Winter inputs buffer streamflow sensitivity to snowpack losses in the Salt River Watershed in Lower Colorado River Basin

Authors: ROBLES, M. - Nature Conservancy; HAMMOND, J. - Colorado State University; KAMPF, S. - Colorado State University; Biederman, Joel; Demaria, Eleonora

Submitted to: Water
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/17/2020
Publication Date: 12/22/2020
Citation: Robles, M., Hammond, J., Kampf, S., Biederman, J.A., Demaria, E.M. 2020. Winter inputs buffer streamflow sensitivity to snowpack losses in the Salt River Watershed in Lower Colorado River Basin. Water. 13(3). https://doi.org/10.3390/w13010003.
DOI: https://doi.org/10.3390/w13010003

Interpretive Summary: Over 40 million people rely on water from the Colorado River, especially for food production. Recent studies have suggested that warming temperatures since the 1980s have reduced Colorado River streamflow, likely due to increased evaporation of snow during the winter. However, it isn’t clear whether this same warming-driven reduction in streamflow occurs in other major rivers of the western US. In this study, we evaluated how warming has impacted the streamflow of the Salt River Basin in Arizona. We found that warmer winter temperatures were associated with more rainfall at the expense of snow, but that warming did not appear to decrease streamflow. This is likely because in winter months, rainfall can generate streamflow more effectively, because plant roots are not taking up water and evaporation rates are low. These results suggest that in the Southwest US, warming-driven increases in rain may be beneficial for water resources, but the accompanying decreases in snowpack will likely have negative effects on forest health.

Technical Abstract: Studies that associate recent declines in Colorado River flow with warmer temperatures have received considerable attention because 40 million people rely on water from the Colorado River basin. However, in other western US river basins, significant warming has not consistently led to declines in streamflow despite losses in snow storage, perhaps due to greater hydrological inputs from winter rain. In winter months, rainfall can potentially generate streamflow more efficiently than snowmelt because flow production occurs before peak sublimation and evapotranspiration losses. The Salt River basin in central Arizona is a promising location to explore this issue because it has a mixed rain and snow climatic regime that is influenced by large individual events. We examined whether warming has led to changes in precipitation phase and how that influenced streamflow patterns at the annual and event-scales in the Salt River basin and its gauged sub-basins from 1966-2011. We developed a basin-scale model that separates snow and rainfall inputs and simulates snow accumulation and melt using temperature, precipitation, and relative humidity. Despite significant warming and simulated snow water equivalent (SWE) losses, annual streamflow did not decline significantly in most sub-basins. Multiple linear regression models showed that Dec-Feb rain-on-snow when combined with peak SWE most consistently explained flow variability, and that flow generation from rain-on-snow inputs was 2-fold greater than SWE. At the event-scale, streamflow was significantly higher in winter and fall versus summer seasons, and 40-70% of the largest cool-season events overlapped with atmospheric rivers. Moreover, magnitudes of winter and fall events were significantly higher from 1966-2011 than 1926-1965. We conclude that SWE losses from warming in the Salt River basin are ameliorated by winter rain-on-snow contributions. With continued warming and SWE losses, mean annual flows may decline, but inter-annual flow variability may increase due to intensifying winter rain-on-snow events.