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Exploration of debris flow initiation in the western United States through the lens of postfire, capillary infiltration, An
Reijm, Cornelis Roelof
Reijm, Cornelis Roelof
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2024
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Abstract
Post-fire debris-flow (PFDF) hazards are an increasingly prevalent problem in the western United States. Current conceptual PFDF models suggest that humid climates like the Pacific Northwest predominantly experience debris flows born from landslides, while more arid regions like the Southwest primarily experience PFDFs driven by infiltration excess, causing surface water runoff. However, runoff-generated debris flows are increasingly documented in the Pacific Northwest and it is unclear how climate change will affect PFDF initiation. Some studies have analyzed PFDF-relevant hydrological responses in the context of environmental influence, but none to date have done so on a multi-regional scale spanning the western United States.
In this study, I use a database to consistently estimate soil hydraulic properties (SHPs) from volumetric time series data measured with a mini-disk infiltrometer, specifically hydraulic conductivity (Kfs), sorptivity (Sfs), and wetting front potential (ψfs). Using these SHP estimates, I simulated 1D capillary (i.e., matric) infiltration for 433 postfire sites using a numerical model and comparatively analyzed them to discern possible regional environmental influences. I also analyzed regional environmental dissimilarity, correlation, and variable importance toward burn-induced changes of Kf s using nonlinear statistical methods to aid simulation interpretation.
Comparing unburned estimates of Kfs to total hydraulic conductivity (Ktotal) across different regions, results show that Kfs for the Pacific Northwest (Pacific NW) only composes ≈4% of Ktotal as compared to ≈ 30% for other regions. This highlights the importance of gravity-driven flow (i.e., non-matric) for post-fire infiltration, which can be driven by an abundance of macropore flow.
Simulations indicate that regional differences in infiltration is partly driven by short-duration rainfall intensity (I15) and by burning, where burned sites in the Pacific NW show no appreciable change in runoff for increased I15. Simulations show a strong association between mean post-fire infiltration changes and increasing belowground biomass density for areas with low mean annual temperature (MAT). The geographic extent of this association encompasses the Pacific NW and Colorado Plateau, implying that the Pacific NW is naturally predisposed to high runoff. Historically, high runoff is not observed, suggesting that either I15 rarely promotes infiltration-excess conditions, or that macropore flow is an important component of post-fire response.
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