By Mitch Tobin
ARIZONA DAILY STAR
To calculate how much growth our water supply can support, hydrologists turn to "water budgets."
It's like accounting that uses gallons rather than dollars.
Here's a crash course.
Planners look at how much water enters the area - through rainfall, snowmelt, the seeping of treated effluent into the ground and the pumping of Colorado River water from Lake Havasu to Tucson.
They also estimate demand for water, making assumptions about population growth, how many gallons each resident will use and what will happen to agriculture and industry.
Evaporation and plants' needs also are factored in.
If demand exceeds supply, the water budget is in deficit. Tucson then turns to its savings account - the aquifers beneath the city and Avra Valley that have stored billions of gallons over thousands of years.
If supply exceeds demand, the budget is in surplus and the aquifers can start to recover.
The confusion and uncertainty arise because hardly any of the components of the budget are known for certain.
Mines' future water use depends on copper prices. People's penchant for golf courses and swimming pools depends on the region's economic performance. Global climate change and advances in treating effluent might affect how much water seeps into the aquifers. And politics will help determine how much Central Arizona Project water we use and whether cities spread into farmland.
"It all depends on your assumptions," said Barbara Tellman of the University of Arizona's Water Resources Research Center.
Tellman is preparing a report for Pima County that looks at attempts to quantify how much growth our water supply can support. Here's how three of those exercises played out:
* A study last fall by the Southern Arizona Water Resources Association, a business-dominated group that lobbied for the CAP, ran through three scenarios that varied how much water mines and farms will use. It assumed the Tucson area would stay at "safe yield" - where ground-water supplies aren't depleted - and that we'd use our full CAP allocation.
The study concluded Tucson could support 1.38 million people if mining and agriculture remain at current uses. But if mining and farming ceased, we could achieve safe yield with 2.28 million residents.
By comparison, the Arizona Department of Economic Security projects Pima County's population will rise to 1.29 million in 2025 and 1.67 million in 2050.
* In 1999, UA's Water Resources Research Center examined 14 scenarios that varied the amount of CAP used and the behavior of three sectors - agriculture, industry and municipal water providers. If all three sectors doubled their water demands and the region only used ground water, our overdraft would be 417,000 acre-feet.
At the other extreme, if agriculture and industry used half as much ground water and a twice-as-large population used only CAP water, we'd have a 99,000 acre-foot surplus.
By comparison, our current overdraft is 170,000 acre-feet.
UA's water budget can be manipulated on the Web at
ag.arizona.edu/AZWATER/sustainability/.
* A 1988 Sierra Club report looked at "optimistic," "moderate" and "pessimistic" supply projections that varied use of CAP water and effluent. It then plugged in different population totals and calculated how much water each person would have if the area stayed at safe yield.
If Tucson's population were
2 million, under the group's optimistic scenario each resident could use 90 gallons per day, about half what people currently consume. Under the pessimistic scenario, each resident could use 17 gallons.
Find out more about the CAP at its own Web site.
Tucson Water is the delivery source for CAP water, and more.
Get the view of opponents of adding CAP water to local mains.
