Scale-dependency of effective hydraulic conductivity on fire-affected hillslopes

Effective hydraulic conductivity (K-e) for Hortonian overland flow modeling has been defined as a function of rainfall intensity and runon infiltration assuming a distribution of saturated hydraulic conductivities (K-s). But surface boundary condition during infiltration and its interactions with the distribution of K-s are not well represented in models. As a result, the mean value of the K-s distribution ((K-S) over bar), which is the central parameter for K-e, varies between scales. Here we quantify this discrepancy with a large infiltration data set comprising four different methods and scales from fire-affected hillslopes in SE Australia using a relatively simple yet widely used conceptual model of K-e. Ponded disk (0.002 m(2)) and ring infiltrometers (0.07 m(2)) were used at the small scales and rainfall simulations (3 m(2)) and small catchments (ca 3000 m(2)) at the larger scales. We compared (K-S) over bar between methods measured at the same time and place. Disk and ring infiltrometer measurements had on average 4.8 times higher values of (K-S) over bar than rainfall simulations and catchment-scale estimates. Furthermore, the distribution of K-s was not clearly log-normal and scale-independent, as supposed in the conceptual model. In our interpretation, water repellency and preferential flow paths increase the variance of the measured distribution of K-s and bias ponding toward areas of very low K-s during rainfall simulations and small catchment runoff events while areas with high preferential flow capacity remain water supply-limited more than the conceptual model of K-e predicts. The study highlights problems in the current theory of scaling runoff generation.