Environmental Science Wildfire as a Catalyst for Hydrologic and Geomorphic Change
Francis K. Rengers
  • LAST REVIEWED: 03 December 2020
  • LAST MODIFIED: 24 April 2019
  • DOI: 10.1093/obo/9780199363445-0112


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.1163886Save Citation »Export Citation » Share 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).

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  • 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)00011-8Save Citation »Export Citation » Share 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.

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  • 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.2011.08.002Save Citation »Export Citation » Share 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.

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  • 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/WF07162Save Citation »Export Citation » Share 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.

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  • 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.2013.03.004Save Citation »Export Citation » Share 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.

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  • 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.

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    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.

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  • 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.2009.12.012Save Citation »Export Citation » Share 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.

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  • 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.2011.01.001Save Citation »Export Citation » Share 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.

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  • 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.2005.10.006Save Citation »Export Citation » Share 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.

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  • 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/WF12161Save Citation »Export Citation » Share 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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Climate Influence on Wildfire and Post-Wildfire Erosion

How does climate and future climate change influence wildfire conditions and post-wildfire erosion? Westerling, et al. 2006 shows that climate change has been an influence on wildfire initiation in the western United States since the 1980s, and Kitzberger, et al. 2007 shows that El Niño and La Niña climate cycles can create conditions that encourage wildfire in the United States. Moreover, climate change is leading to conditions that preferentially encourage wildfire; see Abatzoglou and Williams 2016 and Flannigan, et al. 2009. Local climate conditions, such as the development of atmospheric rivers, deliver large amounts of precipitation to burned areas and can exacerbate post-wildfire erosion (Oakley, et al. 2017).

  • Abatzoglou, J. T., and A. P. Williams. 2016. Impact of anthropogenic climate change on wildfire across western US forests. Proceedings of the National Academy of Sciences 113.42: 11770–11775.

    DOI: 10.1073/pnas.1607171113Save Citation »Export Citation » Share Citation »

    This study points out the ways that climate change is contributing to fire activity: (1) climate influences fuel in fuel-limited environments, (2) climate modulates fuel aridity in flammability-limited environments. This work suggests that climate change increases temperature and the vapor pressure deficit, enhancing fuel aridity and thus wildfire. This affected both the total annual duration of wildfire days and the area (4.2 million hectares) subject to wildfire.

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  • Flannigan, M. D., M. A. Krawchuk, W. J. de Groot, B. M. Wotton, and L. M. Gowman. 2009. Implications of changing climate for global wildland fire. International Journal of Wildland Fire 18.5: 483–507.

    DOI: 10.1071/WF08187Save Citation »Export Citation » Share Citation »

    This paper is basically a review of wildfire and climate interactions. It begins by looking at wildfire throughout the world and feedbacks between fire, carbon, people, and climate. The second half of the paper is dedicated to summarizing future predictions of wildfire that have been made in a variety of other studies. In general, this article will be helpful for readers looking to have a quick snapshot of future global fire and climate prediction patterns.

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  • Kitzberger, T., P. M. Brown, E. K. Heyerdahl, T. W. Swetnam, and T. T. Veblen. 2007. Contingent Pacific–Atlantic Ocean influence on multicentury wildfire synchrony over western North America. Proceedings of the National Academy of Sciences 104.2: 543–548.

    DOI: 10.1073/pnas.0606078104Save Citation »Export Citation » Share Citation »

    The authors look at how climate patterns influence wildfires. In the southwestern US El Nino creates wet conditions, encouraging plant growth. When this is followed by a La Nina, which creates dry conditions, fires appear to synchronize across the region. In the Pacific Northwest El Niño creates warmer conditions, the snowpack warms, and fires increase. The Northwest and southwestern US are typically opposite in their droughts and hence fire regimes. A multidecadal pattern of fire was observed to be controlled by a warm Atlantic Multidecadal Oscillation.

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  • Oakley, N. S., J. T. Lancaster, M. L. Kaplan, and F. M. Ralph. 2017. Synoptic conditions associated with cool season post-fire debris flows in the Transverse Ranges of southern California. Natural Hazards 88.1: 327–354.

    DOI: 10.1007/s11069-017-2867-6Save Citation »Export Citation » Share Citation »

    Shows how atmospheric conditions create the potential for post-wildfire debris flows in southern California. In particular this work shows that atmospheric river conditions (a long-duration high-intensity rainfall event that is common on the US West Coast) were seen in the majority of rain events that led to post-wildfire debris flows from 1980–2014. This work relates large-scale climatic events to burn area response.

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  • Westerling, A. L, H. G. Hidalgo, D. R. Cayan, and T. W. Swetnam. 2006. Warming and earlier spring increase Western U.S. forest wildfire activity. Science 313:940–943.

    DOI: 10.1126/science.1128834Save Citation »Export Citation » Share Citation »

    This study asks the question: is increased wildfire starting in the 1980s due to land-use history or changes in climate? They found that increasing wildfire occurrence is largely due to changes in climate that create warmer springs and longer, drier summers. This change in the hydroclimate leaves forests more vulnerable to wildfire and cannot be easily ameliorated by changes in land management.

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Fire-Induced Water Repellency

After wildfires, water infiltration tends to decrease, producing overland flow runoff during rainstorms. DeBano, et al. 1970 uses a relatively simple experiment to show that burning organic matter can result in a hydrophobic layer below the soil surface. DeBano, et al. 1979 quantifies how hydrophobicity is influenced by different soil temperatures and shows that at high surface temperatures soil hydrophobicity can be destroyed, leaving behind a hydrophobic layer deeper in the soil, an observation also observed by Scott and Van Wyk 1990. More recently, Doerr, et al. 2004 examined how surface soil water repellency was destroyed and found that the duration of heating was important. Doerr, et al. 2006 highlights the general complexity of post-wildfire hydrophobicity in some landscapes and shows that in southeast Australia unburned sandy soils were hydrophobic. But runoff was limited due to rough litter and duff, whereas high-burn severity soils had little water repellency at the soil surface.

  • DeBano, L. F., L. D. Mann, and D. A. Hamilton. 1970. Translocation of hydrophobic substances into soil by burning organic litter Soil Science Society of America Journal 34.1: 130–133.

    DOI: 10.2136/sssaj1970.03615995003400010035xSave Citation »Export Citation » Share Citation »

    The authors experimentally burned soils with different levels of organic matter. The vaporized organic material created a hydrophobic layer below the soil surface. The soils were initially hydrophobic at the soil surface, but after burning, this hydrophobicity moved several centimeters deeper. In addition, soils with litter developed stronger hydrophobic layers than soils without a litter cover. Finally, coarser soils developed stronger hydrophobicity than finer textured soils.

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  • DeBano, L. F., R. M. Rice, and C. C. Eugene. 1979. Soil heating in chaparral fires: Effects on soil properties, plant nutrients, erosion, and runoff. Res. Paper PSW-RP-145. Berkeley, CA: US Department of Agriculture, Forest Service, Pacific Southwest Forest and Range Experiment Station.

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    While this research paper covers soil heating effects, it also offers important insight on soil hydrology in burned environments. Importantly, they show that fire-induced hydrophobicity reaches a maximum at temperatures near 200°C and decreases as temperatures increase closer to 300°C. In addition, soil heating decreases strongly with depth. Consequently, as hydrophobic material volatilizes and moves downward, it may be destroyed higher in the soil column. But it can be preserved deeper in the soil because of the temperature gradient.

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  • Doerr, S. H., W. H. Blake, R. A. Shakesby, et al. 2004. Heating effects on water repellency in Australian eucalypt forest soils and their value in estimating wildfire soil temperatures. International Journal of Wildland Fire 13.2: 157–163.

    DOI: 10.1071/WF03051Save Citation »Export Citation » Share Citation »

    This study used initially hydrophobic soils and showed that once they were heated above 260–340°C the soil water repellency was destroyed. They showed that soil water repellency breakup is related to the duration of heating, and longer duration heating allowed soil water repellency destruction at lower fire temperatures. The article also suggests it may be possible to infer the soil heating temperature range based on the soil hydrophobicity.

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  • Doerr, S. H., R. A. Shakesby, W. H. Blake, C. J. Chafer, G. S. Humphreys, and P. J. Wallbrink. 2006. Effects of differing wildfire severities on soil wettability and implications for hydrological response. Journal of Hydrology 319.1–4: 295–311.

    DOI: 10.1016/j.jhydrol.2005.06.038Save Citation »Export Citation » Share Citation »

    Found that unburned sandy soils, and low to moderate severity burned soils were highly water repellent. However, the highest severity burned soils had little soil water repellency at the soil surface but retained a hydrophobic layer a few centimeters below. Therefore, rainstorms sufficient to wet the surface of a high-severity burned soil could lead to overland flow and high erosion.

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  • Scott, D. F., and D. B. Van Wyk. 1990. The effects of wildfire on soil wettability and hydrological behavior of an afforested catchment. Journal of Hydrology 121.1–4: 239–256.

    DOI: 10.1016/0022-1694(90)90234-OSave Citation »Export Citation » Share Citation »

    Observations from this paper show that after a severe wildfire the surface soil water repellency can be removed. However, soil water repellency deeper in the soil column can increase and thus lead to increased runoff and erosion.

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Post-Wildfire Hydrology

Hydrologic processes are complicated by wildfires. An early paper to recognize that post-wildfire infiltration did not fit theoretical predictions was Imeson, et al. 1992. The general differences in infiltration theory versus observations are well articulated by Ebel and Moody 2013. Soil infiltration is influenced by both a hydrophilic ash layer that can absorb water rapidly (Onda, et al. 2008 and Keesstra, et al. 2014) as well as underlying soil that is water repellent due to either chemically induced hydrophobicity (see Fire-Induced Water Repellency) or hyper-dry conditions, presented by Moody and Ebel 2012. On a large scale, wildfire is capable of homogenizing hydrologic conditions on a landscape Ebel 2012, but on a smaller scale, McGuire, et al. 2018 found that the remaining infiltration heterogeneity can be estimated with an “effective” saturated hydraulic conductivity value based on field measurements. Loáiciga, et al. 2001 showed how reduced infiltration could lead to increased stream flows in burned watersheds, but Moody, et al. 2008 suggested runoff requires rainfall above a threshold. Finally, Cerdà and Doerr 2005 shows different vegetation types influenced hydrologic recovery over time, with shrubs and herbs reducing post-wildfire runoff, whereas trees maintained hydrophobic soils by adding more hydrophobic organic matter to the surface.

  • Cerdà, A., and S. H. Doerr. 2005. Influence of vegetation recovery on soil hydrology and erodibility following fire: An 11-year investigation. International Journal of Wildland Fire 14.4: 423–437.

    DOI: 10.1071/WF05044Save Citation »Export Citation » Share Citation »

    Researchers monitored a burned site for eleven years following a wildfire and explored how vegetation recovery influenced overland flow runoff. They found that tree cover behaved differently than shrub and herb cover. The tree litter produced organic material that increased hydrophobicity leading to enhanced overland flow runoff for ten years after the wildfire, whereas the herb and shrub cover reduced overland flow to near zero by two years after the wildfire. Consequently, the type of vegetation recovery is likely to influence overland flow.

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  • Ebel, B. 2012. Wildfire impacts on soil-water retention in the Colorado Front Range, United States. Water Resources Research 48:W12515.

    DOI: 10.1029/2012WR012362Save Citation »Export Citation » Share Citation »

    This study shows that soil water retention curves on different hillslope aspects are strongly influenced by organic matter. Moreover, landscapes where organic matter is strongly dependent on slope and aspect can show very different soil water retention curves. However, wildfire will largely consume the soil organic matter and can homogenize the aspect-dependent soil water retention curves such that north- and south-facing hillslopes will approach one another.

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  • Ebel, B. A., and J. A. Moody. 2013. Rethinking infiltration in wildfire‐affected soils. Hydrological Processes 27.10: 1510–1514.

    DOI: 10.1002/hyp.9696Save Citation »Export Citation » Share Citation »

    This is a commentary piece that synthesizes the current knowledge governing post-wildfire hydrology and points out a current problem in modeling approaches. They note that infiltration theory would suggest that during particularly dry conditions capillary suction should be high and thus create conditions where water infiltrates quickly. However, in practice extremely dry soils can infiltrate negligible amounts of water due to soil water repellency, which interferes with capillary-driven infiltration. Thus, the article suggests that traditional theory is insufficient to explain current infiltration rates in burned soils. They suggest reexamination of soil-water repellency characteristic curves and utilizing sorptivity as a key parameter to estimate infiltration rates.

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  • Imeson, A. C, J. M. Verstraten, E. J. van Mulligen, and J. Sevink. 1992. The effects of fire and water repellency on infiltration and runoff under Mediterranean type forest. Catena 19:345–361.

    DOI: 10.1016/0341-8162(92)90008-YSave Citation »Export Citation » Share Citation »

    This study uses nearly 150 rainfall simulations to make observations on post-wildfire hydrology. They find that contrary to infiltration theory, not all soils show a decreasing infiltration rate with time. They categorize the infiltration response into four distinct patterns. This work provides a framework for thinking about different ways that soils respond after wildfire.

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  • Keesstra, S. D., J. Maroulis, E. Argaman, A. Voogt, and L. Wittenberg. 2014. Effects of controlled fire on hydrology and erosion under simulated rainfall. Cuadernos de Investigación Geográfica 40.2: 269–294.

    DOI: 10.18172/cig.2532Save Citation »Export Citation » Share Citation »

    Researchers conducted a series of controlled experiments to determine how burning influenced water runoff and erosion. They found that ash significantly delayed overland flow and erosion by both absorbing water and preventing rainsplash from directly impacting the soils. Therefore, the presence of ash after a wildfire can help reduce runoff and erosion.

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  • Loáiciga, H. A., D. Pedreros, and D. Roberts. 2001. Wildfire-streamflow interactions in a chaparral watershed. Advances in Environmental Research 5.3: 295–305.

    DOI: 10.1016/S1093-0191(00)00064-2Save Citation »Export Citation » Share Citation »

    Attempts to quantify the total increase in annual streamflow due to watershed burning using a paired catchment study. This study uses known streamflow data from unburned time periods and uses these data to reconstruct estimates of flow in the absence of a wildfire. The reconstructed estimates are subsequently compared to field measurements. Not surprisingly, they found that burned watersheds can increase stream flows by 30 percent. The methodology in this paper could be expanded on using better models and more historical data.

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  • McGuire, L. A., F. K. Rengers, J. W. Kean, D. M. Staley, and B. B. Mirus. 2018. Incorporating spatially heterogeneous infiltration capacity into hydrologic models with applications for simulating post‐wildfire debris flow initiation. Hydrological Processes 32.9: 1173–1187.

    DOI: 10.1002/hyp.11458Save Citation »Export Citation » Share Citation »

    The researchers in this study sought to define a practical model to describe heterogeneity in soil infiltration on a burned landscape. Using a suite of measurements from a tension infiltrometer the researchers recorded the distribution of field-saturated hydraulic conductivity at a burned study site. From this distribution of infiltration measurements, they found that it was possible to determine an effective saturated hydraulic conductivity value.

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  • Moody, J. A., D. A. Martin, S. L. Haire, and D. A. Kinner. 2008. Linking runoff response to burn severity after a wildfire. Hydrological Processes 22.13: 2063–2074.

    DOI: 10.1002/hyp.6806Save Citation »Export Citation » Share Citation »

    Uses observations of rainfall and runoff to develop a series of empirical equations for post-wildfire runoff. They found a linear relationship between rainfall and runoff and observed that thirty-minute rainfall intensities below 8.5 mm/hr produced no runoff at all, which they show is similar to prior studies. In addition, they develop a metric called dimensionless hydraulic functional connectivity, which incorporates upstream drainage area, burn severity, and local slope. Using this metric, they show a strong linear relationship between runoff and the hydraulic functional connectivity. However, it is unclear how large a role burn severity plays in this relationship versus the drainage area and slope.

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  • Moody, J. A, and B. A. Ebel. 2012. Hyper-dry conditions provide new insights into the cause of extreme floods after wildfire. Catena 93:58–63.

    DOI: 10.1016/j.catena.2012.01.006Save Citation »Export Citation » Share Citation »

    Post-wildfire soil water infiltration is often thought to be dominated by fire-induced soil hydrophobicity. However, this article points out that immediately after a wildfire, for the first few days to weeks, the infiltration may be affected by the hyper-dry conditions imparted by the wildfire. Moody and Ebel present field measurements that suggest that after a wildfire the soil water content is so low that capillary infiltration is disturbed, and the only way for capillary pores to rewet is through molecular diffusion of water vapor. During this hyper-dry period soils can become virtually impermeable thus producing high water runoff.

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  • Onda, Y., W. E. Dietrich, and F. Booker. 2008. Evolution of overland flow after a severe forest fire, Point Reyes, California. Catena 72:13–20.

    DOI: 10.1016/j.catena.2007.02.003Save Citation »Export Citation » Share Citation »

    Uses observations to show that post-wildfire hydrology is influenced by the initial ash layer in a burned area, and as the site experiences rainstorms, the ash, underlying soil, and runoff types co-evolve. Initially, the ash layer absorbs water, but as it fills with water, the underlying soil prevents water from infiltrating except for a few preferential flow paths. However, over time those preferential flow paths can get clogged by ash, and runoff can increase with subsequent rainfall.

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Soil Alteration

Wildfires can influence soil properties through heating. Moody, et al. 2005 shows that simply heating a soil can lead to a reduction in the resistive shear stress to erosion. The soil changes induced by burning are related to burn severity, and Parsons, et al. 2010 provides a systematic guide for describing the severity of burned soils. After a wildfire, Nyman, et al. 2013 identified a loose erodible topsoil layer that is a mixture of soil and ash and that typically overlies more cohesive soils. Balfour, et al. 2014 shows that wood ash from wildfires influences soil erosion and hydrology. Understanding how heat moves through soil during a wildfire requires modeling, and this has been attempted by Campbell, et al. 1995 and Massman 2015.

  • Balfour, V. N., S. H. Doerr, and P. R. Robichaud. 2014 The temporal evolution of wildfire ash and implications for post-fire infiltration. International Journal of Wildland Fire 23:733–745.

    DOI: 10.1071/WF13159Save Citation »Export Citation » Share Citation »

    After a wildfire the top portion of the soil is often blanketed with ash from the fire. This ash is the product of burned litter and duff as well as ash from the higher canopy material that is deposited on top of the soil. This study shows that in some cases high-combustion ash can form a crust on top of the soil that can decrease hydraulic conductivity. However, the ash crust also decreases the effectiveness of aeolian transport.

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  • Campbell, G. S., J. D. Jungbauer Jr., K. L. Bristow, and R. D. Hungerford. 1995. Soil temperature and water content beneath a surface fire. Soil Science 159.6: 363–374.

    DOI: 10.1097/00010694-199506000-00001Save Citation »Export Citation » Share Citation »

    Models soil temperature and water content that were monitored during an experiment. The strongest contribution of this work is the article’s clear mathematical framework explaining a method for computing heat flux in a soil. The results suggest that the heat model approximates the measured temperatures fairly well, however, modeled water content provided worse estimates than modeled temperature.

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  • Massman, W. J. 2015. A non-equilibrium model for soil heating and moisture transport during extreme surface heating: the soil (heat–moisture–vapor) HMV-Model Version 1. Geoscientific Model Development 8.11: 3659.

    DOI: 10.5194/gmd-8-3659-2015Save Citation »Export Citation » Share Citation »

    This work outlines a state-of-the-art approach to modeling temperature and soil moisture. This new model improves predictions of both temperature and soil moisture from prior proposed approaches. The model shows that water content actually increases ahead of the drying front in heated soils, and that rapid evaporation can occur at temperatures between 50–90°C.

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  • Moody, J. A., J. D. Smith, and B. W. Ragan. 2005. Critical shear stress for erosion of cohesive soils subjected to temperatures typical of wildfires. Journal of Geophysical Research: Earth Surface 110.F1.

    DOI: 10.1029/2004JF000141Save Citation »Export Citation » Share Citation »

    In this study, researchers heated soils at different temperatures then used a recirculating flume to measure the shear stress required for soil erosion. Their results suggested that when heated <175°C the soils had variable critical shear stresses, but at slightly higher temperatures (175–275°C) the soil critical shear stress actually increased. At temperatures >275 °C, the critical shear stress reached a minimum and there was little variability.

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  • Nyman, P., G. J. Sheridan, J. A. Moody, H. G. Smith, P. J. Noske, and P. N. J. Lane. 2013 Sediment availability on burned hillslopes. Journal of Geophysical Research: Earth Surface 118:2451–2467.

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    Shows that in burned areas a highly erodible, non-cohesive layer of sediment is often found at the top of soils, and this decreases with depth exponentially for 20 mm. The non-cohesive layer is a mix of ash and sediment. Notes that the top non-cohesive layer also changes in time, and more cohesion appears with time after the wildfire.

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  • Parsons, A., P. R. Robichaud, S. A. Lewis, C. Napper, and J. T. Clark. 2010. Field guide for mapping post-fire soil burn severity, U.S. Department of Agriculture, Forest Service. Rocky Mountain Research Station General Technical Report RMRS-GTR-243. Washington, DC: U.S. Department of Agriculture.

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    This is a practical document that is designed to help Burned Area Emergency Response (BAER) teams from the US Forest Service primarily in the western United States. The document goes into a broad range of techniques used to assess burn severity including remote sensing, vegetation, soil, and hydrology data. The approach described in this field guide helps scientists to understand the changes to the soil due to wildfire.

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Post-Wildfire Sediment Transport Measurements

Researchers have made considerable progress in understanding post-wildfire erosion by making direct measurements. Lane, et al. 2006 shows that hydrologic changes induced by wildfire often lead to overland flow runoff and can generate erosion many times the background erosion rates. Robichaud, et al. 2016 observed that the lack of litter/duff in burned areas is a primary driver of erosion, and on hillslopes unchannelized erosion can account for the majority of the eroded sediment volume (Staley, et al. 2014 and DeLong, et al. 2018). DiBiase and Lamb 2013 shows how dry ravel can occur directly after vegetation is incinerated by wildfire and sediment stored by the vegetation ravels down steep slopes. Berg and Azuma 2010 found that rill erosion is common on burned slopes, but where infiltration rates remain high, Sheridan, et al. 2007 observes that erosion can be dominated by rainsplash instead. Wester, et al. 2014 shows that overall sediment export from watersheds is highly dependent on overall sediment transport connectivity, and Reneau, et al. 2007 points out that in some settings material can by moved downstream and deposited by one storm event and then remobilized in subsequent years by a different event.

  • Berg, N. H., and D. L. Azuma. 2010. Bare soil and rill formation following wildfires, fuel reduction treatments, and pine plantations in the southern Sierra Nevada, California, USA. International Journal of Wildland Fire 19.4: 478–489.

    DOI: 10.1071/WF07169Save Citation »Export Citation » Share Citation »

    Observed that rilling is a common phenomenon after wildfire, and all wildfire sites in their study with more than 60 percent bare soil exhibited rills. However, rills were ephemeral, and most rills were no longer present four to five years following wildfire.

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  • DeLong, S. B., A. M. Youberg, W. M. DeLong, and B. P. Murphy. 2018. Post-wildfire landscape change and erosional processes from repeat terrestrial lidar in a steep headwater catchment, Chiricahua Mountains, Arizona, USA. Geomorphology 300:13–30.

    DOI: 10.1016/j.geomorph.2017.09.028Save Citation »Export Citation » Share Citation »

    Monitors a 0.075 km2 area of high burn severity with five repeat ground-based lidar surveys. They found that 69 percent of the total erosion occurs due to hillslope erosion and only 31 percent of the erosion originated from channelized areas. In addition, they test the sediment volume predictions suggested by previous researchers and found that their measurements of erosion are on the same order of magnitude as two empirical models.

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  • DiBiase, R. A., and M. P. Lamb. 2013. Vegetation and wildfire controls on sediment yield in bedrock landscapes. Geophysical Research Letters 40.6: 1093–1097.

    DOI: 10.1002/grl.50277Save Citation »Export Citation » Share Citation »

    Identifies the role of vegetation in creating sediment dams that store sediment in steep landscapes. They subsequently suggest that after wildfire burns vegetation the loose sediment is released from the vegetation sediment dams and moves downhill as dry ravel. Moreover, the article ties in the loss of soil from dry ravel to increased soil production rates after wildfires.

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  • Lane, P. N., G. J. Sheridan, and P. J. Noske. 2006. Changes in sediment loads and discharge from small mountain catchments following wildfire in south eastern Australia. Journal of Hydrology 331.3–4: 495–510.

    DOI: 10.1016/j.jhydrol.2006.05.035Save Citation »Export Citation » Share Citation »

    This paper provides a dataset of post-wildfire hydrology and erosion measurements. Compares the pre- and post-burned hydrology and erosion characteristics, showing that post-fire erosion was eight to nine times pre-fire erosion. This study provides a compilation of data in different locations where runoff and erosion statistics are needed.

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  • Reneau, S. L., D. Katzman, G. A. Kuyumjian, A. Lavine, and D. V. Malmon. 2007. Sediment delivery after a wildfire. Geology 35.2: 151–154.

    DOI: 10.1130/G23288A.1Save Citation »Export Citation » Share Citation »

    Explores post-wildfire sedimentation in a reservoir, and the researchers document the differential transport of fine versus coarse sediment. They found that ash was primarily transported from the hillslopes to the reservoir within the first year after the wildfire, whereas coarse sediment was transported and stored in fluvial channels initially. In the following years, the amount of ash declined, but coarse sediment was moved into the reservoir during snowmelt events, as opposed to flash floods generated by summer convective storms.

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  • Robichaud, P. R., J. W. Wagenbrenner, F. B. Pierson, K. E. Spaeth, L. E. Ashmun, and C. A. Moffet. 2016. Infiltration and interrill erosion rates after a wildfire in western Montana, USA. Catena 142:77–88.

    DOI: 10.1016/j.catena.2016.01.027Save Citation »Export Citation » Share Citation »

    This study suggests that ground cover is the most important factor influencing infiltration and erosion. The article showed that unburned soil surfaces have high water repellency, and burned soil was water repellent below the surface. They also show that a bare-unburned soil produced a high degree of surface erosion that did not decrease over time; however, the burned soil produced erosion during the first few years after fire and then erosion decreased.

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  • Sheridan, G. J., P. N. Lane, and P. J. Noske. 2007. Quantification of hillslope runoff and erosion processes before and after wildfire in a wet Eucalyptus forest. Journal of Hydrology 343.1–2: 12–28.

    DOI: 10.1016/j.jhydrol.2007.06.005Save Citation »Export Citation » Share Citation »

    Examines post-wildfire hydrology in a wet Eucalyptus forest where background rates of saturated hydraulic conductivity are high. They found that despite an increase in water repellency, the infiltration rates remained high after the wildfire. In this environment, the effect of overland flow is minimized, and most of the erosion is due to rainsplash. This work suggests that the forest-type and soil infiltration rates before a wildfire help to determine the degree of overland flow.

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  • Staley, D., T. Wasklewicz, and J. Kean. 2014. Characterizing the primary material sources and dominant erosional processes for post-fire debris flow initiation in a headwater basin using multi-temporal terrestrial laser scanning data. Geomorphology 214:324–338.

    DOI: 10.1016/j.geomorph.2014.02.015Save Citation »Export Citation » Share Citation »

    Uses ground-based lidar to show the changes in a small basin before and after a long-duration storm (sixty-one hours). They show that the majority of erosion occurs on unchannelized hillslopes, and channelized incision only accounts for 3 percent of the total sediment budget. A benefit of this research is the detailed quantification of erosion; for example, they report a total sediment yield of 383.8 ± 80.7 t ha−1 for a 0.01 km2 basin.

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  • Wester, T., T. Wasklewicz, and D. Staley. 2014. Functional and structural connectivity within a recently burned drainage basin. Geomorphology 206:362–373.

    DOI: 10.1016/j.geomorph.2013.10.011Save Citation »Export Citation » Share Citation »

    The study used ground-based lidar to show connectivity of erosion and hydrology. The rill/gully areas are discontinuous. Erosion was observed to abruptly stop, and then deposition would begin. Moreover, the rill-gully flow networks sometimes received non-local inputs of sediment. In other locations, the contributing area upstream of the rill network was typically erosive, and this sediment routed directly into the downstream rill-gully network that developed.

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Empirically Based Models of Post-Wildfire Erosion

A suite of research papers has provided empirical models that could be used to predict post-wildfire erosion and may have practical applications. Sediment yield has been estimated as a linear function of rainfall (Malmon, et al. 2007), as a power-law function dependent on slope and burn severity (Pelletier and Orem, 2014), and as a power-law function dependent on ground cover, rainfall intensity and upstream drainage area (Wagenbrenner and Robichaud 2013). Moody, et al. 2008 explores how erosion varied in burn areas with different underlying geology and found generally similar results, showing that erosion was a function of hillslope stream power.

  • Malmon, D. V., S. L. Reneau, D. Katzman, A. Lavine, and J. Lyman. 2007. Suspended sediment transport in an ephemeral stream following wildfire. Journal of Geophysical Research: Earth Surface 112.F2.

    DOI: 10.1029/2005JF000459Save Citation »Export Citation » Share Citation »

    This study found that the suspended sediment concentration after a fire increased by two orders of magnitude and was invariant over a 10 km study reach. Their data suggest a strong relationship between the total rainfall and suspended sediment concentration. Moreover, they saw the highest concentration at the flood bore and decreasing sediment concentration afterward. Using this dataset, they derived an empirical equation to estimate sediment yield.

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  • Moody, J. A., D. A. Martin, and S. H. Cannon. 2008. Post-wildfire erosion response in two geologic terrains in the western USA. Geomorphology 95:103–118.

    DOI: 10.1016/j.geomorph.2007.05.011Save Citation »Export Citation » Share Citation »

    Characterizes erosion in terms of erodibility efficiency (i.e., the percentage of the available stream power required to erode sediment). They estimate erosion in channels from DEM differencing of photogrammetry and sediment collection equipment on hillslopes. They found that erosional efficiency was similar in two study areas.

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  • Pelletier, J. D., and C. A. Orem. 2014. How does sediment yield from post-wildfire debris-laden flows depend on terrain slope, soil burn severity class, and drainage basin area? Insights from airborne-LiDAR change detection. Earth Surface Processes and Landforms 39:1822–1832.

    DOI: 10.1002/esp.3570Save Citation »Export Citation » Share Citation »

    Uses airborne lidar to map changes in erosion before and after a wildfire. They focus on a debris flow dominated watershed. They found that sediment volume is proportional to upstream contributing area. In addition, they estimated sediment yield as the total sediment volume divided by the contributing area, and this effectively normalized the erosion by drainage area. They saw that yield increased with both slope and burn severity.

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  • Wagenbrenner, J., and P. Robichaud. 2013. Post-fire bedload sediment delivery across spatial scales in the interior western United States. Earth Surf. Process. Landforms 39.7: 865–876.

    DOI: 10.1002/esp.3488Save Citation »Export Citation » Share Citation »

    The majority of studies on post-wildfire erosion use small hillslope plots, and there is little guidance on how to scale the sediment yield measured at the hillslope scale to predict the erosion at the larger watershed scale. Develops an empirical equation and shows that sediment yield decreased with increasing drainage area and groundcover, and conversely it increased with ten minute rainfall intensity.

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Post-Wildfire Debris Flow Measurements

Recent research has helped to identify and quantify the hazards posed by post-wildfire debris flows. Cannon, et al. 2001 recognized that, unlike debris flows that initiate from landslides, post-wildfire debris flows can start from overland flow runoff erosion. And Santi, et al. 2008 shows how the erosion from debris flows increases in channels in the downstream direction. Nyman, et al. 2015 further helped to document the contribution of different burned landscape features to the sediment delivery that ultimately contributes to debris flows. Langhans, et al. 2017 also notes the presence of small debris flows on hillslopes. Wondzell and King 2003 suggests that runoff-generated debris flows are less common in the Pacific Northwest than in the Rocky Mountain region of the United States due to few high-intensity thunderstorms. Measuring debris-flow timing is difficult but can be done with in situ sensors such as pressure transducers (Kean, et al. 2012), and measurements reveal that debris flows can be triggered within a few minutes of peak rainfall intensities.

  • Cannon, S. H., E. R. Bigio, and E. Mine. 2001. A process for fire-related debris flow initiation, Cerro Grande fire, New Mexico. Hydrological Processes 15:3011–3023.

    DOI: 10.1002/hyp.388Save Citation »Export Citation » Share Citation »

    Links debris-flow initiation to fine sediment (attributed to wood ash) in post-wildfire overland flow. They note that in the first storms, which produced debris flows, the amount of fine material (< 2 mm) was more abundant than in subsequent storms. In addition, their observations suggest that debris flows initiated from dispersed rilling rather than downstream channel scour.

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  • Kean, J. W., D. M. Staley, and S. H. Cannon. 2011. In situ measurements of post‐fire debris flows in southern California: Comparisons of the timing and magnitude of 24 debris‐flow events with rainfall and soil moisture conditions. Journal of Geophysical Research: Earth Surface 116.F4.

    DOI: 10.1029/2011JF002005Save Citation »Export Citation » Share Citation »

    Provides a unique set of measurements documenting twenty-four unique debris flows that occurred after wildfires. The authors used field sensors to document debris-flow properties as well as the triggering rainfall event. One of the most important observations from this study is that debris-flow stage is shown to lag rainfall peaks by only five minutes on average.

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  • Kean, J. W., D. M. Staley, R. J. Leeper, K. M. Schmidt, and J. E. Gartner. 2012. A low‐cost method to measure the timing of postfire flash floods and debris flows relative to rainfall. Water Resources Research 48.5.

    DOI: 10.1029/2011WR011460Save Citation »Export Citation » Share Citation »

    Rapidly instrumenting watersheds to monitor post-wildfire debris flows is challenging because it is difficult to determine whether a debris flow will take place. Therefore, easily deployable field instrumentation is crucial, and this paper shows the utility of using pressure-transducers paired with rainfall data to make field observations of debris-flow timing and magnitude. The authors furthermore show that the basin response time between floods and debris flows is both short (less than thirty minutes) and similar to one another.

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  • Langhans, C., P. Nyman, P. J. Noske, R. E. Van der Sant, P. N. Lane, and G. J. Sheridan. 2017. Post-fire hillslope debris flows: Evidence of a distinct erosion process. Geomorphology 295:55–75.

    DOI: 10.1016/j.geomorph.2017.06.008Save Citation »Export Citation » Share Citation »

    This paper highlights a post-wildfire process, which the authors name hillslope debris flows (these differ from channelized debris flow). They describe the mechanics, identify literature where this process has been observed by other researchers, and develop a suite of experiments to document initiation conditions of hillslope debris flows. There is evidence that hillslope debris flows may occur worldwide, but this research suggests they do not occur in all landscapes.

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  • Nyman, P., H. G. Smith, C. B. Sherwin, C. Langhans, P. N. Lane, and G. J. Sheridan. 2015. Predicting sediment delivery from debris flows after wildfire. Geomorphology 250:173–186.

    DOI: 10.1016/j.geomorph.2015.08.023Save Citation »Export Citation » Share Citation »

    Document debris-flow volumes, locations, and controlling factors in Australia. This paper provides a data compilation including: documentation on sediment yields, the relevant contribution of sediment from hillslopes and channels, and volume estimates from debris flows. The authors also demonstrate relationships between burn severity and debris-flow susceptibility. The tabular data will be especially useful to researchers investigating debris-flow trends.

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  • Santi, P. M., J. D. Higgins, S. H. Cannon, and J. E. Gartner. 2008. Sources of debris flow material in burned areas. Geomorphology 96.3: 310–321.

    DOI: 10.1016/j.geomorph.2007.02.022Save Citation »Export Citation » Share Citation »

    This study investigated the sediment provenance for material that was ultimately moved as debris flows in forty-six burned areas. They found that the volume of material entrained in debris flows grew at an average rate of 2.5 m3 per meter of channel length in the downstream direction. They attribute only 3 percent of the total debris-flow volume to rills.

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  • Wondzell, S. M., and J. G. King. 2003. Postfire erosional processes in the Pacific Northwest and Rocky Mountain regions. Forest Ecology and Management 178:75–87.

    DOI: 10.1016/S0378-1127(03)00054-9Save Citation »Export Citation » Share Citation »

    Points out regional post-wildfire erosion differences between the Pacific Northwest and the Rocky Mountains of the United States. They note that while burned soils in the Pacific Northwest have low enough infiltration capacities to generate surface erosion after wildfire, this process has been rarely documented, unlike in the Rocky Mountains. They suggest that this is due to more convective thunderstorms in the Rocky Mountains.

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Physically Based Numerical Models of Post-Wildfire Erosion

A few models have emerged to help with estimating post-wildfire erosion and flooding. Robichaud, et al. 2007 developed ERMiT to probabilistically estimate erosion after wildfire; this model is subsequently tested by Robichaud, et al. 2016, and the results are shown to reasonably match observations. McGuire, et al. 2016 uses a more physics-based approach to explore different sediment transport mechanisms after wildfire such as rainsplash erosion and overland flow erosion, and McGuire, et al. 2017 creates a physically based numerical model capable of simulating rainfall, runoff, and sediment transport processes. Miller, et al. 2016 shows that it is possible to use GeoWEPP to model overland flow in burned areas over a large scale to estimate erosion, focusing on post-wildfire overland flow as well as dry ravel. Roering and Gerber 2005 shows the utility of a hillslope diffusion model to constrain the dry-ravel portion of post-wildfire erosion. Finally, Kean, et al. 2016 uses a physically based model to show the large differences in depth and shear stress between post-wildfire floods and post-wildfire debris flows.

  • Kean, J. W., L. A. McGuire, F. K. Rengers, J. B. Smith, and D. M. Staley. 2016. Amplification of postwildfire peak flow by debris. Geophysical Research Letters 43.16: 8545–8553.

    DOI: 10.1002/2016GL069661Save Citation »Export Citation » Share Citation »

    Quantifies the difference in discharge and depth between post-wildfire floods and debris flows. The results show that debris flows are much deeper than floods and the shear stress generated by debris flows is commensurately larger. Also uses a model based on runoff to estimate the length of the debris-flow granular front and the tail, which could be useful in both fore- and hindcasting debris-flow depths and surge lengths.

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  • McGuire, L. A., J. W. Kean, 
D. M. Staley, F. K. Rengers, and T. A. Wasklewicz. 2016. Constraining the relative importance of raindrop- and flow-driven sediment transport mechanisms
 in postwildfire environments
 and implications for recovery time scales. Journal of Geophysical Research: Earth Surface

    DOI: 10.1002/2016JF003867Save Citation »Export Citation » Share Citation »

    This study seeks to understand the physical processes that are causing interrill erosion. They show that the majority of interrill erosion results from raindrop detachment of sediment rather than the shear stress of shallow overland flow. This helps to physically explain the observations of several different authors who recently used lidar to show that the majority of erosion on hillslopes occurs via interrill erosion.

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  • McGuire, L. A., F. K. Rengers,
 J. W. Kean, and D. M. Staley. 2017. Debris flow initiation by runoff in a recently burned basin: Is grain-by-grain sediment bulking or en masse failure to blame? Geophysical Research
 Letters 44:7310–7319.

    DOI: 10.1002/2017GL074243Save Citation »Export Citation » Share Citation »

    Despite several competing theories for debris flows initiation in burned landscapes, there are two processes that are frequently cited: (1) sediment bulking as water flows gradually increase in sediment concentration and (2) en masse failure of sediment stored in fluvial channels. The authors compare measured debris-flow data with a physically based numerical model. They found that a model invoking en masse failure, using Mohr-Coulomb criteria for bed failure, best explained the observed data.

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  • Miller, M. E., W. J. Elliot, M. Billmire, P. R. Robichaud, and K. A. Endsley. 2016. Rapid-response tools and datasets for post-fire remediation: Linking remote sensing and process-based hydrological models. International Journal of Wildland Fire 25.10: 1061–1073.

    DOI: 10.1071/WF15162Save Citation »Export Citation » Share Citation »

    Focuses on a new tool called the Rapid Response Erosion Database and reviews several model platforms for post-wildfire hydrology and erosion predictions. They highlight the WEPP model, which has a hillslope version and a watershed version. The Disturbed WEPP version predicts average annual runoff/erosion, and the Erosion Risk Management Tool (ERMiT) is designed for single events. They also highlight the model RAVEL RAT for modeling hillslope ravel.

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  • Robichaud, P. R., W. J. Elliot, S. A. Lewis, and M. E. Miller. 2016. Validation of a probabilistic post-fire erosion model. International Journal of Wildland Fire 25.3: 337–350.

    DOI: 10.1071/WF14171Save Citation »Export Citation » Share Citation »

    This study tests the Erosion Risk Management Tool (ERMiT) against real-world observations. The inputs are topography, soil burn severity, soil texture, local climate, and erosion mitigation treatments. ERMiT outputs the probability of a selected runoff depth and sediment delivery using stochastically generated rainfall intensity and snowmelt. In this study they test ERMiT in eight areas where sediment yield was measured after wildfire and found a reasonable match with site observations.

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  • Robichaud, P. R., W. J. Elliot, F. Pierson, D. E. Hall, and C. A. Moffet. 2007. Predicting postfire erosion and mitigation effectiveness with a web-based probabilistic erosion model. Catena 71:229–241.

    DOI: 10.1016/j.catena.2007.03.003Save Citation »Export Citation » Share Citation »

    This study describes the Erosion Risk Management Tool (ERMiT). ERMiT uses Water Erosion Prediction Project (WEPP) to determine the probability of erosion based on land use, wildfire impact, and types of post-wildfire mitigation. WEPP simulates rill and interrill water runoff and erosion. It uses a module called the Climate Generator (CLIGEN) to estimate precipitation based on historical data. The model ultimately produces a distribution of potential erosion rates.

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  • Roering, J. J., and M. Gerber. 2005. Fire and the evolution of steep, soil-mantled landscapes. Geology 33:349–352.

    DOI: 10.1130/G21260.1Save Citation »Export Citation » Share Citation »

    Observed that within hours of a wildfire, dry ravel moved colluvium downhill on slopes >30°, removing centimeters of soil and exposing bedrock. They used sediment trap data to calibrate a nonlinear hillslope diffusion equation and found that the transport rate coefficient increases ~3.5 times and the critical slope angle decreased. Their model indicates that hillslope erosion after wildfire increased five times above the background levels.

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Statistical and Machine Learning Approaches to Post-Wildfire Debris-Flow Modeling

A suite of tools has been developed to compute important aspects of post-wildfire debris flows. With this unique numerical framework, these tools can test hypotheses and show that often in-channel mass failure of sediment initiates post-wildfire debris flows. In order to understand rainfall-thresholds that lead to debris flows, Staley, et al. 2013 and Staley, et al. 2017 developed regression models, and Kern, et al. 2017 and Nikolopoulos, et al. 2018 tested machine-learning models to predict the occurrence/non-occurrence of debris flows. Gartner, et al. 2014 developed a statistical approach to estimate the volume of post-wildfire debris flow, while Friedel 2011 used a more complicated genetic algorithm approach for volume predictions. In addition, Gartner, et al. 2015 statistically modeled the probability of debris-flow channels switching from erosion to deposition. Riley, et al. 2013 uses large datasets of debris flows to show statistically significant differences between post-fire debris flows and non-fire related debris flows.

  • Friedel, M. J. 2011. A data-driven approach for modeling post-fire debris flow volumes and their uncertainty. Environmental Modelling & Software 26.12: 1583–1598.

    DOI: 10.1016/j.envsoft.2011.07.014Save Citation »Export Citation » Share Citation »

    This study uses a genetic algorithm approach to develop an equation for predicting debris-flow volumes. The approach appeared to marginally improve debris-flow volume predictions compared with an equation developed using a multiple linear regression. This approach has not been tested against many new debris flows to confirm its utility.

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  • Gartner, J. E., S. H. Cannon, and P. M. Santi. 2014. Empirical models for predicting volumes of sediment deposited by debris flows and sediment-laden floods in the transverse ranges of southern California. Engineering Geology 176:45–56.

    DOI: 10.1016/j.enggeo.2014.04.008Save Citation »Export Citation » Share Citation »

    This paper presents two statistical models used to predict debris-flow volumes in the Transverse Range of Southern California. The first model is used to predict volumes regardless of the time since fire, and the second statistical model is used to predict the sediment volume within two years of a wildfire. Currently, this is the debris-flow volume model used in USGS debris-flow assessments.

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  • Gartner, J. E., P. M. Santi, and S. H. Cannon. 2015. Predicting locations of post-fire debris flow erosion in the San Gabriel Mountains of Southern California. Natural Hazards 77.2: 1305–1321.

    DOI: 10.1007/s11069-015-1656-3Save Citation »Export Citation » Share Citation »

    Provides a suite of observations showing where debris flows change from erosional to depositional. They subsequently produced a multiple logistic regression equation that was used to predict portions of the channel network that would be erosional and depositional based on the channel geomorphic characteristics. These data and approach could have important uses in predicted debris-flow inundation locations.

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  • Kern, A., P. Addison, T. Oommen, S. Salazar, and R. Coman. 2017. Machine learning based predictive modeling of debris flow probability following wildfire in the Intermountain Western United States. Mathematical Geosciences 49:717–735.

    DOI: 10.1007/s11004-017-9681-2Save Citation »Export Citation » Share Citation »

    Using a dataset of debris flows measured by the USGS, this study used a variety of machine-learning techniques to determine techniques that were successful in predicting debris flows. They used the following machine-learning approaches in this study: logistic regression, penalized regression, variants on discriminant analysis, support vector machine, neural network, k-nearest neighbor, naïve Bayes, and classification trees. They found that the naïve Bayes and the mixture discriminant analysis provided the best results.

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  • Nikolopoulos, E. I., E. Destro, M. A. E. Bhuiyan, M. Borga, and E. N. Anagnostou. 2018. Evaluation of predictive models for post-fire debris flows occurrence in the western United States. Natural Hazards Earth System Sciences 18:2331–2343.

    DOI: 10.5194/nhess-18-2331-2018Save Citation »Export Citation » Share Citation »

    Explores the ability of a random forest model to discriminate between rainfall events that do/do not produce post-wildfire debris flows. Explored two random forest models with different variables. They found that both models performed well in isolating debris flow from non-debris flow events in burn areas.

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  • Riley, K. L., R. Bendick, K. D. Hyde, and E. J. Gabet. 2013. Frequency–magnitude distribution of debris flows compiled from global data, and comparison with post-fire debris flows in the western US. Geomorphology 191:118–128.

    DOI: 10.1016/j.geomorph.2013.03.008Save Citation »Export Citation » Share Citation »

    Performs a statistical analysis of 988 debris flows across the globe. The study compares post-wildfire versus non-wildfire-initiated debris flows. They suggest that post-fire debris flows are typically smaller in volume than debris flows caused in the absence of wildfire. However, they infer from the distributions that post-fire debris flows have a higher recurrence interval than non-wildfire debris flows.

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  • Staley, D. M., J. W. Kean, S. H. Cannon, K. M. Schmidt, and J. L. Laber. 2013. Objective definition of rainfall intensity–duration thresholds for the initiation of post-fire debris flows in southern California. Landslides 10.5: 547–562.

    DOI: 10.1007/s10346-012-0341-9Save Citation »Export Citation » Share Citation »

    The researchers of this study used a database of debris flows and rainfall in order to determine a rainfall threshold for debris flows. Using receiver operating characteristics (ROC) they minimized the false positives and false negatives of rainfall that would trigger debris flows. This approach was applied to two areas in southern California, making the rainfall thresholds regionally specific. However, the approach could be used in many regions.

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  • Staley, D. M., J. A. Negri, J. W. Kean, J. L. Laber, A. C. Tillery, and A. M. Youberg. 2017. Prediction of spatially explicit rainfall intensity–duration thresholds for post-fire debris flow generation in the western United States. Geomorphology 278:149–162.

    DOI: 10.1016/j.geomorph.2016.10.019Save Citation »Export Citation » Share Citation »

    Using a database of 1,550 documented debris flows in wildfire areas, this study develops a logistic regression model that is effective at predicting the likelihood of debris flows. In addition, the article shows how the model can be used to predict the rainfall prediction thresholds for debris-flow initiation.

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Geomorphic Legacy of Wildfires and Process Recovery

Wildfires create distinct changes to soils and hydrology that can last for at least one hydrologic season after wildfire (Orem and Pelletier 2015) and often several years. For example, Mayor, et al. 2007 shows that wildfire can elevate runoff and erosion for up to six years after a wildfire, but these differences are influenced by the local climate. Noske, et al. 2016 suggests that slope and aspect has a strong influence on the recovery of pre-fire hydrology and erosion processes. Moreover, the way that erosion influences landscapes after a wildfire is highly dependent on the connectivity of sediment transport pathways (Williams, et al. 2016). Finally, wildfires have long-term landscape consequences; for example, Kirchner, et al. 2001 found that decadal-scale sediment yield measurements do not sufficiently explain millennial-scale without accounting for post-fire erosion events. Istanbulluoglu and Bras 2005 shows that fires set the stage for increased erosion/deposition activity that ultimately shapes overall landscape evolution.

  • Istanbulluoglu, E., and R. L. Bras. 2005. Vegetation‐modulated landscape evolution: Effects of vegetation on landscape processes, drainage density, and topography. Journal of Geophysical Research: Earth Surface 110.F2.

    DOI: 10.1029/2004JF000249Save Citation »Export Citation » Share Citation »

    Investigates the long-term influence of wildfire on landscape evolution simulating a period of six hundred thousand years. They model fire recurrence using a probabilistic approach with a mean recurrence interval of two hundred years and subsequently reduce soil cohesion parameters in the model following wildfires. This modeling shows that after a wildfire disturbance there is an increase in landscape dissection, but many convex areas are also infilled with sediment resulting in an overall decrease in basin relief.

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  • Kirchner, J. W., R. C. Finkel, C. S. Riebe, et al. 2001. Mountain erosion over 10-year, 10,000-year, and 10,000,000-year timescales. Geology 29:591–594.

    DOI: 10.1130/0091-7613(2001)029<0591:MEOYKY>2.0.CO;2Save Citation »Export Citation » Share Citation »

    Shows that sediment yield measurements obtained over decades in Idaho may underrepresent sediment yield estimates taken over much longer millennial timescales because rare episodic events make up an oversized proportion of the total sediment yield. In particular the article notes the role of post-wildfire debris flows. They show that a single post-fire debris flow can deliver an order of magnitude more sediment than the total sediment measured over a twenty-five-year period.

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  • Mayor, A. G., S. Bautista, J. Llovet, and J. Bellot. 2007. Post-fire hydrological and erosional responses of a Mediterranean landscape: Seven years of catchment-scale dynamics. Catena 71:68–75.

    DOI: 10.1016/j.catena.2006.10.006Save Citation »Export Citation » Share Citation »

    This study measures post-wildfire runoff and erosion for a period of seven years in paired, burned, and unburned watersheds. They show that runoff and sediment yield increased for the first three years in the burned catchment after wildfire and then began to recover. Moreover, burned watersheds produced more runoff and erosion than unburned watersheds for up to six years after the wildfire. The researchers have identified drought as a key factor in delaying post-wildfire vegetation recovery, which subsequently influenced both erosion and runoff.

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  • Noske, P. J., P. N. J. Lane, P. Nyman, and G. J. Sheridan. 2016. Effects of aridity in controlling the magnitude of runoff and erosion after wildfire. Water Resources Research 52:4338–4357.

    DOI: 10.1002/2015WR017611Save Citation »Export Citation » Share Citation »

    This study follows two catchments (one is polar-facing and the other has an equatorial-facing aspect) over 5 years following a wildfire and the researchers document the evolution of soil hydraulic conductivity, runoff, and soil erodibility. They show that runoff was elevated for four years after the wildfire. However, the sediment erosion rates were shown to decline to background levels by the second year after wildfire. The decline in erosion was attributed to an exhaustion of sediment supply. Moreover, they noticed a large difference in runoff and erosion in the catchments due to the hillslope aspect.

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  • Orem C. A., and J. D. Pelletier. 2015. Quantifying the time scale of elevated geomorphic response following wildfires using multi-temporal LiDAR data: An example from the Las Conchas fire, Jemez Mountains, New Mexico. Geomorphology 232:224–238.

    DOI: 10.1016/j.geomorph.2015.01.006Save Citation »Export Citation » Share Citation »

    Explores the connection between erosion from upland watersheds and the subsequent sediment balance on downstream alluvial fans in two adjacent watersheds. They observed a recovery in one area after one year but persistent erosion in the adjacent basin. The study only encompassed two years of monitoring. In addition, they found that a two-fold difference in rainfall in nearby basins produced a two-fold increase in sediment erosion.

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  • Williams, C. J., F. B. Pierson, P. R. Robichaud, O. Z. Al-Hamdan, J. Boll, and E. K. Strand. 2016. Structural and functional connectivity as a driver of hillslope erosion following disturbance. International Journal of Wildland Fire 25.3: 306–321.

    DOI: 10.1071/WF14114Save Citation »Export Citation » Share Citation »

    Investigates connectivity and defines structural connectivity as the connectivity of surface conditions that are susceptible to runoff and erosion. In contrast they define functional connectivity as the connectivity of runoff and erosion processes. They note that areas with larger amounts of bare ground produce more runoff and erosion, and therefore it is implied that these areas are more connected.

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Remote Sensing

The influence of wildfire can be observed with remotely sensed products. Díaz-Delgado, et al. 2002 and Riaño, et al. 2002 used satellite imagery to show how vegetation recovers after wildfire, and Robichaud, et al. 2007 leveraged imagery to reveal the severity of wildfire on a landscape. Several investigations have been used to show the applicability of remote-sensing metrics to describe different burn severities, and the difference Normalized Burn Severity (dNBR) has been recognized to be a widely useful metric (Cocke, et al. 2005; Eidenshink, et al. 2007; Escuin, et al. 2007; Miller and Thode 2007), which Key and Benson, 2006 explained how to field calibrate. Chen, et al. 2011 observes that burn severity has been seen to influence overall vegetation recovery, and Moody, et al. 2016 demonstrates that burn severity estimated via satellite imagery can be related to soil hydrologic processes.

  • Chen, X., J. E. Vogelmann, M. Rollins, et al. 2011. Detecting post-fire burn severity and vegetation recovery using multitemporal remote sensing spectral indices and field-collected composite burn index data in a ponderosa pine forest. International Journal of Remote Sensing 32.23: 7905–7927.

    DOI: 10.1080/01431161.2010.524678Save Citation »Export Citation » Share Citation »

    These researchers use Landsat imagery to track metrics of vegetation recovery for six years following a wildfire. They show that high burn severity areas recover more slowly than areas with lower burn severities. This approach could be a template for future studies.

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  • Cocke, A. E., P. Z. Fulé, and J. E. Crouse. 2005. Comparison of burn severity assessments using Differenced Normalized Burn Ratio and ground data. International Journal of Wildland Fire 14:189–198.

    DOI: 10.1071/WF04010Save Citation »Export Citation » Share Citation »

    The researchers used the dNBR statistic to map burn severity. Burn severity was not always coincident with tree mortality. Some areas dominated by Aspen groves had lower burn severities but higher overall tree mortality because aspen trees die back at lower temperatures. The ability of the dNBR statistic to accurately document the severity despite differences in tree mortality shows the robustness of this metric. The highest burn severities occurred in areas with high tree density and basal area, as well as fine fuel accumulation.

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  • Díaz-Delgado, R., F. Lloret, X. Pons, and J. Terradas. 2002. Satellite evidence of decreasing resilience in Mediterranean plant communities after recurrent wildfires. Ecology 83.8: 2293–2303.

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    This paper uses the Normalized Difference Vegetation Index (NDVI) to determine how vegetation recovers from wildfire. They are able to demonstrate that areas that are burned multiple times recover more slowly than areas that burn only once. They also showed a strong correlation between rainfall after the fire and NDVI.

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  • Eidenshink, J, B. Schwind, K. Brewer, Z.-L. Zhu, B. Quayle, and S. Howard. 2007. A project for monitoring trends in burn severity. The Journal of the Association for Fire Ecology 3:3–21.

    DOI: 10.4996/fireecology.0301003Save Citation »Export Citation » Share Citation »

    Explains the origin and products delivered from the Monitoring Trends in Burn Severity (MTBS) project. The MTBS project was initiated by the Wildland Fire Leadership Council in 2006 to understand the burn severity of wildfire in the United States as well as trends in wildfire over time, using Landsat imagery starting in 1984. They provide a useful working definition of burn severity and discuss how to convert dNBR data to a burn severity.

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  • Escuin, S., R. Navarro, and P. Fernández. 2007. Fire severity assessment by using NBR (Normalized Burn Ratio) and NDVI (Normalized Difference Vegetation Index) derived from LANDSAT TM/ETM images. International Journal of Remote Sensing 29:1053–1073.

    DOI: 10.1080/01431160701281072Save Citation »Export Citation » Share Citation »

    The researchers in this study sought to determine the most effective remote-sensing indices for mapping burn severity after a wildfire. They found that the dNBR metric provided the best discrimination between burned and unburned pixels. Moreover, they found that the burn severity was best assessed between pixels in the same post-fire image using the NBR from imagery of the burned area.

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  • Key, C. H, and N. C. Benson. 2006. Landscape assessment (LA). In FIREMON: Fire effects monitoring and inventory system. Gen. Tech. Rep. RMRS-GTR-164-CD. Fort Collins, CO: US Department of Agriculture, Forest Service, Rocky Mountain Research Station.

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    This technical report provides a practical guide for estimating burn severity. It shows the specific steps needed to create the dNBR statistic from Landsat imagery. The report also shows how to use a field sampling approach to calibrate and validate the remote sensing analysis.

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  • Kinoshita, A. M., and T. S. Hogue. 2011. Spatial and temporal controls on post-fire hydrologic recovery in Southern California watersheds. Catena 87.2: 240–252.

    DOI: 10.1016/j.catena.2011.06.005Save Citation »Export Citation » Share Citation »

    The researchers here use MODIS data to determine the Enhanced Vegetation Index (EVI) and dNBR. They show that vegetation recovery varies as a function of hillslope aspect. Furthermore, they show how vegetation recovery patterns translate to post-wildfire hydrologic recovery.

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  • Miller, J. D., and A. E. Thode. 2007. Quantifying burn severity in a heterogeneous landscape with a relative version of the delta Normalized Burn Ratio (dNBR). Remote Sensing of Environment 109:66–80.

    DOI: 10.1016/j.rse.2006.12.006Save Citation »Export Citation » Share Citation »

    Shows the utility of the relative differenced Normalized Burn Ratio (RdNBR). This statistic appears slightly more robust in delineating changes and for classifying burn severity than the dNBR statistic. They suggest that dNBR is better for showing vegetation mortality, whereas RdNBR shows the mortality with respect to pre-fire vegetation.

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  • Moody, J. A., B. A. Ebel, P. Nyman, D. A. Martin, C. Stoof, and R. McKinley. 2016. Relations between soil hydraulic properties and burn severity. International Journal of Wildland Fire 25.3: 279–293.

    DOI: 10.1071/WF14062Save Citation »Export Citation » Share Citation »

    This study uses remotely sensed data to correlate the burned severity of a soil with soil hydraulic properties. The article suggests that it is possible to use the dNBR to quantitatively define portions of a burned landscape with different soil hydraulic properties related to the wildfire. They show that below a dNBR value of 420 soil hydraulic properties are largely unaffected, but for dNBR>420 sorptivity decreases exponentially.

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  • Riaño, D., E. Chuvieco, S. Ustin, et al. 2002. Assessment of vegetation regeneration after fire through multitemporal analysis of AVIRIS images in the Santa Monica Mountains. Remote Sensing of Environment 79.1: 60–71.

    DOI: 10.1016/S0034-4257(01)00239-5Save Citation »Export Citation » Share Citation »

    This study uses remotely sensed data to create two indices for measuring vegetation regrowth after wildfire: the Regeneration Index (RI) and the Normalized Regeneration Index (NRI). The NRI showed a good estimate for the time of recovery in both northern mixed chaparral and coastal sage scrub ecosystems. They additionally used the Normalized Difference Vegetation Index to show recovery of the chaparral, but it was not as useful in the coastal sage scrub ecosystem.

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  • Robichaud P. R., S. A. Lewis, D. Y. M. Laes, A. T. Hudak, R. F. Kodaly, and J. A. Zamudio. 2007. Post-fire soil burn severity mapping with hyperspectral image unmixing. Remote Sensing of Environment 108.4: 467–480.

    DOI: 10.1016/J.RSE.2006.11.027Save Citation »Export Citation » Share Citation »

    This study used hyperspectral imagery with 5 m pixel-resolution to identify different ground components (ash, soil, scorched and green vegetation, litter and new litter). The results were compared with measured ground values and reasonable relationships were observed. Typically, Landsat images (30 m pixel resolution) are used to create a dNBR map. However, this approach is potentially better because it has a finer resolution and identifies different physical components of the ground surface.

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