Possible Climate Shift Could Worsen Water Deficit In The Southwest

If these climate experts are right - and it may take another 10 years of data gathering to settle the question - the American Southwest could be poised at the beginning of a drought that could last 10 years or longer, says Barbara Morehouse.

Morehouse directs the University of Arizona's Climate Assessment Project for the Southwest (CLIMAS) , a program in UA's Institute for the Study of Planet Earth.

CLIMAS researchers have analyzed how a drought similar to the one that occurred here in the 1950s would affect Phoenix and Tucson water supplies in the year 2025.

The Pacific Decadal Oscillation is a fairly regular pattern of high and low pressure systems over the northern Pacific Ocean, off the coast of Alaska and Canada. The PDO operates on a 20- to 30-year time scale -- much longer than the better-known but short-term El Nino and La Nina events. Shifts in the PDO regime occurred in 1925, 1947 and 1977. Some climatologists believe that the PDO shifted again around 1995.

Researchers know that the PDO correlates with relatively wetter and drier periods in western North America, Morehouse said. And new research by University of Washington climate researchers suggests that the PDO enhances El Nino and weakens La Nina conditions in one phase, then weakens El Nino and enhances La Nina conditions in its alternate phase.

Since 1977, as the new research would predict, the American Southwest has been blessed with wetter winters during El Nino years and and not-so-dry winters in La Nina years. Winter precipitation is nature's major means of watering this region. So if the PDO did indeed shift to its alternate phase in 1995, Morehouse noted, the Southwest may be short on renewable water for the next several decades.

How severe that drought will be and how long it will last still is not clear.

Most of the Southwest experienced a prolonged and severe drought during the 1950s, Morehouse noted. If a drought of that magnitude and duration occurred today, "the consequences could be at least as severe," she added.

CLIMAS scientists have modeled the effects of prolonged drought on urban water supplies in Phoenix and Tucson based on the 10-year drought conditions that occurred in the 1950s and the Arizona Department of Water Resources estimates of Phoenix and Tucson water needs in 2025.

Even assuming full availability of Central Arizona Project (CAP) water, Phoenix' water demand could exceed its renewable water supply by 39 percent, according to CLIMAS. That is a 15-percent increase above the 24 percent overdraft already projected for Phoenix for 2025, Morehouse noted.

Tucson is projected to have a 15-percent groundwater overdraft in the year 2025. A 10-year drought comparable to the 1950s drought would increase that deficit by another 10 percent, according to the same CLIMAS study. The project is funded by the National Oceanic and Atmospheric Administration.

"In raw numbers, this would translate to an additional 3.78 million acre feet of groundwater overdraft for the Phoenix area over the next ten years, and an additional 380,000 acre feet of groundwater overdraft for Tucson during that time period," Morehouse said. "These are significant amounts of water, especially given current infrastructure and water policy considerations in the two metroplexes.

"The Tucson/Phoenix area has experienced considerable growth and change in the past several decades, a time when conditions have been relatively wet. Assuming that these conditions will continue into the indefinite future flies in the face of everything we have learned about the ancient and recent climate history of the Southwest.

"We all should begin thinking seriously about the impacts of extended dry conditions, and what viable alternatives exist for coping, what contingency plans we need," Morehouse said.