Changes in Snow Hydrology After a Severe Wildfire
Loading...
Authors
Koshkin, Arielle Levin
Issue Date
2022
Type
Thesis
Language
Keywords
Fire , Forests , Snow , Water
Alternative Title
Abstract
Most freshwater in the western United States used for agriculture, municipalities, and industrial consumption originates from headwaters in snow-dominated watersheds. During the winter months, snowpacks serve as natural water towers providing flood control, and during the spring snowmelt is a major contributor to reservoirs for agricultural and municipal water supplies, and hydroelectric power generation. However, wildfires are increasing in intensity, size, frequency, and duration in snow-dominated watersheds and are burning higher in elevation, well into the seasonal snow zone. After a wildfire, the burned trees shed black carbon and charred woody debris which decreases the snow albedo and increases snowmelt rates resulting in an earlier snow disappearance date. The loss of canopy cover causes an increase in solar radiation reaching the snow surface and together these effects lead to an increase in net shortwave radiation. The increase in the geographical overlap between fire and snow poses unique and emerging challenges for managing snow-dominated watersheds. This study investigates the post-wildfire impacts of the Creek Fire that burned nearly 44% of the Upper San Joaquin Watershed (USJW) in the south-central Sierra Nevada in 2020. The goal of this research is to quantify and understand post-fire impacts on snowpack ablation as a function of burn severity in a snow-dominated watershed. Using three spatial scales, ground-based, airborne, and satellite data, we measured a diverse set of snowpack properties along a burn severity gradient (unburned forest, moderate burn severity, and high burn severity). Snow depth was derived from lidar from multiple overflights of the Airborne Snow Observatory (ASO). Concurrent with the ASO overflights, we collected ground-based measurements of snow depth, SWE, snow albedo, and black carbon concentrations. Burn severity was computed from Landsat 8 data using the Differenced Normalized Burn Ratio. Results highlight that the nuance of burn severity matters when understanding the impacts of fire on snow. Higher burn severity is associated with higher black carbon concentrations, lower albedo, increased net shortwave radiation, and subsequently, lower snow depths during ablation and earlier snow disappearance compared to moderate burn severity and unburned areas. These impacts are seen most notably at mid-elevations (1500 -2500 m). To understand the impacts of wildfires on snow hydrology and water resources, regional and broad-scale variability of these impacts on snow-water storage and snowmelt timing needs to be taken into account to improve water resource management and forecasting models. As fires become more prevalent, water resources from snowpacks will become more threatened, especially in high severity burned areas. It is important that runoff forecast models reflect the post-fire changes in alpine watersheds.