Spatial variability of steady-state infiltration into a two-layer soil system on burned hillslopes

Rainfall–runoff simulations were conducted to estimate the characteristics of the steady-state infiltration rate into 1-m2 north- and south-facing hillslope plots burned by a wildfire in October 2003. Soil profiles in the plots consisted of a two-layer system composed of an ash on top of sandy mineral soil. Multiple rainfall rates (18.4–51.2 mm h−1) were used during 14 short-duration (30 min) and 2 long-duration simulations (2–4 h). Steady state was reached in 7–26 min. Observed spatially-averaged steady-state infiltration rates ranged from 18.2 to 23.8 mm h−1 for north-facing and from 17.9 to 36.0 mm h−1 for south-facing plots. Three different theoretical spatial distribution models of steady-state infiltration rate were fit to the measurements of rainfall rate and steady-state discharge to provided estimates of the spatial average (19.2–22.2 mm h−1) and the coefficient of variation (0.11–0.40) of infiltration rates, overland flow contributing area (74–90% of the plot area), and infiltration threshold (19.0–26 mm h−1). Tensiometer measurements indicated a downward moving pressure wave and suggest that infiltration-excess overland flow is the runoff process on these burned hillslope with a two-layer system. Moreover, the results indicate that the ash layer is wettable, may restrict water flow into the underlying layer, and increase the infiltration threshold; whereas, the underlying mineral soil, though coarser, limits the infiltration rate. These results of the spatial variability of steady-state infiltration can be used to develop physically-based rainfall–runoff models for burned areas with a two-layer soil system.