Soil burn severity and climatic analysis of post-wildfire soil hydraulic properties from across the western continental United States

Abstract

Rainfall infiltration into soil is a key factor in both landscape development and generation of rainfall-runoff hazards like flash flooding and debris flows in recently burned areas. However, infiltration capacity is spatially and temporally variable, which makes its prediction challenging, particularly in burned environments. The extent of fire-prone areas in the western United States is growing, and locations where historically wildfires were rare are burning more frequently, highlighting a need for additional study of post-wildfire soil hydrology and associated hazards. In this work, I have compiled and re-analyzed new and existing post-wildfire Mini Disk infiltration datasets from two dozen wildfires across the western United States. This compilation contains field saturated hydraulic conductivity (KFS) estimates from different soil burn severity classes and collection times post-burn. I quantified the impact of three common methodologies for KFS estimation on the re-processed compilation, and assessed the overall fit of burned KFS estimates to normal family statistical distributions. To test if the observed variability in post-wildfire infiltration behavior can be explained by other landscape and climatological factors, I analyzed these datasets in conjunction with soil burn severity, climatological, and environmental data from each site. My results show that cumulative infiltration (CI), cumulative linearization (CL), and differentiated linearization (DL) methods for KFS estimation produce significantly different outcomes at different spatial and temporal scales, and estimates produced using these methods should not be directly compared if precision is required. Additionally, KFS estimates from burned environments do not show strong linear correlations with climatic and other environmental variables; however, the average change in KFS estimates between burn severity classes does show an inverse linear relationship with both 15-minute duration rainfall intensity for a 2-year recurrence interval storm, and pre-wildfire soil moisture. Better understanding of how post-wildfire infiltration behavior relates to regional climatic variables and burn conditions will be valuable in post-wildfire hazard prediction and modeling under the current regime of rapidly changing wildfire behavior.