Simulated unsaturated flow processes after wildfire and interactions with slope aspect

A series of virtual experiments were conducted using a one‐dimensional numerical model of unsaturated flow (Hydrus‐1D) to investigate the factors responsible for shifts in subsurface hydrologic response following wildfire. These virtual experiments used a series of well‐characterized experimental plots in the area affected by the 2010 Fourmile Canyon Fire near Boulder, CO, USA, that cover north and south facing slope aspects for plots both affected and unaffected by wildfire. Inverse estimation of soil‐hydraulic parameters allowed establishing “foundation simulations” that served as the basis for virtual experiments to test findings from interpretation of field and laboratory data and extend understanding beyond what could be gleaned from data alone. The numerical model virtual experiments showed that loss of transpiration because of vegetation combustion/mortality caused soils to be wetter at depths greater than 5 cm on both north and south facing slopes, which agrees with field observations. Loss of tree canopy interception contributed to wetter subsurface conditions and loss of litter/duff increased evaporation, drying soils, in the top few cm on north facing slopes. On south facing slopes, at depths shallower than 3 cm, the simulations did not reproduce the observed trends of drier soils after wildfire, which suggests that more robust methods are needed to simulate evaporation and soil‐water retention at soil‐water contents less than 0.05 cm 3 cm −3 . Simulated bottom boundary flux, a proxy for groundwater recharge, was greater in north facing burned plots by a factor of 3.5–5.2 and greater on south facing plots by a factor of 10.6–12.7, relative to unburned plots.