Temporal changes in rainfall intensity–duration thresholds for post-wildfire flash floods in southern California

Tao Liu,Luke A Mcguire,Nina Oakley,Forest Cannon

doi:10.5194/nhess-22-361-2022

Abstract

Abstract. Rainfall intensity–duration (ID) thresholds are commonly used to assess flash flood potential downstream of burned watersheds. High-intensity and/or long-duration rainfall is required to generate flash floods as landscapes recover from fire, but there is little guidance on how thresholds change as a function of time since fire. Here, we force a hydrological model with radar-derived precipitation to estimate ID thresholds for post-fire flash floods in a 41.5 km2 watershed in southern California, USA. Prior work in this study area constrains temporal changes in hydrological model parameters, allowing us to estimate temporal changes in ID thresholds. The results indicate that ID thresholds increase by more than a factor of 2 from post-fire year 1 to post-fire year 5. Thresholds based on averaging rainfall intensity over durations of 15–60 min perform better than those that average rainfall intensity over shorter time intervals. Moreover, thresholds based on the 75th percentile of radar-derived rainfall intensity over the watershed perform better than thresholds based on the 25th or 50th percentile of rainfall intensity. Results demonstrate how hydrological models can be used to estimate changes in ID thresholds following disturbance and provide guidance on the rainfall metrics that are best suited for predicting post-fire flash floods.

Highlights

Heightened hydrological responses are common within and downstream of recently burned areas, resulting in an increased likelihood of flash floods
It may take more than a decade for hydrological responses to return to pre-fire levels, yet there is limited guidance on how the magnitude and utility of rainfall ID thresholds change with time since fire
Given the increased frequency and size of fires in many geographic and ecological zones (e.g. Gillett et al, 2004; Westerling et al, 2006; Kitzberger et al, 2017), it is of growing importance to quantify the best metrics for assessing flash-flood potential in the immediate aftermath of fire as well as how these metrics change throughout the recovery process (e.g. Ebel, 2020)

Summary

Introduction

Heightened hydrological responses are common within and downstream of recently burned areas, resulting in an increased likelihood of flash floods. Since the hydrologic impacts of fire are transient, rainfall ID thresholds associated with flash floods are likely to change as a watershed recovers (Ebel and Martin, 2017; Ebel and Moody, 2017; Moreno et al, 2020; Ebel, 2020). It may take more than a decade for hydrological responses to return to pre-fire levels, yet there is limited guidance on how the magnitude and utility of rainfall ID thresholds change with time since fire. Given the increased frequency and size of fires in many geographic and ecological zones (e.g. Gillett et al, 2004; Westerling et al, 2006; Kitzberger et al, 2017), it is of growing importance to quantify the best metrics for assessing flash-flood potential in the immediate aftermath of fire as well as how these metrics change throughout the recovery process (e.g. Ebel, 2020)

Objectives

Results

Discussion

Conclusion