Synergistic effects of water repellency and macropore flow on the hydraulic conductivity of a burned forest soil, south-east Australia

Research shows that water repellency is a key hydraulic property that results in reduced infiltration rates in burned soils. However, more work is required in order to link the hydrological behaviour of water repellent soils to observed runoff responses at the plot and hillslope scale. This study used 5 M ethanol and water in disc infiltrometers to quantify the role of macropore flow and water repellency on spatial and temporal infiltration patterns in a burned soil at plot (<10 m(2)) scale in a wet eucalypt forest in south-east Australia. In the first summer and winter after wildfire, an average of 70% and 60%, respectively, of the plot area was water repellent and did not contribute to infiltration. Macropores (r > 0.5 mm), comprising just 5.5% of the soil volume, contributed to 70% and 95%, respectively, of the field-saturated and ponded hydraulic conductivity (K(p)). Because flow occurred almost entirely via macropores in non-repellent areas, this meant that less than 2.5% of the soil surface effectively contributed to infiltration. The hydraulic conductivity increased by a factor of up to 2.5 as the hydraulic head increased from 0 to 5 mm. Due to the synergistic effect of macropore flow and water repellency, the coefficient of variation (CV) in K(p) was three times higher in the water-repellent soil (CV = 175%) than under the simulated non-repellent conditions (CV = 66%). The high spatial variability in K(p) would act to reduce the effective infiltration rate during runoff generation at plot scale. Ponding, which tend to increase with increasing scale, activates flow through macropores and would raise the effective infiltration rates at larger scales. Field experiments designed to provide representative measurements of infiltration after fire in these systems must therefore consider both the inherent variability in hydraulic conductivity and the variability in infiltration caused by interactions between surface runoff and hydraulic conductivity. Copyright (C) 2010 John Wiley & Sons, Ltd.