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Widening hydrograph spread with declines in snow across the western United States is modulated by sub-basin heterogeneity
Bazlen, Kyla A.
Bazlen, Kyla A.
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2025
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Abstract
Mountain snowpacks are a critical source of water, supplying the majority of runoff in the western U.S. Seasonal snowpacks act as natural water towers, storing water during the winter and releasing it during the spring and summer when demand is highest. The quantity, timing, and distribution of streamflow throughout the water year all determine water availability for both ecosystems and human use. However, declining snow accumulation and shifting melt dynamics are changing hydrograph characteristics and affecting water availability in ways that are not yet fully understood. In particular, changes to the width of the streamflow distribution can affect water resource allocation, but have only been minimally investigated. This study investigates the relationship between snow water equivalent (SWE), streamflow center of timing (CT), and the width of the streamflow distribution, operationalized as the standard deviation of timing (SDoT) in snow-dominated basins across the western United States. Furthermore, I examine how hydrologic partitioning and spatially heterogenous responses to warming control SDoT and CT using the East River, CO as a case study basin. Using empirical analysis of gauged streamflow from 95 minimally disturbed basins and a spatially distributed SWE reanalysis product, I evaluate the relationship between SWE and SDoT, the role of basin climate in this relationship, and the performance of statistical models under different snow drought conditions. I then apply a physics-based hydrologic model to disentangle shifts in the distribution of individual runoff generation components with warming and assess how these changes are reflected in the channel. At 85% of gages across the western United States, low peak SWE is associated with a wider SDoT, with the strongest effect observed in basins with low precipitation. In the East River, a simulated 2ºC of warming induces changes in CT and SDoT for groundwater, interflow, and surface runoff, but interannual and spatial responses are highly variable. At the basin outlet, the channel exhibits less than one day change in SDoT. This stability results from opposing responses to warming: SDoT widens in the cases of surface water, interflow, and low elevation groundwater SDoT, but narrows for groundwater contributions from high elevation HRUs, which produce the majority of streamflow and counterbalance declining SDoT from other sources. Overall, results indicate that declining peak SWE may alter streamflow distributions and water availability, and that the degree of future impacts will depend on basin-specific hydrologic processes.
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