Wildfire impacts on western United States snowpacks

Abstract

Mountain snowpacks provide 53–78% of water used for irrigation, municipalities, and industrial consumption in the western United States. Snowpacks serve as natural reservoirs during the winter months and play an essential role in water storage for human consumption and ecosystem functions. However, wildfires across the West are increasing in severity, size, and frequency, progressively putting snowpacks at risk as they burn further into the seasonal snow zone. Following a fire, snow disappears 4–23 days earlier and melt rates increase by up to 57%. In a high burn severity fire in the Oregon Cascades, the black carbon and charred woody debris shed from burned trees onto the snowpack decreased the snow albedo by 40%. Canopy cover loss causes a 60% increase in solar radiation reaching the snow surface. Together, these effects produce a 200% increase in net shortwave radiation absorbed by the snowpack. This mini-review synthesizes the implications of wildfire for snow hydrology in mountainous watersheds with the primary aim to characterize wildfires' varied influences on the volume and timing of water resources across time scales (daily to decadal), space (plot to watershed) and burn severity (low to high). The increase in the geographical overlap between fire and snow poses unique challenges for managing snow-dominated watersheds and highlights deficiencies in research and operational snow hydrologic modeling, emphasizing the need for additional field and remote-sensing observations and model experiments.

Midwinter Dry Spells Amplify Post‐Fire Snowpack Decline

“…Unlike dust, which has primary absorption in the visible wavelengths (He et al., 2019), black carbon is a “gray” absorber and can absorb throughout the solar spectrum. We translated our broadband snow albedo measurements to net shortwave radiation using the Beer‐Lambert Law (Hellström, 2000; Monsi & Saeki, 1953) following Koshkin (2022) and Landsat‐based estimates of leaf area index (LAI; a proxy for vegetation canopy) in the Caldor Fire perimeter. With a 70% reduction in Caldor LAI (Figure S1 in Supporting Information ), post‐fire net shortwave radiation increased from 6.12 W m −2 (unburned) to 67.68 W m −2 (high severity) during the MWDS.…”

“…Dust deposition could further accelerate wildfire‐induced midwinter melt (Huang et al., 2022). Similar to dust‐on‐snow (Skiles & Painter, 2019), in post‐fire environments, radiative forcing‐induced positive feedbacks likely occur between grain size growth, albedo decline from melt‐driven LAP accumulation, and larger‐scale albedo decline as the land surface becomes snow‐free (Huang et al., 2022; Koshkin, Hatchett, & Nolin, 2022; Sterle et al., 2013; Warren, 1982). Despite some of the lowest midwinter albedos on record, further investigation of why our remotely sensed WY2022 albedo values did not decline to values as low as measured in‐situ are warranted.…”

“…Decreases in both albedo (Figures 2b–2d) and canopy (Figure S1 in Supporting Information ) increase the net shortwave radiation and accelerate snowmelt during MWDS (Figure 3c; Gleason et al., 2013). Additional radiation reduces snow covered area (Figure 4c) directly and indirectly through positive feedbacks (Koshkin, Hatchett, & Nolin, 2022). Similar results associated with local and long‐range transport and deposition of fire‐generated LAPs occurs in seasonally snow‐covered regions (Gleason et al., 2019; Uecker et al., 2020) and glacial environments (Aubry‐Wake et al., 2022).…”

“…The MODIS data is available from the Zenodo repository: (Rittger & Hatchett, 2023; https://doi.org/10.5281/zenodo.7522988). Spectrometer data is available from the Zenodo repository (Koshkin, Nolin, & Hatchett 2022; https://doi.org/10.5281/zenodo.7545408).…”

Midwinter dry spells amplify post-fire snowpack decline

“…Unlike dust, which has primary absorption in the visible wavelengths (He et al, 2019), black carbon is a "gray" absorber and can absorb throughout the solar spectrum. We translated our broadband snow albedo measurements to net shortwave radiation using the Beer-Lambert Law (Hellström, 2000;Monsi & Saeki, 1953) following Koshkin (2022) and Landsat-based estimates of leaf area index (LAI; a proxy for vegetation canopy) in the Caldor Fire perimeter. With a 70% reduction in Caldor LAI (Figure S1 in Supporting Information S1), post-fire net shortwave radiation increased from 6.12 W m −2 (unburned) to 67.68 W m −2 (high severity) during the MWDS.…”