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Unit 5: Fuel Moisture

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Natural Fuels   ::   Environmental Factors   ::  Timelag  ::   Dead Fuels   ::   Ignition & Combustion   ::   Exercises

Fuel Moisture Timelag

Timelag : An indication of the rate a fuel gains or loses moisture due to changes in its environment, or the time necessary for a fuel particle to gain or lose approximately 63 percent of the difference between its initial moistture content and its equilibrium moisture content.

This gain or loss of moisture does not occur at a constant rate. When conditions change, fuels respond quickly at first. The change in moisture content becomes slower as the fuel moisture gets closer to the equilibrium moisture content. In nature, fuel takes five time lag periods for 95 percent of the change to occur, but most of the change occurs in the first time lag period.

The time lag of fine fuels is short, and they reach their equilibrium moisture content quickly. Heavy fuels have a longer time lag. They will usually not reach an equilibrium moisture content since environmental conditions do not stay constant. However, it is still worthwhile to classify fuels according to their time lag.

Timelag and Fuel Size Relationship
Timelag and fuel size relationship.

Remember, time lag is related to fuel size. This is illustrated in the figure above. On the horizontal axis, we have the size of branchwood in inches of diameter. The vertical axis gives us time lag in days. Fuels of 1.4 inches in diameter have a time lag of 48 hours or 2 days. Fuels 2 inches in diameter have a time lag of 4 days, and so on. This means that if the air was kept at a constant point drier than the fuels, it would take 4 days, time for 2-inch branchwood to lose 63 percent of the difference between its initial weight and the equilibrium moisture content.

As discussed earlier, wildland fuels come in many shapes and sizes, and we will never see a fuel complex of homogenous fuel. A pure grass stand comes closest to being a homogenous fuel. The wide variety of fuel components and changes in the weather make it virtually impossible for an entire complex to be at equilibrium moisture content at the same time.

Reaction time of fuels to wetting and drying
Reaction time of fuels to wetting and drying

The graphic above illustrates the time lag concept by showing the reaction times of two different size fuels to wetting and drying. The fuels are 1/2-inch sticks and a 12-inch log. During a typical fire season with a week of dry weather, the fuel moisture in 1/2-inch dead fuels will be consider-ably less than the moisture content of a 12-inch log. This is because the time lag period is much shorter in the 1/2-inch sticks.

If the fuels experience a day with precipitation, the moisture content of both will go up, but note the rates at which they absorb moisture. The 12-inch log is still gaining moisture after the rain has stopped, perhaps because of free water and wet soils resulting from the rainfall. The 1/2-inch sticks gain moisture rapidly but also lose it rapidly when temperatures and relative humidity return to normal.

For the purpose of predicting fire behavior, it is acceptable to use estimates for the moisture content of the fuel sizes which contribute most to fire spread. Dead fuels are grouped into four size classes based on time lag: 1-hour fuels which are up to 1/4-inch in diameter; 10-hour--1/4 to 1-inch in diameter; 100-hour--1- to 3-inches in diameter; and 1000-hour--3- to 6-inches in diameter. Thousand hour fuels are used in the National Fire Danger Rating System, but not for making fire behavior predictions.

Although it's helpful to have current estimates of fuel moisture in each of the four categories, we are most concerned with the 1-hour group, which includes all fine or small fuels up to 1/4-inch in diameter. This is the group that mostly determines whether a fire will start and continue to spread. This is also the group that is constantly changing with changes in relative humidity. It is possible to predict these changes, and thus fire behavior, for different periods of the day and night.

Daily relationship of relative humidity to fine dead fuel moisture
Daily relationship of relative humidity to fine dead fuel moisture

The figure above shows the daily relationship of relative humidity to fine, dead fuel moisture. With no major airmass changes, relative humidity typically rises during the night with lowering temperatures until it reaches the highest humidity just about sunrise. Relative humidity then usually starts to drop with rising temperature until the lowest humidity is reached during midafternoon. The fine, dead fuel moisture curve follows the relative humidity curve with a short time lag of about 1 hour. Before you have finished this unit, you will be able to estimate fine, dead fuel moisture content for various times of day or night, given atmospheric conditions and other site factors.

Copyright 2008, by the Contributing Authors. Cite/attribute Resource . admin. (2005, November 07). Unit 5: Fuel Moisture. Retrieved January 07, 2011, from Free Online Course Materials — USU OpenCourseWare Web site: This work is licensed under a Creative Commons License Creative Commons License