Fuel Moisture Inputs in Fire Spread
Learn how fuel moisture inputs are used in Fire Spread simulations, including additional inputs available with U.S. fuel models.
Fuel moisture inputs
Fuel moisture describes the amount of water contained in vegetation relative to its oven-dry weight. Drier fuels generally ignite and burn more readily, while higher moisture reduces the energy available for combustion and can limit fire spread.
For dead fuels, the 1-hour, 10-hour, and 100-hour classes describe different fuel particle sizes and how quickly they respond to changing weather conditions. Smaller fuels respond rapidly; larger fuels change moisture more slowly.
The effect of each moisture input on a simulation depends on the fuel model and the amount of fuel assigned to that size class.
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Category |
Default Values |
Effect on simulation |
Custom Values |
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1-hour Dead Fuel Moisture - The moisture content of fine dead fuels, approximately 0-0.25 in (0-0.6 cm) in diameter, such as fine twigs, needles, and other small dead material. These fuels exchange moisture rapidly with the surrounding atmosphere, which is why they are referred to as 1-hour fuels. |
The default is derived from soil-moisture data from the DWD ICON Seamless weather model, which is used as a proxy for dead fuel moisture. |
Because 1-hour fuels have a high surface-area-to-volume ratio and constitute an important part of the fine fuel load in many fire behavior fuel models, their moisture can have a strong influence on predicted surface fire behavior. The exact sensitivity depends on the affected fuel type. |
You can override the default to represent observed or expected local fuel conditions. We recommend providing a custom 1-hour fuel moisture value where reliable local information is available. |
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The following 10-hour, 100-hour, live herbaceous, and live woody moisture inputs are currently applicable only to simulations using U.S. fuel models. |
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10-hour Dead Fuel Moisture The moisture content of dead fuels, approximately 0.25-1 in (0.6-2.5 cm) in diameter. Because these particles are larger than 1-hour fuels, their moisture responds more slowly to changes in atmospheric conditions. |
When no independent 10-hour value is provided, it’s estimated as: 10-hour moisture = 1-hour moisture + 1% |
10-hour moisture value is applied to the 10-hour fuel load defined by the selected fuel model. Its influence therefore depends on how much 10-hour fuel that model contains and on the moisture of the other fuel components. |
You can override the derived value when measured or locally estimated 10-hour fuel moisture is available. |
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100-hour Dead Fuel Moisture The moisture content of dead fuels, approximately 1–3 in (2.5–7.6 cm) in diameter. These larger fuel particles respond substantially more slowly to changes in atmospheric moisture than 1- or 10-hour fuels |
When no independent 100-hour value is available, it’s estimated as: 100-hour moisture = 1-hour moisture + 2% |
The value is applied to the 100-hour fuel load in the selected fuel model. Its influence on predicted fire behavior depends on the amount of 100-hour fuel represented by that model. |
You can override the derived value when measured or locally estimated 10-hour fuel moisture is available. |
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100-hour Dead Fuel Moisture The moisture content of dead fuels, approximately 1–3 in (2.5–7.6 cm) in diameter. These larger fuel particles respond substantially more slowly to changes in atmospheric moisture than 1- or 10-hour fuels |
When no independent 100-hour value is available, it’s estimated as: 100-hour moisture = 1-hour moisture + 2% |
The value is applied to the 100-hour fuel load in the selected fuel model. Its influence on predicted fire behavior depends on the amount of 100-hour fuel represented by that model. |
You can override the derived value when measured or locally estimated 100-hour fuel moisture is available. |
Live Herbaceous Fuel MoistureThe moisture content of living herbaceous vegetation, such as grasses and forbs. Unlike the dead fuel time-lag classes, this is a vegetation category rather than a particle-size class. |
Unless a custom value is provided, a default value of 30% is used. |
In dynamic fuel models, live herbaceous moisture also represents the degree of curing of herbaceous vegetation. At 120% moisture or above, the herbaceous fuel is treated as fully green; between 30% and 120%, an increasing proportion is transferred from the live to the dead fuel category; at 30% or below, it is treated as fully cured. As a result, this input can have a strong effect on predicted spread and intensity in fuel models containing herbaceous fuels. |
Adjust this value to reflect the observed or expected curing and moisture state of local herbaceous vegetation. |
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Live Woody Fuel Moisture The moisture content of living woody vegetation, primarily the foliage and fine living material of shrubs and similar woody fuels. It is distinct from the dead 1-, 10-, and 100-hour fuel classes. |
Unless a custom value is provided, a default value of 60% is used. |
Live woody moisture affects the contribution of live woody fuels to predicted surface fire behavior. Its importance depends on whether, and how much, live woody fuel is present in the affected fuel type. |
Adjust this value to represent observed or expected moisture conditions in local live woody vegetation. |