In dry places like California, 90 percent of rain is used by plants. This water, green water, keeps ecosystems and crops growing, but it is the other 10 percent of the rain, blue water, that supplies rivers, reservoirs, hydropower, and daily water use.
"We need to understand how ecosystems change the water cycle,” says Sally Thompson, CEE Assistant Professor of Surface Hydrology, “Because in water-scarce places, plants are by far the biggest users of water.”
200 miles from the Berkeley campus, Thompson Lab PhD student Gabrielle Boisramé stands in an alpine meadow in Yosemite National Park. Tall, blackened tree trunks, and young willows surround her as she plunges a sensor into the mud at her feet.
“There’s something odd about this meadow,” she says. “Those dead trees are lodgepole pines. Everywhere else in this basin, they are really dry forests. But send a fire through and the dry forest turns into a wetland.”
(L-R) Blackened tree trunks; Gabrielle Boisramé
Boisramé comes to the Illilouette Creek Basin in Yosemite to study the effects of fire on the hydrology of the Sierra Nevadas–California’s major water source.
“The Illilouette is a unique experiment,” she says, “In almost all of California fire is put out as soon as it starts. But naturally, these forests burned every 6 years. Here, the National Park Service is letting that happen again.”
Boisramé sees 2 exciting things about this experiment.
- The first is that the fires stay small–less than 15 mi2. (For comparison, the Rim Fire that burned northern Yosemite in fall 2013 was 400 mi2.) Frequent fires are actually safer.
- The second is that the change in fire seems to have led to more blue water. Flow increased after the fire came back. Could the change in fire regime have delivered a win-win, that is, a more natural ecosystem, less risk, and better water supply? Boisramé notes that more research needs to be done to say, but the possibility is tantalizing
At Blue Oak Ranch Reserve, near San Jose, undergraduate Taylor Winchell is grasping what looks like a giant Q-tip made of plastic and metal. He bends over, holding it parallel to the ground, and walks it rapidly across a grid.
“This is an EMI–an electromagnetic induction sensor,” Winchell says, “I’m using it to see how soil water is varying across these sites.”
Although most of the water used in the San Francisco Bay comes from the Sierras, state water planners are urging Bay Area utility companies to rely more on local water, including the streams flowing from the East Bay hills.
Taylor Winchell with EMI
“The trouble with relying on these hills for water,” says Winchell, “is that as the climate warms, the vegetation out here will change, so we don’t know how much water these plants will be using in future or how much will be left for us.”
By understanding how native plants use water now, Winchell aims to build a picture of local water supplies in the Bay Area in a warmer climate.
Back in the Thompson Lab, PhD student David Dralle analyzes images of the same research site via GIS.
“Where different kinds of plants grow on the hills makes a big difference to flow.” He points to a string of equations. “This is modeling work for now,” he says, “But if the forests stay in place, the stream flow at the end of winter will last twice as long as if they do not.
"Trees do not use less water than grass, but they force it to leave the soil more slowly and keep the streams flowing for longer."
GIS images of Blue Oak Ranch Reserve
California has a tremendously variable climate that influences our economy and quality of life.
“We can’t control the weather,” says Thompson, “But we can make strategic decisions about land and water management to help us manage its extremes. Our goal is to develop the knowledge base that we need to make those decisions.”
by Sally Thompson
CEE Environmental Engineering