Hydrologic Response of Atmospheric River Events in the Salt-Verde river basins, Arizona

Prolonged droughts and population growth are of great concern in the Southwest U.S. where human and ecosystems compete for already stressed water resources. The Salt-Verde river basin in northeastern Arizona constitutes the water supply for the greater Phoenix area, the 12th largest metropolitan area in the country, for two Native American tribes that strongly rely of the basin’s natural resources for their livelihood, and for numerous wilderness areas. Summer monsoonal precipitation quickly re-evaporates and leaves the basins, consequently the region is critically dependent on winter precipitation for its water resources. Winter precipitation is modulated by Atmospheric Rivers (ARs), narrow atmospheric water vapor corridors, which bring a large percentage of winter precipitation and are responsible for 60% of the most intense precipitation events. Despite our increasing understanding of the atmospheric processes leading to AR storms in the region, the hydrologic impacts are still not well understood. This study evaluates the contribution of AR-related precipitation to winter (October through March) snow accumulation, streamflows, and soil moisture in the basins during the period 1979-2009. We identified the occurrence of 112 AR events during the 30-year period using Integrated Water Vapor Transport (IWV) estimates from MERRA. Our results show that passage of AR events over the basins lasted, in average, 21(20) hours and that contributed approximately 25% of the total seasonal precipitation. During wet winter years, as it is the case of the 1992-1993 season, that contribution reached 60%. We found a strong relationship between the occurrence of high peak flows and increases in runoff coefficients and AR events in both basins.