Why are fires in the West growing larger this year?

Drought — fuel moisture — energy release component

Observed precipitation
Observed precipitation during the 30 days before August 23, 2021.

There are a number of ways to analyze the behavior of wildland fires using data that is easily available. The amount of moisture in the live and dead vegetation is a critical factor in determining how readily it will burn, because it has to be cooked off before the grass, brush, or woody vegetation will vigorously combust.

The amount of precipitation over days, weeks, months, and years affects how wildfires burn. The map above depicts precipitation during the 30-day period ending August 23, 2021.

The Drought Monitor is one way of using an index to express how the precipitation compares to normal for an area. As you can see below most of California is in either Exceptional Drought (the highest level of drought) or Extreme Drought. The only areas in California that are not, are a tiny sliver in the extreme northwest corner, and the five southernmost counties. Both drought categories can also be found in areas of Oregon and Idaho which I will get to later.

Drought Monitor, August 17, 2021

Extended drought lowers the moisture content of both live and dead vegetation. When that occurs, it takes less energy out of a fire to cook off the moisture, and that energy instead goes toward enhanced combustion of the material and then preheating and igniting nearby vegetation, resulting in faster spread of the fire.

The observed precipitation map at the top of the page shows that most of California received less than 1/10 inch in the 30-day period. This, and the multi-year drought has led to the 1,000-hour time-lag fuels, woody material 3 to six inches in diameter, being extremely dry. Fuel monitoring stations in the foothills of the Sacramento Valley and the Northern Sierras are finding moisture levels lower than kiln-dried lumber, which is usually 8 to 12 percent. Both stations recently have been recording levels around 6 percent, which is near and sometimes below the lowest levels ever recorded for the date (the red lines on the charts). The Incident Management Team on the Caldor Fire said the 1,000-hour fuels are at three percent moisture.

1,000 hour fuel moisture Sacramento Valley-Foothills fire

In these charts, “Min” is the historic minimum for the date. “Max” is the historic maximum for the date.

1,000 hour fuel moisture, Northern Sierras

Knowing the moisture content of the fuel is an ingredient in determining another index, the Energy Release Component (ERC) which can help predict the intensity and rate of spread of a fire. It is defined as a number related to the available energy (BTU) per unit area (square foot) within the flaming front at the head of a fire. The ERC is considered a composite fuel moisture index as it reflects the contribution of all live and dead fuels to potential fire intensity. As live fuels cure and dead fuels dry, the ERC will increase and can be described as a build-up index. The ERC has memory. Each daily calculation considers the past 7 days in calculating the new number. Daily variations of the ERC are relatively small as wind is not part of the calculation.

Since mid-May the ERCs at two locations in Northern California have been flirting with the historic daily highs, either slightly above or slightly below. This is consistent with the observed fire activity this year on several large fires in the northern part of the state. The Dixie Fire is closing in on three-quarters of a million acres, and the Caldor Fire in nine days has blackened 117,000 acres. Fire Behavior Analysts at the fires are describing historically low fuel moistures.

Here is an excerpt from the recent Fuel Model Summary for the Caldor Fire:

There is a heavy dead and down component with drought-stressed fuels. Live fuels are cured to levels normally seen in late September, and fuels are extremely receptive to spotting. Fuel moistures are historically low. Northern California remains under a Fuels and Fire Behavior Advisory. ERC’s are above the 97th percentile. 100 hr and 1000hr fuels are below the 3rd percentile.

These fires are primarily fuel-driven. They are burning very well with gentle breezes. When the wind increases above 10 mph, they are hauling ass.

ERC Sac Valley-Foothills fire
Energy Release Component, Northern Sierras fire

Now let’s visit two other states, south-central Oregon and Idaho’s southern panhandle. The 30-day precip map shows that both areas had significant amounts which is reflected in the charts below. The ERCs which were above average and at times close to historic highs, dropped in August and the 1,000 hours rose. The fire activity also slowed in Washington, Oregon, Idaho, and Montana.

1,000-hour time lag fuel moisture, south-central Oregon
1,000-hour time lag fuel moisture, south-central Oregon. (The Northwest Geographic Area really needs to change the range of the Y axis, so that the highest number is 20 or 30 percent.)
Energy Release Component, south-central Oregon.
Energy Release Component, south-central Oregon.
1,000-hour time lag fuel moisture Idaho
1,000-hour time lag fuel moisture, Southern Idaho panhandle.
Energy Release Component Idaho
Energy Release Component, Southern Idaho panhandle.

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Author: Bill Gabbert

After working full time in wildland fire for 33 years, he continues to learn, and strives to be a Student of Fire.

15 thoughts on “Why are fires in the West growing larger this year?”

  1. While I agree with the description of the role of fuel moisture in the very first paragraph, I do take issue with the assumption behind this statement in the fourth paragraph, “… it takes less energy out of a fire to cook off the moisture, and that energy instead goes toward enhanced combustion of the material and then preheating and igniting nearby vegetation, resulting in faster spread of the fire.”
    Recent research has been casting doubt on the “preheat and ignite” theory of fire spread, and finding support for the “flame impingement and ignition” view. Think of a canopy fire running through a forest- whole trees don’t torch because they were preheated until they ignited, they were lit by massive tongues of flame from the adjacent burning trees, moved by rolling vortices of turbulent gasses generated internally beneath the smoke plume.
    I’m not trying to generate controversy with this comment, just sharing how my own thoughts have changed after reading publications from the Missoula Fire Sciences Laboratory (USFS Rocky Mountain Research Station), and watching the videos they have posted. I invite all those interested in this topic to investigate what researchers like Mark Finney and his associates have published.

  2. I would like to see a study that looks at large fire occurrence over a given period of time in the north west to include N. Calif, say 20 years or more and it’s correlation in creating the desired mosaic conditions that would prevent these large destructive fires in the future, it seams to me that we have had a great many very large fires in the last decade or so. Or am I way off base with this idea, it seams to me that we are going to continue trending in this direction, if these trends hold true for the next 10-20 years will it make a difference if fire severity.

    How many acres need to be treated, I am sure it’s in the millions.

    How long is fire history a deterrent in slowing the spread of fire, I realize that there a great many factors to consider when trying to answer this question.

    I recall seeing aerial photographs in Mexico, because of how fire in managed there is a well defined mosaic pattern, they do not appear to have these large conflagrations, fire often run there course and go out naturally. They do not have the suppression resources that we have and interface issues like we do.

    Is what we are seeing unprecedented concerning the fire environment or is it just the fires that are unprecedented.

    The fire return interval has been out of balance for a very long time, and fuels management will never be enough to fix it, however these very large fire may be around for a while longer to be sure…

    It would be nice to see an upside to these large fires.

    it’s obvious that I am not a fire ecologist, it’s just an idea.

  3. One aspect to these extreme fuel conditions that bears further thought is the term “resistance to control”. Most firefighters with any time on the ground know what I’m referencing, but the general public who love to armchair quarterback would have no idea just how difficult it is to suppress and hold what you have suppressed, when fuel and fuel beds reach these conditions.

    It doesn’t take much time under these conditions for fire to become embedded in the fuel bed, whether that be deeper in a duff layer or established in larger 100 hour and 1000 hour fuels. You might be able to get some flanks knocked down with the use of air support, but in order to continue to hold that line, it takes boots on the gound….lots of boots !!
    And once we are in the 1000’s of acres category…now we are talking tens or hundreds of miles of fireline.
    It sounds daunting because it is. At best, point protection and indirect tactics can have some benefit.
    I have felt for a long time that the suppression agencies are not upfront and honest about just how limited their capabilities are when fire and fuel conditions hit the extreme levels.
    The cold hard fact is, if you choose to live in the woods, you need to take the effort to fireproof your property and be able to defend it. I wouldn’t rely on the government to save your a**
    They will if they can, but they get overwhelmed pretty quickly .
    Just recently, my wife’s cousin, his son, and a bunch of friends successfully defended several family homes at twentyfivemile creek, when that fire went up in a hurry……way to go Phil !!
    Me…I just spent the past several days on my bobcat grappling and power raking almost an acre of woods around my place. I will still be sweating bullets if our short but critical fire season goes critical next spring….but hopefully I’ll be ready for it….not going down without a fight.

  4. It’s nice to see an adult conversation about a serious issue without comments about “unvaccinated folks raking the forest”.

  5. Yes, it has been hot; record Hot. Yes, it has been dry; very DRY. There you have it.

    I have lived in the region for going on 50 years and the heat waves we experienced this year were completely off the charts. It felt like somebody had turned on a giant oven and fan then aimed it towards the ground.

    Very sorry to hear about the passing of Firefighter Ruiz Carapia in Oregon on the Gales Fire. Many positive thoughts to his family. To the tens of thousands of Firefighters working on so many wildfires from California and into Canada; thank you and be careful.

    1. Humm, that’s odd because I see lots of logging signs, logging trucks, timber harvesters and equipment along with satellite maps that show lots of checkboard plots from tree harvests up and down the Sierra Range. You might be confused between “Spotted Owls” and Japanese / Chinese lumber ships that started purchasing larger logs several decades ago.

  6. These fires are just a snapshot of what will come much worse for many reasons. What will be a huge factor is the millions of the trees already weakened and dead from the Pine Beetle , extreme heat, drought, disease and other stressors that haven’t fell and dropped to the ground yet, creating a tremendous amount of duff and kindling that will spread fire quickly and smolder for along time. No amount of logging (what is the cost to helicopter one tree in the middle of a forest?) ot “raking the leaves” will prevent it.
    The best way to stop these mega fires is prevention but when that fails and nature unleashes a 1000 lightning strikes we must use cutting edge technology to detect and efficiently extinguish them. Artificial Intelligence can scan and predict much more quickly than now and fleets of drones can carry retardant and fly when manned aircraft can’t. This should be part of the multi trillion dollar infrastructure package before Congress.

  7. Mr. Gabbert, the work you done since your retirement is amazing. This article you have provided on the reason we are experiencing thease mega fires is great. And by the way you were my daughters first foreman, Dawn Essex when she started her career with the Forest Service at Cameron Station in 1982. She retired a couple years ago. Her son is an engineer up at Camp Ole.

  8. Our local force supervisor was tickled when he increased commercial thinning on the forest from 4 million board feet up to 8 million board feet.. He also had to admit that in that same year the total volume of trees in the forest increased by 88 million board feet. This has been the case for the last 25 to 30 years. Even the environmentalists are bemoaning the fact that 90% of the saw mills in this area have been shut down and sold for scrap iron. When we do have a timber sale there are very few places to take it.
    Fuels, weather, and topography influence fire behavior. The Bootleg fire here in Oregon is a perfect example of that. Live, overcrowded, dead, dying, and down fuels has created a situation which, in combination with the hot, dry weather, exploded like a powder keg until the weather changed or the fuels ran out.

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