Fire Danger Index Research Articles

Increased extreme fire weather occurrence in southeast Australia and related atmospheric drivers

There is evidence suggesting that fire activity in southeast Australia has increased in the past two decades. There is also anecdotal evidence that pyrocumulonimbus events have become more common, although the observed record is short. We explore the extent to which possible changes in fire and pyrocumulonimbus occurrence in southeast Australia can be explained by changes in the number of extreme (above the 95th percentile) days per year of two fire-weather indices, the Forest Fire Danger Index (FFDI) and the continuous Haines index (C-Haines). For the period 1958 through 2020, we show that there is a dependence between the number of extreme FFDI days per year and burned area for forested parts of southeast Australia. To a lesser extent, there is a relationship between the FFDI and pyrocumulonimbus occurrences. We find an increase in the occurrence of extreme FFDI days from the late 1990s, with up to 24 more extreme days per year. In the southwest of the state of Victoria, there are concomitant increases in temperature and wind speed, and decreases in relative humidity. We show that the structure of C-Haines has not changed, but extreme days more frequently coincide with extreme FFDI days. This suggests that possible observed increases in pyrocumulonimbus occurrences may partly be explained as a consequence of the increases in co-occurring extreme FFDI and C-Haines days, rather than due to changes in the atmospheric stability. We also find that changes in the atmospheric circulation are consistent with those seen in the fire weather indices. Increased surface and mid-tropospheric pressure since 1998 are consistent with a poleward shift of the storm track. In particular, an increase in anticyclonic anomalies in the Tasman Sea is indicative of enhanced blocking activity. Although blocking frequency in these longitudes has decreased overall, the proportion of extreme FFDI days that are considered blocked has increased.

Fire foci in South America: Impact and causes, fire hazard and future scenarios

Fire is used in the management of pastures, renewal and expansion of areas, and agricultural activities in South America (SA). The objectives of this study were: i) to identify the countries and regions with the highest number of fire foci in SA, and ii) to evaluate the spatial dynamics of fire foci based on the Meteorological Fire Danger Index (MFDI) and future scenarios through numerical simulations. Fire foci time series comprised 21 years (1998–2018) from the BDQueimadas database. Cluster Analysis (CA), descriptive and exploratory statistics were employed. Fire foci maps for SA were made in 10-km pixel dimensions. MFDI was used to assess fire danger via SPEEDY (Simplified Parametrizations, primitivE-Equation DYnamics) model simulations. Three simulations were performed: control scenario (1980–2015), RCP2.6 scenario (optimistic - 2016 and 2050), and RCP8.5 scenario (pessimistic - 2015 and 2050). Regionally, three homogeneous groups of fire foci (G1, G2 and G3) and one atypical (NA - Not Grouped) were identified for Brazil via CA. The highest fire foci occurred in Brazil (62.72%), followed by Bolivia (9.03%), Argentina (8.28%), Venezuela (6.11%), Paraguay (5.94%), and Colombia (3.87%), respectively. The highest density of fire foci occurred in the MATOPIBA region, the confluence of Maranhão, Tocantins, Piauí, and Bahia, - (agricultural frontier), and also in the Cerrado-Amazon transition and the states of Mato Grosso and Mato Grosso do Sul in Brazil, followed by Paraguay, Bolivia, Venezuela, and Argentina. The countries and regions of Brazil do not change, only intensify from year to year, and such fire foci variability may be associated with the El Niño-Southern Oscillation (ENSO) phases. The control scenario identified in east-central Brazil, western Bolivia, Paraguay, and northern Argentina. The optimistic scenario showed an improvement in some countries and a worsening in the territorial distribution in Brazil, Venezuela, and Colombia. The pessimistic scenario identified increased degradation compared to the previous scenarios in almost all SA countries.

A prescribed fire cost model for public lands in south-east Queensland

Prescribed fire is a management tool in many Australian and international ecosystems, where it can benefit biodiversity conservation and potentially reduce the risk of extreme wildfires threatening highly valued assets. However, the economic efficiency of prescribed fire is poorly understood. The aim of this study was to progress this understanding by improving estimates of prescribed fire costs. A generalized linear prescribed fire cost model for south-east Queensland has been developed from a dataset of 527 prescribed fire on public land over the period 2004 to 2015, to estimate prescribed fire costs per hectare as a function of environmental predictors. The best model explained 88% of the variance, and significant predictor variables were prescribed fire burned area, fire vegetation group (FVG), forest fire danger index (FFDI), fuel quantity, distance to the nearest building, distance to the nearest Queensland Parks and Wildlife Service and Partnerships (QPWS&P) base, and distance to the nearest freshwater body. Prescribed fire costs per hectare were negatively related to prescribed fire burned area, distances to the nearest building and nearest freshwater body and the FFDI, but positively related to fuel quantity and distance to nearest QPWS&P base. Prescribed fire costs also varied significantly between some FVGs. Prescribed fire burned area had the strongest influence on prescribed fire costs (83% of variation in cost explained). Prescribed fire cost for riparian, foredune and beach ridge vegetation was significantly higher than for heath, while prescribed fire cost for melaleuca vegetation was significantly lower than heath. Higher costs were associated with prescribed fires within 500 m of the nearest building. The model can support the estimation and justification of annual operational budgets for prescribed fire.

Fire Danger Harmonization Based on the Fire Weather Index for Transboundary Events between Portugal and Spain

Portugal and Spain have a cross-border cooperation protocol on wildfires response for a buffer strip of 25 km for each side of the border. In spite of the success of this collaboration, there are issues to be improved, since Portuguese and Spanish authorities use different methodologies to assess the daily fire danger. A methodology to harmonize fire danger and its interpretation by the Portuguese and Spanish Civil protection authorities in the transboundary buffer strip area is hereby presented. The fire danger index used is the Canadian Fire Weather Index (FWI), which requires input from meteorological data and gives an indication of fire intensity. The fire danger class is an important decision support tool for preventing and fighting wildfires. Since the meaning of FWI values change from region-to-region according to its specific characteristics, a calibration process was performed based on statistical data of the daily FWI values, the number of fires and burned area between 2005 and 2013. The results of the FWI calibration and harmonization of the data for the five danger classes minimizes the fire danger discrepancies across the border. This methodology has the potential to be reproduced in other areas.

Investigating the drivers of the unprecedented Chernobyl Power Plant Wildfire in April 2020 and its effects on 137Cs dispersal

In this study, we explore the conditions that led to the unprecedented wildfire that occurred in the Chernobyl Exclusion Zone in April 2020 and the effect of this fire on 137Cs dispersal, as wildfires are important drivers of 137Cs resuspension, with potentially harmful consequences for the receiving ecosystems. We characterised the historical wildfire record between 2000 and 2020 using the MCD64A1.006 MODIS Burned Area Monthly Global 500 m dataset and assessed the climatic conditions associated with these wildfire events using ERA5-Land reanalysis data. We also examined fire danger indices at the time of these wildfires. We then explored the widespread effects of the April 2020 wildfire on 137Cs resuspension and subsequent deposition using the NOAA-HYSPLIT model, concluding that the impacts of such resuspension on areas further afield were minimal. Results show that climatic conditions leading to severe wildfires are increasing, especially during March and April. High soil moisture, relative humidity and extreme temperature anomalies are associated with the largest wildfires on record, and fire risk indices at the time of the April 2020 fire were higher than for other large fires on record. We have estimated that 3854 GBq of 137Cs resuspended during the CPPF, with atmospheric transport dominant over Russia, Ukraine, Moldova and Kazakhstan. The observed increase in large wildfires will have implications for wildfire-driven soil erosion processes, which will further exacerbate the effects of atmospheric-driven 137Cs redistribution.

Evaluation of different parameter estimation techniques in extreme bushfire modelling for Victoria, Australia

Generalised Extreme Value (GEV) distribution can be used effectually to model extreme climatic events like bushfire. The major predicament of using GEV distribution is accurate determination of three parameters (location, scale, and shape); nevertheless, there are no specific guidelines to identify the most apposite parameter estimation technique of the GEV distribution for bushfire studies. In this study, influence of different GEV parameters estimation techniques were investigated in Victoria, Australia for extreme bushfire event modelling, using annual maximum forest fire danger index (FFDI), which is a combination of manifold climatic and fuel variables to indicate the potential for bushfires to propagate, and withstand suppression. Four GEV parameters estimation methods namely: Maximum Likelihood Estimation (MLE), Generalised Maximum Likelihood Estimation (GMLE), Bayesian and L-moments were used for two different timescale data (full data set and last 10 years of full dataset) to estimate the GEV distribution parameters. The return levels of FFDI for different Average Recurrence Interval (ARI) were estimated using the above mentioned four methods and two timescales. The study demonstrates that Fréchet (type II) extreme value distribution is pertinent for modelling the annual maximum FFDI for most of the selected stations; nonetheless, GEV distribution parameters can vary considerably due to variation in the length of the data series. Several applied statistical parameters namely: Mean square error (MSE) and Mean absolute error (MAE) were used to identify the most pertinent parameter estimation technique of the GEV distribution. The study reveals that L-moments can be used, even in the presence of the smaller data set. In addition, L-moments is the most appropriate parameters estimation technique of GEV distribution because of the presence of the lowest MSE and MAE values for most of the stations. The outcomes of this research are pivotal for Victorian public and private stakeholders to forecast the severity and intensity of imminent bushfire events due to recent bushfire events in fire prone areas.

Estimates of temporal-spatial variability of wildfire danger across the Pan-Arctic and extra-tropics

Satellite-based hot-spot analysis for the Pan-Arctic, shows that Asia experiences a greater number of fires compared to North America and Europe. While hot spots are prevalent through the year in Asia, Europe (North America) exhibits marked annual (semi-annual) variability. The Potential Fire Danger index (PFIv2) demonstrates increased fire incidence vulnerability across the Arctic and extra-tropics. Though not significant in northwestern North America and eastern Asia, significant-positive trends across the mid-latitudes of Eurasia and Siberia are clear. PFIv2 accurately matches the regional distribution of observed fires, while the worldwide used Fire Weather Index does not. Fire danger has evolved as distributions of short-interval precipitation events and background vegetation characteristics change. In conjunction with increased population and expanded infrastructure, frequent extreme events may increase pressure for new settlements that lead to greater fire exposure across the Pan-Arctic. Thus, PFIv2 may be useful for decision planners and danger managers to anticipate and minimize the adverse effects of indiscriminate fire use.

Prediction of Meteorological Parameter Time Series Data for the Forest Fire Early Warning System

A forest fire early warning system must be developed to reduce the impact of greater community losses. One effort to develop an early warning system is to use a forest fire hazard index as a potential assessment guide. The main factor which is a parameter in the fire hazard index calculation method is the meteorological parameter. In general, to know today’s fire hazard index is calculated from today’s weather conditions, but the need for an early warning system is to know the future fire hazard index. Based on a series of meteorological conditions data held for thirty-six months, using the backpropagation algorithm, it is estimated that the meteorological conditions will be several months to come. Several meteorological parameters have their respective roles, the unknown contribution of which is calculated. In this study, each parameter will be measured by predicting time series data and compared with the results of calculations. The method of calculating the forest fire index used is the McArthur Forest Fire Danger Index with the meteorological parameter elements are temperature, relative humidity, wind speed, and drought factor. Each parameter was trained in artificial neural networks and tested its predictions to produce accuracy for data series temperatures of 91.67%, the relative humidity of 83.33%, and wind speed of 50%.

Measuring a Fire. The Story of the January 2019 Fire Told from Measurements at the Warra Supersite, Tasmania

Non-stand-replacing wildfires are the most common natural disturbance in the tall eucalypt forests of Tasmania, yet little is known about the conditions under which these fires burn and the effects they have on the forest. A dry lightning storm in January 2019 initiated the Riveaux Road fire. This fire burnt nearly 64,000 ha of land, including tall eucalypt forests at the Warra Supersite. At the Supersite, the passage of the fire was recorded by a suite of instruments measuring weather conditions and fluxes (carbon, water and energy), while a network of permanent plots measured vegetation change. Weather conditions in the lead-up and during the passage of the fire through the Supersite were mild—a moderate forest fire danger index. The passage of the fire through the Supersite caused a short peak in air temperature coinciding with a sharp rise in CO2 emissions. Fine fuels and ground vegetation were consumed but the low intensity fire only scorched the understorey trees, which subsequently died and left the Eucalyptus obliqua canopy largely intact. In the aftermath of the fire, there was prolific seedling regeneration, a sustained reduction in leaf area index, and the forest switched from being a carbon sink before the fire to becoming a carbon source during the first post-fire growing season.

ИСТОЧНИКИ ДАННЫХ ГЛОБАЛЬНОГО МОНИТОРИНГА ВЛАЖНОСТИ ПОЧВЫ СРЕДСТВАМИ ДИСТАНЦИОННОГО ЗОНДИРОВАНИЯ ПОВЕРХНОСТИ ЗЕМЛИ

A review of modern tools of global monitoring of soil moisture by means of remote sensing of the Earth’s surface is presented. The characteristic features of the use of orbital radiometers and radars of C, X and L microwave bands for estimating the volumetric soil moisture at a depth of 5 cm and the root layer of vegetation are considered. A review of the capabilities of satellite gravimetry to assess the land water equivalent thickness is made. A number of sources have been proposed for obtaining estimates of soil water content from satellite based radiometric devices and orbital gravimetric systems. Based on the analysis of scientific research papers, the complexity of monitoring the level of fire danger indices in forests is shown, and the prospects of assessing soil moisture in agricultural regions using microwave orbital instruments are demonstrated, and the adequacy of calculating the moisture content in soil at a depth of up to one meter using satellite gravimetry is described.

Downscaled GCM climate projections of fire weather over Victoria, Australia. Part 2

This paper applies the Multivariate Adaptive Constructed Analogs (MACA) statistical downscaling method to 12 general circulation models to produce 21st century projections of fire weather variables over Victoria, Australia, under two emissions scenarios. The statistically downscaled model data accurately replicate the observed distributions of meteorological variables over the contemporary period, but underestimate fire danger extremes in some models. Under each climate scenario, both mean and extreme fire danger are expected to increase. Though there is variation across Victoria, the 12-model average by year for RCP8.5 indicates a 10–20% increase in extreme (99th percentile) Forest Fire Danger Index across the state, with the greatest change projected in the north-west region. At five geographically and climatologically different locations across Victoria, there is a 50–200% increase in the number of days per year exceeding the threshold for the Victorian Very High or higher fire danger rating by the end of the century compared with the start. The high-end warming (RCP8.5) scenario shows increased temperature to be the main driver of heightened fire danger. Changes in temperature, humidity and precipitation during spring and early summer both increase the length of the fire season and may reduce springtime opportunities for prescribed burning.

Downscaled GCM climate projections of fire weather over Victoria, Australia. Part 1

Anthropogenic climate change is expected to cause an increase in fire danger over south-eastern Australia during the 21st century, primarily driven by increased surface temperature. Studies of future fire weather in Victoria, Australia, have so far mostly utilised direct output from general circulation models, which have inadequate resolution for resolving the dynamics of local fire danger and are prone to substantial biases that may affect the seasonality of dry fuels. In this paper, we assess the ability of the Multivariate Adaptive Constructed Analogs (MACA) method to downscale output from general circulation models over Victoria, and replicate statistical attributes of fire danger indices. We find that climatological descriptors of meteorological variables of wind, temperature and humidity are captured extremely well, and fields on extreme fire days are well captured. We find that the method works very well for statistically downscaling fire weather elements over Victoria and provides a vehicle to assess the regional variation of fire weather projections over Victoria.

Fire weather effects on flammability of indigenous and invasive alien plants in coastal fynbos and thicket shrublands (Cape Floristic Region).

BackgroundGlobally, and in the Cape Floristic Region of South Africa, extreme fires have become more common in recent years. Such fires pose societal and ecological threats and have inter alia been attributed to climate change and modification of fuels due to alien plant invasions. Understanding the flammability of different types of indigenous and invasive alien vegetation is essential to develop fire risk prevention and mitigation strategies. We assessed the flammability of 30 species of indigenous and invasive alien plants commonly occurring in coastal fynbos and thicket shrublands in relation to varying fire weather conditions.MethodsFresh plant shoots were sampled and burnt experimentally across diverse fire weather conditions to measure flammability in relation to fire weather conditions, live fuel moisture, fuel load and vegetation grouping (fynbos, thicket and invasive alien plants). Flammability measures considered were: burn intensity, completeness of burn, time-to-ignition, and the likelihood of spontaneous ignition. We also investigated whether the drying of plant shoots (simulating drought conditions) differentially affected the flammability of vegetation groups.ResultsFire weather conditions enhanced all measures of flammability, whereas live fuel moisture reduced burn intensity and completeness of burn. Live fuel moisture was not significantly correlated with fire weather, suggesting that the mechanism through which fire weather enhances flammability is not live fuel moisture. It furthermore implies that the importance of live fuel moisture for flammability of evergreen shrublands rests on inter-specific and inter-vegetation type differences in fuel moisture, rather than short-term intra-specific fluctuation in live fuel moisture in response to weather conditions. Fuel load significantly increased burn intensity, while reducing ignitability. Although fire weather, live fuel moisture, and fuel load had significant effects on flammability measures, vegetation and species differences accounted for most of the variation. Flammability was generally highest in invasive alien plants, intermediate in fynbos, and lowest in thicket. Fynbos ignited rapidly and burnt completely, whereas thicket was slow to ignite and burnt incompletely. Invasive alien plants were slow to ignite, but burnt with the highest intensity, potentially due to volatile organic composition. The drying of samples resulted in increases in all measures of flammability that were comparable among vegetation groups. Flammability, and by implication fire risk, should thus not increase disproportionately in one vegetation group compared to another under drought conditions—unless the production of dead fuels is disproportionate among vegetation groups. Thus, we suggest that the dead:live fuel ratio is a potentially useful indicator of flammability of evergreen shrublands and that proxies for this ratio need to be investigated for incorporation into fire danger indices.

Observed and expected changes in wildfire-conducive weather and fire events in peri-urban zones and key nature reserves of the Czech Republic

Recent drought and a surge in days with weather conditions conducive to wildfire occurrence during 2015-2019 reminded the Czech Republic that it is not immune to this type of natural hazard. Although Central Europe has not been at the center of such events, observed climate data and climate projections suggest a tendency toward more years with wet and mild winters and dry and hot summers. To fill the existing knowledge gap, we used an ensemble of 9 fuel aridity metrics, including 3 dedicated fire weather indices, and evaluated their level of agreement with actual fire occurrence and their temporal trends. The analysis included peri-urban zones of the 36 largest cities and towns in the Czech Republic (home of 3.8 million inhabitants) and the 29 largest protected areas (covering 13.7% of the territory). Fire weather climatology, based on both the Fire Weather Index and the Forest Fire Danger Index, agreed well with the long-term frequency of fires both in peri-urban zones and within protected areas. Future projections based on regional and global model ensembles indicated a significant increase in fuel aridity and an increase in the area affected by fire-conducive conditions both around urban areas and within protected regions. In particular, the area affected by days with very high risk fire weather conditions is likely to increase significantly relative to the past 60 yr. However, the magnitude of the projected change depends to a large degree on the selected fire weather metric and whether RCM- or GCM-based scenarios are used.

Climate change driven changes of vegetation fires in the Czech Republic

With rising temperatures and decreasing soil water, the frequency of vegetation fires is increasing globally. We analyzed the impact of climate change on the frequency of vegetation fires that required intervention by the Czech Fire Rescue Service in the Czech Republic between 1971 and 2015. We analyzed spatial patterns across administrative regions and evaluated the role of climate and other contributing factors that could explain spatiotemporal shifts during the study period. Our study has shown that the frequency of vegetation fires has increased most in the warmest and driest areas of the Czech Republic countryside. The increase between 1991 and 2015 is about 70% in comparison with the 1971–1990 period. In all administrative regions, a statistically significant relationship of the frequency of vegetation fires and weather conditions has been demonstrated. In all regions of the study area, extreme occurrences of vegetation fires were due to a combination of drought and heat waves. We conclude that population growth at the wildland-urban interface causes a greater frequency of vegetation fires. However, the main reason for the statistically significant increase in the frequency of vegetation fires is the ongoing climate change, manifested by an increase in values of the Fire Danger Index and heat wave occurrence.

Increasing concurrence of wildfire drivers tripled megafire critical danger days in Southern California between1982 and 2018

Wildfire danger is often ascribed to increased temperature, decreased humidity, drier fuels, or higher wind speed. However, the concurrence of drivers—defined as climate, meteorological and biophysical factors that enable fire growth—is rarely tested for commonly used fire danger indices or climate change studies. Treating causal factors as independent additive influences can lead to inaccurate inferences about shifting hazards if the factors interact as a series of switches that collectively modulate fire growth. As evidence, we show that in Southern California very large fires and ‘megafires’ are more strongly associated with multiple drivers exceeding moderate thresholds concurrently, rather than direct relationships with extreme magnitudes of individual drivers or additive combinations of those drivers. Days with concurrent fire drivers exceeding thresholds have increased more rapidly over the past four decades than individual drivers, leading to a tripling of annual ‘megafire critical danger days’. Assessments of changing wildfire risks should explicitly address concurrence of fire drivers to provide a more precise assessment of this hazard in the face of a changing climate.

Improving firefighter tenability during entrapment and burnover: An analysis of vehicle protection systems

When attempting to suppress severe wildfire the possibility for firefighting crews to be overrun by wildfire, known as entrapment and burnover, remains a catastrophic and all too common occurrence. While improvements have been made to vehicle protection systems to increase the safety of firefighters caught in burnover, the potential effectiveness of these systems remains limited. This study involved systematic analysis of 62 historical entrapment and burnover reports from the USA, Australian and New Zealand from 1978 to 2020 (Phase 1), and 135 simulated wildfires encompassing the 99th percentile of Australian fire weather conditions, fuel structures and terrain (Phase 2). Analysis of historical entrapments identified existing vehicle protection systems have failure points well below the reported Fire Danger Index associated with the majority of house loss during wildfire events in Australia. Increasing the performance threshold of vehicle protection systems to the historical mean fire line intensity identified at the point of entrapment increased efficacy, and, prevented vehicle protection systems being overwhelmed in simulations regardless of Fire Danger Index and up to windspeeds of 55 kmh−1. In order to further improve firefighter protection during entrapment and burnover it is recommended the radiant heat flux performance threshold of vehicle protection systems are increased.

ERA5-based global meteorological wildfire danger maps

Forest fires are an integral part of the natural Earth system dynamics, however they are becoming more devastating and less predictable as anthropogenic climate change exacerbates their impacts. In order to advance fire science, fire danger reanalysis products can be used as proxy for fire weather observations with the advantage of being homogeneously distributed both in space and time. This manuscript describes a reanalysis dataset of fire danger indices based on the Canadian Fire Weather Index system and the ECMWF ERA5 reanalysis dataset, which supersedes the previous dataset based on ERA-Interim. The new fire danger reanalysis dataset provides a number of benefits compared to the one based on ERA-Interim: it relies on better estimates of precipitation, evaporation and soil moisture, it is available in a deterministic form as well as a probabilistic ensemble and it is characterised by a considerably higher spatial resolution. It is a valuable resource for forestry agencies and scientists in the field of wildfire danger modeling and beyond. The global dataset is produced by ECMWF, as the computational centre of the European Forest Fire information System (EFFIS) of the Copernicus Emergency Management Service, and it is made available free of charge through the Climate Data Store.

A Fire Danger Index for the early detection of areas vulnerable to wildfires in the Eastern Mediterranean region

Wildfires continue to be a major factor of disturbance to Mediterranean ecosystems, and are often associated with significant losses of properties and human lives. Fast fire detection and suppression within the first few minutes after ignition are crucial to successfully managing wildfires and preventing their potentially catastrophic consequences. In this study, remote-sensing methods and data were integrated wih fire behavior simulation and field data to develop a Fire Danger Index (FDI) that can be used to detect the areas most vulnerable to wildfires. This FDI will be integrated into an automatic fire detection system that utilizes optical and thermal land cameras and an unmanned aerial vehicle. The FDI was calculated for a nature reserve in Southern Greece based on fire behavior, pyric history, and anthropogenic influence. Fire behavior was estimated using the FlamMap fire simulation model, while the fuel types to include in the model were determined using state-of-the-art remote-sensing methods and field data. The pyric history was represented by point data on fire occurrences over a period of 40 years. The anthropogenic influence was estimated based on an inverse relationship of this influence with the Euclidean distance from roads and settlements. The calculated FDI demonstrated that a large part of the reserve, including its most ecologically important ecosystems, is highly vulnerable to wildfires. Integrating the FDI into the automatic fire detection system is expected to significantly improve its detection accuracy.

A Conceptual Interpretation of the Drought Code of the Canadian Forest Fire Weather Index System

The Drought Code (DC) was developed as part of the Canadian Forest Fire Weather Index System in the early 1970s to represent a deep column of soil that dries relatively slowly. Unlike most other fire danger indices or codes that operate on gravimetric moisture content and use the logarithmic drying equation to represent diffusion, the DC is based on a model that balances daily precipitation and evaporation. This conceptually simple water balance model was ultimately implemented using a “shortcut” equation that facilitated ledgering by hand but also mixed the water balance model with the abstraction equation, obscuring the logic of the model and concealing two important variables. An alternative interpretation of the DC is presented that returns the algorithm to an equivalent but conceptual form that offers several advantages: The simplicity of the underlying water balance model is retained with fewer variables, constants, and equations. Two key variables, daily depth of water storage and actual evaporation, are exposed. The English system of units is eliminated and two terms associated with precipitation are no longer needed. The reduced model does not include or depend on any soil attributes, confirming that the nature of the “DC equivalent soil” cannot be precisely known. While the “Conceptual Algorithm” presented here makes it easier to interpret and understand the logic of the DC, users may continue to use the equivalent “Implemented Algorithm” operationally if they wish.