Modeling the effects of surface storage, macropore flow and water repellency on infiltration after wildfire

•Infiltration after wildfire is initially controlled by storage in wettable surface material.•Steady state infiltration is controlled by the hydraulic conductivity of ash or the soil.•Soil hydraulic conductivity is restricted by repellency and macropore availability.•The maximum strength of repellency in the soil is a function of monthly weather.•Macropore flow is initially low after wildfire but increases during recovery.

SummaryWildfires can reduce infiltration capacity of hillslopes by causing (i) extreme soil drying, (ii) increased water repellency and (iii) reduced soil structure. High severity wildfire often results in a non-repellent layer of loose ash and burned soil overlying a water repellent soil matrix. In these conditions the hydraulic parameters vary across discrete layers in the soil profile, making the infiltration process difficult to measure and model. The difficulty is often exacerbated by the discrepancy between actual infiltration processes and the assumptions that underlie commonly used infiltration models, most of which stem from controlled laboratory experiments or agricultural environments, where soils are homogeneous and less variable in space and time than forest soils. This study uses a simple two-layered infiltration model consisting of surface storage (H), macropore flow (Kmac) and matrix flow (Kmat) in order to identify and analyze spatial-temporal infiltration patterns in forest soils recovering from the 2009 Black Saturday wildfires in Victoria, southeast Australia. Infiltration experiments on intact soil cores showed that the soil profile contained a region of strong water repellency that was slow to take on water and inactive in the infiltration process, thus restricting flow through the matrix. The flow resistance due to water repellent soil was represented by the minimum critical surface tension (CSTmin) within the top 10 cm of the soil profile. Under field conditions in small headwaters, the CSTmin remained in a water repellent domain throughout a 3-year recovery period, but the strength of water repellency diminished exponentially during wet conditions, resulting in some weather induced temporal variation in steady-state infiltration capacity (Kp). An increasing trend in macropore availability during recovery was the main source of temporal variability in Kp during the study period, indicating (in accordance with previous studies) that macropore flow dominates infiltration processes in these forest soils. Storage in ash and burned surface soil after wildfire was initially high (∼4 mm), then declined exponentially with time since fire. Overall the study showed that the two layered soil can be represented and parameterized by partitioning the infiltration process into surface storage and flow through a partially saturated and restrictive soil layer. Ash, water repellency and macropore flow are key characteristics of burned forest soils in general, and the proposed model may therefore be a useful tool for characterizing fire impact and recovery in other systems.