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
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