Wildfires Impacts on Spring Flows and Summer Low Flows Across the Western United States
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Authors
Van Dusen, Hannah
Issue Date
2023
Type
Thesis
Language
Keywords
Alternative Title
Abstract
Forested watersheds supply essential drinking water to much of the western United States. In recent years, forest fires have increased in size, frequency, and severity, and have climbed to higher elevations into the seasonal snow zone. Most previous work focused on annual scales of hydrologic response after a forest fire and indicated increased water yields. However, effective water management requires understanding seasonal flow patterns, so it is critical to understand how forest fires impact both seasonal high and low flows. I investigated post-fire changes in high and low flows in watersheds across the western U.S. Using multiple linear regression to account for interannual climate variability, I quantified post-fire changes in the 7-day summer low flow, the 7-day spring peak flow, and the timing of the freshet. This study includes 95 watersheds that were burned between 2000 to 2020 and 156 unburned control watersheds. Watersheds were disaggregated based on aridity, percent burned area, and snowmelt contribution. Our results show significant increases in summer low flows and spring peak flows for the first five years after a fire. The post-fire freshet occurs 7 days earlier in snow-dominated watersheds compared with unburned watersheds. “Wet watersheds,” where mean annual PET/P < 1, have a significant post-fire residual increase in summer low flows whereas “dry watersheds,” where mean annual PET/P > 1, do not show any significant post-fire increases. Snow-dominated watersheds have a significant increase in summer low flows for at least the first six years after a fire whereas rain-dominated watersheds only show a significant increase for the first two years after a fire. Post-fire changes to summer low flows are amplified by the percentage of the watershed burned, especially in snow-dominated watersheds. Disaggregating watersheds by aridity, percent basin burned, and snowmelt contribution is crucial for understanding post-fire hydrology on a regional scale. For effective water management, we must not only understand how extremes of the hydrographs change after fires, but also how they vary across watersheds.