SIERRA VISTA — “Arizona’s water myth is that people believe they can pump ground water without affecting surface water.”

That assessment by hydrologist T. Allen J. Gookin’s was made some years ago, as legal proceedings to determine the San Pedro River’s navigability as a waterway before the Arizona Navigable Stream Adjudication Commission.

For decades, the meandering river has been the subject of numerous studies and has been proclaimed by many scientists as probably, the most studied river in the Southwest.

A host of federal, state and local governments and agencies, along with environmental groups, have all come to the same basic conclusion in their analyzations over the past three decades — the root of the health of the river grows up from the aquifer.

Now, a new study by Laura E. Condon, University of Arizona Department of Hydrology and Atmospheric Sciences, and Reed M. Maxwell, Groundwater Modeling Center at the Colorado School of Mines, offers another substantiation of the connection.

The authors produced the report, “Simulating the sensitivity of evapotranspiration and stream flow to large-scale water depletion.”

“Groundwater pumping has caused marked aquifer storage declines over the past century,” according to the report. “In addition to threatening the viability of groundwater-dependent economic activities, storage losses reshape the hydrologic landscape, shifting groundwater surface water exchanges and surface water availability. A more comprehensive understanding of modern groundwater-depleted systems is needed as we strive for improved simulations and more efficient water resources management.”

Their study hinged on isolating impacts of “decreased groundwater storage on the hydrologic landscape and start to unravel the hydrologic differences between modern, depleted, groundwater systems and natural watersheds.” They produced a modeling simulation which shows the impact groundwater levels to land surface disturbances.

In their research, they found roughly 200 cubic miles of water was withdrawn from aquifers across the United States over the 20th century. Depending on a finite source with an infinite–use state of mind leads to groundwater losses. Combined with varying climate conditions, the problem is exacerbated.

“The long–term storage losses caused by a century of groundwater development can be viewed as a large–scale reshaping of the integrated hydrologic landscape. This will influence watershed response to both natural and human perturbations moving forward. This study seeks to isolate the impact of decreased groundwater storage on the hydrologic landscape and start to unravel the hydrologic differences between modern depleted groundwater systems and natural watersheds,” they say.

Hydrologist Laurel Lacher stated in a 2011 report, “In general, the simulations predict that in the absence of any major water use changes in the basin, much of the San Pedro and Babocomari rivers will cease to have perennial baseflow over the next century due to widespread impacts of projected ground water pumping.”

USGS hydrologist Bruce Gungle and other USGS researchers noted in a 2017 report, “It should be obvious that a watershed perennially in deficit will likely never see an increase in natural water discharge to the river. Even if groundwater pumping were to stop today and the groundwater budget balance was positive for decades to come, the effects of the pumping over the past century would eventually capture surface flow from the river.”

The study continues, “Even if pumping were immediately stopped the cone of depression would continue to propagate for decades or more. Without significant mitigation measures, it is likely too late already to prevent declining water levels from reaching the San Pedro Riparian National Conservation Area from Charleston to Tombstone.”

However, hydrologists have concurred and have cautioned continuous groundwater pumping from the aquifer will turn the San Pedro River’s perennial reaches to intermittent or ephemeral and will end up dry like the Santa Cruz River to the west.

Recharge efforts

For the San Pedro Basin Aquifer, the study becomes one more affirmation of the importance of groundwater to surface water. Numerous modeling and ground-based data all show the connectivity of the basin aquifer and the river and various ways to sustain them are being tried.

For instance, Sierra Vista set up the Environmental Operations Park which recharges Sierra Vista’s A–quality treated effluent. The project helped raise “groundwater levels in a critical area that is both supporting San Pedro River flows and protecting the river from municipal groundwater pumping centers. Through the park’s 11 recharge basins, an average of 2,000 acre–feet is recharged annually, plus another 700 acre-feet of incidental recharge occurs across its wetlands.”

Cochise County constructed its first water recharge project off State Highway 92 in Palominas in 2014. The $1.1 million, 280–acre project was constructed with a recharge goal of 98–acre feet annually. However, monitoring records from July 2015 to July 2018 indicate a total of only 82.1–acre feet has been recharged.

Cy Miller, of JE Fuller Hydrology and Geomorphology, provided an update of county data collected from a number of monitoring sites for the Cochise Conservation and Recharge Network (CCRN) to the Upper San Pedro Partnership during the June meeting.

“Groundwater levels generally declined over the monitoring period from 2014. Palominas recharge facility showed an increase (mound) in groundwater elevations following Hurricane Odile in September 2014 at monitoring wells near the facility, but followed regional declines thereafter. Additional continued monitoring is recommended to provide a record from which hydrologic modeling methods can be refined,” he added.

Though the report acknowledged CCRN annual monitoring records show less runoff than the models predicted, the historical period of record does show high flows in other locations in the Sierra Vista Subwatershed. So other stormwater projects could “take advantage of available water in the best predicted locations and downstream of the greatest magnitudes of runoff.”

Stormwater recharge facilities are dependent on precipitation which varies from year to year, he continued. “CCRN anticipates that recharge project benefits will be relevant over the long-term, regardless of annual local precipitation variability.”

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