The Prescott area water managers are over a barrel. And the barrel looks to be running dry. 

The folks who keep the water coming out of our taps know that our use of the current supply isn’t sustainable. The Quad-Cities have been sucking up water from under Chino Valley and Prescott Valley, and the water levels there have been dropping since at least 1990. They’re going to have to look somewhere else. For years, water managers and elected officials in these booming cities have been looking to the Big Chino for salvation from the drying of their water barrel—but those aquifers may not to be sustainable for our needs either, and that’s just the beginning of the trouble.

Artwork by CJ Eder

How much water can the City of Prescott pump out of the Big Chino aquifers? The answer hides underground. But the Big Chino groundwater basin isn’t a tank; you can’t just open the lid and look. Besides, “How much water is in there?” isn’t the only question. There’s also “Where is it, exactly? Where does it go, how fast does it move? How much comes in from rain or snow?” The questions become political, and hot: “What about the river at the aquifer’s downhill end? How much of the Verde headwaters come from the Big Chino aquifer systems? How will pumping affect the Verde’s flow?”

Clearly, the managers need to know as much as possible about the likely consequences of their decisions. What everyone would like is guaranteed predictions about causes and their effects. What we have is science. The word descends from the Latin scire, “to know.” But on the way to knowledge, there is much groping and guesstimating, much arguing over the meaning of results. There are more questions than answers about this complex rock-and-water system. Still, more answers come in every day, more pieces of the puzzle fall into place. The blurry picture begins to resolve. Because of the political temperature of the issue, a lot of science is taking place. The Big Chino area is one of the most researched groundwater basins in the country.

This does not, of course, stop people from disputing about it. But as new evidence accumulates and the blurry picture sharpens, those watching must be ready to put aside their old conclusions in the light of the new data.

Here in Prescott, the dispute grinds on. Prescott officials, elected and appointed, express confidence that their pipeline project will not impact the Verde headwaters. Other observers, many of them uncommonly well informed, are concerned. So I asked some of those Concerned Observers, “What are the data that worry you?” And I asked some of the Confident  Water Managers “What are the data that give you assurance?”

Water in, water out

One worrisome fact is basic. Although aquifers have their complexities, they still function as physical systems. Water flows into the ground; water flows out. Before human intervention (“predevelopment,” the experts call it), the same quantity that flows in from rain and snow and streams (recharge) will also flow out, as streams and springs (discharge), balancing over time. True, certain aquifers might also lose water to evaporation, and then there are plants—nature’s small pumps that pull water from the ground and release it to the air. Estimates of this loss, called evapotranspiration (ET), vary wildly for the Big Chino, but the Concerned Observers believe it to run at about 15 percent. So, one unit of water into the aquifer, maybe .85 of a unit out at the stream.

It’s when humans haul their turbine pumps into this system that the Concerned Observers get anxious. Among this group are two retired US Geological Survey (USGS) scientists, geologist Ed Wolfe and hydrologist Bill Meyer, two Prescott area residents who have followed the issues, reviewed the research, and served on the local water groups. Wolfe has expressed his concern this way: “The introduction of pumping by man removes water that otherwise would have naturally discharged from an aquifer, so that the amount of water withdrawn by wells eventually results in an equal reduction in discharge.”

Meyer adds, “The above concept is fully… supported by field observation. It has complete acceptance in the literature and is taught in college. [It] explains why pumpage from the Prescott Active Management Area has reduced natural discharge from Del Rio Springs by about one-half of its original value and why ADWR [Arizona Department of Water Resources] has predicted that the spring will cease to flow in 2025.” I have heard both Wolfe and Meyer patiently repeat the basic principle in meetings whenever possible.

Beyond this basic evidence, scientific studies by disinterested research organizations—one study building upon another—provide confirmation for the likelihood that Prescott’s planned water withdrawal will, sooner or later, substantially reduce the upper Verde’s flow.

A tale of three studies, and the Infamous Clay Plug

OK, truth: way more than three hydrological studies have appeared over the decades. We’ll look at three that stand as landmarks in the dispute over Prescott’s thirst for a new water source, studies the Concerned Observers mention as validating their anxieties. The Confident Water Managers challenge their readings of the results. The city officials have also commissioned studies from private consultants. Along the way we’ll compare the results.

Science builds on science; for example, a 1976 USGS study by Wallace and Laney used a lot of existing data to show that the groundwater in the Big Chino basin does move down the valley toward the Verde headwaters. And the first of our three studies of concern, a 1994 US Bureau of Reclamation (USBR) study, built on that and other research to produce two main conclusions.

Its first conclusion was that “there is no continuous impediment to the flow of water from the northern part of the valley to the upper Verde River.” This refuted a finding by a private consulting firm, Water Resource Associates (WRA), in a 1990 Prescott-commissioned study, that a formation of clay deposits occupying the center of the Big Chino Wash actually divided the long valley in two, blocking the groundwater to the northwest of it from the Verde headwaters. WRA concluded, “Based on this geologic formation and the large distance between [Prescott’s then-optioned water ranch] and the Verde River headwaters, it seems very unlikely that pumping the proposed quantities of groundwater would have any impact on the Verde River.”

But the USBR research created two mathematical groundwater models that compiled existing data on the rock types and movement of water through them, and both models indicated that groundwater could travel the length of the valley. Deposits of sand and gravel surround the clay formation, allowing groundwater to flow around it, and probably under it. So, although people talked for years (and some still do) about a “clay plug” protecting the river from pumping on the far side of it, the evidence is persuasive: No clay plug.

The second conclusion of the 1994 USBR study was “…the flows in the Verde River can be accounted for by the known recharge sources in the Big Chino Valley…and from [local] precipitation.” Wolfe and Meyer have reviewed both this and the 1990 WRA study. They point out that the WRA consultants also constructed models, but that because they modeled only the upper 30 percent of the aquifer, in only the upstream part of the valley, they couldn’t show how groundwater behaved as it flowed down the basin and out at the Verde River. But the USBR models, extending from the upper Big Chino Valley to the Verde headwaters, simulated the entire sequence of rocks above the basement floor of the basin. This permitted modeling the whole extent of groundwater movement, and it allowed them to estimate recharge and discharge to the aquifer. This is how they calculated that the Verde River baseflow roughly equaled all known recharge sources. Water out, water in.

Another puzzle piece

USGS geologist Laurie Wirt spent years studying the Big Chino’s rock and water interactions. She participated in three reports, each building on the last, based on the years of research she did with other scientists. In 2000 USGS published the first report, which Wirt wrote with now-retired USGS hydrologist Win Hjalmarson. He comments on its reception: “ADWR was real picky with some of our methods, and some of their concern was justified. However, we clearly said our analysis was based on existing data, which was incomplete, but showed the connection between the BC aquifer and the baseflow of the Verde River.”

To get both more precise and more comprehensive information, Wirt participated in another ongoing USGS study, with reports published in 2002 and 2005 and with Victoria Langenheim as the principal researcher. Picture a small plane, armed with a magnetometer, flying a grid pattern over the Big Chino to record magnetic variations in the terrain below; meanwhile hundreds of stations on the ground measured tiny gravity variations characteristic of different rock types. These “x-ray vision” techniques have been combined with others to develop a clearer geophysical image of the basin—to map the location of faults, to outline where tongues of lava lolled across the “basin-fill” deposits of sand, gravel and clay deposits and hardened into basalt. Gradually, the contours of the ancient bedrock canyon at the basin’s bottom, of the limestone layers spreading across the bedrock, are coming into focus.

As a next step, Wirt wanted to provide a “more detailed understanding of… the relation between major aquifers and the upper Verde River.” So over the next two years she and her team combined the 2002 geophysical data, plus other geological statistics, with chemical information they collected from the main aquifer units in and near the basin and from upper Verde springs water. Sampling trace elements in key locations allowed her to model how the water moved through the aquifer.

These separate lines of evidence—geophysical, geological, geochemical—gave extra support to her 2005 principal finding: At river mile 10, the Big Chino basin-fill aquifer and the limestone lying under it contribute 80 to 86 percent of the river’s baseflow. Up to 14 percent comes from the Little Chino basin—home of Prescott’s and Chino Valley’s overdrawn wellfields, and of the disappearing Del Rio Springs. Between none and 6 percent of it originates in a regional limestone aquifer outside the Big Chino, running from under the Big Black Mesa, north of Paulden, heading southeast and visible in Hell Canyon. This “Wirt et al. 2005” report provides the second line of worrisome evidence.

Modeling complexity

While Laurie Wirt was working up her results, the third study was underway. Kyle Blasch led a USGS team to produce a “study combin[ing] climatic, surface-water, groundwater, water-chemistry, and geologic data to describe the hydrogeologic systems within the upper and middle Verde River watersheds and to provide a conceptual understanding of the groundwater flow system.” Yes, in some ways it overlapped the Wirt study, and like Wirt, Blasch took off from a Victoria Langenheim geophysical effort, this one published in 2005 as a companion to the Blasch opus.

Some of the elected officials facing this flood of science were finding the results perplexing: Do these two USGS studies reach the same conclusions about the Big Chino contribution to the Verde? Equally perplexed, I emailed John Hoffman, one of the Blasch study participants who also worked with Wirt. His response: “There are small differences in estimated contributions to baseflow between the two studies; however, the differences are not unexpected. The studies were conducted at different scales, with different study areas, and used different methods. Given the differences in the investigations, the findings are remarkably similar.” One of Blasch’s conclusions is, “Ground-water outflow from the Big Chino Valley occurs only as base flow in the Verde River.” This confirms that basic principle of the Concerned Observers, as expressed by Win Hjalmarson—that “the export of water stored in the Big Chino aquifer will remove water that would otherwise have discharged from the river’s headwater springs.”

The Blasch report is designed to form the basis for a mathematical model, a powerful scientific tool. A model is a series of related mathematical formulas that describe the parts of a system. You can vary X—say, rainfall in one area—and the model will tell you what happens to well levels A and B. You can input valid data from all studies. Users can develop scenarios for different levels of water pumping and show the probable effects throughout the system. So when Blasch and his team have finished their model, in December of 2007, we will be on our way to science-based predictions of Big Chino pumping impacts.

Click to enlarge.

Strictest Confidence

These three studies, published by the federal agencies and available on the internet, provide the conclusions that worry the Concerned. So I asked the Confident, in the persons of Jim Holt, senior project manager for Prescott’s pipeline, and John Munderloh, Prescott Valley’s water resources manager, about the data that give them confidence. Munderloh mentioned “a hydrologic model that Prescott had contracted to Southwest Groundwater Consultants (SGC) for the area of the Big Chino Water Ranch.” Holt noted that while it covered only the extreme upper area of the Big Chino basin, it was currently “the only groundwater model that exists for any portion” of the basin. In acknowledging that the model “doesn’t in itself predict potential impacts to the Verde River because it does not encompass that,” he added that “in fact there is no work of science out there that is predictive regarding pumping impacts on the upper Verde.”

SGC created their mathematical hydrologic model in connection with reports they did for Prescott in 2004 and 2005, to evaluate the physical availability of groundwater under the city’s Big Chino Water Ranch. The consultants concluded that Prescott could withdraw up to 12,704 acre-feet/year, the full amount legally available, and still meet ADWR’s requirements. Given that this pumping will ultimately reduce the flow of the Verde by that (or almost that) amount, one major conclusion of SGC’s model is that this drop would take more than 1,000 years.

I asked the water managers if they had any doubts about the conclusions of the USGS studies. Munderloh responded, “There are always reasons to investigate more thoroughly information that’s presented.” The great volume of data that went into the Wirt and Blasch reports, he said, would require thorough review. The cities have engaged Montgomery & Associates, a “well-renowned hydrologic firm,” to produce a review due for release in late March. At the Prescott City Council meeting approving this review, Holt had said that the two USGS reports contained data and conclusions that could impact water resource management strategies. Because the reports were not designed to consider the water management needs and interests of the Prescott Active Management Area communities, he said, Montgomery “would evaluate [their] hydrologic significance… and would verify or otherwise identify assumptions or conclusions in concert or conflict with other reports, and facilitate understanding the implications [for] long term management strategies.”

Munderloh also advised caution in the use of the Blasch-led model that will appear at the end of this year. He said we should “anticipate a number of iterations [run-throughs]” before seeing reliable cause-and-effect conclusions. “I don’t want to give the public false expectations that this model will be the final answer …[for] hydrologic knowledge in the area,” he commented. The model will allow us to tell where we need more data, but “I don’t know if it will be able, in the scale of a 15 million acre-feet storage capacity aquifer, to really tell what’s happening with our pumping. It’ll wait to be seen.”

Dueling hydrologists

One difference between a report commissioned from a consultant and one performed by a government agency is peer review. All the USGS scientists and many other sources I spoke with echoed Bill Meyer’s comment: “The USGS has one of the most highly respected peer review processes in the scientific community, and all of their reports are required to be peer reviewed prior to publication by the USGS.” This means that research is turned over to disinterested specialists in the same field and submitted to questioning, nitpicking, and suggestions for improvement, and the researchers address these comments before the report goes public. Laurie Wirt’s final study, completed in 2004, wasn’t actually in the public’s hands until 2005 due to lengthy back-and-forth with reviewers.

Research commissioned from a private consulting firm, however, does not undergo such review. While Prescott is now hiring a firm to scrutinize the two already-reviewed USGS reports, it has fallen to others to critique the SGC reports and model. In 2005, when Prescott officials were meeting with US Fish and Wildlife Service (FWS) staff to discuss plans to mitigate any impacts to the river, FWS field representative Steven Spangle wrote to the city, “We feel there are limitations in using this model as a predictive tool for estimating changes in spring discharge at the Verde River.” Limitations included that “the values used for recharge are unrealistic in quantity”—unrealistically large—resulting from the model’s construction with “far too many cells representing mountain-front recharge to the basin.” Also, the artificially cut off southern boundary caused the modeling to go wonky at that point. As a result, “the quantification of groundwater flows across the southern model boundary [toward the river] should not be considered accurate.”

FWS notes that “the authors appear to put much weight into the theory that most groundwater contributions to the Verde River originate from the [Big] Black Mesa area [north]east of the Big Chino Basin.” This was the area that both Wirt and Blasch had actually found to make a negligible groundwater contribution. FWS cited “a lack of corroborating evidence” for this, one of SGC’s most important findings.

When geologist Ed Wolfe and hydrologist Bill Meyer look at the SGC model that inspires the city’s confidence, they experience the opposite of confidence. They find that the model’s use for prediction is “severely restricted” by its geographic limitation and by not even addressing the “problem of most concern,” the possibility that pumping could cut the river’s baseflow. They say the model overestimates evapotranspiration  to the point that thirsty plants would leave no groundwater at all to reach the river, and it makes up for this deficit with the Big Black Mesa theory mentioned above. Meyer and Wolfe identify a lack of information about essential features of the system, such as the basin’s geologic framework and this framework’s hydraulic properties—how it handles water. Critically, these lacks make the model’s “calibration and verification” (essential steps for reliable prediction) impossible.

When science collides with desire

The Prescott area municipalities and Yavapai County have generously helped fund the public agencies’ research described here. But ordinary citizens may be forgiven for expressing some doubt about the private studies commissioned by the City of Prescott. The work of private consultants never undergoes the scrutiny of peer review, and these consultants have drawn conclusions that don’t hold up in the face of the evidence presented by other researchers.

To recap the consultants' conclusions: In 1990 the Water Resource Associates study found that an underground “clay plug” blocked the Big Chino aquifer’s flow to the Verde, concluding that the Big Chino isn't the upper Verde's source and thus “it seems very unlikely” that Prescott’s pumping from the aquifer would harm the river. But a 1994 Bureau of Reclamation hydrologic study refuted the clay plug theory. Then in 1998 Prescott hired another consultant (SGC), who concluded that the Prescott AMA was in safe yield; almost immediately the Arizona Department of Water Resources blew that one out of the water, finding that the data show we are not, and declaring we are instead mining groundwater. Now Prescott has new research from SGC that concludes the Verde River's source instead flows from a completely different aquifer. But the latest comprehensive and peer-reviewed USGS studies show that the aquifer in question actually contributes a negligible amount of groundwater to the river.

Considering the barrel they’re stretched over, the water managers must be praying for good news. If not Big Chino water, then what? But Win Hjalmarson believes that the burden of proof should lie with City of Prescott, which “must conclusively demonstrate… that its pumping will have no detrimental effect on Verde River flows and ecology  before any pumping and exportation of ground water from Big Chino Valley begins.” Moreover, Hjalmarson and others insist, the city must develop plans for mitigating any impact they do have on the river. The water managers have responses to this issue, as we’ll see in an upcoming story.

The future of science

Fortunately, much more scientific investigation is underway, by a variety of research groups collaborating with government entities—from local elected officials to federal agencies. One desired result is a water budget, quantifying water that enters and leaves the system; some of the research listed here has begun these calculations, as have interested observers, and we’ll present some of those later. And the Blasch model and others will permit graphic displays of alternative scenarios that make possibilities visible to the end users of the science.

As for these end users—the elected officials who must make policy—they struggle like all of us to understand the implications. Laurie Wirt and Kyle Blasch have presented their research to Yavapai County’s Water Advisory Committee (WAC). I was there; it was tough. The committee’s technical advisory group created a “white paper” to digest the information in the Blasch report, but are they still at work on one for Wirt’s research. For months they have danced carefully around how, exactly, to present it without interpretation, because interpretation becomes political. Still, the WAC wants to know “what it all means.”

Jerome Vice-Mayor Jane Moore, a WAC member, sees the studies as “a great education.” She knows there is much she doesn’t understand, but she takes a positive approach. “I certainly can't say it's been easy, but I find it extremely interesting.” She tries to listen closely to speakers and ask questions—also to read and research apart from meetings, and she understands how hard it is for many WAC members to find time for this. A responsibility of the WAC, she believes, will be to bring the science to the public. The most important thing she has gleaned from the science? “That there is a finite amount of groundwater that we need to manage responsibly, and come to some agreement of what ‘responsibly’ means.”

For more information, including links to the research mentioned here, Read It Here's WaterWord.

(Candace lives in and reports from Prescott. Contact her at This email address is being protected from spam bots, you need Javascript enabled to view it .)

Comments (2)

The Honorable : Steven Ayres : http://Prescott Bravo, Candace, this is great work. March 09, 2007 Citizen : Kim Gould : http://Clarkdale I'm an AZ native - lived in the VV since 1967 - love the river and naturally very concerned with the current Prescott pipe line issue. I was helping my son do some research on the Verde and came across your WaterWord articles - so glad I did. Absolutely excellent work. Bravo - and a standing ovation! April 22, 2007

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