Causes and consequences of fire-induced soil water repellency

A wettable surface layer overlying a water-repellent layer is commonly observed following a fire on a watershed. High surface temperatures 'burn' off organic materials and create vapours that move downward in response to a temperature gradient and then condense on soil particles causing them to become water repellent. Water-repellent soils have a positive water entry pressure h(p) that must be exceeded or all the water will runoff. Water ponding depths h(o) that exceeds h(p) will cause infiltration. but the profile is not completely wetted. Infiltration rate and soil wetting increase as the value of h(o)/h(p), increases. The consequence is very high runoff, which also contributes to high erosion on fire-induced water-repellent soils during rain storms. Grass establishment is impaired by seeds being eroded and lack of soil water for seeds that do remain and germinate. Extrapolation of these general findings to catchment or watershed scales is difficult because of the very high temporal and spatial variabilities that occur in the field. Copyright (C) 2001 John Wiley & Sons, Ltd.