The Hydrologic Cycle

Return to Land & Water Conservation

900x600-master_0019_Edgar Canyon Jessica Fraver

Key to the Desert Rivers Program’s success is engaging ranchers and farmers as stewards of the water on their land. Through small acts they can directly impact the health of the whole hydrologic system. The scientific processes and technologies that can measure the consequences of their discrete actions on riparian habitat and river flow are crucial for sustained success.

Amy McCoy of AMP Insights notes the “obvious and confounding factor” that sets her work in Arizona apart from her work in the Pacific Northwest: observable river flow. “It sounds so obvious,” she says, “but it’s really perplexing to think about how to restore a river that has no water in it.” The very thing they’re trying to measure is invisible.

Dr. Paul A. “Ty” Ferre, a professor at the University of Arizona Department of Hydrology and Water Resources, is using technology and calculations to measure the impact of the Trust’s water transactions. “You can’t really do the accounting drop for drop,” Ferre says. “There are lots of things naturally changing in the background.” He means natural growth of vegetation or the response of the land to fire. He calls this background noise and explains that you can’t take out 100 gallons of water and expect to see it appear right away in the stream.

Despite the variability and complexity inherent in natural conditions, Ferre emphasizes that it’s critical that these measurements happen in the field as opposed to in a lab. He wants to understand exactly how much water plants are absorbing under real field conditions.

“By taking measurements, we’re trying to find the locations along the river where helping people to use less water will have the most important, measurable and immediate impacts,” Ferre says.

But in order to do that, Ferre needs to figure out how much water is being used by the plant – and essentially lost to the local system – and how much water is being returned to the water table. The work to isolate those numbers is currently being done with the use of temperature sensors placed in the ground at Double Check Ranch.

“The sensors measure ground temperature at a few different depths,” says Ferre, explaining that throughout each day, during sunrise and sunset, the ground becomes hotter and colder. Though this cycle of hot and cold continues to move down into the ground, there is less temperature change at greater depths. “But if there’s water moving down through the ground, it will carry some of that heat signal with it. So by looking at the temperature variation near the surface and at a greater depth, we can actually figure out how much water is moving into the ground.”