Abstract
Abstract. The current work evaluates the spatial and temporal variability in snow after a large forest fire in northern California using Moderate Resolution Imaging Spectroradiometer (MODIS) snow-covered area and grain size (MODSCAG). MODIS MOD10A1 fractional snow-covered area and MODSCAG fractional snow cover products are utilized to detect spatial and temporal changes in snowpack after the 2007 Moonlight Fire and an unburned basin, Grizzly Ridge, for water years (WY) 2002–2012. Estimates of canopy-adjusted and non-adjusted MODSCAG fractional snow-covered area (fSCA) are smoothed and interpolated to provide a continuous time series of average daily snow extent over the two basins. The removal of overstory canopy by wildfire exposes more snow cover; however, elemental pixel comparisons and statistical analysis show that the MOD10A1 product has a tendency to overestimate snow coverage pre-fire, muting the observed effects of wildfire. The MODSCAG algorithm better distinguishes subpixel snow coverage in forested areas and is highly correlated to soil burn severity after the fire. Annual MODSCAG fSCA estimates show statistically significant increased fSCA in the Moonlight Fire study area after the fire (P < 0.01 for WY 2008–2011) compared to pre-fire averages and the control basin. After the fire, the number of days exceeding a pre-fire high snow-cover threshold increased by 81%. Canopy reduction increases exposed viewable snow area and the amount of solar radiation that reaches the snowpack, leading to earlier basin average melt-out dates compared to the nearby unburned basin. There is also a significant increase in MODSCAG fSCA post-fire regardless of slope or burn severity. Regional snow cover change has significant implications for both short- and long-term water supply for impacted ecosystems, downstream communities, and resource managers.
Highlights
The last several decades have been marked by distinct increases in large-wildfire frequency as well as fire duration and season across the western US (Westerling et al, 2006)
Non-canopy-adjusted MODIS snow-covered area and grain size (MODSCAG) and MOD10A1 difference maps for the Moonlight Fire area show a distinct difference in fractional snow-covered area (fSCA) after the fire (Fig. 2)
An elemental pixel comparison (EPC) and linear regression of fSCA and soil burn severity shows a stronger correlation of non-canopyadjusted MODSCAG fSCA to soil burn severity (r = 0.56) than MOD10A1 fSCA (r = 0.43)
Summary
The last several decades have been marked by distinct increases in large-wildfire frequency as well as fire duration and season across the western US (Westerling et al, 2006). Soil and vegetation change after fires result in increased flooding, mass wasting, increased runoff intensities, long-term changes in energy and water budgets, and increased air pollutants (Swanson, 1981; Kattelmann et al, 1983; Stednick, 1996; Webb et al, 2012). Vegetation recovery significantly controls long-term hydrologic conditions; elevated discharge has been observed for nearly 10 years post-fire (Kinoshita and Hogue, 2011). Forest canopy considerably influences snowpack properties and snowmelt response (Faria et al, 2000). Given the dependency of the western US on snowpack and mountain runoff for water supply and the assumption of stationarity, under which water reservoir systems are designed and managed (Milly et al, 2008), minimal forest structure alterations will have critical implications for regional and state water resources and management
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