Mention tamarisk, or “saltcedar” to a westerner and you’ll likely get a non-family-friendly reply. This invasive tree has taken over riparian areas across the region. Tamarisk is to the American west what the Eastern redcedar is the southern plains and kudzu is to the south. One of the many knocks against saltcedar is the amount of water these devil shrubs are supposed to suck up each day. But a recent U.S. Geological Survey study revealed a surprise about the common belief that each tamarisk uses 200 gallons of water daily…

From this story in High Country News:
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.