SE Valley Times

The Sun Corridor in Arizona, located in the semi-arid Southwestern U.S., is a region characterized by rapid growth and significant physical constraints. Despite its mountainous terrain, it encompasses a large valley that includes one of the fastest-growing metropolitan areas in the country. The Phoenix metropolitan area faces challenges due to prolonged drought and fluctuating seasonal water availability, making future planning essential.

A team of researchers from Arizona State University (ASU), Salt River Project (SRP), and Universidad de Santiago de Compostela in Spain are using weather modeling to better understand water availability in Arizona’s uplands and its sources. This research aims to aid future planning for the Phoenix metro area and guide predictions for upcoming precipitation seasons.

The Salt-Verde watershed, situated north to northeast of Phoenix, feeds into a series of rivers that supply approximately half of the municipal, industrial, and agricultural water needs for over 5 million residents in the Phoenix area. Unlike many other rivers in the Western U.S., these rivers are less sensitive to warming temperatures, making winter precipitation forecasts crucial.

The team's study focuses on winter precipitation within the Salt-Verde watershed. Their findings are detailed in "Modeling Salt-Verde Watershed Winter Precipitation Using Convection-Permitting WRF-Simulations with Water Vapor Tracers," published in the Journal of Geophysical Research — Atmospheres.

Using the Weather Research and Forecasting model (WRF) with an advanced refinement technique rarely applied elsewhere globally, researchers identified a surprising source of wintertime precipitation for Arizona. They discovered that sea surface temperature anomalies across an area bounded by 140 degrees west longitude and 100 degrees west longitude—excluding regions typically associated with El Niño/La Niña—are primary sources of moisture for Arizona's winter precipitation.

"We have identified the main moisture source region for wintertime precipitation across Arizona’s mountains," said Matei Georgescu, an associate professor at ASU’s School of Geographical Sciences and Urban Planning. "This is a critical step in allowing us to improve seasonal precipitation forecasts for Arizona."

Bohumil Svoma, a meteorologist at SRP and co-author of the paper, noted: "The importance of evaporation from the Pacific Ocean near North America for Salt-Verde precipitation was surprising." He added that while El Niño events usually bring wet winters to Arizona, this central Pacific moisture source proved more significant than previously thought.

Previous research led by Joseph Karanja indicated no robust relationship between standard El Niño/La Niña metrics and historical wintertime precipitation variability in the Southwestern U.S. The new study simulated different types of El Niño/La Niña winters using historical data through WRF models to examine their performance against actual rainfall data.

By masking individual geographical regions during simulations, researchers determined how much water sourced from specific areas contributed to precipitation over the Salt-Verde watershed. They found that local sea evaporation between meridians 140 degrees west and 100 degrees west off North America's West Coast played a crucial role.

"Our work sheds new light by highlighting that we need greater attention on sea surface temperatures closer to the U.S.-Baja coastline," Georgescu stated.

Francisco Salamanca-Palou emphasized: "Even during La Niña winters when tropical sea surface temperatures are colder than average, more than half of Arizona's rainfall originates from local sea evaporation."

This tool can help water managers understand storm or seasonal water sources better. While applicable elsewhere, its significance is clear for Arizona's climate projections.

"Climate model projections of winter precipitation in Arizona are highly uncertain," Svoma remarked. "Detailed research about current climate conditions is essential for advancing our understanding."

Co-authors include Joseph Karanja (ASU), James Walter (SRP), Damian Insua-Costa (Universidad de Santiago de Compostela), Gonzalo Miguez-Macho (Universidad de Santiago de Compostela). The study was funded by SRP.