Researchers Unlock Key to Winter Precipitation Origins, Aiding Future Water Planning

A breakthrough by researchers led by Arizona State University has pinpointed the main moisture sources for Arizona’s winter precipitation, reshaping water management strategies for the growing Phoenix metropolitan area amid a prolonged drought.

In a groundbreaking study, researchers from Arizona State University, in collaboration with Salt River Project (SRP) and Universidad de Santiago de Compostela, Spain, have uncovered new insights into the origins of winter precipitation in Arizona. This discovery holds significant potential for improving water management and planning in the rapidly growing Phoenix metropolitan area.

The Phoenix area, one of the fastest-growing metropolises in the United States, relies heavily on the Salt-Verde watershed. This watershed is crucial for supplying nearly half of the region’s municipal, industrial and agricultural water. Understanding the sources of winter precipitation that feeds this watershed is vital for future water security, especially amid prolonged drought conditions and the challenges posed by fluctuating seasonal water availability.

Using the Weather Research and Forecasting (WRF) model, researchers identified the main regions that contribute to Arizona’s wintertime precipitation. Contrary to common assumptions, the dominant source area is not the equatorial tropical Pacific regions typically associated with El Niño and La Niña. Instead, the critical moisture source lies in a specific area between 140°W and 100°W off the North American West Coast and south of 40°N.

“We have identified the main moisture source region for wintertime precipitation across Arizona’s mountains,” Matei (Matt) Georgescu, an associate professor at ASU’s School of Geographical Sciences and Urban Planning and director of the Urban Climate Research Center, said in a news release. “This is a critical step in allowing us to improve seasonal precipitation forecasts for Arizona and potentially other regions across the globe as well.”

The research provides a fresh perspective by challenging the traditional focus on El Niño/La Niña regions, leading to more accurate seasonal precipitation forecasts.

Bohumil (Bo) Svoma, a meteorologist at SRP and co-author of the paper, emphasized the importance of this finding. “The importance of evaporation from the Pacific Ocean near North America for Salt-Verde precipitation was surprising,” he said. “Since El Niño events tend to result in wet winters for Arizona, it was surprising that this central Pacific moisture source is not as important for Arizona precipitation.”

The study’s innovative approach involved reconstructing historical precipitation patterns using the WRF model across three types of El Niño/La Niña winters. The simulations validated the model’s accuracy by matching historical rainfall data, revealing a consistent pattern where warmer winters yielded more precipitation, and cooler winters resulted in less.

Francisco Salamanca-Palou, the study’s lead author and an assistant research professor of geographical sciences and urban planning at ASU, noted the broader implications of this work.

“Even for the cold (La Niña) winter, where tropical sea surface temperatures are colder than average, more than half of the water that fell over Arizona originates from what we define as local sea evaporation, which we bound as between meridians 140° W and 100° W off the North American West Coast and south of 40° N,” she said.

The research team’s findings have significant implications for water resource management in Arizona. By pinpointing the primary sources of winter precipitation, water managers can better understand and predict water availability, aiding in more effective long-term planning.

“This research is critical for long-term water supply planning in Arizona,” Svoma added.