Growing impact of wildfire on western US water supply

Abstract

Streamflow often increases after fire, but the persistence of this effect and its importance to present and future regional water resources are unclear. This paper addresses these knowledge gaps for the western United States (WUS), where annual forest fire area increased by more than 1,100% during 1984 to 2020. Among 72 forested basins across the WUS that burned between 1984 and 2019, the multibasin mean streamflow was significantly elevated by 0.19 SDs (P < 0.01) for an average of 6 water years postfire, compared to the range of results expected from climate alone. Significance is assessed by comparing prefire and postfire streamflow responses to climate and also to streamflow among 107 control basins that experienced little to no wildfire during the study period. The streamflow response scales with fire extent: among the 29 basins where >20% of forest area burned in a year, streamflow over the first 6 water years postfire increased by a multibasin average of 0.38 SDs, or 30%. Postfire streamflow increases were significant in all four seasons. Historical fire–climate relationships combined with climate model projections suggest that 2021 to 2050 will see repeated years when climate is more fire-conducive than in 2020, the year currently holding the modern record for WUS forest area burned. These findings center on relatively small, minimally managed basins, but our results suggest that burned areas will grow enough over the next 3 decades to enhance streamflow at regional scales. Wildfire is an emerging driver of runoff change that will increasingly alter climate impacts on water supplies and runoff-related risks.

Wildfire impacts on western United States snowpacks

“…Concurrent with declining snowpacks, wildfire characteristics are changing. From 1984 to 2020, area burned across the western U.S. increased 1,150%, and in 2020 alone, nearly 6% of California's forested area burned (Williams et al, 2022). From 1984 to 2011, the top 10% of fires by size in California increased by 2 km 2 yr −1 without a significant increase in the number of fires (Dennison et al, 2014).…”

Spruce Beetle Outbreak Increases Streamflow From Snow‐Dominated Basins in Southwest Colorado, USA

“…As the fraction of Western forests affected by bark beetles and other disturbances grow and increasingly influence hydrology (Williams et al., 2022), understanding variability in hydrologic response across basins and ecosystems will become increasingly important for streamflow forecasting, adaptative management, and water resource allocations. Our findings suggest the importance of both snow‐related and growing season ET processes to postbeetle streamflow; however, answering process‐related questions and diagnosing the effects of basin characteristics will require continued research.…”

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Disturbance of mountain forests, which supply critical streamflow to downstream water users, remains an active area of research (NRC, 2008) with substantial implications on large-scale water availability and carbon budgets (Williams et al, 2016(Williams et al, , 2022Zhang et al, 2017). Recent insect outbreaks have caused widespread forest mortality in snow-dominated basins in the Western US (Goeking & Tarboton, 2020;Meddens et al, 2012).

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Global premature mortality by dust and pollution PM2.5 estimated from aerosol reanalysis of the modern-era retrospective analysis for research and applications, version 2

“…Although PM 2.5 shows similar spatial patterns between 2019 and 2020, differences in the magnitude are evident in some regions. Notably, PM 2.5 in the American West was significantly higher in 2020 than 2019, presumably due to the record-breaking wildfires in 2020 (Williams et al, 2022). In both years, the PM 2.5 often exceeded 25 μg m -3 in the dust belt (North Africa and Middle East), India, and eastern China, implying a tremendous benefit of achieving the WHO AQG in these dusty and polluted regions.…”