The timing and magnitude of changes to Hortonian overland flow at the watershed scale during the post-fire recovery process

Authors: LIU, T. - University Of Arizona; MCGUIRE, L.A. - University Of Arizona; WEI, H. - University Of Arizona; RENGERS, F.K. - Us Geological Survey (USGS); GUPTA, H. - University Of Arizona; JI, L. - University Of Arizona; Goodrich, David - Dave

Submitted to: Hydrological Processes
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/5/2021
Publication Date: N/A
Citation: N/A

Interpretive Summary: Wildfires often result in significant changes not only to the vegetation in the watershed but also the soils. These changes, in-turn, can result in a substantial change in the how a watershed produces runoff from rainfall events. What might have been an ordinary runoff event from a given rainfall can become a very damaging flood after a wildfire. Investigators from the University of Arizona, the U.S. Geological Survey and the USDA, Agricultural Research Service (ARS) used the ARS KINEROS2 watershed model to estimate how the burned Arroyo Seco watershed recovered from the 2009 Station wildfire in the San Gabriel Mountains, California, USA. They used 13 observed storms and runoff events between 2008-2019 to infer watershed properties by calibrating the model to observations at the watershed outlet. They found that hillslope infiltration values were lowest (low infiltration values result in more runoff) in the first year after the fire. The infiltration values increased over the next three years and stabilized after 6 years of recovery. The other parameter estimated by the model was the hydraulic roughness in the channels of the watershed. Roughness controls how quickly runoff gets to the watershed outlet (the higher the roughness the slower the runoff response). Temporal variations in channel roughness were qualitatively similar to those observed for hillslope infiltration. Overall, our results provide additional constraints on the timescale for watershed recovery following severe wildfires in chaparral-dominated ecosystems.

Technical Abstract: We used the KINEROS2 hydrological model to quantify watershed-scale temporal changes in hillslope saturated hydraulic conductivity (Ksh) and channel hydraulic roughness (nc) resulting from a wildfire. The upper Arroyo Seco watershed (41.5 km2) burned during the 2009 Station fire in the San Gabriel Mountains, California, USA. We used 13 storms between 2008-2019 to infer watershed hydraulic properties by calibrating the model to observations at the watershed outlet. Modeling indicates Ksh is lowest in year 1 following the fire. Ksh then increases at an average rate of approximately 5 mm/hr/yr over the first 3 years of recovery before plateauing after 6 years of recovery and decreasing slightly in later years. The estimated values for Ksh in the first year of recovery are similar to those obtained in previous studies on much smaller watersheds (<1.5 km2) following the Station fire, suggesting hydrologic changes detected here may also apply to low-order watersheds. Temporal variations in Ksh are attributed to the dynamics of short-term fire-induced soil water repellency (SWR) as well as the decay and then gradual strengthening of natural SWR, throughout 10 years of recovery. Temporal variations in nc were qualitatively similar to those observed for Ksh. Post-fire observations suggest changes in nc are due to changes in grain roughness and vegetation in channels. Overall, our results provide additional constraints on the timescale for hydrologic recovery following severe wildfires in chaparral-dominated ecosystems and suggest that relationships between saturated hydraulic conductivity, hydraulic roughness, and time since burning may be non-monotonic.