Observed Hydrologic Impacts of Landfalling Atmospheric Rivers in the Salt and Verde River Basins of Arizona, United States

Atmospheric rivers (ARs), narrow atmospheric water vapor corridors, can contribute substantially to winter precipitation in the semiarid Southwest U.S., where natural ecosystems and humans compete for over-allocated water resources. We investigate the hydrologic impacts of 122 ARs that occurred in the Salt and Verde river basins in northeastern Arizona during the cold seasons from 1979 to 2009. We focus on the relationship between precipitation, snow water equivalent (SWE), soil moisture, and extreme flooding. During the cold season (October through March) ARs contribute an average of 25%/29% of total seasonal precipitation for the Salt/Verde river basins, respectively. However, they contribute disproportionately to total heavy precipitation and account for 64%/72% of extreme total daily precipitation (exceeding the 98th percentile). Excess precipitation during AR occurrences contributes to snow accumulation; on the other hand, warmer than normal temperatures during AR landfallings are linked to rain-on-snow processes, an increase in the basins' area contributing to runoff generation, and higher melting lines. Although not all AR events are linked to extreme flooding in the basins, they do account for larger runoff coefficients. On average, ARs generate 43% of the annual maximum flows for the period studied, with 25% of the events exceeding the 10 year return period. Our analysis shows that the devastating 1993 flooding event in the region was caused by AR events. These results illustrate the importance of AR activity on the hydrology of inland semiarid regions: ARs are critical for water resources, but they can also lead to extreme flooding that affects infrastructure and human activities. Plain Language Summary Within the Salt and Verde River basins in the semiarid northeastern Arizona, natural ecosystems and humans compete for over-allocated water resources. The basins are highly dependent on winter precipitation for ecosystem functioning, water supply, fire suppression, and in the case of extreme events, for water quality, flood control and dam safety. Atmospheric Rivers (ARs) are narrow corridors of concentrated water vapor that bring copious amount of rainfall from the Pacific into the region and contribute to its hydrology by increasing snow pack and replenishing soil moisture in the basins. ARs are also responsible for extreme precipitation and streamflow events, such as the 1993 flooding event, which caused human casualties and economic losses. These results illustrate the importance of AR activity on the hydrology of inland semiarid regions: ARs are critical for maintaining natural ecosystem health, but they can also lead to extreme flooding that affects infrastructure and human activities.