Determining the proportion of streamflow that is generated by cold season processes versus summer rainfall in Utah, USA

Study region: The state of Utah, western USA. Study focus: Analysis of annual hydrographs from most rivers in the western U.S. suggests that snowmelt provides the majority of streamflow in these areas. In this study, we attempt to quantify what proportion of Utah’s streamflow comes from cold season processes. Previous estimates have ranged from around 60%–80% of total annual streamflow, with uncertainty stemming from complexities related to rain-on-snow events, the geographic and temporal distribution of precipitation events, and the partitioning of rainfall/snowmelt into streamflow. New hydrological insights: The proportion of streamflow in the western U.S. that is snowmelt in origin is well-understood to be quite high but has rarely been explicitly quantified. Our data indicate that 98%–99% of streamflow in Utah watersheds originates from melting snow and associated processes. The contribution of summer-month precipitation to annual streamflow in Utah is extraordinarily low: typically 1%–2%. Using soil moisture and well depth data we also demonstrate that vast areas of Utah watersheds are incapable of producing event flow from summer precipitation due to consumptive losses from evapotranspiration and other factors. This analysis is likely representative of areas within other western states that have cool continental climates, abundant snowfall, long-duration snowpacks (meaning that substantial portions of the snowpack do not typically melt during the winter), and sedimentary bedrock