Thermal effects on soil water repellency: experimental evidence and relevance for post-fire slope stability

Abstract

Wildfires are among the most impactful natural phenomena affecting ecosystems, with a concerning increased frequency driven by climate change and human activity. Beyond the immediate destruction of vegetation, wildfires trigger a series of cascading effects within the slopes, significantly modifying shallow processes and compromising their stability. Wildfires significantly alter the hydrological and geo-mechanical properties of soils, often increasing the susceptibility to shallow landslides and debris flows. Several authors have reported that the development of soil water repellency plays a crucial role in controlling post-fire infiltration dynamics and runoff generation. However, depending on vegetation coverage and organic matter content, soil water repellency may already characterize slopes prior to wildfires. Laboratory studies have shown that the onset of soil water repellency is strongly influenced by granulometry, organic matter content and temperature rise, whereas its breakdown occurs at higher temperatures. Yet, the direct contribution of temperature-induced changes in soil water repellency to slope failure processes remains unclear. The present study investigates how temperature influences the development and breakdown of soil water repellency, and how this property influences soil infiltration capacity. Controlled thermal treatments were conducted on soil samples in laboratory, coupled with the assessment of the hydrophobic behaviour through Water Drop Penetration Time (WDPT) tests and the evaluation of the hydraulic conductivity with a permeameter after heating. Results reveal a temperature dependent response only in samples containing organic matter. Differences in thermal conductivity among the tested materials produced heterogeneous responses to heating, suggesting a non-uniform alteration even after prolonged exposure to elevated temperatures. These findings indicate that temperature exerts a complex control on soil wettability and, consequently, on post-fire infiltration behaviour.