A water hog, redeemed

"A big tamarisk can suck 73,000 gallons of river water a year. For $2.88 a day, plus water bounty, Lolo rips tamarisk all winter long."

So begins Paolo Bacigalupi's "The Tamarisk Hunter," a short story set in a dystopic future when humans must fight tamarisk for every drop of water. The story might be made up, but "73,000 gallons" a year is based on the belief that each tamarisk plant can guzzle up to 200 gallons of water a day.

As it turns out, that number is simply wrong. Last Wednesday the U.S. Geological Survey reported that tamarisk consume about the same amount of water as native cottonwoods and willows — 32 gallons a day. It's put a big dent in the idea that replacing tamarisk with native trees saves water.

Ever since the invasive plant (also known as saltcedar) began taking over riparian zones in the West, people have tried to get rid of it using fire, beetles and the occasional camel. Tamarisk displaces native vegetation; it decreases biodiversity and chokes up trails. Of course ripping up tamarisk frees up water for streams — but what happens if native trees grow in their place?

"I think one of the reasons why (our study is) surprising is because the value 200 gallons per day has been printed so many times in the popular press," says Pat Shafroth, a USGS research ecologist who helped prepare the report.

So where did that 200 number come from? The USGS traced it back to a paper published in 1987 whose authors never described how they got that result. A later study from 2007 calculated 32 gallons per plant.

Large tamarisk can grow to be two feet in diameter; most hover around 2-4 inches wide, and some will have dozens of branches coming off a central trunk. So it's hard to tell how much water a "typical" tamarisk needs, says Tim Carlson, Research and Policy Director of the Tamarisk Coalition, a Colorado-based group that works to control the plant. Soil conditions, salinity and climate also affect water use. So in general, scientists calculate a plant's average yearly water need based on the amount of land it covers. An acre covered in tamarisk, for example, would probably use enough water to cover that same land area to a depth of three feet.

Regardless of the actual numbers, what matters is that an acre of willows or cottonwoods would use the same amount of water. The Tamarisk Coalition got similar results in a study published last year. But Carlson and Shafroth point out that tamarisk can tap into deeper groundwater, which means they can live farther from riparian zones where native trees can't survive. And if you remove tamarisk there, the plant could be replaced by native grasses that need much less water.

Even in cases like this, there's no good research that proves water is saved in the long run, says Carlson. "But clearly there is the potential for savings…(and) common sense would tell you (the same)."