Snow drought and streamflow drought in western North America in the context of a warming climate

In western North America, snowpack supplies much of the water used for irrigation and for municipal and industrial uses, and snowmelt recharges groundwater and provides ecosystem-sustaining baseflow during low flow periods. Continued climate warming is expected to have large impacts on snowmelt hydrology, with subsequent impacts on low flows and snow and streamflow drought regimes. This research combined two separate methodologies, a data-driven (downward) approach and a process-based (upward) approach, to improve our understanding of snow drought and streamflow drought in the context of a warming climate. The data-driven approach combined observed hydroclimatic time series with multiple statistical methods, including bivariate and partial correlation and temporal and spatial analogs. The process-based approach combined climate change projections and hydrological modelling. The two approaches yielded consistent results that, together, illustrate that snow drought, low flows, and streamflow drought are sensitive to winter climate conditions, particularly precipitation and thawing degrees. In the context of climate warming, increased winter season thawing degrees leads to increased warm (temperature-driven) snow drought, shorter and less severe winter low flows, longer and more severe summer low flows, and increased summer streamflow drought risk. Further, both approaches showed that the response of snowmelt hydrology to climate warming is non-linear, and regions with winter temperatures near 0°C exhibit substantially larger impacts from +2°C of warming compared to regions with winter temperatures far below 0°C. Temperature-driven shifts in snow drought, low flows, and streamflow drought regimes will have widespread implications for surface water supply security. Increased frequency of warm snow droughts will likely lead to an increased frequency of mid-winter melt events, which will create challenges for water management. As summer low flow periods become more severe and snow-drought related summer streamflow droughts become more frequent, the potential for more severe summer water shortages increases. The most severe shortages will likely occur due to the co-occurrence of warm and dry conditions.