Engineer who worked on plans for nuclear-powered airplane later developed the fire spread model

Throwback Thursday: the origin of the model for predicting the spread of wildland fires

Today we may take it for granted that tools are available that can estimate how a fire, unplanned or prescribed, will spread across a landscape. It is not an exact science because there are far too many variables than can realistically be accounted for, at least with the technology available today. But in 1972 when Dick Rothermel and others developed the Forest Service’s first quantitative, systematic tool for predicting the spread and intensity of forest fires, it introduced a new era in fire management. And surprisingly, it is still the main tool being used today. Many researchers have produced alternative models, but none have made it into the hands of firefighters on a widespread basis.

Dick Rothermel fire research Ember Award
For his research and contributions to understanding how fires spread, Dick Rothermel was given the Ember Award at caonference in Missoula in 2014. Photo by Bill Gabbert.

After Mr. Rothermel developed the mathematical model, others used the information to make the concept more user-friendly and to analyze complex scenarios. Behave, software burned onto a custom made chip in a hand-held Texas Instruments 51 calculator, and later BehavePlus for personal computers, became must-have tools for fire behavior analysts. FARSITE added the ability to predict spread across variable terrain, vegetation, and weather. Rare Event Risk Assessment Process (RERAP) estimates the risk that a fire will reach a particular place before it dies. FireStem estimates tree mortality based on fire behavior and intensity. And there are many others.

When Mr. Rothermel began researching the behavior of wildland fires, he had just been downsized from a shuttered Department of Defense program that had been attempting to develop a nuclear-powered airplane.

Below is an excerpt from an excellent article by Gail Wells for the March, 2008 edition of Fire Science Digest, a publication of the Joint Fire Science Program.

[Jack] Barrows, [the first director of the fire laboratory in Missoula when it opened in 1960], went looking for researchers. He learned that General Electric was closing a laboratory in Idaho Falls where engineers had been working on a defense project to develop a nuclear-powered airplane. The government scrapped the program in 1961, and a handful of highly trained engineers and scientists were suddenly up for grabs.

“GE wanted to see that we got as good a placement as we could,” Rothermel recalls. “So we all wrote resumes, and Jack got hold of these, and he said it was like a Sears and Roebuck catalog of people.” Barrows hired four of the GE scientists: Hal Anderson, a physicist; Stan Hirsh, an electrical engineer; Eric Breuer, a technician; and Dick Rothermel.

Their hiring represented a departure from Forest Service custom. Up until that time, fire research had been pretty much the domain of foresters, who are used to looking at their work through the lenses of biology and silviculture. Gisborne was a forester; Barrows was a forester. But Barrows recognized that fire is a physical process, and that physical scientists and engineers could contribute much to the emerging science of fire behavior.

Rothermel, then barely into his 30s, was glad to join Barrows’s staff. He had a bachelor’s degree in aeronautical engineering from the University of Washington. During the 8 years since he’d graduated, he had worked in the engineering of nuclear systems in Albuquerque and then in Idaho. (Rothermel later went on for a master’s degree in mechanical engineering from Colorado State University.)

“I had the option of staying on [at GE] and working on a lot of programs, but with the cancellation of the atomic-powered airplane, nothing sounded that appealing,” he says. “And then I heard about this laboratory, and they said they had two wind tunnels and a combustion lab where you could control the atmosphere, temperature, and humidity. I thought, “Wow, that’s an opportunity!” Rothermel worked with Hal Anderson to get the new lab’s equipment calibrated and running smoothly. Then they began a set of experiments in the wind tunnel and combustion chamber, testing the effects of wind and moisture on various fuels and determining how fast a fire would spread under different conditions.


Given their training, it made sense to Rothermel and Anderson to approach the task as an engineering problem. Says Rothermel: “The idea was, if we could develop a way of describing the fuels, the weather, the topography, and something about the fire, and be able to put that into what we call a mathematical model, and if we described all these things properly, the model would integrate it and produce answers. It would tell you the resulting fire intensity, rate of spread, flame length, these sorts of things.”

Rothermel, Anderson, and Bill Frandsen, another physicist on the project, adapted an approach developed by an early Forest Service fire researcher, Wally Fons, which turned on the concept of conservation of energy. A fire spreads by igniting a series of little fires in the fuel ahead of it. The ignitions are driven by convection, radiation, and conduction. Even if it’s unknown which mode is operating in a given instance, the rate of heat transfer can be measured. The researchers reasoned that if they knew how much fuel was ahead of a fire, how big and how densely packed the fuel particles were, and how much moisture the fuel contained, then they could figure out how much energy would be needed to transfer enough heat to bring the fuel up to the ignition point. They could then calculate the rate of ignition that would carry the fire as it spread. The model would also have to account for the critical variables of wind speed and slope of the ground.

Because of the limitations of wind tunnels and combustion chambers, the model is forced to make certain assumptions that don’t hold in real life. For example, it assumes that the fuel is continuous and evenly distributed and burns uniformly. It further assumes that the fire is carried primarily by dead plant material and that only moisture will stop it.

The Rothermel model “describes very well a fire burning in a field of wheat,” says Bret Butler, a mechanical engineer at the Fire Sciences Lab whom Rothermel hired in 1992. “As you get further away from that uniformity, the less accurate it becomes.”

More significantly, the researchers had no basis for modeling the endless spatial variability that actually exists in a forest. So there was no way to simulate a fire’s movement through clumpy, discontinuous trees and shrubs. There was also no way to model a crown fire, one that leaves the surface and moves up into the crowns of trees. These were significant and universally acknowledged shortcomings.

Fire research scientists throughout the world are working on developing more accurate surface-fire spread models, but at this point all of them are too complicated to be used in an operational system. The beauty of Rothermel’s model, says Butler, “is that it’s simple—it can be run quickly with a low-capability computer.”

(end of excerpt)

What made me think of Mr. Rothermel was a graphic distributed on Twitter today by the National Weather Service. It is a fancy, colorized version of the figure in his 1972 paper that depicts how heat is transferred in a fire.

wildfire research dick rothermel
Graphic distributed by @NWS that is based on Dick Rothermel’s 1972 paper.

But of course Mr. Rothermel’s contributions are far more complex than this graphic.

Below is a screenshot from his paper where he describes Propagating Flux, just one of many elements of his mathematical fire spread model.

rothermel propagating flux

And here is his summary of equations for the model:

Summary equations Rothermel's 1972 paper fire model
Summary of equations from Rothermel’s 1972 paper.

Epilogue 1: The current administration has expressed a desire to zero-out the budget for the Joint Fire Science Program, the organization that published the 2008 article. 

Epilogue 2: Mr. Rothermel was one of the 655 attendees at the Fire Continuum Conference in Missoula last month. 

Robot walks through vegetation fire

With “Cassie”, the University of Michigan is testing the limits of what a robot can do.

Above: Cassie, a robot developed by the University of Michigan, is seen walking through a prescribed fire in this screen shot from the video below.

The folks at the University of Michigan’s robotics lab have shown that a small robot can walk through a slowly spreading, very low-flame-length prescribed fire.

During the next few years robots are not going to replace wildland firefighters. But as was seen as far back as 2009 they might be able to carry heavy loads. For firefighters this could include hauling fuel, hose, pumps, or drinking water to hot shot crews, or resupplying spike camps.

Below is an excerpt from an article at Wired:

You might notice the Cassie still walks a bit gingerly. But [Jessy] Grizzle and his team are constantly tweaking the biped’s algorithms, then testing it all out in the real world … that’s sometimes on fire. It still struggles with larger obstructions like fallen tree limbs, but these are the kinds of challenges that are going to push the platform forward. Theoretically, you could outfit a Cassie—which would set you back a few hundred thousand dollars, by the way—to see straight through the smoke with lidar. It could see things no human firefighter could.

“I think it is an interesting demonstration of the ability to get robots out of the lab and into the real world, with a view toward robots that are able to perform useful tasks and get humans out of harm’s way,” writes Caltech’s Aaron Ames, one of a handful of roboticists who’s using the Cassie platform to study robotic bipedal locomotion. “We are still a long ways from autonomous firefighting robots, but the robots of today—and the dynamic walking control algorithms that have been developed recently—take an important step in this direction.”

Diversity of structure in a forest can make it more resilient to fire

Above: A slab of wood from the Stanislaus-Tuolumne Experimental Forest showing a history of very frequent fires, some of them as little as four years apart. Screen grab from the USFS video.

The U.S. Forest Service found some old research plots in the Sierras that have been measured over time dating back to the days of old growth. The evidence suggests that a diversity of species, density, and structure can make a forest more resilient to fire and attacks by insects.

More research indicates some forests are not growing back after wildfires

Rim Fire, August 21, 2013.
Rim Fire, August 21, 2013. Photo by Robert Martinez.

Recent research in the Rocky Mountains has found what others also determined in a 2013 study in Oregon — significant decreases in post-fire tree regeneration. In a paper titled “Evidence for declining forest resilience to wildfires under climate change”, eight researchers noted reductions in tree regeneration in the 21st century.

Below are some excerpts:

Annual moisture deficits were significantly greater from 2000 to 2015 as compared to 1985–1999, suggesting increasingly unfavourable post-fire growing conditions, corresponding to significantly lower seedling densities and increased regeneration failure. Dry forests that already occur at the edge of their climatic tolerance are most prone to conversion to non-forests after wildfires. Major climate-induced reduction in forest density and extent has important consequences for a myriad of ecosystem services now and in the future.

Climate change is already affecting multiple ecosystem properties, leading to shifts in species composition and state changes (Walther et al. 2002; Donato et al. 2016). In the US Rocky Mountains, we documented a significant trend of reduced post-fire tree regeneration, even over the relatively short period of 23 years covered in this analysis. Our findings are consistent with the expectation of reduced resilience of forest ecosystems to the combined impacts of climate warming and wildfire activity. Our results suggest that predicted shifts from forest to non-forested vegetation (e.g. Bell et al. 2014) may be underway, expedited by fire disturbances (Kemp 2015; Donato et al. 2016; Harvey et al. 2016; Johnstone et al. 2016; Rother & Veblen 2016).

Regeneration failures, as measured by both seedling presence/absence and regeneration thresholds, occurred across all forest types (Figs 3 and 4d). Low-elevation forests, dominated by tree species near the warm, dry edge of their climatic tolerance may be particularly vulnerable to shifts to non-forest vegetation, because of the absence of any tree species that could reestablish under warmer, drier conditions (Harvey et al. 2016). Meanwhile, moist forest types may experience a shift in species dominance and a decrease in tree density. And while only 15% of the moist forest sites we studied lacked seedling after 21st-century fires, 35% of these sites did not meet the recruitment threshold. This represents a substantial increase (300%) relative to the 1985–1999 period, highlighting the impacts of warming in moist forests as well.

Wildfire problem to increase in coming decades

Research projects substantial increases in area burned across western North America, with implications for land managers and policy makers.

Above: Projected change in annual area burned for the period 2010–2039, with red colors indicating areas with the greatest increase in area burned annually in wildfires, and dark blue the least.

By Susan McGinley, University of Arizona

The massive wildfires that burned in California, Oregon, Montana, Idaho, British Columbia and other parts of North America in 2017 in many cases exhibited a disturbing trend: a marked increase in the amount of area burned.

The Thomas Fire, which consumed 281,893 acres in California’s Santa Barbara and Ventura counties in December, was the largest in the state’s history. The Nazko Complex Fire in British Columbia burned more than 1 million acres, the largest ever recorded for the province.

Thomas Fire
Thomas Fire, Ventura, CA, Los Padres National Forest, 2017. USFS photo.

That trend will continue in coming decades across the western U.S. and northwestern Canada, though not uniformly, according to a recent study. UA professor Don Falk and Thomas Kitzberger from the Universidad Nacional del Comahue in Argentina, who started working on the research as a visiting scholar at the UA, were co-investigators on the study that also included Thomas Swetnam from the UA and Leroy Westerling of the University of California, Merced.

While it may have been an exceptional year in some respects, Falk’s and Kitzberger’s predictions suggest that years like 2017 are likely to become more common over time. States in the interior Western U.S., in particular, may be faced with large increases in total wildfire area burned, potentially beyond anything that has been experienced in the past.

Their research paper, “Direct and indirect climate controls predict heterogeneous early-mid 21st century wildfire burned area across western and boreal North America,” was published in the journal PLOS ONE in December as the 2017 fire season was ending. The results project where the greatest increases in area burned are likely to occur across the Western U.S. and Canada in coming decades. It suggests that large fires years such as the recent ones in southern and northern California may become more common.

A Model to Measure and Project Fire Activity

“We used 34 years of climate data to calibrate area burned in 1,500 grid cells across western North America, so we could capture the different ways that seasonal climate regulates fire in different regions,” said Falk, a professor in the School of Natural Resources and the Environment in the UA College of Agriculture and Life Sciences.

The key measurement, annual area burned, is a combination of fire size, frequency and variability from year to year. Area burned does not necessarily indicate fire severity, the ecological effects in a burned area.

Taking into account geographic variation, the study data focused on fire occurrence, seasonal temperatures and snowpack. The seasonal climate variables that turned out to be driving the amount of area burned were summer temperatures during fire season, spring temperatures and rainfall, and winter temperatures. Winter and spring conditions regulate snowpack, which can delay the onset of the fire season.

The team built a statistical model for wildfire area burned in each of the grid cells studied, and then tested it with data for actual area burned since 2010 to validate their predictions. It did not project the extent of area burned beyond the mid-21st century, as climate and vegetation changes become more uncertain later in the century.

Findings for western and northern North America show that about half the states and provinces are projected to have a large increase — five or more times the current levels — in total wildfire area burned. Others may see smaller increases, indicating there is no “one-size-fits-all” model. Increases in area burned are unevenly distributed across the study area, with the strongest increases projected in the interior western region.

Heads-Up for Land Management

“Ultimately, this means that the large fire seasons of recent years, such as the one just ending, are likely to occur more frequently, affecting ecosystems, communities and public safety,” Falk said. “These will be billion-dollar fire years. We’re just not ready for fire impacts of this kind, including post-fire effects from flooding after fire.”

The total cost of the 2017 fires in California alone is projected to exceed $180 billion. This includes not only the immediate costs of firefighting, but also the much larger costs, including:

  • Landscape rehabilitation;
  • Medical and hospital costs;
  • Insurance losses and the costs of replacing thousands of homes and other buildings;
  • Lost economic productivity from the destruction of businesses;
  • Repair and replacement of key infrastructure such as roads, power lines and dams; and
  • Weeks of lost income by employees.

Across the U.S., public land managing agencies are being stretched to their limits by the current scale of wildfire. The U.S. Forest Service spends more than half of its entire budget on wildfire response, leaving little for other key elements of its mission such as recreation, ecosystem restoration, research and public education.

Knowing about future regional variation in the projected annual area burned can help land managers and policy makers prepare for the possibility of extremely large fire years. Falk pointed out that seasonal climate changes also are having the effect of making the fire season longer, so there is additional time for more acreage to burn. In years when seasonal climate drives lengthy fire seasons, fire management resources may be stretched to the limit.

“Wildfires act as a multiplier of other forces such as climate change, exposing more and more areas not only to the immediate effects of fire, but also to the resulting cascade of ecological, hydrological, economic and social consequences,” Falk said. “We hope that this research will be a wake-up call to public agencies and legislatures at all levels of government that the fire problem is not going to get any smaller in coming decades.

“If anything, we need a serious, fact-based national dialogue about how to sustain our forests and woodlands through smart management and policy.”

Study shows firefighters’ exposure to smoke increases disease risk

Depending on the type of work performed and the number of years of exposure, the increased risk can be 22 to 39 percent.

Above: Smoky conditions on the Legion Lake Fire in Custer State Park in South Dakota, December 12, 2017. Photo by Bill Gabbert.

Originally published at 6:02 p.m. MT, February 6, 2018.

After collecting data from wildland firefighters in the field, a group of researchers concluded that firefighters’ exposure to smoke can increase the risk of mortality from lung cancer, ischemic heart disease, and cardiovascular disease. In this first section we cover what is in vegetation fire smoke, and after that we have details about the additional mortality risk faced by firefighters who can’t help but breathe the toxic substances.

What is in the air that firefighters breathe?

There have been many studies about smoke dating back to the 1988 NIOSH project at the fires in Yellowstone National Park. Most of them confirmed that yes, wildland firefighters ARE exposed to smoke and in most cases they quantified the amount.

In 2004 Timothy E. Reinhardt and Roger D. Ottmar  found a witches’ brew of methyl ethyl bad stuff that firefighters are breathing. All of these are hazardous to your health:

  • Aldehydes (volatile organic compounds); can cause immediate irritation of the eyes, nose, and throat, and inhalation can cause inflammation of the lungs. Short-term effects include cough, shortness of breath, and chest pain. The most abundant aldehyde in smoke is formaldehyde. When formaldehyde enters the body, it is converted to formic acid, which also is toxic.
  • Sulfur dioxide (SO²); causes severe irritation of the eyes, skin, upper respiratory tract, and mucous membranes, and also can cause bronchoconstriction. It forms sulfuric acid in the presence of water vapor and has been shown to damage the airways of humans.
  • Carbon monoxide (CO); As CO is inhaled it displaces O2 as it attaches to red blood cells and forms COHb. COHb reduces the ability of the blood to carry oxygen and causes hypoxia (a condition in which the body does not receive sufficient oxygen). Due to their strenuous work, wildland firefighters often have increased respiratory rates, which will increase the amount of CO being inhaled when smoke is present. COHb has a half-life (the time it takes half of the COHb to dissipate from the body) of about 5 hours. Symptoms of CO exposure include headaches, dizziness, nausea, loss of mental acuity, and fatigue. Prolonged, high exposure can cause confusion and loss of consciousness
  • Particulate matter; Respirable particulates are a major concern as they can be inhaled into the deeper recesses of the lungs, the alveolar region. These particles carry absorbed and condensed toxicants into the lungs
  • Acrolein; may increase the possibility of respiratory infections. It can cause irritation of the nose, throat, and lungs. Long-term effects can include chronic respiratory irritation and permanent loss of lung function if exposure occurs over many years.
  • Benzene; can cause headaches, dizziness, nausea, confusion, and respiratory tract irritation. Although the human body can often recover and repair damage caused by irritants, prolonged exposure from extended work shifts and poorly ventilated fire camps can overwhelm the ability to repair damage to genes and deoxyribonucleic acid (DNA).
  • Crystalline silica; can cause silicosis, a noncancerous lung disease that affects lung function. But OSHA classifies it as a carcinogen.
  • Intermediate chemicals; have been shown to cause a variety of health problems including bronchopulmonary carcinogenesis, fibrogenesis, pulmonary injury, respiratory distress, chronic obstructive pulmonary disease (COPD), and inflammation.

One of the more recent research efforts, from 2009 to 2012, was led by George Broyles of the U.S. Forest Service, National Technology and Development Program, in Boise, Idaho. They collected data in 11 fuel models in 17 states on initial attack, prescribed burns, and large project fires. The group measured carbon monoxide (CO) with electronic datalogging dosimeters and particulate matter using air pumps and filters.

carbon monoxide exposure firefighters
Data from the 2009-2012 wildland firefighter study led by George Broyles. “TWA” stands for Time Weighted Average. CO is carbon monoxide. OEL is Occupational Exposure Limits.

Monitoring carbon monoxide (CO) can be important, and is also fairly easy to do and not terribly expensive. Researchers have found that it can be a surrogate for the primary irritants of concern in wildland smoke near the combustion source. If CO is present, it’s almost certain that the smorgasbord of nasty stuff is in the air.

wildfire smoke monitoring firefighters
Jon Richert displays the various devices the National Technology Development Center research crews use to measure the amount of smoke firefighters deal with during wildfire suppression. This equipment was used in 2016 in a different but similar study than the one described in this article.
Diffusion tube
Diffusion tube.

Electronic CO monitors are available for $100 to $300. Another option is the little disposable CO monitors called diffusion tubes. With the holder they are about the size of a dry erase marker. Many are made by Drager, and for eight hours can record the cumulative CO. You can’t get an instantaneous reading, but the total hourly exposure can be monitored. They cost about $13 each. If one or two people on the crew carry them it can provide a heads up if the air quality is really bad.

What are the health effects of smoke exposure on a wildland fire?

Employers in most if not all workplaces are required to minimize hazards and provide a safe working environment. But of course it is impossible to totally eliminate all risks to firefighters. A cynic might assume that leadership in the wildland fire community may be hesitant to ask the question if they don’t want to hear the answer.

In spite of numerous studies confirming that yes, there is smoke where wildland firefighters work, there has been little in the literature that quantifies the effects on a person’s health. A new study published in August, 2017 contains a preliminary analysis addressing that question.

It is titled Wildland Fire Smoke Health Effects on Wildland Firefighters and the Public – Final Report to the Joint Fire Science Program. The authors are Joe Domitrovich, George Broyles, Roger D. Ottmar, Timothy E. Reinhardt, Luke P. Naeher, Michael T. Kleinman, Kathleen M. Navarro, Christopher E. Mackay, and Olorunfemi Adetona.

They used the field data collected in the 2009 to 2012 George Broyles study to extrapolate the physical and health effects on humans. The authors actually came up with numbers that indicate firefighters’ relative mortality risk for lung cancer, ischemic heart disease, and cardiovascular disease.
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