Pore Scale Connections, Contributions, and Mechanistic Controls on Infiltration in Water Repellent Soils.

At variable scales, fire can both remove and induce repellency in soil materials. Consequently, fractions and regions of wettable and non-wettable soil can coexist at equally variable scales, according the nature of burning and the distribution of repellent constituents horizontally, vertically, and temporally. Fractional wettability, or the spatial heterogeneity of repellency, is commonly used in the multiphase flow literature and describes pore scale differences in the wettability of surfaces. Its applicability as a concept in the characterization of repellency and interpretation of larger scale infiltration processes, has however, been touched upon only marginally heretofore. As it relates to soil water repellency in post-fire soils, there is an opportunity to utilize it conceptually in the interpretation of water repellency data such that greater insight can be gained on the contribution and effect of variably repellent fractions in overall system responses during infiltration events and larger scale erosional/depositional processes. This work investigates the wettability of ash/charred near-surface duff materials and attempts to gain insight into how mm-scale observations of differences in fractional wettability at the near-surface can manifest during infiltration in deeper profile post-wildfire materials. Infiltration responses were investigated using tension infiltrometers in the field and laboratory using different cm scale soil profiles that included undisturbed field condition, mechanically layered, and homogenized water repellent profiles. Fractional wettability was investigated through observations using Axisymmetric Drop Shape Analysis, WDPT/MED data, and early time tension infiltrometer data. Statistical approaches and relational results were found useful in discerning coarse differences in fractional wettability between different materials. It was also observed that a repellent layer overlain by wettable ash/charred materials experiences quicker infiltration and wettable system responses than repellent materials with no wettable ash/charred materials at the surface. Utilizing different MED-solutions also presents opportunities for greater insight into fractional wettability and its contribution to larger scale / longer term infiltration processes.