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.


One comment

  1. And did you know that the water Tucson drinks is between 800 years and 10,000 years old?  That is the measured time (using isotopes  – specifically carbon 14) since our water fell as rain or snow prior to recharging the aquifer.

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