This unit of fuel moisture is divided into FOUR main sections. Click on the subjects below to link you directly to that part of the chapter.

FUEL MOISTURE

Fuel moisture (FM) content is the quantity of water in a fuel particle expressed as a percent of the ovendry weight of the fuel particle. FM content is an expression of the cumulative effects of past and present weather events - and must be considered in evaluating the effects of current or future weather on fire potential. FM is computed by dividing the weight of the "water" in the fuel - by the oven dry weight of the fuel and then multiplying by 100 to get the percent of moisture in a fuel.

FM is measured in both live and dead fuels. Dead fuel can have fuel moistures that range from 1% (extremely dry) to a value that ranges from 25-40% where the fire will not spread, known as the moisture of extinction (see below for detailed explanation). However in live fuels, the range of fuel moisture varies from 30 (very dry) to over 300 percent. Some examples of the variability of live fuel moisture are as follows: new foliage about 300%; mature growth 100%; curred fuels about 30%.

LIVE FUEL moistures are much slower to respond to environmental changes - and are most influenced by things such as long drought period, natural disease and insect infestation, annuals curing out early in the season, timber harvesting, and changes in the fuel models due to blowdown from windstorms and icestorms. Also, blowdown from windstorms in itself should be classified as a dead fuel.

DEAD-FUEL moisture is the moisture in any cured or dead plant part - whether attached to a still-living plant or not. Dead fuels are made up of hygroscopic materials - that allows them to absorb water vapor from the air - as well as release moisture into the air. It is within these hygroscopic materials where changes in the vapor pressure between water bound in the dead fuels and the surrounding atmosphere influence the exchange of moisture between the fuel and surrounding air through evaporation or condensation.

Fuel moistures are also strongly influenced by the aspect of the slope upon which they exist. Additionally weather parameters vary greatly from slope to slope. Fire Danger ratings may range from low on a northern exposure to extreme on a southern exposure for the same day/time. Most often fire suppression tactics are dependent upon which aspect of the slope the fire is burning. See the graphs that illustrate these changes.

TIME LAG FUEL MOISTURE

The rate of penetration of moisture into a fuel is a function of fuel size or the surface to volume ratio...and slows down with increasing fuel size and/or distance from the surface. Because 10 hour fuels respond faster to weather changes than a larger 100 or 1000 hour fuel, 10 hour fuels reflect weather changes in a more practical time frame. The National Fire Danger Rating System (NFDRS) thereby uses the forecasted 10 hour fuel moisture value to help determine the level of fire danger.

One timelag period is the amount of time necessary for a fuel component to lose (or gain) 63% of the difference between the current level of moisture within the fuel and the equilibrium moisture content, assuming constant atmospheric conditions (temperature and relative humidity) for an extended period of time. The larger the fuel, the more time needed to reach the equilibrium moisture content with the atmosphere. This is the "timelag" concept in a nutshell.

The table below is an example of a timelag calculation through 3 timelag periods. Notice how values are carried from one timelag period to the next. Note: the redder the colors become, the drier the fuel.

Notice the rate of change has decreased logarithmically with an increasing number of timelag periods. The fuel moisture went from 28% to 13.8% to 8.6% to 6.7%. Each successive period gets closer to equilibrium with the atmosphere - or 5.5%. If the fuel is a 10 hour fuel the total amount of time needed to complete 3 timelags is 30 hours. However if this is a 1 hour fuel, the total amount of time needed to complete 3 timelags is 3 hours. The reverse of this is also true when considering a dry fuel moisture relative to a saturated environment.

The relationship between fuel sizes and timelag can be seen in graphs 1, 2 and 3. Dead Fuel Moistures are influenced most by both direct and indirect elements. Graph 4 illustrates these influences.

Studies show how average fuel moisture change with fuel sizes. The larger the fuel, the slower it will respond to change. Applications of Larger fuels are used more for climatic studies and have higher saturation levels with the ability to hold more water/surface area.

MOISTURE OF EXTINCTION

The moisture of extinction (MOE) varies significantly from one geographic area to another and is based upon the type of fuel and climate. MOE takes into account all fuel class sizes but is heavily weighed toward the 1 hour fuels (<1/4 inch). For example the MOE of grass in western Washington is about 40%, Cascades and Puget Sound 30% and in eastern Washington 25%.

APPLICATIONS OF FUEL MOISTURES

A variety of applications exist from knowing the fuel moisture (both dead and alive) with its respective fuel size and model. This knowledge assists with the prediction of fire behavior through well established algorithms, nomograms and studies of fire history and climatology. For example, in western Washington if a 1000 hour fuel drops to 20% or less, the potential for large timber wildfires becomes very large. However in eastern Washington, the potential for large timber wildfires does not exist until the 1000 hour fuel drops to 12% or less. The reason for this is the variation in fuel models for each region (fir in the west and pine in the east).

LONG TERM planning applications focus on studying TRENDS of fuel moisture. Some longer term applications involve studying seasonal, yearly and historical differences and similarities, comparing (via satellite imagery) fuel condition from slope to slope, zone to zone, and current to past conditions. Many financial long term planning decisions are based on this also, such as fire suppression funding.

SHORT TERM planning is typically a real-time tool that reflects a fuel's receptivity to fire. To logically determine whether a fuel is receptive to fire involves using NFDRS calculations. This system of calculations and predictions uses input of weather conditions, and fuel characteristics (such as fuel model and moisture) to determine various levels of fire prediction indices. Examples of the initial level of output are: fire ignition, fire spread, and energy release components. These components are used to determine levels of RISK FACTORS - such as lightning-caused and man-caused Fire Risk.

One very important planning index that is based upon fuel model, fuel moisture, current and forecasted weather parameters is the BURNING INDEX (BI). Mathematical expressions of the BI are employed in all forest regions whether other kinds of danger ratings are utilized or not. In southern California, BI values for certain fuel models at different times of the year were the first level in the decision tree of issuing Red Flag Warnings or Fire Weather Watches. If BI's did not support fire, no Red Flag could be issued...iregardless of weather conditions. This decision was unanimously agreed upon among all fire agencies and the NWS.

All of these calculated indices and components determine FIRE LOAD - that aids in both the LONG and SHORT term planning applications, better financial planning and land management activities. This enhances a more efficient and effective distribution of resources on a daily level and in most cases leads to a much faster response time to an incident - with a more appropriate supply of resources.

FORECASTING fuel moistures for the NFDRS is both an art and science. Studying the behavior of fuels and their reaction times per weather changes over a period of time is perhaps the best means to truly learn how to forecast fuel moisture (just as a more experienced forecaster learning to forecast for extended periods). Albeit, pattern recognition has earned little respect among the research community, the concept is significant to better forecasting in longer time frames. Understanding the timelag concept is the most important tool for forecasting fuel moistures. Once that is understood, completion of the "Intermediate Wildland Fire Behavior Course - S-290" and observation of the fuel moisture exposed to other phenomena will enhance a forecaster's ability to forecast fuel moisture.

In Washington, and this may be the only exception, fuel moistures are not forecast. John Werth in Olympia, has studied the algorithms to determine that the NFDRS system already calculates a 10 hour fuel moisture based upon the previous 24-hour max/min temperature and relative humidity and the current temperature and humidity. From this value, NFDRS then can calculate out 24 hours the expected 10 hour fuel moisture.

Some general rules of thumb do apply to determining fuel moistures.These serve as a basic guideline when conditions are not changing and no other means of determining FM exist.

1. For every 1000 feet of elevation gain, the 10 hour fuel moisture will increase by 1%.

2. In summer and fall after 1000 hour fuels have had time to adjust to the drier environment associated with more critical fire weather conditions, a factor of 1% can be added or subtracted from a 1, 10, 100 or 1000 hour fuel to determine an adjoining fuel size. For example if you know the 10 hour fuel is 5% you can assume the one hour is 4%, the 100 hour is 6% and 1000 hour is 7%.