Dynamic soil water repellency and infiltration in post-wildfire soils

Abstract

The changing nature of soil water repellency and its apparent affect on hydrologic processes in the vadose zone are regularly reported upon, yet the fundamental mechanisms governing these changes over time and how they are manifested during infiltration are poorly understood. To address this knowledge gap, we investigate dynamic soil water repellency and report upon the concerted roles of contact angle dynamics and fractional wettability in post-wildfire materials collected approximately 1.5years post-fire. A term commonly used in the multiphase flow literature, fractional wettability describes proportional spatial variability in contact angles in bulk media. Twenty two laboratory column experiments were used to measure 1D infiltration rates and moisture contents over time. Tension infiltrometers were used to mechanically maintain a negative pressure throughout water repellent layers. Those data were analysed in conjunction with advancing contact angle data to better understand infiltration processes in these materials and the propensity for them to change with time. The hydrophobic materials expressing low fractional wettability experienced the longest delays in infiltration which we attribute to slow contact angle dynamics. Materials with greater fractional wettability expressed faster rates of contact angle change through much shorter delays in infiltration and unique infiltration rate behaviours. These findings were used to develop a conceptual model that integrates contact angle dynamics and fractional wettability and explains their combined effect on soil-water processes in complex water repellent media.