UNEXPECTED POST-FIRE SOIL HYDROLOGIC RESPONSE IN NORTHERN CALIFORNIA (USA)

Wildfires change California’s landscapes overnight. The burned hillslopes wildfires leave behind are potentially much more susceptible to runoff and sediment generation. Rains that once soaked into the soil may now cause floods and debris flows—as evident by the disastrous 2018 Montecito Debris flows in Southern California—as well as threaten water quality. Previous studies found post-fire reductions in soil infiltration in forested landscapes and several locations in Southern California. The prevailing belief is that fires can reduce effective infiltration capacity by various mechanisms (e.g. increased water repellency, loss of vegetation, etc.). However, the soil response data are limited for Northern California where there are few examples of post-fire floods and debris flows. In order to better understand the hydrologic response to wildfires in Northern California, and how the response might differ from other fire-prone regions, we conducted repeat infiltration tests to 1) track soil recovery to pre-fire infiltration rates and 2) compare burned and unburned sites following 2017, 2019, and/or 2020 wildfires. Test sites were chosen based on their burn severity, slope angle, vegetation, and lithology. We used a variety of methods—including tension disk, falling head, and dual-head infiltrometers—and repeat test sites to monitor hydrologic change though time. After several seasons of post-fire hydrological observations our field research identified a complex response to fire related to vegetation and lithology. We found, compared to unburned sites and long-term trends, chaparral test sites had a 5-10x increase in infiltration rates immediately post-fire (with the possible exception of serpentinite-derived soils), grassland sites showed no effect from fires, and forested sites had infiltration rates decrease by half. Our data suggest that, contrary to Southern California, some soils soak up even more water after fires than before burning and that soil hydrologic recovery can happen within 1-2 years. Understanding the interaction between vegetation, lithology, and hydrologic changes is key to identifying areas of elevated risk post-fire and helping prioritize post-fire mitigation using scarce resources.