The mechanisms by which a precipitation shift from snow to rain affects streamflow in the contiguous US: large sample data analysis

Climate warming has caused in a significant decrease for the snowpack, and earlier snowmelt onsets for many mountainous regions like the western United States. Changes in seasonal streamflow patterns have been observed, and these are mainly caused by those changes in snowpack. However, there are also changes in annual streamflow, which are associated with snow processes, but are not yet explained. To explore potential causes of a previously-recognized pattern of reduction in annual streamflow in snow-dominated catchments, we analyzed the sensitivity of seasonal streamflow to the average annual snow fraction in 253 snow-dominated catchments in the continental US. We find that places (and years) with higher mean annual snow fraction tend to have higher seasonal streamflow (mainly in spring, summer, and winter). We found that 91% (spring), 57% (summer), 22% (autumn) and 51% (winter) of these catchments showed a positive sensitivity of streamflow to snow fraction.

In order to explore the potential hydrological processes by which a change in snow fraction affects the seasonal streamflow for different places, we summarized four potential mechanisms according to a literature review. The mechanisms are described as (i) inputs exceed threshold, (ii) demand-storage competition, (iii) water-energy synchrony, and (iv) energy partitioning. We considered the effects that each mechanism would be expected to have on the seasonal runoff, and then associated this to the observed changes. How the snow change affects season runoff is a mixed combination of the four mechanisms for catchments in the west part of the continental US. The first and second mechanisms could be applied to explain the changes for catchments in the central part of the continental US. There are some catchments where we could only use the third mechanism to explain the changes we observed, and a few catchments could only use the third mechanism or the fourth mechanism to explain their changes, and these catchments are scattered around the continental US. These findings could provide evidence for how changes in snow affect hydrology, which may help society to better understand the effect of climate warming and help support water management for places which rely on snowmelt water.