Computer modeling shows where Arizona's winter precipitation originates

The Sun Corridor in Arizona in the semi-arid Southwestern U.S. is a land of seeming unlimited growth that is constantly colliding with physical constraints. It is mountainous but also home to a large valley that includes one of the fastest growing metropolitan areas in the U.S.

While experiencing explosive growth, the Phoenix metropolitan area faces an uncertain future due to prolonged drought and fluctuating seasonal water availability. Planning for the future, especially in terms of water, has long been both a challenge and a staple of life in Arizona.

Now a team of researchers working with a weather model have uncovered new details of water availability in Arizona's uplands and where that water comes from. The work may help in future planning of the Phoenix metropolitan area, as well as provide guides for coming precipitation seasons.

The Arizona uplands is home to the Salt-Verde watershed, which feeds the Arizona reservoir system and helps provide critical water resources needed to support the 5 million plus residents and businesses of Phoenix. The Salt-Verde watershed, located north-northeast of Phoenix, feeds a series of rivers in Arizona that provide about half of the municipal, industrial and agricultural water supply to the Phoenix metro area.

The rivers in this watershed are less sensitive to warming temperatures than many other important rivers in the western U.S. Therefore, warming temperatures are not an effective predictor of future Salt-Verde streamflow making the future of winter precipitation extremely important to understand.

The new work—carried out by a team of researchers from Arizona State University; Salt River Project (SRP), an Arizona utility; and Universidad de Santiago de Compostela, Spain—focuses on the precipitation in the Salt-Verde watershed during the winter months. It represents a new approach to improved precipitation predictability for Arizona and the Salt-Verde watershed.

Results of the work are reported in a paper titled "Modeling Salt-Verde Watershed Winter Precipitation Using Convection-Permitting WRF-Simulations with Water Vapor Tracers" in the Journal of Geophysical Research: Atmospheres.

The researchers applied a model (the Weather Research and Forecasting model, or WRF) with an important refinement that has been used in only a few locations elsewhere in the world to improve understanding of precipitation predictability. This new work pinpoints where wintertime precipitation for Arizona comes.

One surprising finding is that it is not the area normally associated with El Niño/La Niña, (equatorial tropical Pacific). The team found that the sea surface temperature across an area bounded by 140°W and 100°W, excluding the El Niño/La Niña areas, is the dominant source area of water for wintertime precipitation for Arizona.

Moreover, this work indicates that if sea surface temperature anomalies, or deviation from an average value of this newly pinpointed region is known, then that information can be used as a guide to wintertime precipitation forecasting for Arizona.