Multi‐Stage Soil‐Hydraulic Recovery and Limited Ravel Accumulations Following the 2017 Nuns and Tubbs Wildfires in Northern California. Issue 6 (26th June 2022)

• Record Type: Journal Article
• Authors: Perkins, Jonathan P.; Diaz, Carlos; Corbett, Skye C.; Cerovski‐Darriau, Corina; Stock, Jonathan D.; Prancevic, Jeff P.; Micheli, Elisabeth; Jasperse, Jay
• Is Part Of: Journal of geophysical research. Volume 127:Issue 6(2022)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2022-06-26
• Subjects: wildfire -- hydraulic conductivity -- ravel -- post‐fire debris flows -- California -- post‐fire recovery; Geomorphology -- Periodicals; 551.3
• DOI: 10.1029/2022JF006591
• Languages: English
• ISSNs: 2169-9003

Abstract

Abstract: Wildfire can impact soil‐hydraulic properties by reducing saturated hydraulic conductivity and sorptivity, making recently burned landscapes prone to debris flows and flash floods. The post‐fire hazard window can range from years to decades. In Northern California, where wildfire frequency is steadily increasing, the impact and soil‐hydraulic recovery from wildfires is unknown. Following the October 2017 Nuns and Tubbs fires in the Northern Bay Area of California, we established 41 monitoring sites for repeat tension‐disc infiltrometer measurements of field‐saturated hydraulic conductivity ( K fs ) over 3.5 years. Our site arrays, which encompass grasslands, chaparral, and oak and conifer forests across a range in lithology, show a marked decrease in K fs following the wildfires and a swift partial recovery following the initial post‐fire rainy season. Our time series reveals a complex path to soil‐hydraulic recovery marked by distinct seasonal stages. Analysis of changing K fs, sorptivity, and infiltration model residuals collectively suggests that these stages are related to transitions between soil‐hydraulic processes like structural soil sealing from rainsplash, thermal cracking of bare soil, and vegetation regrowth. While soil infiltration rates were strongly impacted by the 2017 fires, dry ravel estimates are an order of magnitude less for similar slopes than the 2009 Station fire in the San Gabriel mountains of Southern California, suggesting that limited ravel flux may insufficiently load channels for debris flows that initiate from within‐channel failure. Our analysis suggests that burned landscapes in the Northern Bay Area of California may experience rapid soil‐hydraulic recovery and limited pathways toward post‐fire debris flow initiation. Plain Language Summary: Landscapes that once rarely experienced severe burning are now being threatened by increasing wildfire frequency and intensity. Burning soils decreases their ability to absorb rainfall, which increases the risk of post‐wildfire hazards such as flash floods and debris flows. In Northern California it is unclear how wildfires have impacted soils and how long the landscape is in a state of elevated hazard potential. Following the devastating October 2017 wildfires in the Northern Bay Area of California, U.S. Geological Survey scientists partnered with local groups to survey soil recovery and hillslope sedimentation. We found that soil infiltration rates were substantially reduced, but only for the first rainy season following burning. By the next summer, opening of water pathways by fine roots and surface cracks led to substantial increases in water transport rates. We also found that for the same surface slopes, sediment released from hillslopes by burned vegetation is much less compared to the arid debris‐flow‐prone landscapes of Southern California. The post‐wildfire hazard window in the Northern Bay Area of California may therefore be relatively brief, and sedimentation‐related hazards comparatively low. Key Points: Soil hydraulic conductivity was reduced substantially by the 2017 Nuns and Tubbs fires, but only for the first rainy season following fire Infiltration time series analysis reveals distinct stages of soil‐hydraulic recovery in sites ranging from grasslands to conifer forests Post‐fire ravel yields are comparatively low, potentially buffering the landscape from within‐channel debris flow initiation