Rainfall intensification amplifies exposure of American Southwest to conditions that trigger postfire debris flows

June 12, 2024

A new paper entitled “Rainfall intensification amplifies exposure of American Southwest to conditions that trigger postfire debris flows” was recently published in Nature’s journal npj Natural Hazards. This work was authored by Matt Thomas (U.S. Geological Survey), Allison Michaelis (Northern Illinois University/CW3E affiliate), Nina Oakley (California Geological Survey/CW3E affiliate), Jason Kean (U.S. Geological Survey), Victor Gensini (Northern Illinois University) and Walker Ashley (Northern Illinois University).

Postfire debris flows pose a threat to life, property, and infrastructure in many mountainous areas of the Southwest. When areas of steep terrain with susceptible geologic and hydrologic characteristics are burned at moderate to high severity, short duration (<1 hour), high-intensity rainfall can trigger postfire debris flows. For some areas, debris-flow triggering rainfall intensities can be as low as 0.25 inches in 15 minutes. Minimum triggering thresholds in the Southwest are often less than the 2-year recurrence interval at the 15-minute duration. In a changing climate, with increasing wildfire activity and rainfall intensification, the question arises, how will postfire debris flow hazard change? Previous work demonstrated that debris flow probability is most sensitive to rainfall intensification. Thus, this study focuses on exceedance of estimated debris-flow rainfall thresholds across the Southwest using downscaled climate model projections to address this question.

In this work, authors use dynamically downscaled (3.75 km) convection-permitting simulations of short duration (15-min) rainfall for a historic period (1990-2005) and late century period (2085-2100) using both the RCP 4.5 and RCP 8.5 emission scenarios, developed by Gensini et al. (2022 ). Model output at high spatial and temporal resolutions relevant to postfire debris flows are very recent; we are just beginning to have sufficient information to address the questions of postfire debris-flow hazard changes in a warming climate. Statistical methods were applied to allow for comparison between the downscaled model output and rainfall thresholds for debris flows issued by the USGS for burn areas across the Southwest.

Figure 1. (Fig. 4 from Thomas et al., 2024): a) Box and whisker plots that track changes in the exceedance ratio of 15-minute rainfall intensity thresholds for the HIST, FUT4.5, and FUT8.5 simulation scenarios for mountainous terrain throughout California and Colorado (Figs. 2, 3, Supplementary Fig. 1), as well as for 175 burned areas across the American Southwest (i.e., Arizona, California, Colorado, Nevada, New Mexico, and Utah) where the U.S. Geological Survey issued rainfall thresholds as part of emergency assessments of postfire debris-flow hazards between 2014 and 2022 (Fig. 1a, Supplementary Fig. 1). The boxes in (a) are bound by the first and third quartile, with the median indicated by a line, and the whiskers extend from the boxes to the farthest data point within 1.5 times the inter-quartile range. The bar plots illustrate differences in the seasonality of rainfall threshold exceedance for mountainous terrain throughout (b) California and (c) Colorado.

Results indicate that while seasonality of over threshold precipitation events remains similar in climate projections for the regions studied (b and c above), both the magnitude (a, above) and frequency (below) of threshold exceedances increases in both RCP 4.5 and 8.5 scenarios compared to historic. A greater magnitude of threshold exceedance indicates the potential for larger volume, more damaging debris flows. An increased frequency of over-threshold events indicates we may expect more opportunities for debris flows to occur before burn areas can sufficiently recover.

For California in particular, the largest increases in threshold exceedances per year are most prominent in the North Coast Range and Klamath Mountain geomorphic provinces in the RCP 4.5 scenario. Threshold exceedance frequency further increases in these areas and expands to include the Sierra Nevada province in the RCP 8.5 scenario (figure below). These areas historically have experienced less frequent postfire debris-flow activity than Southern California, where damaging events are commonplace. Communities, emergency managers, and weather forecasters in the Northern California areas projected to see an increase in over-threshold precipitation may be less accustomed to contending with postfire debris flow hazards as compared to their counterparts in Southern California. This study highlights the increased hazard potential and benefits of prefire planning, education, and outreach for postfire hazards.

Figure 1. (Fig. 2 from Thomas et al., 2024): State of California with an overlay of scatter plots that reflect 15-min rainfall intensity threshold (equivalent to a one-year recurrence interval under the present climate) exceedances in mountainous terrain for the WRF-BCC HIST, FUT4.5, and FUT8.5 simulation scenarios. Several areas are labeled for reference, including “KLM” (Klamath Mountains), “NCR” (Northern Coast Ranges), “Sierra Nevada” (SNV), “SCR” (Southern Coast Ranges), “TRV” (Transverse Ranges), and “PNR” (Peninsular Ranges). State boundaries provided by U.S. Census Bureau.

This work addresses the CW3E Strategic Plan priority area of “Monitoring and Projections of Climate Variability and Change”, and the area goal of advancing understanding and projections of extreme precipitation events. This work provides insight into how short-duration rainfall intensities conducive to postfire debris flows may change in the future in frequency, magnitude, space, and seasonality. The work also addresses the CW3E core value of Collaboration, as authors represent university as well as state and federal agencies.

Thomas, M.A., Michaelis, A.C., Oakley, N.S. et al. Rainfall intensification amplifies exposure of American Southwest to conditions that trigger postfire debris flows. npj Nat. Hazards 1, 14 (2024). https://doi.org/10.1038/s44304-024-00017-8