In This Article Wildfire as a Catalyst for Hydrologic and Geomorphic Change

  • Introduction
  • General Overviews
  • Climate Influence on Wildfire and Post-Wildfire Erosion
  • Fire-Induced Water Repellency
  • Post-Wildfire Hydrology
  • Soil Alteration
  • Post-Wildfire Sediment Transport Measurements
  • Empirically Based Models of Post-Wildfire Erosion
  • Post-Wildfire Debris Flow Measurements
  • Physically Based Numerical Models of Post-Wildfire Erosion
  • Statistical and Machine Learning Approaches to Post-Wildfire Debris-Flow Modeling
  • Geomorphic Legacy of Wildfires and Process Recovery
  • Remote Sensing

Environmental Science Wildfire as a Catalyst for Hydrologic and Geomorphic Change
by
Francis K. Rengers
  • LAST MODIFIED: 24 April 2019
  • DOI: 10.1093/obo/9780199363445-0112

Introduction

Wildfire has been a constant presence on the Earth since at least the Silurian period, and is a landscape-scale catalyst that results in a step-change perturbation for hydrologic systems, which ripples across burned terrain, shaping the geomorphic legacy of watersheds. Specifically, wildfire alters two key landscape properties: (1) overland flow, and (2) soil erodibility. Overland flow and soil erodibility have both been seen to increase after wildfires, resulting in order-of-magnitude increases in erosion rates during rainstorms with relatively frequent recurrence intervals. On short timescales, wildfire increases erosion and leads to natural hazards that are costly and threatening to society. Over longer timescales, wildfire-induced erosion can account for the majority of total denudation in certain settings with long-term implications for landscape evolution. There is a special focus on debris flows in this document because they are the most destructive geomorphic processes observed to follow wildfires after high severity burns. To date, research on post-wildfire debris flows has focused on: the provenance of sediment moved in debris flows, the hydrologic and soil properties required to produce debris flows, and debris flow initiation mechanisms. Herein we highlight the relevant research articles showing the current state of progress in debris flow research as well as pointing to the fundamental research on post-wildfire hydrology and erosion necessary for understanding how water and sediment behave after wildfires.

General Overviews

Many review articles exist that synthesize the current knowledge of post-wildfire hydrology and erosion. Doerr, et al. 2000 provides a relevant overview of soil water repellency that is important for understanding how fire and non-fire processes influence soil hydrology. Scott 2010 provides a review of charcoal and shows many ways to investigate charcoal to understand fire characteristics. Sedimentation processes and soil changes are a major focus in several reviews (Moody and Martin 2009; Mataix-Solera, et al. 2011). Bowman, et al. 2009 offers an important overview of the influence of wildfire on the planet and through geologic time. Shakesby 2011 provides a regional review of fire observations that are specific to the Mediterranean region, and general reviews of post-wildfire hydrology and erosion are highlighted in Santi, et al. 2013; Shakesby and Doerr 2006; Moody, et al. 2013; and Williams, et al. 2014.

  • Bowman, D. M., J. K. Balch, P. Artaxo, et al. 2009. Fire in the Earth system. Science 324.5926: 481–484.

    DOI: 10.1126/science.1163886E-mail Citation »

    This is a review used to illustrate how fire has shaped the Earth over time. Points to interactions and feedbacks between climate, humans, and ecosystems in shaping fire regimes. This is a good paper to put the role of fire into a long-term context (i.e., geological time scale).

  • Doerr, S. H., R. A. Shakesby, and R. Walsh. 2000. Soil water repellency: Its causes, characteristics and hydro-geomorphological significance. Earth-Science Reviews 51.1–4: 33–65.

    DOI: 10.1016/S0012-8252(00)00011-8E-mail Citation »

    Many researchers have established the fact that hydrophobicity in soils results after a wildfire; however, this review article helps to show the broadest possible perspective on the phenomenon of soil hydrophobicity. They show that hydrophobicity exists in many soils prior to wildfire due to decaying organic compounds, but this can be exacerbated by heat during wildfires. This article provides an important primer on a topic that is intimately associated with wildfire.

  • Mataix-Solera, J., A. Cerdà, V. Arcenegui, A. Jordán, and L. M. Zavala. 2011. Fire effects on soil aggregation: A review. Earth-Science Reviews 109.1: 44–60.

    DOI: 10.1016/j.earscirev.2011.08.002E-mail Citation »

    Wildfire affects soil stability, and this article has summarized the conditions under which burned soil decreases (and sometimes increases) in soil stability. They find that low-severity fires generate little change in soil stability. High severity fires can cause soil disaggregation by removing the organic content. However, in some soils high-severity fire can recrystallize minerals (e.g., Fe and Al oxyhydroxides) or thermally fuse clays, and consequently soil aggregation can increase.

  • Moody, J. A., and D. A. Martin. 2009. Synthesis of sediment yields after wildland fire in different rainfall regimes in the western United States. International Journal of Wildland Fire 18:96–115.

    DOI: 10.1071/WF07162E-mail Citation »

    This synthesizes post-wildfire erosion (within two years of a fire) measured throughout the western United States. They conclude that post-fire sediment yield from channels (240 t/ha) is greater than sediment yield from hillslopes (82 t/ha); however, there is large variability in their measurements. Suggests that sediment yield is not correlated with slope or sediment erodibility, and they contextualize erosion measurements in locations with similar rainfall patterns.

  • Moody, J., R. Shakesby, P. Robichaud, S. Cannon, and D. Martin. 2013. Current research issues related to post-wildfire runoff and erosion processes. Earth-Science Reviews 122:10–37.

    DOI: 10.1016/j.earscirev.2013.03.004E-mail Citation »

    This paper is structured to focus on emergent research topics. First, it identifies similarities/differences in post-wildfire hydrologic and geomorphic responses in different regions. Second, it explores the relationships between fire and soil hydraulic properties. Third, it asks how meso-scale rainfall patterns influence burned areas. Fourth, it explores what influences flood and debris-flow magnitude and timing. Fifth, it provides suggestions on standard measurements of post-wildfire hydrology and geomorphology.

  • Santi, P. M., S. Cannon, and J. DeGraff. 2013. Wildfire and landscape change. In Treatise on geomorphology. Edited by J. Shroder, L. A. James, C. P. Harden, and J. J. Clague, 262–287. Geomorphology of Human Disturbances, Climate Change, and Natural Hazards. Vol. 13. San Diego, CA: Academic Press.

    E-mail Citation »

    Presents a thorough review of wildfire effects on hydrology and geomorphology with a focus on post-wildfire hazards. Compiles a set of figures summarizing important trends, such as forest infiltration rates before/after fire, the change in vegetation tensile strength before/after fire, and soil loss estimates with time since fire.

  • Scott, A. C. 2010. Charcoal recognition, taphonomy and uses in paleoenvironmental analysis. Palaeogeography, Palaeoclimatology, Palaeoecology 291.1–2: 11–39.

    DOI: 10.1016/j.palaeo.2009.12.012E-mail Citation »

    This paper is an expansive review on charcoal generally, and it will be a useful reference for anyone seeking to use charcoal to understand paleo-fire. In particular, the article outlines how to estimate fire temperature from charcoal reflectance, as well as the different stages of charcoal that form at different fire temperatures. Also discusses charcoal transport processes.

  • Shakesby, R. A. 2011. Post-wildfire soil erosion in the Mediterranean: Review and future research directions. Earth-Science Reviews 105.3: 71–100.

    DOI: 10.1016/j.earscirev.2011.01.001E-mail Citation »

    This review focuses on fire in the Mediterranean region specifically, which has a unique history due to the longtime presence of human cultivation and landscape influence. Shakesby notes that soil loss after wildfire in this region is not significant compared to other land-use practices such as tillage and forest management practices. Soil water repellency also appears to be inconsistent throughout the region, with high-water repellency in unburned plantation areas.

  • Shakesby, R., and S. Doerr. 2006. Wildfire as a hydrological and geomorphological agent. Earth-Science Reviews 74.3: 269–307.

    DOI: 10.1016/j.earscirev.2005.10.006E-mail Citation »

    This paper remains the most complete review of wildfire effects on hydrology and geomorphology to date. They include some helpful tables with data pulled from prior publications. Tables include: fire intensity, burn severity relationships, post-wildfire runoff measurements, and post-wildfire erosion measurements at several scales. They also have figures showing post-wildfire erosion processes as well as recovery measurements.

  • Williams, C. J., F. B. Pierson, P. R. Robichaud, and J. Boll. 2014. Hydrologic and erosion responses to wildfire along the rangeland–xeric forest continuum in the western US: A review and model of hydrologic vulnerability. International Journal of Wildland Fire 23.2: 155–172.

    DOI: 10.1071/WF12161E-mail Citation »

    This is a general review of current post-wildfire hydrologic and erosion knowledge. In particular, the article points out that the breadth of post-fire hydrologic changes is often not recorded in observational studies. Researchers typically focus on one aspect (e.g., overland flow, rainsplash, or channelized flow). They point out five grand challenges for understanding post-wildfire hydrologic and erosional susceptibility. This paper offers several launching points for new research.

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