SOIL-HYDRAULIC PROPERTIES AND INFILTRATION TIMESCALES IN WILDFIRE-AFFECTED SOILS AND ASH

Soil-hydraulic property data were gleaned from the literature for wildfire-affected soils, including ash, and unburned soils. These data were used to calculate metrics and timescales of hydrologic response related to infiltration and surface runoff generation. Sorptivity (S) and the Green-Ampt wetting front parameter (Ψ_f) were significantly lower while field-saturated hydraulic conductivity (K_fs) was not significantly lower in burned soils compared to unburned soils. The magnitude and duration of the influence of capillarity was greatly reduced in burned soils, causing faster ponding times in response to rainfall. Ash had large values of S and K_fs, but intermediate values of Ψ_f, compared to unburned and burned soils indicating ash has long ponding times in response to rainfall. The ratio of S²/K_fs was nearly constant (~100 mm) for unburned soils but more variable in burned soils. The changes in this ratio after wildfire suggest that unburned soils have a balance between gravity and capillarity contributions to infiltration that may depend on soil organic matter while burning shifts infiltration more towards gravity contributions by reducing S. The changes in S and K_fs in burned soils act synergistically to reduce infiltration and accelerate and amplify surface runoff generation. Synthesis of these findings identifies areas for future research. First, short timescales of capillary influences on infiltration indicate that better measurements of infiltration at times less than 1 minute are needed to accurately characterize S in burned soils. Second, using parameter values, such as Ψ_f, from unburned areas could produce substantial errors in hydrologic modeling when used without adjustment for wildfire effects, causing parameter compensation and underestimation of K_fs. Third, more thorough post-wildfire measurement campaigns that capture soil-structural changes, organic matter impacts, quantitative water repellency effects, and soil-water content, along with soil-hydraulic properties, could drive the development of better techniques for numerically simulating infiltration into soil and ash in burned areas.