SEEING THE FOREST WITHOUT THE TREES: UNSATURATED ZONE FLOW CHANGES AFTER WILDFIRE

Wildfire affects the hydrologic cycle, yet we know little about changes in unsaturated zone flow in fire-affected soils. This investigation combines field observations, laboratory characterization, and numerical modeling to discern the dominant changes in the unsaturated zone after wildfire. Well characterized experimental plots covering north- and south-facing slope aspects for plots both affected and unaffected by wildfire within the area impacted by the 2010 Fourmile Canyon Fire near Boulder, CO USA are considered. The unsaturated zone flow model Hydrus-1D is used for the numerical simulations. 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. 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.