Unsaturated Zone Flow Changes After Wildfire: A Virtual Experiment Perspective

Wildfire is a frequent disturbance event in the Western U.S. and other regions worldwide. It is well known that wildfire impacts the hydrologic cycle, yet the accompanying changes in unsaturated zone flow are poorly understood. This effort uses unsaturated zone flow simulation for well characterized experimental plots covering north- and south-facing slope aspects for plots both affected and unaffected by wildfire to improve understanding. Comparisons to observed soil-water content and matric potential data establish 'foundation simulations' that lay the groundwork for virtual experiments testing hypotheses developed from interpretation of field and laboratory data. The virtual experiments with the numerical model then extend understanding beyond what could be gleaned from data alone. Unsaturated zone flow is simulated with Hydrus-1D and the field site for this work is within the area affected by the 2010 Fourmile Canyon Fire near Boulder, CO USA. Preliminary work shows 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. Loss of interception by the tree canopy also contributes to wetter subsurface conditions on north-facing slopes. On south-facing slopes, at depths less than 3 cm, the soil was drier after wildfire because of decreases in soil-water retention, confirming hypotheses from field and laboratory measurements.