Water Vapor Sorption Behavior of Wildfire-Burnt Soil

Abstract

Wildfires and associated wetting-induced slope stability issues (i.e., erosion, shallow landslides, and debris flows) are common problems all around the world. The water-retention mechanism of the burnt soil after a severe wildfire is adsorption followed by capillary condensation as saturation increases. During this time, soil is more susceptible to runoff-dominated erosion and associated debris flows. The water vapor sorption behavior of wildfire-burnt soil and wildfire ash is not fully known. This study investigates the evolution of water vapor sorption behavior of wildfire-burnt soil over a year and the impact of wildfire ash on the sorption behavior of burnt soil. Soil samples were collected from the surface and from 50-cm depth, and ash samples were collected from the surface at varying times after the 2019 Williams Flats Wildfire in Colville Indian Reservation, Washington State. Soil water retention curves of the surface soil and 50-cm soil were measured using a potentiometer. Hysteretic water vapor sorption isotherms were obtained along adsorption and desorption paths using a dynamic water vapor sorption analyzer. Several different parameters including maximum adsorbed water content, degree of hysteresis, specific surface area, and transition relative humidity were calculated from water vapor sorption isotherms and used to evaluate the sorption behavior of wildfire-burnt soil and wildfire ash. The results indicate that (1) wildfire ash is hydrophilic, has an active surface, and contributes to water retention; and (2) spatial redistribution of ash may result in fluctuations in the water retention of burnt soil over time.