François Pimont

Validation of FIRETEC wind-flows over a canopy and a fuel-break.

The wildfire model FIRETEC simulates the large coherent eddies of the wind-flows induced by the canopy. It has been qualitatively validated in its ability to simulate fire behavior, but there is still a need to validate physical submodels separately. In the present study, the dynamics and turbulence of the flow simulated by FIRETEC are validated in a manner similar to other air-flow models without fire, through comparison with measurements associated with flows within continuous and discontinuous forests captured through in situ and wind-tunnel experiments with neutral thermal stratification. The model is shown to be able to reproduce accurately all essential features of turbulent flow over both forests. Moreover, a short sensitivity study shows that the model is not very sensitive to uncertain parameters such as vegetation drag coefficient. Finally, it is shown in the discontinuous forest case that wind gusts on fuel-breaks can be very strong and significantly higher than in surrounding canopies, even if their directions are more stable. These results and others briefly reviewed in the present paper allow better understanding of wind-flow perturbations induced by fuel-breaks. This new validation added to previous ones confirms the ability of FIRETEC for investigating effects of fuel-break design on fire propagation.

Impacts of tree canopy structure on wind flows and fire propagation simulated with FIRETEC

Introduction Forest fuel management in the context of fire prevention generally induces heterogeneous spatial patterns of vegetation. However, the impact of the canopy structure on both wind flows and fire behavior is not well understood.Material and methods Here, a coupled atmosphere wildfire behavior model, HIGRAD/FIRETEC, was used to investigate the effects of canopy treatment on wind field and fire behavior in a typical Mediterranean pine ecosystem.Discussion First, the treatment-induced winds were simulated with the model. We observed that with decreasing cover fraction the wind velocity increased within the treated zone. The wind spatial variability increased when the vegetation was aggregated into larger clumps. Fire simulations indicated that a decrease of fire intensity occurred after several meters of propagation in the treated zone. This intensity decrease was significant with a cover fraction below 25%, but negligible with a cover fraction greater than 50%. The treatment also induced a more significant inclination of the plume away from vertical. The size of the tree clumps did not show significant effects on fire behavior.Conclusion This study was a preliminary investigation of wind/fire interaction over various canopy treatments, by using a physically based model. It gives some practical considerations for discerning the appropriate cover fraction and open perspectives for further investigations.

Fuel bulk density and fuel moisture content effects on fire rate of spread: a comparison between FIRETEC model predictions and experimental results in shrub fuels

Fuel bulk density and fuel moisture content effects on fire rate of spread were assessed in shrub fuels, comparing experimental data observed in outdoor wind tunnel burns and predictions from the physically-based model FIRETEC. Statistical models for the combined effects of bulk density and fuel moisture content were fitted to both the experimental and the simulated rate of spread values using non-linear regression techniques. Results confirmed a significant decreasing effect of bulk density on rate of spread in a power law in both laboratory burns and simulations. However, experimental data showed a lesser effect than simulations, suggesting a difference in the effective drag. Fuel moisture content effect was highly consistent, showing a similar exponential relationship with rate of spread in laboratory and in simulations. FIRETEC simulations showed similar orders of magnitude with predictions of two field-based empirical models, finding a significant correlation between rate of spread values. The study confirms the efficacy of the combined approach through experimental data and simulations to study fire behaviour.

Exploring three-dimensional coupled fire–atmosphere interactions downwind of wind-driven surface fires and their influence on backfires using the HIGRAD-FIRETEC model

The obstruction of ambient winds and the possible existence of indrafts downwind of a wildfire are aspects of coupled fire–atmosphere interaction influencing the effectiveness of a backfiring operation. The fire-influenced winds behind a headfire as well as their influences on backfire spread are explored using the three-dimensional HIGRAD-FIRETEC model. Fires are simulated under weak to strong wind speeds and in shrubland and grassland fuel types. The importance of three-dimensionality in the simulation of such phenomena is demonstrated. Results suggest that when fire–atmosphere interaction is constrained to two-dimensions, the limitations of air moving through the head fire could lead to overestimation of downwind indrafts and effectiveness of backfiring. Three-dimensional simulations in surface fuels suggest that backfires benefit from the obstruction of ambient winds and potentially the existence of an indraft flow in only a limited range of environmental conditions. Simulations show that flows are most favourable when the wildfire is driven downslope by a weak wind and the backfire is ignited at bottom of the slope. Model simulations are compared with backfiring experiments conducted in a dense shrubland. Although this exercise encountered significant difficulties linked to the ambient winds data and their incorporation into the simulation, predictions and observations are in reasonable agreement.

Coupled slope and wind effects on fire spread with influences of fire size: a numerical study using FIRETEC

Wind and slope are commonly accepted to be major environmental factors affecting the manner in which wildfires propagate. Fire-line width has been observed as having a significant effect on fire behaviour in some experimental fires. Most wildfire behaviour models and fire behaviour prediction systems take wind and slope effects into account when determining the rate of spread, but do not take into account the influence of fire width on the coupled effects of slope and wind. In the present study, the effect of topographic slope on rate of spread under weak (1 m s–1), moderate (5 m s–1) and strong (12 m s–1) wind speeds is investigated for two different initial fire widths (20 and 50 m) in a typical Mediterranean garrigue, using the coupled atmosphere–fire HIGRAD-FIRETEC model. The results show non-trivial combined effects and suggest a strong effect of fire width under low-wind conditions, especially for steep slopes. Simulated spread rates were compared with predictions of existing models of operational systems and a reasonable agreement was found. Additional exploratory simulations of fire behaviour in small canyons are provided. These simulations show how combined effects of wind, slope and fire-front size can induce different fire behaviours that operational models could fail to predict and provide insight that could be valuable for analysis of blow-up fires. These preliminary results also suggest that 3D physically based models could be used in the future to investigate how operational models can include non-local effects of fire propagation.

Effect of vegetation heterogeneity on radiative transfer in forest fires

Wildland fires are driven by the heat transferred from the fire source to the unburned fuel bed and this transfer is likely to be affected by the spatial heterogeneity of fuel element distributions at different scales from shoot to stand. In a context of theoretical fire modelling, we investigated the impact of a departure from randomness of fuel distributions on the radiative transfer of energy. Our methodology was derived from the approach developed for solar radiation in heterogeneous canopies or clouds and was modified to suit an analysis of fire behaviour. Some fine and coarse fuel distributions for several Mediterranean fuel types were derived from field measurements and plant architecture modelling. A comparison of the average irradiances in different fuels showed whether heterogeneity effects were significant or not. Results showed that both marked spatial variability in fuel distribution (low cover fraction and large clumps) and a high vegetation density were required to provide significant effects. The radiative transfer in heterogeneous maritime pines and in dense shrub stands was significantly affected by heterogeneity, mainly at crown and shoot scales. Less pronounced effects were observed in Aleppo pine stand and light shrubs. In terms of fuel modelling, the 2-m resolution used in a fire model such as FIRETEC seems to be sufficient for the fuel types investigated here, with the exception of dense small clumps in shrublands. An effective coefficient was proposed for these latter cases.

Estimating Leaf Bulk Density Distribution in a Tree Canopy Using Terrestrial LiDAR and a Straightforward Calibration Procedure

Leaf biomass distribution is a key factor for modeling energy and carbon fluxes in forest canopies and for assessing fire behavior. We propose a new method to estimate 3D leaf bulk density distribution, based on a calibration of indices derived from T-LiDAR. We applied the method to four contrasted plots in a mature Quercus pubescens forest. Leaf bulk densities were measured inside 0.7 m-diameter spheres, referred to as Calibration Volumes. Indices were derived from LiDAR point clouds and calibrated over the Calibration Volume bulk densities. Several indices were proposed and tested to account for noise resulting from mixed pixels and other theoretical considerations. The best index and its calibration parameter were then used to estimate leaf bulk densities at the grid nodes of each plot. These LiDAR-derived bulk density distributions were used to estimate bulk density vertical profiles and loads and above four meters compared well with those assessed by the classical inventory-based approach. Below four meters, the LiDAR-based approach overestimated bulk densities since no distinction was made between wood and leaf returns. The results of our method are promising since they demonstrate the possibility to assess bulk density on small plots at a reasonable operational cost.

Live fuel moisture content (LFMC) time series for multiple sites and species in the French Mediterranean area since 1996

Live fuel moisture content (LFMC), the ratio of water mass to dry mass of living shoots, is a primary driver of wildfire activity (Chandler et al. 1983; Dennison andMoritz 2009; Nolan et al. 2016) and fuel flammability (Marino et al. 2012; Rossa et al. 2016; Fares et al. 2017 Ruffault et al. 2018). LFMC is an input variable in several fire behavior models (Sullivan 2009; Alexander and Cruz 2013) and is often implicitly accounted for in fire hazard indices in Mediterranean areas (e.g. Viegas et al. 1999; Ruffault and Mouillot 2017). Despite the importance of LFMC for a wide range of wildfire research studies, its estimation at stand to landscape scales is still highly uncertain, because LFMC results from complex interactions between the antecedent and concurrent weather and several biological mechanisms that influence water content (i.e. plant water relations) and dry matter accumulation (i.e. carbon allocation at the leaf level) (Turner 1981; Jolly et al. 2014). There is therefore a need for robust and longterm LFMC datasets to improve our understanding of LFMC variations and refine our predictions. In 1996, the French organization in charge of protection of the Mediterranean forest (DPFM) initiated the systematic measurement of LFMC to improve its operational fire danger rating system during the fire season. Weekly measurements have been performed in various sites and shrub species over the fire-prone French Mediterranean. This operational network, called the “Reseau Hydrique” (what could be translated as “hydric network”) has been operated by the National Forest Service (Office National des Forêts (ONF)) since then. To date, the “Reseau Hydrique” produced a dataset that includes 584 “Sites × Years”, on 24 species, with 7 to 20 measurement dates per year. In addition, rainfall amounts during the fire season have been recorded since 2009 on some sites. The raw dataset is currently available on demand via the Reseau Hydrique website but, in its current form, cannot be easily used for scientific purposes for several reasons: (i) the database is not referenced (i.e. does not have a DOI); (ii) information is in French only; (iii) the labels and names of sampling sites and species names are not always consistent; (iv) outliers, duplications and inconsistencies in LFMC data have not been corrected; (v) measurement uncertainties (confidence levels) are not provided.

Modeling thinning effects on fire behavior with STANDFIRE

Key messageWe describe a modeling system that enables detailed, 3D fire simulations in forest fuels. Using data from three sites, we analyze thinning fuel treatments on fire behavior and fire effects and compare outputs with a more commonly used model.ContextThinning is considered useful in altering fire behavior, reducing fire severity, and restoring resilient ecosystems. Yet, few tools currently exist that enable detailed analysis of such efforts.AimsThe study aims to describe and demonstrate a new modeling system. A second goal is to put its capabilities in context of previous work through comparisons with established models.MethodsThe modeling system, built in Python and Java, uses data from a widely used forest model to develop spatially explicit fuel inputs to two 3D physics-based fire models. Using forest data from three sites in Montana, USA, we explore effects of thinning on fire behavior and fire effects and compare model outputs.ResultsThe study demonstrates new capabilities in assessing fire behavior and fire effects changes from thinning. While both models showed some increases in fire behavior relating to higher winds within the stand following thinning, results were quite different in terms of tree mortality. These different outcomes illustrate the need for continuing refinement of decision support tools for forest management.ConclusionThis system enables researchers and managers to use measured forest fuel data in dynamic, 3D fire simulations, improving capabilities for quantitative assessment of fuel treatments, and facilitating further refinement in physics-based fire modeling.

Correction to: “Modeling thinning effects on fire behavior with STANDFIRE”

The original article shows unit errors in Table 2: The torching index (TI) and crowning index (CI).