From The Arizona Republic (Alex Devoid):
The restoration site is one of three south of the U.S.-Mexico border, in the riparian corridor along the last miles of the Colorado River. There, in the delta, a small amount of water has been reserved for nature, returned to an overallocated river whose flow has otherwise been claimed by cities and farms.
Although water snakes through an agricultural canal system to irrigate the restoration sites, another source is increasingly important for restoring these patches of nature in the delta’s riparian corridor: groundwater.
Scientists who monitor restoration in the delta wrote in a November 2018 report that a shallow water table, one with higher groundwater levels, is essential to the survival of riparian vegetation in the broad expanses of the delta.
“The trees, the cottonwoods and willows, they need to be connected to the groundwater directly,” said Osvel Hinojosa-Huerta, a scientist at the Cornell Lab of Ornithology who has worked to restore pockets of the delta for over 20 years with the non-profit Pronatura Noroeste. “They have shallow roots so the groundwater level is very important to their survival.”
But groundwater levels have declined, much like the river’s surface flows that once flowed into the Gulf of California. Some of the declines may be due to drought, some to overpumping and some to the loss of agricultural runoff as farmers become more efficient.
Historically, so much water poured into the aquifer that it overflowed, creating a vast wetland, said Eloise Kendy, a scientist at the Nature Conservancy who worked on the 2018 study.
Now, researchers warn that a groundwater “depletion zone,” where levels have dropped too far to support riparian vegetation, is extending both upriver and downriver from an area near the border. Such zones are created when the groundwater pumped out exceeds what is replaced, either naturally or through artificial recharge.
“If (groundwater levels) continue to go down there’s not going to be enough water to fulfill the restoration objectives,” Hinojosa-Huerta said, while stressing there’s currently still “amazing opportunities for restoration” in key areas in of delta.
Although restoration efforts in the delta have shown progress, habitat within the riparian corridor is more and more vulnerable to declining groundwater levels, according to a 2017 report that assessed the vulnerability and sustainability of the region under different scenarios.
When groundwater levels are deeper, trees like cottonwoods need more time to sink their roots into the water table, which means they’ll need to be irrigated for longer, Hinojosa-Huerta said. In some cases, they’ll have to be irrigated forever.