Evaluating the Persistence of Post-Wildfire Ash: A Multi-Platform Spatiotemporal Analysis

Abstract

As wildland fires amplify in size in many regions in the western USA, land and water managers are increasingly concerned about the deleterious effects on drinking water supplies. Consequences of severe wildfires include disturbed soils and areas of thick ash cover, which raises the concern of the risk of water contamination via ash. The persistence of ash cover and depth were monitored for up to 90 days post-fire at nearly 100 plots distributed between two wildfires in Idaho and Washington, USA. Our goal was to determine the most ‘cost’ effective, operational method of mapping post-wildfire ash cover in terms of financial, data volume, time, and processing costs. Field measurements were coupled with multi-platform satellite and aerial imagery collected during the same time span. The image types spanned the spatial resolution of 30 m to sub-meter (Landsat-8, Sentinel-2, WorldView-2, and a drone), while the spectral resolution spanned visible through SWIR (short-wave infrared) bands, and they were all collected at various time scales. We that found several common vegetation and post-fire spectral indices were correlated with ash cover (r = 0.6–0.85); however, the blue normalized difference vegetation index (BNDVI) with monthly Sentinel-2 imagery was especially well-suited for monitoring the change in ash cover during its ephemeral period. A map of the ash cover can be used to estimate the ash load, which can then be used as an input into a hydrologic model predicting ash transport and fate, helping to ultimately improve our ability to predict impacts on downstream water resources.

Chemical Characteristics of Wildfire Ash Across the Globe and Their Environmental and Socio-Economic Implications

“…The only significant correlation observed by the authors among any of the analysed factors, both organic and inorganic contaminants, was with pH. In addition, high burn severity often leads to higher ash loads (e.g., Lewis et al, 2021;Santín et al, 2015b) and both, the chemical characteristics and the ash loads, need to be considered when determining ash environmental contamination potential (Santín et al 2015b).…”

Evaluating a New Relative Phenological Correction and the Effect of Sentinel-Based Earth Engine Compositing Approaches to Map Fire Severity and Burned Area

“…Because of this variety of approaches, there is a need for a formal evaluation of the effect of compositing techniques (i.e., average or minimum) and compositing periods on their accuracy in mapping fire perimeters and fire severity. Such an evaluation is particularly lacking for immediate postfire assessments of fire impacts (hereafter termed "fire severity" following [1], [2] or [54], which propose the use of this term for initial severity assessments), which are fundamental for post-fire erosion emergency planning (e.g., [8,9,56]) but have received less attention from compositing analysis studies.…”

The Accuracy of Gililawa Darat Wildfire Spread Estimation Using Burn Severity and WRF-Sfire Model

“…Although, EVI as a modified version of NDVI that has been adapted for soil color and aerosol scattering and has improved sensitivity to areas which are rich in biomass (Ellsworth, 2012) could not outperformed NDVI due to the dry condition of the region. In addition, gray, which belongs to the blue spectral range, can be reflected from ash and char (Lewis et al, 2021) also could not be identified due to very little grey ash in the burned area.…”