The state of Tucson’s water supply is always a concern. So how are we doing? Recently, Docents at the Arizona Sonora Desert Museum had an update by Wally Wilson, chief hydrologist at Tucson Water. The reason is that we Docents often have to explain to museum visitors what all those rectangular ponds are doing in Avra Valley just west of the museum. The following material is taken from his talk.
Tucson gets water from four sources: pumped groundwater, water transported from the Colorado River via the Central Arizona Project canal (CAP), water reclaimed from sewers, and water treated from former industrial usage (Tucson Airport Reclamation Project, TARP). Water is measured in Acre-feet (AF). One AF is 325,851 gallons and one acre-foot will serve four residences in Tucson for a year. Mr. Wilson presented the following graph on water usage (as of 2011):
Notice that total water usage has been declining and has reached the level it was in 1994. That was surprising to me. Perhaps our conservation efforts are paying off. Mr. Wilson noted that average residential use in Tucson is about 90 gallons per day per capita (versus 200 in Scottsdale). Tucson is conserving groundwater by using more and more CAP water. This graph shows that our groundwater use has declined to what it was in 1959 in spite of our increasing population.
In 2011, CAP supplied 64% of our water while groundwater supplied 20%. The remainder was made up of reclaimed water. Total production in 2011 was 120,350 AF. In 2013, Mr. Wilson expects CAP will supply 80% of our needs allowing us to decrease primary groundwater pumping.
Below is a map of the CAP system. It consists of ponds to recharge the aquifer, wells to pump the water, a treatment plant, and a reservoir which stores 60 million gallons.
There are three recharge areas which Tucson Water fondly calls CAVSARP, SAVSARP, and PMRRP. These are the areas featuring recharge ponds filled with CAP water and wells to reclaim the water after it recharges the aquifer.
Why put the water in ponds to sink into the aquifer rather than treating it and pumping it directly to consumers? There are several reasons. When we first began to receive CAP water it was treated and sent to households, but the water wreaked havoc with some of our old plumbing. The current system is plan B and it has several advantages besides being kinder to plumbing.
Water from the ponds sinks into the ground at the rate of about 1.5 feet per day. As it travels 300- to 400 feet to the water table, soil filters out any viruses and bacteria that may be in the water. This filtering method is much less expensive than disinfecting the water in a treatment plant. The water still goes through the Hayden-Udall treatment plant for filtering and chlorination.
Some numbers: CAVSARP recharges 70,000 to 80,000 AF/year and recovers 70,000 AF/year. SAVSARP is permitted to recharge 60,000 AF/year and recovers about 15,000 AF/year. We are still ramping up to use our total CAP allocation. PMRRP is permitted to recharge at the rate of 30,000 AF/year and recovers water at 14,000 AF/year.
Tucson Water claims that it loses about less than 2% of the water due to evaporation from the recharge ponds. The overall CAP system loses about 5% of its water due to evaporation. Most of that occurs in Lake Pleasant which acts as a storage buffer between supply and demand.
Mr. Wilson says Tucson will have plenty of water through 2050 and beyond because we are banking water in the recharge system (and we still have the groundwater). Tucson Water is also pursuing additional sources of renewable water such as water owned by Indian Tribes. For more information see http://cms3.tucsonaz.gov/water .
See also my older posts on our water supply: