Hydrological recovery after a severe wildfire in a chaparral dominated, mountainous watershed

Abstract:

We used the KINEROS2 hydrological model to quantify the temporal changes in hillslope saturated hydraulic conductivity (K_sh) and channel hydraulic roughness (n_c), resulting from the 2009 Station Fire in the upper Arroyo Seco (41.5 km²) watershed in the San Gabriel Mountains, California, USA. We used tipping bucket rain gauge records for 13 rainfall-runoff events between 2008-2019 to infer watershed hydraulic properties. Model performance was evaluated using Kling-Gupta Efficiency. We see a decrease in K_sh immediately after the fire, followed by an increase over several years to values that exceed the pre-fire condition, returning after ten years of recovery to its approximate pre-fire value. These temporal variations in K_sh can be attributed to the dynamics of short-term fire-induced soil water repellency (SWR) and the gradual strengthening, throughout the ten-year recovery period, of natural SWR. Temporal variations in hydraulic roughness, n_c, were qualitatively similar to those observed for K_sh. Post-fire observations suggest these changes in n_c are due to changes in grain roughness and vegetation. We also observed the minor effect of hillslope hydraulic roughness and capillary drive on simulated hydrographs in the studied watershed. Overall, our results provide additional constraints on the time scale for hydrologic recovery following severe wildfires in chaparral dominated ecosystem, and suggest that relationships between saturated hydraulic conductivity, hydraulic roughness, and time since burning may be non-monotonic.