Post-fire Impacts on Spring-flow and Low-flow Hydrology in Snow-dominated Watersheds

Seasonal snow is a critical water resource for the Western US; however, in recent years, not only has climate change made snowpacks vulnerable and streamflow increasingly more difficult to forecast, but forest fires have increased in size, frequency, severity, and elevation. Previous work has shown that post-fire conditions affect snow water equivalent (SWE) and lead to earlier snow disappearance in burned watersheds. Nevertheless, it is paramount for the management of water resources to understand how fire impacts seasonal snow and if these impacts translate to changes in streamflow. In this work, we investigated post-fire streamflow timing and magnitude of high and low flows. We analyzed streamflow patterns in burned and unburned basins which were classified as snow-dominated, rain-dominated, and mixed watersheds from 2000-2020. The study area is the western United States. To quantify post-fire changes in streamflow, we used three hydrologic metrics: (1) the change of magnitude of summer and winter low flows, (2) the date of spring melt onset (freshet), and (3) the slope of the spring freshet limb of the hydrograph. The snow-dominated burned watersheds experienced earlier spring freshet compared with rain-dominated or mixed watersheds and unburned, control watersheds. As the burned area increases into the seasonal snow zone, the steeper and flashier the rising limb of the hydrograph. Post-fire changes to spring streamflow are amplified by the percentage of the watershed that was burned and the fraction of burn in the seasonal snow zone. Post-fire low-flow magnitude increases in snow-dominated watersheds, whereas rain-dominated watersheds do not show a clear change of low-flows. Our results point to the need to consider fire impacts on seasonally snow-covered watersheds in water management - especially as both fire and seasonal snowpacks are experiencing long-term changes due to climate change.