much of it along the way to runoff, seepage, or evaporation—drip irrigation uses a system of hoses that delivers water directly to the
plants' roots. It allows farmers to use no more water than their crops actually
need, an alluring prospect in a world of diminishing usable water supplies and a
growing number of mouths to feed. This promise of getting more "crop per drop"
has caused drip irrigation to become the standard prescription for making
agriculture work in arid regions, both in the American Southwest and the dry
parts of the developing world.
But what if drip irrigation isn't
actually that big of an improvement over traditional irrigation methods? That's
the conclusion of a new paper by Frank Ward and Manuel Pulido-Velazquez that models the hypothetical results of varying
levels of drip-irrigation subsidies on water use in the Rio Grande basin of
Colorado, New Mexico, and Texas. The question is not whether drip irrigation is
efficient—it unquestionably is—but whether traditional irrigation actually wastes as
much water as critics say it does. Some of the water used in sprinkler or flood
irrigation evaporates and is lost from the watershed. But a lot of it runs off
into streams or percolates into the ground, where it helps recharge the basin's
aquifers. If these return flows are taken into account, traditional irrigation
doesn't look as bad.
The broader point is that in regions suffering from
water shortages, there's a big difference between consumptive and
non-consumptive uses of water. Letting the water run while you brush your teeth
is not actually that big of a problem, because the water just ends up back in a local
river to be used by people downstream. Irrigation water that flows back into
creeks or soaks into the ground isn't lost, it's just temporarily relocated.
Evaporation is the only way that a watershed really loses water.
Unfortunately, while some U.S. cities—like Las Vegas—get
credit for water they return to its source, the end-users of water do not. A Las
Vegas resident gets charged the same amount for a gallon of water used to take a
shower, which eventually goes back into the Colorado River, or a gallon of water
used to water the lawn, which probably does not. If water utilities wanted to
get serious about discouraging the consumptive use of water, they would charge
differential rates for water from outdoor spigots, which is far more likely to
be lost to evaporation than water used indoors, or give customers some kind of
credit for the water they return through their sewer pipes. But for now, at
least, water is too cheap to make the extra metering costs worthwhile.
--Rob Inglis, High Country News
6 comments
But what about fresh water that drains to the sea and gets salinated? I, like 90%+ of Australia's population, live on the coast. It isn't desert here, but it's a hell of a lot drier than it was ten years ago before the onset of the Great Australian Drought which is beginning to look less like an aberration than a new, permanent state of climactic affairs. Here in Geelong we've been on Stage IV water restrictions for years and after the driest spring on record, Melbourne's in a near panic heading into summer with catchments at their lowest levels in history. The thing is, water down my drain has about 2 kilometers to run before it drains into Corio Bay, and as I say, the same goes for everyone else in Oz's capital cities. It seems to me that once it hits the sea, that water is gone-daddy-gone. Do I have this wrong?
- aeromonas
November 24, 2008 at 4:38pm
An important adverb is missing from the sentence about letting the water run while brushing one's teeth: eventually. Yes, the water *eventually* returns to rivers and *eventually* is available downstream for tooth-brushing. But this is a flimsy excuse for wasting it. It is this kind of "It all comes out in the wash" and "Oh, it's not that big of a deal" thinking that creates crises.
What is not consumed today is still available for consumption today; what is consumed or wasted is gone. Multiply your 2-gallon tooth-brushing session by 250 million brushers, times twice a day, and that is a lot of water that, while we may *eventually* have available again "downstream" is, currently, down the drain, unavailable for brushing or drinking or irrigating.
And, while the water you waste may come back to one person downstream, the number of people in the stream increases every day, so wasting water becomes even more egregious and just plain ignorant.
- onze
November 24, 2008 at 4:52pm
aero -
Maybe I am misundertanding you, but if water is so hard to come by, why is it allowed to drain into Corio Bay at all? If it can't be recycled for drinking, can't it at least be sent out to irrigate the fields?
- dhauck
November 25, 2008 at 12:50pm
Water recycling--sewage back to potable--is being closely examined in Australia and is already being trialed in a few locales. (It's been in place for a long while in Windhoek, Namibia, where I lived for a while last year.)
As for why water down the drain can't be sprayed on fields, well, I suppose it could be, but it isn't. The water-sewage system around here was built at a time when the region was water replete. As I said, unlike Phoenix and Las Vegas, coastal Australia isn't desert. Sydney, Melbourne, Adelaide, Perth and my secondary Victorian city of Geelong all grew up at a time when rain-fed reservoirs easily served the citizenry's needs. What's happened is, a nearly nationwide drought has come and stayed. When my house was built, it didn't really matter if I dumped 2 liters down the sink to mix with my poop, get partially treated and dumped into the sea. Now it does matter. A lot.
- aeromonas
November 26, 2008 at 7:49am
Ah, I gotcha. Thanks.
- dhauck
November 26, 2008 at 12:34pm
basmanroselaw.blogspot.com
- basman
January 6, 2009 at 12:43pm