Causes and consequences of fire-induced soil water repellency

A wettable surface layer overlying a water-repellent layer is commonly obse rved following a fire on a watershed. High surface temperatures 'burn' off organic materials and create vapours that move downward in response to a te mperature gradient and then condense on soil particles causing them to beco me water repellent. Water-repellent soils have a positive water entry press ure 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 n ot completely wetted. Infiltration rate and soil wetting increase as the va lue of h(o)/h(p), increases. The consequence is very high runoff, which als o 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 the se general findings to catchment or watershed scales is difficult because o f the very high temporal and spatial variabilities that occur in the field. Copyright (C) 2001 John Wiley & Sons, Ltd.