Classifying Moisture Transport during Large Precipitation Events in the Western United States

Recent studies have revealed that total annual streamflow in several watersheds across the western United States is largely driven by the occurrence of a few large precipitation events each year. In mountain snowpack-sourced river basins, the effects of these events, often observed in localized headwater subbasins, can make the difference in a year resulting in well above or below average streamflow basin-wide. With the growing research interests in the roles atmospheric rivers contribute to hydrology, particularly among coastal watersheds, a similar level of study among continental watersheds may offer applicable insights for a variety of interests, including water supply forecasting.

In this study, a synoptic classification of integrated vapor transport is performed relative to days that resulted in large precipitation events (LPEs) observed in subbasins across the Upper Colorado River Basin. The classification reveals a latitudinal trend in moisture advection during LPEs, with southwesterly flow prevailing among LPEs observed in southern subbasins and westerly flow common among LPEs in the northern subbasins. These preferences between subbasins are corroborated through storm trajectory analyses and exhibit significant correlations to streamflow, suggesting potential links that warrant further investigation. These results, if reliable in a changing climate, could inform actionable forecast improvements from streamflow model adjustments to lowering uncertainties in longer-term seasonal projections.