The San Pedro River is one of the only free-flowing waterways in Arizona. Though it’s more of a small stream than a river, it nourishes a vibrant ecosystem known as a riparian area. Full of tall trees, it serves as a major flyway for migrating birds and provides a ribbon of green in an otherwise barren landscape. In this Science Update, you’ll hear about the research being done to keep the San Pedro flowing.
Giving trees a stress test. I'm Bob Hirshon and this is Science Update.
Water is a prized commodity in Arizona’s dry, desert climate. And as populations there increase, the demand for water grows—threatening to dry up the few rivers left, and possibly starving out the plants and wildlife living on the banks.
Tom Maddock and his group at the University of Arizona are studying how groundwater pumping affects the health of these riverbank ecosystems. Maddock says his grad student, Kate Baird, just finished a field study on a site right along the San Pedro River, south of Tucson.
“Which jokingly became known as the tree torture experiment. Where we actually raised and lowered the water table underneath the trees using wells.”
Instruments on the trees measured their response to the water deprivation. Baird says the results are now being put into models that can predict how pumping affects tree life in the area.
“So we look at things both on a small scale, and then with that data try to improve the larger scale models and questions.”
And with that information, cities, landowners, and environmental groups can make more informed decisions on how to allocate Arizona’s precious water. For the American Association for the Advancement of Science, I'm Bob Hirshon.
Making Sense of the Research
The San Pedro ecosystem is a kind of ecosystem called a riparian system. That’s basically a thin ribbon of plant and animal life—mostly trees—that springs up around a river in a desert landscape. Because they’re tied so tightly to a limited source of water, these ecosystems are especially sensitive to human disturbances.
And we’re not just talking about sticking a pump in the river and draining it. Around the San Pedro, just digging a well in your backyard (and drawing water from it) can lower the water level in the river. That’s because some of the water that gets pumped out of the ground might have ended up in the river otherwise. In extreme cases, this can actually dry up the river. That’s what happened to the Santa Cruz River in downtown Tucson. According to Madduck, the San Pedro is a fairly small stream, so it wouldn’t take much to dry it out completely.
The tree torture experiment is a model of what might happen to the nearby trees if the water level in the river were to drop. Among other things, Madduck and Baird noticed air bubbles forming inside the trees, which can cause permanent damage. They also found out that the trees "shut down," letting out less moisture through their leaves into the air, indicating that the trees were unable to obtain enough water to meet their biological needs. Madduck also points out that the trees serve as homes to many kinds of animals and birds, which would be harmed if the trees were to disappear.
This research actually taps in to some difficult political issues. For one thing, pumping groundwater from wells usually isn’t regulated as strictly as pumping directly from streams. So a lot of landowners feel it’s their right to pump from wells if they want to, and the law is generally on their side.
In Arizona, where water is scarce, access to water can even pit one town against another. Right now, the city of Prescott wants to pump water out of the ground, but another group is arguing against it, because they say the pumping will cut off the water supply to the Verde River, which provides water for the city of Phoenix. Unfortunately, in resolving situations like these, science can sometimes take a back seat to politics and power.
Now try and answer these questions:
- What is the goal of the tree torture experiment?
- Why are scientists concerned about trees in these particular ecosystems?
- How can pumping water out of the ground affect the trees near a river several miles away?
- Often, environmental concerns conflict with the interests of landowners. The right to pump groundwater on one’s property, versus its effects on nearby rivers, is one example. How do you think these conflicts should be handled? What are some of the challenges in coming up with solutions?
- Access to water has become an increasingly volatile issue in politics. Why do you think that is? What factors have driven up the demand for water over the past several decades?
National Geographic’s Geography Action! offers Rivers, a site devoted to rivers, and developed for educators and students.
EconEdLink has a lesson in which the economics of a water shortage are discussed: Tapped Dry: How Do You Solve a Water Shortage?
For younger students, the city of Phoenix offers Water Info For Kids, a site on water use and conservation in a desert climate.