Predicting post-fire debris flow grain sizes and depositional volumes in the Intermountain West,United States

Post-fire debris flows represent one of the most erosive consequences associated with increasing wildfire severity and investigations into their downstream impacts have been limited. Recent advances have linked existing hydrogeomorphic models to predict potential impacts of post-fire erosion at watershed scales on downstream water resources. Here we address two key limitations in current models: (1) accurate predictions of post-fire debris flow volumes in the absence of triggering storm rainfall intensities and (2) understanding controls on grain sizes produced by post-fire debris flows. We compiled and analysed a novel dataset of depositional volumes and grain size distributions (GSDs) for 59 post-fire debris flows across the Intermountain West (IMW) collected via fieldwork and from the literature. We first evaluated the utility of existing models for post-fire debris flow volume prediction, which were largely developed for Southern California. We then constructed a new post-fire debris flow volume prediction model for the IMW using a combination of Random Forest modelling and regression analysis. We found topography and burn severity to be important variables, and that the percentage of pre-fire soil organic matter was an essential predictor variable. Our model was also capable of predicting debris flow volumes without data for the triggering storm, suggesting that rainfall may be more important as a presence/absence predictor, rather than a scaling variable. We also constructed the first models that predict the median, 16th percentile, and 84th percentile grain sizes, as well as boulder size, produced by post-fire debris flows. These models demonstrate consistent landscape controls on debris flow GSDs that are related to land cover, physical and chemical weathering, and hillslope sediment transport processes. This work advances our ability to predict how post-fire sediment pulses are transported through watersheds. Our models allow for improved pre- and post-fire risk assessments across diverse ranges of watersheds in the IMW.     

Effects of 2003 wildfires on stream chemistry in Glacier National Park,Montana

Changes in stream chemistry were studied for 4 years following large wildfires that burned in Glacier National Park during the summer of 2003. Burned and unburned drainages were monitored from December 2003 through August 2007 for streamflow, major constituents, nutrients, and suspended sediment following the fires. Stream‐water nitrate concentrations showed the greatest response to fire, increasing up to tenfold above those in the unburned drainage just prior to the first post‐fire snowmelt season. Concentrations in winter base flow remained elevated during the entire study period, whereas concentrations during the growing season returned to background levels after two snowmelt seasons. Annual export of total nitrogen from the burned drainage ranged from 1·53 to 3·23 kg ha^?1 yr^?1 compared with 1·01 to 1·39 kg ha^?1 yr^?1 from the unburned drainage and exceeded atmospheric inputs for the first two post‐fire water years. Fire appeared to have minimal long‐term effects on other nutrients, dissolved organic carbon, and major constituents with the exception of sulfate and chloride, which showed increased concentrations for 2 years following the fire. There was little evidence that fire affected suspended‐sediment concentrations in the burned drainage. Sediment yields in subalpine streams may be less affected by fire than in lower elevation streams because of the slow release rate of water during spring snowmelt. Published in 2008 by John Wiley & Sons, Ltd.     

The Influence of Culture on Wildfire Mitigation at Peavine Métis Settlement,Alberta, Canada

This article presents findings of a study that explored how culture influenced support for wildfire mitigation in Peavine Métis Settlement, an Aboriginal community located in Alberta, Canada. Community-based research was completed using interviews, focus groups, and participant observation. The results show that cultural factors appeared to influence wildfire mitigation preferences. Participants indicated the current state of the forest was not natural, and that mitigation activities would likely improve forest health. Participants supported Settlement Council-led wildfire mitigation activities at both the residential and community level due to a preference for communal action and collective problem solving. Participants also were found to distrust “outsiders” and preferred programs developed by members of their own community. The results of this study show that wildfire mitigation programs based on local culture can be well supported in an Aboriginal community.     

Magnetic enhancement in wildfire‐affected soil and its potential for sediment‐source ascription

Intense rainfall following wildfire can cause substantial soil and sediment redistribution. With concern for the increasing magnitude and frequency of wildfire events, research needs to focus on hydrogeomorphological impacts of fire, particularly downstream fluxes of sediment and nutrients. Here, we investigate variation in magnetic enhancement of soil by fire in burnt eucalypt forest slopes to explore its potential as a post‐fire sediment tracer. Low‐frequency magnetic susceptibility values (χ_lf) of <10 µm material sourced from burnt slopes (c. 8·0–10·4 × 10^?6 m³ kg^?1) are an order of magnitude greater than those of <10 µm material derived from long‐unburnt areas (0·8 × 10^?6 m³ kg^?1). Susceptibility of anhysteretic remanent magnetization (χARM) and saturation isothermal remanent magnetization (SIRM) values are similarly enhanced. Signatures are strongly influenced by soil and sediment particle size and storage of previously burnt material in footslope areas. Whilst observations indicate that signatures based on magnetic enhancement show promise for post‐fire sediment tracing, problems arise with the lack of dimensionality in such data. Magnetic grain size indicators χ_fd%, χARM/SIRM and χ_fd/χARM offer further discrimination of source material but cannot be included in numerical unmixing models owing to non‐linear additivity. This leads to complications in quantitatively ascribing downstream sediment to source areas of contrasting burn severity since sources represent numerical multiples of each other, indicating the need to involve additional indicators, such as geochemical evidence, to allow a more robust discrimination. Copyright © 2005 John Wiley & Sons, Ltd.     

Effects of gradient,distance, curvature and aspect on steep burned and unburned hillslope soil erosion and deposition

Wildfire denudes vegetation and impacts chemical and physical soil properties, which can alter hillslope erosion rates. Post‐wildfire erosion can also contribute disproportionately to long‐term erosion rates and landscape evolution. Post‐fire hillslope erosion rates remain difficult to predict and document at the hillslope scale. Here we use ²¹⁰Pb_aex (lead‐210 mineral‐adsorbed excess) inventories to describe net sediment erosion on steep, convex hillslopes in three basins (unburned, moderately and severely burned) in mountainous central Idaho. We analyzed nearly 300 soil samples for ²¹⁰Pb_aex content with alpha spectrometry and related net sediment erosion to burn severity, aspect, gradient, curvature and distance from ridgetop. We also tested our data against models for advective, linear and non‐linear diffusive erosion. Statistically lower net soil losses on north‐ versus south‐facing unburned hillslopes suggest that greater vegetative cover and soil cohesion on north‐facing slopes decrease erosion. On burned hillslopes, erosion differences between aspects were less apparent and net erosion was more variable, indicating that vegetation influences erosion magnitude and fire drives erosion variability. We estimated net soil losses throughout the length of unburned hillslopes, including through a footslope transition to concave form. In contrast, on burned hillslopes, the subtle shift from convex to concave form was associated with deposition of a post‐fire erosion pulse. Such overall patterns of erosion and deposition are consistent with predictions from a non‐linear diffusion equation. This finding also suggests that concave sections of overall convex hillslopes affect post‐disturbance soil erosion and deposition. Despite these patterns, no strong relationships were evident between local net soil losses and gradient, curvature, distance from ridgetop, or erosion predicted with advection or diffusion equations. The observed relationship between gradient and erosion is therefore likely more complex or stochastic than often described theoretically, especially over relatively short timescales (60–100 years). Copyright © 2016 John Wiley & Sons, Ltd.     

Geomorphic influences on the distribution and accumulation of pyrogenic carbon (PyC) following a low severity wildfire in northern New Mexico

The distribution, transport, and accumulation of wildfire‐generated pyrogenic carbon (PyC) has important consequences for contaminant transport and carbon cycling, but a conceptual model for PyC accumulation and loss that includes geomorphic processes is lacking. In this study we quantified PyC concentration in soil samples collected from the Jemez Mountains of New Mexico before and after the 2013 Thompson Ridge (TR) fire, and developed a conceptual model describing PyC redistribution. Pre‐fire samples were fortuitously collected 4 years before the TR burn and post‐fire samples were collected at the same locations 15 months following the TR fire. Samples were collected from the O and A horizon, with sites representing a range of slope angle, aspect, burn severity, and geomorphic setting. PyC was determined by a modified chemo‐thermal oxidation method to compare PyC to total organic carbon (TOC). The mean PyC/TOC ratio was significantly higher post‐fire than pre‐fire (0.14 vs 0.12), indicating increased PyC sequestration. O horizon PyC concentrations were more variable and more responsive to fire than the A horizon. Soil horizon, watershed, and geomorphic setting proved to be the most influential factors in predicting PyC concentration changes. PyC concentrations increased most on hillslopes and in low‐severity burn areas, suggesting higher rates of PyC production or post‐fire accumulation. Burn patchiness appears to facilitate PyC accumulation, with lower severity patches trapping PyC mobilized from high severity patches. While PyC content had greater point scale variance following the fire, the fire also homogenized pre‐fire PyC differences between soil horizons and among watersheds within the burn perimeter, differences that appear to develop over time between fires. The O horizon is a larger sink for PyC in the short term following fire, but based on pre‐fire concentrations the A horizon appears to be a more stable sink for PyC. Copyright © 2018 John Wiley & Sons, Ltd.     

Quartz luminescence sensitivity variation in the Chinese loess deposits: the potential role of wildfires

The sensitivity of optically stimulated luminescence (OSL) of quartz grains has been applied to trace the provenances of sediments in various environments. Here, we studied the OSL sensitivity of quartz grains in four sections (Jingyuan, Huanxian, Xifeng and Luochuan) along a west–east transect on the Chinese Loess Plateau (CLP). The quartz OSL sensitivity in the palaeosol units is higher overall than that in the loess units, and it shows an increasing trend from the west CLP to the east CLP. In palaeosol units, it increases by a factor of ~20 along the west–east transect; while in loess units it increases by a smaller degree of 3–5. The change of provenances and the change of distances between the provenances and the loess sections cannot fully account for the large degree of sensitivity variation between the loess and palaeosol units in the central and eastern CLP. We propose that wildfires may have played an important role in enhancing the quartz OSL sensitivity in the palaeosol units in the central and eastern CLP. Quartz OSL sensitivity has the potential to be used as a proxy for palaeowildfires.