16-3 Ash Mapping After Wildfire: A Geospatial Approach to Hydrologic Response

Session: Wildland Fire: An Agent of Geomorphic, Ecologic, and Societal Change

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Abstract:

The effects of wildfire ash on post-fire hydrologic response have gained recent attention, particularly as more wildfires have impacted municipal drinking water sources within forested watersheds. Soil properties can be altered post-fire via soil heating and organic matter consumption, but also through the formation of an ash crust, soil pore sealing, or changes in soil-ash chemistry. Secondary hydrologic impacts can include reduced infiltration and elevated runoff, and water quality degradation is likely if runoff, soil erosion, or ash transport occurs. We collected targeted field and Earth Observation (EO) data after wildfires to improve our capacity to predict the impacts of ash contamination on water supplies and municipal end-users. Several hundred point-scale ash cover, depth, color and spectral reflectance measurements were made across more than 20 wildfires from 2017–2024 in the western US. Geospatial layers and timescale spectral indices were assembled over each wildfire at the landscape scale with the objective of mapping ash cover and ‘load,’ the ash available for transport via wind or water. Significant predictor variables include soil burn severity, ecoregion, landcover class, canopy and understory vegetation. Aboveground biomass combined with burn severity are prerequisites for predicting ash generation. In addition to ash load, the physical and chemical characteristics of ash are crucial for evaluating hydrologic impact on water sources. Ash color is indicative of temperature reached and combustion completeness, which determines chemical composition, particle size, and bulk density. We found elevated pH, fining of ash and soil particles, and contaminants and metals including hydrocarbons, Zinc, Copper, Lead, Cadmium, Chromium, Nickel, Arsenic, and Aluminum in our ash samples. Ash persistence on site is often short (< 4 months) and is largely driven by precipitation events, leading to subsequent water degradation. Ash load and contaminant values can be input into physically based watershed models to predict ash transport and potential input into waterways. Tangible benefits from these efforts include the ability of land and water managers to apply a more calculated approach to the mitigation of potential hydrologic impacts of post-fire ash on vulnerable watersheds.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-4751

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