How much water is there?

The answer depends in part on how much you are willing to pay. There continues to be some valid concern about our water supply. These concerns generally cite our current drought conditions and population growth. Tony Davis of the Arizona Daily Star has written a series of articles on the subject, articles that generally sound an alarm. For instance, see Tucson’s source of water runs low and Contrasting views on what to do about dwindling water .

To put such articles in perspective, however, consider this:

The Tucson area currently uses about 350,000 acre-feet of water per year. An acre-foot is 325,851 gallons, enough to supply three-to six family residences for a year (the number of residences depends on who’s doing the estimation). For that 350,000 acre-feet of current usage, we withdraw about 256,000 acre-feet from our groundwater supply. The Central Arizona Project (CAP) provides about 65,000 acre-feet and the rest is from use of effluent and incidental recharge. Natural recharge to the aquifers is about 60,000 acre-feet per year, much less than the amount we withdraw.

Estimates from the University of Arizona imply that our groundwater supply, at projected rates of usage, represents about a 200-year supply. Our CAP allocation is 314,000 acre-feet per year. That would seem to cover our needs, but the CAP supply is subject to natural variation of droughts, and the whims of politics. For more details, please read my blog from last June: Water Supply and Demand in Tucson. For a perspective on droughts, see my article: Drought in the West.

Our CAP supply is drawn from the Colorado River. Currently our Colorado River reservoirs stand at 55% capacity, the same as last year at this time. We are not gaining on the amount stored because water released for electrical generation and river health about equals inflow to the system. See: Bureau of Reclamation weekly water report. See also: Bureau of Reclamation forecasted use for 2010. In contrast, the Salt River system, supplying Phoenix, stands at 97% capacity. The BR report says that our “water year” precipitation is 82% of normal in the Colorado River basin and 122% of normal in the Gila River system. Snowpack is put at 83% and 244% respectively.

The point of this article is that our water policy must be based on facts rather than on perceptions. Conservation measures must also be based on facts rather than on “feel-good” ideas of the day.

The groundwater supply mentioned above counts just the aquifers down to about 1200 feet, but depth to bedrock in the Tucson and Avra Valleys is as much as 15,000 feet deep in places, so the valleys contain more water. That deeper water, however, would be more expensive to pump and process.

A related, but important concern is not just the ultimate water resource, but also the distribution system, how to get the water to the customer. Current peak summer water demand in Tucson is greater than maximum well pumping capacity of 143 million gallons per day. How much water is there? That depends on how much you are willing to pay.

Biosphere 2 Ready for New Research

Biosphere 2, that grand experiment with a checkered history, is being readied for new research conducted by the University of Arizona. Tuesday evening, Dr. Travis Huxman discussed plans for the facility with a group of about 30 people at the Cushing Street Bar.

Huxman, who has a doctorate in biological sciences, and is an associate professor of ecology and evolutionary biology at the U of A, is the new director of the Biosphere 2 research program.

For those who may be unfamiliar with Biosphere 2, here is some background. The concept was to construct a self-contained biosphere to investigate what would be needed to colonize other planets, such as Mars. The main structure, built near the town of Oracle, AZ, is a 3.15 acre greenhouse which was to be a self-sustaining ecosystem containing several plant biomes and an “ocean” to grow fish. The facility was built with $150 million in private funds in the late 1980s.

In September of 1991, a group of “biospherians” (four men and four women) entered the greenhouse for a planned two-year stay. It was intended that they depend only on what was inside the enclosure. As noted in a Wikipedia article: “All seven ecosystems of Earth exist within the confines of Biosphere II. They are a rainforest, a desert, a savanah, a marsh, a farmland (in an area called the Intensive Agriculture Biome), and a ‘human habitat’.” [I guess the ocean makes seven.] “Thus, it contains soil, air, water, animals, and plants. About 4,000 plants and animals were introduced to Biosphere II, and the ocean contained 900,000 gallons of water. It was hoped that these provisions would give the ecosystems enough material to be self-sustaining.”

As with many experiments, things didn’t go as planned. One of the main problems was that organic-rich soil consumed too much oxygen. The original oxygen content of 20.9% dropped to 14.5% after 18 months. That’s the equivalent of an altitude of 13,400 feet, and the biospherians suffered from high-altitude effects. Because they were in a greenhouse, the daily fluctuation of carbon dioxide was about 600ppm (current atmospheric concentration is about 390 ppm). During the day, with strong sunlight, plants revved up photosynthesis and used up carbon dioxide, but respired it back at night. There was also a seasonal variation in carbon dioxide, and wintertime levels reached about 4,000 ppm.

This first phase ended in September, 1993 as planned. After a 6-month transition, another group of seven people entered the greenhouse, but injuries and social problems caused abandonment of the project in 1994.

Columbia University took over in 1995 and operated the facility until 2003. Columbia “broke the seal” and formed a flow-through system to test effects of carbon dioxide among other things.

Through all of this, the facility was open for tours and derived much of its operating revenue from visitors. By 2006 the property was zoned for urban development and in 2007 sold to a developer who had planned houses and a resort hotel. However, the University of Arizona took over management responsibilities in June, 2007. And that brings us back to Huxman.

Huxman said that U of A research will “focus on environmental challenges of the day.” And by that he meant they would study initially, at least, the relationship between carbon, water, and energy, essentially photosynthesis, and how it can be applied to current issues.

Huxman mentioned solar power and the smart grid system since apparently Biosphere 2 gets some of its electricity from solar collectors. He said that with a smart grid system, the power company can turn off an individual’s solar system, which might generate power to the common grid in order to protect workers doing repairs on the lines. Biosphere 2 will not be a participant in the smart grid system so as to prevent such power outages. This will allow researchers to better control variables and also test software that manages smart grids.

Huxman says that under U of A management, Biosphere 2 will be better committed to a relationship between science and society, and that even now visitors can watch graduate students conducting experiments.

One of the planned projects is to build a model of a watershed to study the dynamics of how water gets to plants and how soil structures evolve. He wants to know how water gets into the aquifers. (A geologist could tell him that most aquifer recharge occurs at the mountain front.) After the “naive” model is working, they will introduce plants to see how that changes the soil structure. Once they learn from the model, they plan to try it outside in the real world.

They will also study ways to stabilize mine tailings.

Who is paying for all this? According to Huxman, major funding is coming from the facility owners and foundations. Much of the operating budget will come from visitor admissions; a minor part comes from the University and from corporations.

Will they be successful? Only time will tell. You can visit Biosphere 2. You can get information from www.B2science.org , email to info@B2science.org or call 520-838-6200. Currently admission price is $20 for adults. Lower prices are available for seniors and children.

And, by the way, the Cushing Street Bar has Guinness on tap.

Water Supply and Demand in Tucson

With a growing population and predictions of drought, will there be enough water in Tucson in the future? In this essay I review the supply and demand. The numbers are taken mainly from the Arizona Department of Water Resources (ADWR) website and from “Water in the Tucson Area: Seeking Sustainability” a 1999 report published by the Water Resources Research Center at the University of Arizona (WRRC). This review is confined to the Tucson Active Management Area (TAMA), an area of 3,866 square miles, which includes the Tucson Basin and the Avra/Altar Valleys – the areas from which we pump our water. CAP is the Central Arizona Project which imports water from the Colorado River. Most of the numbers refer to acre-feet (AF) of water. One acre-foot is 325,851 gallons.

The Demand

 In 1999, total usage in the Tucson Active Management Area was 323,000 AF according to WRRC. Municipal usage was 154,000 AF which included 17,000 AF used by golf courses (35% was effluent from the sewer plants), and 20,000 AF used by “turf” facilities such as parks, schools, cemeteries (33% was effluent). Agriculture used 132,000 AF (of which 20,000 AF came from imported CAP water). Mines used 39,207 AF, sand and gravel operations used 5,167 AF and “other” industrial use totaled 4,026 AF. Sewer treatment plants produced 70,000 AF per year and are projected to produce 115,000 AF by 2025. Currently 84% of effluent discharge is released into the Santa Cruz river channel where it infiltrates into a shallow aquifer. (Alert readers might notice that these official figures from 1999 add up to more than 323,000 AF, so some categories must have been counted twice.)

By 2003, total usage increased to about 350,525 AF. This is projected to rise to 396,000 AF by 2025 assuming increased municipal and industrial demand, and decreased agricultural use. Natural recharge provides only about 60,000 AF per year. In 2003, municipal usage totaled 185,199 AF. Municipal use includes all domestic and small business consumption. Industries used 47,430 AF; agriculture used 102,959 AF; Indians used 14,196; all others used 3,705 according to WRRC.

This total usage is about 169 gallons per day per capita, with residential use pegged at 110 gallons per day per capita, a figure that has remained constant for many years. In contrast, the Phoenix area uses 238 gallons per day per capita, but gets 73% of its water from “renewable” resources such as rivers, CAP, and effluent.

 The Supply

 In 2003, groundwater supplied 256,233 AF, CAP supplied 64,554 AF, use of effluent supplied 11,360 AF. The rest was due to incidental and natural recharge.

Tucson gets most of its water by mining groundwater stored in aquifers down to 1200 feet deep in the Tucson and Avra Valley basins. This is mainly fossil water deposited during the wet Pleistocene glacial periods. However, there is even more water in deeper aquifers, but as depth increases, water quality decreases, and water becomes briny with salts and toxic metals.

The center parts of Basin and Range valleys such as the Tucson Basin, are filled with porous sediments. The volume of these sediments is typically, 30 miles long, 5 miles wide, 1 mile thick — 150 cubic miles. The upper part of the aquifer consists of young water from the glacial epochs which is fresh, while deeper parts of the basin contain progressively older and saltier waters. The majority of water in the deeper parts of the Tucson basin contains high amounts of dissolved salt, gypsum, boron and lithium; difficult and expensive to treat into drinking water and expensive to pump.

 The 1999 WRRC report states that, “In 1940, when Tucson began to increase its groundwater pumping, these aquifers held approximately 70 million AF of groundwater at depths less than 1,200 feet below the surface.” This resource is equivalent to all the water in Lake Mead and Lake Powell combined. Since 1940, 10% of this groundwater has been withdrawn. Simple arithmetic implies at that rate, the remaining groundwater supply shallower than 1200 feet could last about 150- to 200 years. This time will be extended by increasing use of CAP water and effluent.

Pursuant to the 1980 Groundwater Management Code, Arizona was divided into five Active Management Areas to receive water from the Colorado River. The Tucson Active Management Area is allocated about 314,000 AF per year. Tucson area entities are to receive 215,000 AF of which the City of Tucson is allocated 138,920 AF per year. The statutory goal is to reduce dependence on groundwater and to make water usage “sustainable” by 2025. Tucson is currently using only a small portion of the allotment and political pressure is mounting to use it, or lose it. The recharge project in Avra Valley will eventually use all of our allotment. There are some problems associated with depending entirely on the anticipated CAP supply. The original allotments were meted out during years of abnormally high flow in the Colorado River so that these allotments amount to 150% to 200% of normal flow. There is also the danger that if the Biodiversity Treaty were ever ratified, then endangered species in the Colorado River delta and the Gulf of California would have precedence for much more water than they are currently receiving. The Treaty would supercede state laws.

The Avra Valley recharge project, called the Clearwater Renewable Resource Facility (CRRF), is a political compromise. It would be more efficient to use CAP water directly, but in 1992, when Tucson Water tried that, the improperly treated water gave many customers discolored, evil-smelling water as it dissolved the encrustation built up in the water delivery pipes and in home plumbing. Due to the damage, Tucson Water was sued and stopped delivery in 1994.


It seems that we have an adequate groundwater supply for about 200 years and our CAP allocation, if it materializes, will, alone, cover most of our projected needs. However, it isn’t quite that simple because having the water in the ground is quite different from getting enough of it into the distribution system at a reasonable cost and without causing dangerous subsidence. Current peak summer water demand in Tucson is greater than maximum well pumping capacity of 143 million gallons per day. The shortage is currently made up from small storage facilities filled by wells during non-peak periods and from summer monsoon rains. This seasonal shortfall is why Tucson Water promotes its “Beat the Peak” program. The question is: how shall we continue to meet this demand and at what price.

In more recent years Tucson Water seems to have gotten its act together. Their “Water Plan: 2000-2050http://www.tucsonaz.gov/water/docs/waterplan.pdf) seems to be on the right track to provide water for the long run. However, if we are to continue to “Beat the Peak” we must have more water available on a daily basis. Imagine what will happen to property values if the city cannot deliver sufficient water. Depending solely on recharge will not solve the delivery-rate problem. We need more wells (outside the Central Well Field) and perhaps some reservoirs or underground storage. And, there should be a plan to use properly treated CAP water directly in emergencies.

A recent WRRC report estimates that water resources in TAMA could support a population of 2.3 million people if proper conservation measures are used. However, those conservation measures must be sensible for there are already some unintended consequences. For instance, current city code requires new housing to have low-flow toilets. Harvesting gray water is also encouraged. But, according to the people at the sewer treatment plants, neighborhoods using these conservation methods do not send enough water through the system to properly flush the solids to the sewer plant.

And, we will eventually need to recycle the sewer effluent that currently goes down the Santa Cruz. Despite the “yuck factor” this is a growing resource and if treated properly can be merged with other sources of drinking water.


Arizona Department of Water Resources: http://www.azwater.gov/azdwr/

Water Resources Research Center : http://ag.arizona.edu/azwater/