Hydrologic Impacts of Wildfire on a Small Sub-alpine Southwestern U.S. Watershed: A Simplified Modeling Approach

A Precipitation-Runoff Modeling System (PRMS) based on the National Hydrological Model (NHM) framework was initially developed to determine the extent to which an NHM-based PRMS model can assess effects of fire on watershed-scale hydrologic response. Pre- and post-fire models were calibrated using observed streamflow for a small subalpine Southwestern U.S. watershed located in a monsoon-dominated summer precipitation regime. Streamflow component and parameter response between the two models were then compared. Cumulative runoff was much greater for simulated post-fire conditions. However, if streamflow was partitioned with soil moisture, the models demonstrated that the largest differences between streamflow efficiency for pre- and post-fire watershed response were at moderate antecedent soil moisture conditions. For example, when precipitation occurred in the watershed at low and moderate antecedent soil moisture conditions, the post-fire modeled watershed generated substantial runoff (higher runoff efficiency) whereas the pre-fire model generated almost none (near-zero runoff efficiency). When antecedent soil moisture conditions were high, the pre- and post- fire models demonstrated an almost equivalent ability (though both were highly variable) to generate runoff at all precipitation event sizes (equivalent range of runoff efficiency). In this watershed, the post-fire landscape runoff was increasingly sensitive to soil moisture across all soil-moisture conditions whereas the pre-fire landscape was only responsive to soil moisture at the highest soil moisture-levels. Study results indicate that the rainfall-runoff relationship to soil moisture may be a key aspect of landscape vulnerability to post-fire flooding. Practical implications of this study are that soil moisture may be a useful parameter for monitoring post-fire landscape response and recovery, and that post-fire recovery efforts should be oriented toward establishing pre-fire soil-water-balance equilibrium processes.