  This note presents a comprehensive characterization of physical and
mechanical properties of water repellent (hydrophobic) soil collected from
Cleveland National Forest in California immediately after the Holy Fire, 2018,
and delineates comparisons with chemically induced hydrophobic sand in the
laboratory. Hydrophobicity is a particle surface characteristic that governs
different levels of attraction between water molecules and solid particles.
Wildfires can cause different levels of hydrophobicity in shallow soil layers
based on fire severity, vegetation, and chemical structure of the soil. Natural
and chemically induced regular and hydrophobic sands are characterized by grain
size distribution, water retention curve, water contact angle and electron
microscopic imaging, including the relationship between water entry value and
the drop contact angle in hydrophobic soil. Comparative knowledge of natural
and chemically induced hydrophobic soil properties will help future research to
better predict soil behavior and improve insights into post-wildfire soil
erosion and mudflow mechanisms. This note contributes to a database of
wildfire-induced hydrophobic soil with detailed properties and assesses the
applicability of laboratory made hydrophobic soils for studying mudflows by
comparison to the natural water repellent soil collected from the burned site.
