COLORADO RIVER DELTA, Mexico — The workers were bundled up, some of them in hoodies, spread out across a muddy field on an unusually rainy morning at the Laguna Grande restoration area. Their shovels stood upright, wedged in the soft ground as they planted cottonwood and willow trees and other riparian vegetation.

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.

READ MORE: An Unnatural Wonder: Journey into the heart of a river

Balancing river, groundwater

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.

The groundwater below one restoration site called Miguel Aleman is already too deep for the cottonwood and willow trees to reach, he said, adding that he expects these trees will need less water as they develop stronger root systems over the years. Most of the vegetation at this site, like the Mesquite and Palo Verde, have deeper root systems capable of reaching the water table.

The surface water that’s dedicated to nature can help mitigate potential groundwater declines, scientists wrote in the 2017 report. Government agencies should be aware that riparian habitats are vulnerable to these declines, they wrote, and that dedicated and protected water for nature is needed.

While this water helps to extend the shallow groundwater season in some areas, the November report showed that the water used to irrigate at Miguel Aleman produced only small peaks in groundwater levels because it’s applied so precisely and in such small amounts. And surface water released upstream several years ago from Morelos Dam, the last diversion structure on the river, only raised the water table temporarily.

The restoration sites depend on the water that's dedicated to nature. In the event of a water shortage on the Colorado, Hinojosa-Huerta said, it’s unresolved how water reductions will be shared within Mexico.

In the face of climate change and overallocation, such a shortage will likely happen in the future.

Consequences of a shortage

These forces combine to threaten groundwater in several ways.

Not only is climate change-induced drought expected to decrease the amount of water flowing into the delta, it could cause farmers to irrigate more efficiently, which would limit the excess runoff water that seeps into the aquifer, according to the 2017 report. The same is true if farmers are driven to fallow their fields.

“A lot of the agricultural runoff is inadvertently helping to protect or restore some of these riparian areas in the delta,” Hinojosa-Huerta said.

The Laguna Grande restoration area continues to have a high water table, largely due to excess irrigation water that drains into the aquifer, Kendy said.

Optimizing irrigation to use less water can have unintended consequences on riparian areas, but Hinojosa-Huerta welcomes it to reduce the big-picture risk of a shortage. He just wants a portion of the water it saves to be legally set aside for nature.

The 2017 report also notes a likely uptick in groundwater pumping for irrigation if less surface flows are available from the Colorado River.

The November 2018 report attributes pumping near the U.S.-Mexico border with creating the “depletion zone” that has dried out a 26-mile stretch of the Colorado River in the delta. Groundwater near a river can augment the surface flows; pumping the groundwater can reduce those flows.

The pumping near the border has also altered the flows of groundwater beneath a section of the riparian corridor along the river. While it used to flow roughly southwesterly, groundwater now flows towards the depletion zone from all directions.

It’s a result of lots of pumping on both sides of the border, Hinojosa-Huerta said, but scientists don’t know which side is pumping more.

“There is a lot of uncertainty on really how much water is being used on both sides of the border,” he said.

The two countries have built trust while managing the Colorado River, he said, but they’re not there yet with groundwater. That lack of trust contributes to the dearth of available groundwater data, he said, adding that he hopes groundwater management will be the next big step the two countries take in negotiations.

“If pumping continues and if it continues to decrease groundwater levels then, yeah, restoration is going to be more difficult,” Hinojosa-Huerta said.

Kendy is on a bi-national science team that works to inform water negotiations. She describes having only a “little sliver” of a view into the aquifer, but she’s seeing the water table deepen and the affected area is widening.

Another restoration site, called El Chausse, is closer to the depletion zone, Kendy said, but she doesn’t have fine enough data to know whether groundwater is starting to decline beneath it.

'Water is very scarce in the delta'

As the water table drops, Kendy said, there are two possible approaches: Apply more water or plant plants that don’t require a shallow water table.

Scientists, along with advisers to bi-national water negotiators, talk about these options differently.

Along the river, it doesn't make “good sense” in terms of water efficiency to try to establish native vegetation in certain places where the water table is very low, said Karl Flessa, one of the scientists who worked on the November report, which monitored the water that's designated for nature.

Hinojosa-Huerta agrees that the water table is too low in some areas to try wetland or riparian restoration.

“Upland restoration” could work, but that’s not what historically grew in the river channel within the riparian corridor, he said. This type of restoration could include planting palo verde, creosote and ironwood, which grow farther upland from the riverbed because they don't need a shallow water table.

The two co-chairs of a bi-national working group that advises water negotiators dismiss implications that low groundwater levels limit restoration. The group conducts environmental analyses for the International Boundary Water Commission.

Jennifer Pitt, the U.S. co-chair who also works for the National Audubon Society, tries to explain that, at least right now, water doesn't necessarily limit restoration work.

“Right now the restoration work that we are doing is not water limited," she said. "I mean, it is water limited. Water is very scarce in the delta.”

Her point is that if groundwater levels drop beneath the restoration sites, restoration managers can simply irrigate more, she said. Therefore the construction of restoration sites, the acre-by-acre farming of nature, is not limited by existing water.

Working in uncharted territory

While a shallow water table frees restoration sites from irrigating in perpetuity, the plants ultimately don’t care where they get the moisture from, said Patrick Shafroth, a research ecologist at the U.S. Geological Survey.

The viability of a restoration site really depends on the ability to irrigate, he said. A site with a deep water table isn’t necessarily inviable. A higher water table gives a restoration site the ability to reduce irrigation or cut it off.

Gabriela Caloca, the Mexican co-chair for the advisory group, isn’t worried about low groundwater levels.

“That doesn't affect us,” she said, adding that all of the restoration sites are irrigated.

What will affect the sites, she said, is if the water designated for nature isn’t delivered.

Negotiators and their advisers knew future shortages were likely when they set aside more water for nature, Pitt said.

“With Colorado River flow declines likely to exceed 20-30% or more by 2050, basin-wide shortages that impact Mexico’s Colorado River water allocation are almost certain,” according to the 2017 report.

In the event of a shortage, the environmental flows will be reduced proportionately to Mexico’s total water deliveries, based on a bi-national agreement between the U.S. and Mexico.

A proportionate reduction in the amount of water available for nature seems tolerable to Pitt. At least there is water reserved for nature, she said.

But there has never been a shortage before.

“So we don’t have any operating experience,” she said. “We haven't seen any written rules explaining how shortages would be implemented within the Mexicali Valley, but we do have this language ... that seems pretty clear.”

Despite the unprecedented nature of a shortage and the declining water table in certain areas, she and Caloca are not overly concerned. And Caloca asserts the restoration sites’ claim over the water designated for nature.

“We are the owners of the water,” she said.

Environmental coverage on azcentral.com and in the Arizona Republic is supported by a grant from the Nina Mason Pulliam Charitable Trust. Follow the azcentral and Arizona Republic environmental reporting team at environment.azcentral.com and at OurGrandAZ on Facebook, Twitter and Instagram.

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