A lower water table could be harming desert ecosystems

The ongoing drought has increased focus on groundwater management around the state, with concerns that drawing too much from aquifers can cause damaging land subsidence and decreased capacity.

Ecosystems may also be at risk from extensive overdrafting, the practice of removing more water from the ground than what returns.

An example of that can be seen in the desert areas around the Coachella Valley, which relies primarily on an enormous underground aquifer for water.

For decades, humans have drawn more out of the ground than has flowed back in, leading to significant declines in the water table. Scientists believe the lowered levels have caused declines in honey mesquite, an important tree for desert plant and wildlife communities.

On a recent Thursday, researchers from the UC Riverside Center for Conservation Biology checked on stands of the trees.

At one site along the San Andreas Fault, dead and dying mesquite trees sprawled for hundreds of feet.

The San Andreas fault lines block groundwater and form an underground reservoir, where the honey mesquite plant thrives. But because of development and the drought, much of that groundwater is being used.

“In the past it was a very healthy, luxuriant stand, and right now it’s dying back very, very rapidly. The most likely explanation for that is the groundwater is being pulled away for suburban use, and there’s not sufficient recharge to bring it back up to the previous levels,” said Cameron Barrows, a research ecologist with the center.

Michael Allen, director of the center, said the mesquite themselves bring water to their surroundings in multiple ways. First, they can cause dunes to form around them, essentially creating an above-ground reservoir.

“The sand itself holds water. Just like any sand box, when you dig down in it, there’s a water layer,” Allen said. “It’ll keep accumulating more and more sand, and then the surface will dry out, but it’ll hold the water in the dune. And so that way it creates its own reservoir.”

He said another method of watering happens as the mesquite’s long roots draw water from the aquifer up nearer the surface.

“Little plants that do not have deep roots are still extracting deep soil water through this process of hydraulic redistribution, so these plants not only maintain themselves, but they maintain this entire community through this hydraulic redistribution process,” Allen said.

The scientists said loss of the trees could in decreases in carbon sequestration and biodiversity.

“There’s dozens and dozens of other species that find really high numbers within, in particular, these mesquite stands, because they are relatively lush, relatively wet areas for animals and plants to survive,” Barrows said. “So when the mesquite goes, we lose biodiversity, we lose a lot of these species.”

Even mesquite that appeared to be healthy showed indications of water stress in reduced leaf size.

“When we see plants that have ample access to water, the leaves have twice the size, and this is something that researchers are finding all over California. There’s a group looking at blue oaks in Central California, and they’re finding much, much reduced leaf size,” Barrows said.

Cameron Barrows is an associate research ecologist for UC Riverside's Center for Conservation Biology. Barrows holds a healthy piece of honey mesquite, left, and leaves from a dying mesquite, right. Barrows says the dying mesquite plant leaves are half the size they should be.

Water declines

The region around the Coachella Valley is segmented by fault lines, causing separate subbasins to form. Scientists said overuse has led to general declines in the region.  

“Since about the 1970s, there’s been a general decline upwards of 100 feet or so,” said Justin Brandt, a geophysicist with the U.S. Geological Survey.

Brandt said areas around the Palm Desert have seen land subside by two feet in 14 years.  That subsidence could result in a permanent decrease in the capacity for the aquifer to store water. Currently, the aquifer is estimated to be able to hold more than 9 trillion gallons.

Well data from the Coachella Valley Water District (CVWD) show the water table in the region around the mesquite stand has declined about 60 feet since the 1970s. District officials said, though, that the general decline doesn't apply to the mesquite since stands typically grow along fault lines. Faults act as largely impenetrable barriers to groundwater movement and push levels much nearer to the surface. 

"If you get away from the fault, conditions are completely different. You can't evaluate the water levels along a fault using a well that is not along the fault," said Steve Bigley, director of environmental services for the Coachella Valley Water District. 

Bigley said data from a well along the fault shows water has only declined there by 10 to 15 feet. 

Well data from CVWD

Bigley said it's more likely that off-highway vehicle use has caused the mesquite declines.

Barrows disputed hat claim, saying he and colleagues have conducted an analysis of vehicle tracks and could find no correlation between the vehicles and mesquite death. 

Though the well data might show only a modest decline, Barrows said it might still explain the cause of the mesquites' death. He pointed out the difference in survival rates among trees at differing elevations. 

"The low areas are still green, but just as they gain a little bit of elevation over there, everything’s dead. The same thing on this side — a little bit higher elevation. So the difference between having a live mesquite and a dead mesquite looks like it’s only about three or four meters,” he said.

Hope for the future?

Groundwater declines in parts of the region have begun to stabilize. Efforts to send water diverted from the Colorado River have even increased aquifer levels in areas near recharge facilities. CVWD Steve Bigley said it's a sign that conservation programs, increases in recycled water use, and securing of supplemental water are working to keep the aquifer at a sustainable level. 

“We developed a plan, we implemented programs, and we have eliminated overdraft in that subbasin. This is sustainable groundwater management, so that’s why I say we are in great shape,” Bigley said. 

Barrows and Allen, however, expressed concern that a warmer climate future means droughts will be more severe and that less water will be available for recharge. The State Water Project only delivered five percent of its promised allocations in 2014. So far this year, it expects to deliver 20 percent. 

“This drought that we’re in right now is just probably a harbinger of what we’re likely to see as the climate change that is predicted becomes more established. And so this is giving us a chance to see how well we can adapt and how the plants and animals that are found in our environment can adapt or not adapt as we go forward," Allen said.

Bigley said CVWD has accounted for a warmer future, lower water allocations and a larger population and still projects an increase in average change in water storage in the aquifer. 

All agree on the importance of rain for the future success of life in the valley. At another stop, Allen and Barrows point to a young mesquite tree that was established after a storm last year. The location for the tree did not sit along a fault line, and water levels are estimated to be more than 100 feet below ground at the spot. 

Barrows said a lot would have to go right for the young tree to persist. 

“If we have a couple more wet years, and if a dune forms around it, so it creates its own above-ground aquifer, then it might be able to survive,” he said.

Researchers at UC Riverside's Center for Conservation Biology are a little hopeful after finding this new healthy mesquite plant growing in the Coachella Valley.

An earlier version of this story included a chart of well data from a different location. It has been updated.