Southwest warned to plan for dry decade

According to researchers, Arizona could be headed for an even drier phase in climate

February 21, 2000
Web posted at: 12:22 PM EST (1722 GMT)

By Robinson Shaw

It's too early to know if North America is shifting into a new phase of the Pacific Decadal Oscillation, but it's not too early for the Southwest to plan for an extended period of dry conditions, according to the Climate Assessment Project for the Southwest.

Climatologists won't know for sure whether a shift in the PDO has really happened, or if the shift is for better or worse, until 10 years worth of data is collected. But it's not too early to begin thinking about the possible implications of such a shift: The Southwest could be moving into a decade or longer dry phase, according to Barbara Morehouse, director of the University of Arizona's Climate Assessment Project for the Southwest.

CLIMAS is a program of the university's Institute for the Study of Planet Earth. The National Oceanic and Atmospheric Administration funds the project.

The PDO is a fairly regular pattern of high and low pressure systems over the northern portions of the Pacific Ocean, off the coast of Alaska and Canada. The pattern operates generally on a 20- to 30-year time scale, and correlates with relatively wetter or drier periods in the western portion of North America.

Shifts in the PDO regime occurred in 1925, 1947 and 1977. Some climatologists believe that another shift began around 1995.

"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.

Research conducted by Nate Mantua and colleagues at the University of Washington's Climate Impacts Group suggests that PDO phases can combine with El Niño or La Niña to influence precipitation in the West, particularly in winter. Winter precipitation is typically responsible for most of the water reserves in the Southwest.

Water could become a rare commodity in the Southwest with a severe drought

The positive phase of the PDO tends to enhance El Niño conditions and weaken the effects of La Niņa, while the PDO negative phase can enhance the effects of La Niña and weaken the effects of El Niño. Thus, when the PDO is in its positive phase, as it has been since 1977, the Southwest tends to experience wetter El Niño winters, but relatively normal La Niña winters.

Some climatologists believe that conditions have shifted to the negative PDO phase, causing drier La Niña winters, and normal El Niño winters. Thus, the onset of a negative PDO phase could cause the Southwest to experience generally drier conditions over the next several decades, likely producing a decrease in renewable water supplies. How dry and for how long is unclear.

Most of the region experienced a prolonged and severe drought during the 1950s in the midst of the last PDO negative phase. If a drought of similar magnitude and duration occurred today, the consequences could be at the least severe, according to Morehouse.

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

Even assuming full availability of Central Arizona Project 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 (lack of water) already projected for Phoenix for 2025, said Morehouse. The CAP is Arizona's single largest resource for renewable water.

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.

"In raw numbers, this would translate to an additional 3.78 million acre feet of groundwater overdraft for the Phoenix area over the next 10 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.

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El Niño Study
The Climate Assessment Project for the Southwest

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