Post-fire Runoff Response in Northern California: Testing current models used to predict post-fire runoff magnitude and debris flow triggering thresholds.

Abstract:

The Carr and Delta fires burned over 1100 square kilometers within Shasta and Trinity Counties in northern California during the summer of 2018. The burned areas reside within the Klamath Mountains geomorphic province and contain highly dissected mountainous terrain that supports relatively high unit area biomass composed of mixed conifer forests, oak woodland forests, and chaparral. Average annual precipitation within the burn areas range from about 1000 to 2000 millimeters. Models used to predict post-fire hydrogeomorphic response are typically derived from southern California, particularly the Transverse and Peninsular ranges, where empirical models have been developed to estimate post-fire flood flows and likelihood, volume, and triggering rainfall intensity of post-fire debris flows. However, the Transverse and Peninsular ranges differ substantially from the Klamath Mountains in terms of vegetation, rainfall, and underlying hydrogeomorphic processes. To improve our understanding of post-fire flooding and runoff-induced debris flows in northern California, and to test the applicability of existing post-fire models outside of southern California, we initiated a study to quantify and monitor rainfall and runoff response in watersheds burned at moderate to high soil burn severity using a network of telemetered and non-telemetered rain gages and stream gages. Rainfall and stream gage data from the first winter following the fires were compiled and correlated to field observations used to characterize the type of flows (e.g. flood flow, hyperconcentrated flow, and debris flows) that occurred. Results show that current empirical models underestimate post-fire runoff and overestimate the likelihood of debris flows being triggered in northern California. These results illustrate the importance and need to continue to advance post-fire research across geomorphic provinces with diverse climatic and geologic conditions. Information obtained from such studies will improve our knowledge on post-fire runoff response to better assess downstream impacts to life safety, infrastructure, and natural resources, and inform the use of best management practices and techniques to mitigate those impacts.