PARAMETERIZING POST-FIRE HYDROLOGIC MODELS WITH MINIDISK INFILTRATION MEASUREMENTS TO ASSESS DEBRIS-FLOW HAZARDS

Wildfire can alter soil-hydraulic properties, often resulting in an increased prevalence of Horton overland flow and greater runoff ratios. Mini disk tension infiltrometers (MDI) provide a low cost means of estimating soil-hydraulic properties following wildfire, particularly in remote areas or in situations where there are time or resource limitations. However, the small (point-scale) footprint associated with MDI measurements makes it challenging to translate the results to hillslope and watershed scales where water-related hazards, such as post-fire debris flows, initiate. Here, we design numerical experiments to estimate soil hydrologic parameters at the hillslope scale based on MDI measurements and demonstrate that these estimates also provide reasonable approximations for small watershed-scale simulations. We estimate watershed-scale hydrologic parameters for four different burned sites based on MDI measurements at each site. Rainfall-runoff modeling using a kinematic wave flow model paired with a Green-Ampt infiltration model suggest that watershed-scale hydrologic parameters derived from MDI measurements are most sensitive to the average recurrence interval of the rainstorm and relatively less sensitive to the temporal distribution of rainfall within the storm. Upscaled effective values for wetting front potential and saturated hydraulic conductivity are lower than the arithmetic mean as determined by MDI measurements in the field, and are better approximated by the median or geometric mean of MDI measurements, particularly for storms with recurrence intervals less than two years that commonly initiate post-fire debris flows. At all four burned sites, using the proposed upscaling method to estimate watershed-scale soil hydrologic parameters as opposed to the mean of the MDI measurements improves the ability of a hydrologic model to identify storms that are likely to produce debris flows. Results highlight the importance of spatial scale on runoff generation in post-fire settings and help expand our ability to use MDI data to parameterization of post-fire hydrologic models.