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¯ ), 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 comparedK S¯between methods measured at the same time and place. Disk and ring infiltrometer measurements had on average 4.8 times higher values ofK S¯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.