Quantifying the impact of soil water repellency on overland flow generation and erosion:: a new approach using rainfall simulation and wetting agent on in situ soil

The conventional view of soil water repellency is that it promotes overland flow and soil erosion, but this is not always borne out by observations. This study aimed to isolate the effects of repellency on long-unburnt and recently burnt terrain on) infiltration, overland flow and erosion at the small plot scale (0-36 m(2) . Rainfall simulations (30 min; intensity 100 mm h(-1)), using untreated water. and water treated with surfactants to eliminate repellency, were conducted on in situ repellent soils in fire-prone Eucalyptus globultis plantations, north-central Portugal at (i) a long-unburnt site with and without litter, and (ii) a recently burnt site. On long-unburnt terrain, the mean overland flow coefficient (33%) was 16 times higher and mean slopewash was 23 times higher under repellent compared with wettable conditions. On recently burnt terrain, no overland flow was recorded under wettable conditions, while under repellent conditions the mean coefficient was 70%. The water storage capacity of the litter layer under 10-year-old eucalyptus stands for dry antecedent conditions was at least 3 mm water depth per cm litter depth. implying at least a delay to the onset of overland flow. Severe repellency (36% ethanol) was found to persist through a 30-min storm (100 min h(-1)) when a litter layer was present. A continuous wetting front was observed in the upper similar to 1 cm of exposed soil, indicating a breakdown in repellency at the time of observation. Below similar to 1 cm, repellent, dry soil conditions generally persisted through the simulated storm event. A major implication is that prediction of hydrological impacts of repellency must also take into account the infiltration characteristics of any litter layer and any non-repellent soils, if present. Copyright (c) 2007 John Wiley & Sons, Ltd.