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Table of Contents
Introduction
This module provides information about fuel moisture and its importance for fire ignition and combustion. The module is part of the S-290 Intermediate Wildland Fire Behavior course and offers more detailed information about concepts introduced in other units of the course.
At the end of this module, you should be able to:
- Describe the relationships between relative humidity, wind, and the moisture content of fine and large fuels
- Explain how the amount and duration of precipitation and soil moisture affect the moisture content of fine and large fuels
- Define the fuel moisture timelag concept and its value to firefighters and fire managers
- Describe how fuel moisture is determined for dead fuels in each of the four timelag categories
- Define critical live fuel moisture and the thresholds for various fuel types
- Identify three methods for obtaining live fuel moisture
Fuel Moisture Definition
Fuel moisture content is the amount of water in a fuel, expressed as a percent of the oven dry weight of that fuel.
Fuels can be weighed before and after drying in an oven. Fuel moisture percent is determined by dividing the difference between the wet and dry weights by the dry weight.
If there were no moisture in the fuels, the fuel moisture content would be zero percent.
Significance
A wide range of moisture contents can occur within fuel complexes, as most fuel complexes contain a combination of dead and live fuels.
The fuel moisture content in natural fuels is an important factor determining fuel availability for fire ignition and combustion. There are cases in which not all fuels in a fuel complex are involved in the flaming front or consumed by fire, requiring that personnel in the field be able to determine which fuels might be responsible for fire spread.
Every fuel complex—from a sparsely populated desert, to a rain forest with lush vegetation, to parched timberlands—can be viewed as a potential fire environment. Analysis of an environment's ability to burn and carry fire depends on the fuel available as well as the fuel moisture content.
Identify each statement below as true or false. (Use the selection box to choose the best answer.)
Generally, when fuel moisture content is high, fires do not start as easily. When they do start, they burn poorly. When fuel moisture content is low, fires start easily and spread and burn rapidly.
Fuel moisture contents are usually some value between very high or very low. The values fluctuate with changes in weather, including temperature, humidity, and precipitation. Values can depend on seasons, and also on aspect, elevation, degree of curing, and other local variations.
Effects on Combustion
When a fuel burns, it is actually gases that begin burning. In a solid organic fuel, such as typical wildland fuels, combustible gases are generated at around 400°F.
Water turns into a gas at 212°F. As a fuel is heated, it will begin to give off water vapor as the temperature approaches 212°F. The process of evaporating moisture consumes energy from the ignition source. Water vapor will also dilute the combustible gases.
A fuel will not burn until the moisture in it has been heated and converted to vapor. If the ignition source has sufficient energy, it will evaporate enough fuel moisture that the fuels’ combustible gases can sustain combustion.
Fuel moisture is therefore important in starting and spreading wildland fires and must be continuously monitored. The higher the fuel moisture content, the slower the burning process because there is more moisture to expel.
Comparing Live and Dead Fuel Moistures
Though the fuel moistures of live and dead fuels are measured in the same way, the moisture content of live fuels is usually much higher than for dead fuels.
Live fuel moisture is the moisture in all living plants. Because a living cell can hold up to three times its weight in water, moisture contents in live fuels can range from 30 to 300 percent. The variations depend on species, season, and aspect.
Dead fuel moisture is the moisture remaining in dead plants, including annual grasses, dead woody plants, litter, and slash. Moisture contents in dead fuels can range between 2 and 30 percent. These values are strongly influenced by changes in weather and topography and can change quickly over time and space.
Dead Fuel Moisture
Dead fuel moisture can influence short-term fire behavior changes during a burn period. Dead fuel moisture depends to a large degree on weather factors, including temperature, humidity, and precipitation. These weather factors and expected changes are included in a fire weather forecast and can help you anticipate dead fuel moisture changes in the field.
Equilibrium Moisture Content
If the moisture content in the atmosphere remains constant for a period of time, the vapor pressures of fuels and the air will move toward becoming equal. This point, referred to as the equilibrium moisture content, is reached when there is no net gain or loss of moisture between fuels and the surrounding air.
Timelag and Fuel Size
A fuel’s timelag is defined as the time needed for a fuel particle to lose about 63 percent of the difference between the initial moisture content and the equilibrium moisture content.
Timelags are common in any transfer of heat or moisture. In the case of fuel moisture, fuels require a period of time to approach the equilibrium moisture content.
Changes in fuel moisture content start quickly and begin to slow as the values near the equilibrium moisture content. In nature, it takes five timelag periods for 95 percent of the fuel moisture change to occur, but most of the change occurs in the first timelag period.
Fine fuels have a large surface-area-to-volume ratio. These fuels have a short timelag and can reach their equilibrium moisture content quickly.
Large fuels have a longer timelag and will not reach an equilibrium moisture content because environmental conditions do not stay constant for a long enough duration.
Consider the graph below. Branchwood size (in inches of diameter) is plotted on the horizontal axis and timelag in days is plotted on the vertical axis.
We can observe that fuels 1.4 inches in diameter have a timelag of 2 days. Fuels 2 inches in diameter have a timelag of 4 days.
If the air was kept at a constant value drier than the fuels, it would take 4 days for the 2-inch branchwood to lose 63% (or about 2/3) the difference between its initial moisture and equilibrium moisture content.
Question
What is the timelag for a branch 8 inches in diameter? (Choose the best answer.)
The correct answer is c.
Branchwood with a diameter of 8 inches has a timelag of about 40 days.
Question
Can the fuel moisture content of the 8-inch branch reach an equilibrium moisture content? (Choose the best answer.)
The correct answer is b.
The branchwood will not reach an equilibrium moisture content because the environmental conditions will not stay constant for 40 days.
Size Classes
Because fuel complexes are mostly non-homogeneous, fuel moisture contents are simplified by grouping fuels into four size classes based on timelag.
- 1-hour timelag fuels have diameters of 0 to ¼ inch.
- 10-hour timelag fuels have diameters of ¼ inch to 1 inch.
- 100-hour timelag fuels have diameters between 1 inch and 3 inches.
- 1000-hour timelag fuels have diameters between 3 and 8 inches.
Fuels larger than 8 inches in diameter are referred to as 10,000-hour timelag fuels.
On the fireline during a burn period, changes in the 1-hour fuel moisture are easier to track than changes in the fuel moisture of larger dead fuels. With more experience, firefighters can develop a subjective assessment of 10-, 100-, and 1000-hour fuel moistures in the field.
The 1000-hour fuels are used the National Fire Danger Rating System, but not for making fire behavior predictions. These fuel moistures provide an indicator of drought and often correlate closely with the fire danger for an area.
Reaction Times
The timelag concept can be illustrated by comparing the reaction times of two different fuel sizes to wetting and drying.
The first fuel is a ½-inch diameter stick. The second fuel is 12-inch diameter log. During a typical fire season with a week of dry weather, the fuel moisture in the ½-inch dead stick will be considerably less than the moisture content of the log because of the shorter timelag of the stick.
Now, we’ll look at the fuel moisture contents after it has rained for a day.
Based on the graph shown here, which fuel size reacts more quickly to precipitation? (Choose the best answer.)
The correct answer is a.
The 1/2-inch stick will react more quickly than that of the 12-inch log.
Fuels with short timelags gain moisture more rapidly than larger, longer timelag fuels. However, the shorter timelag fuels will also lose this moisture more rapidly once temperatures and relative humidity return to the drier conditions.
A larger fuel can continue to gain moisture after the rain has stopped, in part because of the surrounding wet soils.
Wildland fuels come in many shapes and sizes. The wide variety of fuel components and continual changes in the weather make it virtually impossible for an entire fuel complex to be at an equilibrium moisture content at the same time.
Factors Affecting Dead Fuel Moisture
Dead fuel moisture is directly influenced by temperature, relative humidity, precipitation, and wind. Each of these variables can be affected by topography and other factors.
Temperature/Humidity
Water moves from higher concentrations to lower concentrations in an effort to establish balance.
Fuels are therefore constantly exchanging moisture with the surrounding air. Fuels gain moisture during periods of high humidity and precipitation. Fuels lose moisture to the air when the air is dry and humidity is low.
Moisture exchange between dead fuels and the air is affected by:
- differences in vapor pressure or the force of water vapor molecules in fuels and the air
- the presence or absence of wind
- the size of the fuels
- the compactness of the fuels
- the proximity of the fuels to damp soil
Precipitation Amount and Duration
The amount and duration of the precipitation are important to consider when estimating dead fuel moisture content. Precipitation can raise fine dead fuel moisture more rapidly than any other factor.
Fine dead fuels react very rapidly to precipitation and reach their saturation points quickly. Additional rainfall has little effect on these fuels, though more rainfall can wet the soils in contact with the fuels and maintain the increased fuel moisture for a longer time period.
Large dead fuels react more slowly to precipitation as much of the rain runs off the fuel. These fuels absorb moisture through the duration of the precipitation, and overall duration is more important than the precipitation amount.
Fuel moisture responses (vertical axis) are plotted versus precipitation duration (horizontal axis). This chart was prepared for average western fuel situations with standing and down, dead fuels.
- The purple dashed line represents the 1-hour timelag (fine) fuels.
- The orange dashed line represents 10-hour timelag fuels.
- The solid blue line represents 100-hour (large) fuels.
Based on the graph of fuel moisture versus precipitation, which of the following are true for this situation? (Choose all that apply.)
The answers are all of the above.
As precipitation falls, the moisture content of the shortest timelag fuels increases most rapidly, while the moisture content of longer timelag fuels increases more slowly.
The amount of rainfall in a given time period affects fuel moisture of horizontal litter and duff more than the fuel moisture of vertical grasses.
Larger dead fuels are affected by both rainfall duration and amount. Having free water on the surfaces and surrounding soils of larger fuels increases the absorption rate.
Cumulative rainfall totals over time provide information about drought conditions, which significantly affect live fuel moisture contents.
Wind
Wind influences fuel moisture by helping fuels reach an equilibrium moisture content with the atmosphere at a faster rate. Winds speed up the drying and evaporation process by moving air as well as by affecting temperature. During calm conditions, air next to the fuels tends to become saturated with water, slowing the evaporation rate of moisture from the fuel. Wind continually replaces this saturated air with drier air, speeding up the evaporation process.
During daytime heating, wind can bring in cooler air to replace warm air layers immediately adjacent to the fuels. This cooler air has a higher relative humidity and also acts to lower the fuel-surface temperature. Both of these factors reduce the drying of fine dead fuels.
At night, winds can cause turbulent mixing and prevent surface air temperatures from reaching the dewpoint. The increase of surface fuel moisture would therefore be restricted.
Some wind types, particularly foehn winds, lead to rapid drying of dead fuels and are referred to as drying winds. Foehn winds bring warm and extremely dry air able to desiccate fuels.
If the wind keeps warm, dry air flowing at a rapid enough rate, the air will not become moist through contact with the surface during either the day or night.
A moist wind, on the other hand, brings a continuous supply of moisture to increase the dead fuel moisture content.
Wind influences are strongest on fine dead fuels and have a lesser effect on larger timelag fuels.
Shading
In direct sunlight, the ground can heat much more quickly than the surrounding air, resulting in very high ground surface temperatures. Ground surface temperatures can reach 160°F in unshaded areas but be considerably less in shaded areas. Relative humidity drops as temperatures increase. In this example, showing an open, unshaded area in which the surface temperature has reached 160°F, the fine dead fuel moisture has dropped to 3 percent, compared to 8 percent in the shaded area.
Aspect
Dead fuel moisture responds very strongly to the timing and duration of solar radiation on sunny days. The Sun's radiation warms the soil, increases the surface temperature, and decreases relative humidity. Because of their orientation to the Sun, south aspects in the northern hemisphere will have higher temperatures, lower relative humidities, and lower fuel moistures compared to other aspects. During the summer, level ground will receive intense heating similar to that received on south aspects.
These changes in heating occur during the daytime hours each day, making aspect an important influence for finer dead fuels. However, the cumulative results of the heating differences result in lower soil moistures on south aspects, decreasing live fuel moistures on these aspects compared to slopes facing other directions.
After sunset, temperature differences between aspects will vanish. During the night, surface inversions and the effects of thermal belts can cause temperature differences in valley bottoms. In the absence of larger-scale weather events, by early morning the temperature and relative humidity will have moderated to a large extent, resulting in the highest fine dead fuel moisture values of the day.
During the day, east aspects reach their lowest fuel moisture contents by early afternoon. Southwest aspects tend not to experience the lowest fuel moisture contents until later in the afternoon.
If all other factors were equal, on which aspect would a fire most easily start based on fuel moisture values? (Choose the best answer.)
The correct answer is a.
If all other factors were equal, a fire would most easily start on the south aspect where the lowest average fuel moisture contents are typically found.
Elevation
Temperatures typically decrease about 3.5°F for every 1000 feet of elevation rise. As temperature decreases with elevation, relative humidity and fine dead fuel moisture increase.
Fine dead fuel moisture contents have been determined for a range of elevations for given temperatures and relative humidities. In this example, a gain in elevation from 1000 to 6000 feet results in a rise in fuel moisture from 4 percent to 8 percent.
Combined with later snowmelt dates, later curing dates, and higher green-to-dead fuel ratios, higher elevations can result in dramatically different fine dead fuel moisture contents. These differences can be significant to fire ignition and spread rates, though fires can still start at higher elevations.
Slope
Slope steepness affects fuel moisture content by changing the amount of solar radiation received. The angle at which solar radiation hits various surfaces changes throughout the day and with the season.
Which slope in the diagrams shown here is receiving the most intense solar radiation? (Choose the best answer.)
The correct answer is a.
A slope oriented perpendicular to incoming radiation receives more heating than slopes that are nearly parallel to the rays.
The steepness or percent slope on north aspects is particularly important as there might be times of the year when those slopes receive no direct solar heating at all.
Assessing Dead Fuel Moisture
The process of determining fuel moisture depends on the timelag category.
Remember that a fuel's timelag reflects the amount of time needed for a fuel to approach an equilibrium moisture content. Finer fuels have shorter timelags and are most sensitive to short-term weather and environmental changes. Knowledge of changes in weather therefore provides guidance for determining changes in the moisture content of short timelag fuels. Other means are generally necessary to determine the moisture content of larger fuels.
Estimating Fine Dead Fuel Moisture (FDFM)
The 1-hour fuel group contains the fuels that mostly determine whether a fire will start and continue to spread. This group includes all fine and small fuels up to ¼-inch in diameter.
The fuel moisture content of 1-hour fuels constantly changes with relative humidity. These changes can be predicted for different periods of the day and night.
As long as there are no major air mass changes, relative humidity will typically rise during the night, reaching its highest value just before sunrise. Relative humidity then begins to decrease with rising temperatures, reaching its lowest value in the mid-afternoon.
The fine dead fuel moisture curve follows the relative humidity curve with a timelag of about 1 hour. The moisture exchange rate for surface litter and fine dead fuels lying on the ground, can be slightly slower because the fuel compactness reduces air circulation.
1-hour Fuel Moisture and Relative Humidity
Relative humidity strongly influences the fuel moisture of 1-hour fuels. In certain cases, the fuel moisture of 1-hour fuels can be approximated by dividing the relative humidity by 5. For instance, a relative humidity of 45 percent translates to a 1-hour fuel moisture of 9 percent. A 20-percent relative humidity translates to a 1-hour fuel moisture of only 4 percent.
Fine Dead Fuel Moisture Tables
Eight inputs are needed to calculate the fine dead fuel moisture:
These inputs can be entered into the fine dead fuel moisture worksheet.
- dry bulb temperature
- relative humidity
- month
- time of day
- degree of shading
- slope
- aspect
- site location
The temperature and relative humidity are required to determine the reference fuel moisture, which is a basic fuel moisture estimate provided by the Appendix B table. The other inputs help determine the fuel moisture correction, which is needed to adjust the reference fuel moisture to your specific location.
RFM Table
Once you know the dry bulb temperature and relative humidity, you can use Table 2 of the NWCG Fireline Handbook Appendix B to determine the reference fuel moisture (RFM). Table 2 is for use during daytime hours.
View a larger version of the table in a new browser window/tab
The reference fuel moisture (RFM) can be read by locating the intersection of the appropriate dry bulb temperature column and relative humidity row. This value should then be entered into line 6 of the Fine Dead Fuel Moisture Worksheet.
Correction Tables
The fuel moisture correction (FMC) value depends on month, time of day, shading, site location, slope, and aspect.
Table B provides correction values for May, June, and July. These are the only months for which this particular table is valid. The other months of the year are covered in Tables C and D.
The top section of the table provides values for unshaded surface fuels while the bottom part is for shaded surface fuels. The numbers labeling each of the columns represent the time of day.
Note that the "B", "L", and "A" columns correspond to any differences in elevation from the location of the temperature and humidity measurements. "B" denotes an elevation 1000-2000 feet below the measurement location. "L" corresponds to an elevation approximately level (+/- 1000 feet) with the location. "A" denotes an elevation 1000-2000 feet above the measurement location. These location adjustments help determine a fuel moisture correction specific to a fire’s location, and offset the effects of the atmosphere's lapse rate, which can be between 3.5 and 5.5 °F per 1000 feet (see Unit 6: Atmospheric Stability for more details about the lapse rate).
Following the proper selections for aspect and slope and for time of day and elevation to their point of intersection provides a fuel moisture correction (FMC) value. The FMC value is added to the RFM to obtain an adjusted fine dead fuel moisture (FDFM).
RFM + FMC = FDFM
Table C is valid for February, March, April/August, September, and October.
Table D is valid for November, December, and January.
FDFM Practice
Use this reference guide to find the reference fuel moisture, fuel moisture correction, and fine dead fuel moisture in each of the following situations.
Questions
You use your belt weather kit to measure a temperature of 82°F and a relative humidity of 12 percent. What is the resulting RFM? (Choose the best answer.)
The correct answer is a.
The reference fuel moisture for a temperature of 82°F and 12 percent relative humidity is 2 percent.
You measure a temperature of 66°F and a humidity of 41 percent. What is the resulting RFM? (Choose the best answer.)
The correct answer is b.
The reference fuel moisture for a temperature of 66°F and 41 percent relative humidity is 6 percent.
On February 10, you are on a 20 percent on west aspect at 1600 hours. The temperature and humidity measurements are at a similar elevation as the fire. There is no shading from vegetation and less than 10 percent cloud cover. What is the FMC? (Choose the best answer.)
The correct answer is c.
The fuel moisture correction for these conditions is 2 percent.
Now let's apply the reference and fuel moisture correction tables to estimate the Fine Dead Fuel Moisture (FDFM) in each of the following situations. The worksheet can help you track the input values and find the appropriate value for the conditions.
Situation 1
On August 8, 1450 hours, you stand on a south aspect with 40 percent slope. Fuels are 30 percent shaded and temperature of 89°F and 14 percent relative humidity were measured 1200 feet below the fire location. What is the fine dead (1-hour) fuel moisture? (Choose the best answer.)
The correct answer is b.
Remember that the reference fuel moisture and fuel moisture correction should be added to give the fine dead fuel moisture values.
Situation 2
On May 24, 1145 hours, you measure a 72°F temperature and 38 percent relative humidity on a north aspect with approximately 60 percent vegetative shading under partly cloudy skies. The slope is 15 percent and the fire is 1,900 feet below your elevation. What is the fine dead (1-hour) fuel moisture? (Choose the best answer.)
The correct answer is c.
Remember that the reference fuel moisture and fuel moisture correction should be added to give the fine dead fuel moisture values.
Situation 3
On January 6 at 1600 hours, you measure a temperature of 45°F and relative humidity of 8 percent. You are on a grassy east aspect with 10 percent slope and the fire is 2,900 feet above your location. Some clouds are present, covering less than 40 percent of the sky. What is the fine dead (1-hour) fuel moisture? (Choose the best answer.)
The correct answer is d.
The elevation difference between your temperature and humidity measurements and the fire is too large. You must acquire a temperature and relative humidity reading within +/- 2,000 feet of the site for which you want to determine fuel moisture.
Estimating Fuel Moisture in Larger Dead Fuels
The 10-hour fuel moisture content can be important in fire behavior predictions, but plays a lesser role than the 1-hour fuels that are the primary carrier of fire. The moisture of 100-hour and 1000-hour fuels provides information about drought conditions and the overall severity of a fire season The moisture contents of these larger fuels, including 10-hour, 100-hour, and 1000-hour fuels, respond more slowly to changes in the environment. They cannot be easily estimated from current weather conditions and generally must be determined in other ways.
Using Moisture Probes
The moisture levels of wildland fuels can be sampled using handheld instruments designed to measure moisture in wood and other building materials.
The sensors on handheld probes can be inserted directly into a branch or log. These probes are particularly useful for assessing the moisture of 100- and 1000-hour fuels.
Field Sampling Techniques
Fuel moisture content can be determined for any category of fuel based on samples collected in the field.
The fuel sample is weighed prior to being dried in an oven, and then is weighed again. The difference between the wet and dry weights, divided by the dry weight and multiplied by 100, gives the fuel moisture content in percent.
Fire Behavior Thresholds
Fine dead fuel moisture can change significantly from the beginning of the burn period to the end, especially under strong night-time inversions, after precipitation events, and with foehn wind events. Generally, 1-hour fuel moisture changes are limited to several percent over short time periods due to changes in temperature and relative humidity.
Differences resulting from changes in aspect are generally limited to 2 to 3 percent, while moving from sunny to shaded areas can raise 1-hour fuel moistures 3 to 5 percent.
Forecasted relative humidity is usually given in a range of 5 percent. In some cases, the timing of air mass changes can result in relative humidity changes of 10 to 20 percent. These ranges are important to keep in mind because a change of 5 percent in relative humidity (e.g., from 35 to 30 percent) results in a 1 percent change in 1-hour fuel moisture.
Effect on Fire Behavior
Fire spread models suggest that a 1 percent change in 1-hour fuel moisture can result in a 10 percent change in the rate of fire spread. By comparison, some fuel type changes can result in a 1500 percent change in the rate of spread.
Monitoring 1-hour fuel moisture is particularly important for providing insight on ignition, flame length, and crown fire potential.
Severe Fire Behavior Potential
A number of tools offer interpreted thresholds for fuel moisture and associated fire behavior. Examples include the summary of "Severe Fire Behavior Potential Related to Relative Humidity and Fuel Moisture Content", which can be found in the Incident Response Pocket Guide. The table summarizes the likelihood of ignition and overall burning conditions for various 1-hour and 10-hour fuel moistures. For 1-hour fuel moistures greater than 20 percent, there is little likelihood of ignition, though winds can cause possible spotting.
The table should be used with caution, because it does not represent the specific conditions that you might find on a given fireline. Note that this table is only relevant for the western U.S. The fuel moisture numbers presented represent approximate ranges and other factors can influence the actual burning conditions. However, the general descriptions offered in the table can alert firefighters to possible hazards or changes in the wildland fire environment. The guidelines do not consider the influence of larger and live fuel moistures on fire behavior. The fire environment you find on the fireline, including fuels, terrain, and wind, combine with temperature and relative humidity to produce the fire behavior you observe.
Probability of Ignition
The Incident Response Pocket Guide also includes a Probability of Ignition table, which provides the probability that a fire will start for a given set of fine dead fuel moisture, temperature, and shading conditions.
As is the case with the Severe Fire Behavior Potential table, the Probability of Ignition table offers general information and is not specific to the conditions you might find on a particular fireline. Remember to take into account all factors, including fuels, topography, and weather, to anticipate the likely fire behavior at a specific incident. Fine dead fuel moisture thresholds that work in Utah may not work in Florida, while thresholds that work in Wisconsin may not apply in California.
Moisture of Extinction
Fire spread models reference a "moisture of extinction" that identifies a dead fuel moisture level (in percent) above which active fire spread is not predicted.
- "Grass" fuels generally need the lowest 1-hour fuel moistures to produce active spread because they generally produce the least amount of heat when they burn.
- "Shrub" and "Timber Understory" fuels can burn actively at more moderate 1-hour fuel moistures, especially when the live fuels are stressed and/or when a large share of the shrub fuels themselves are dead.
- "Timber Litter" and "Slash/Blowdown" fuels can also burn actively under more moderate fine dead fuel moistures, depending on fuel moisture in the larger fuel categories.
Using Tools Exercise
Use the Severe Fire Behavior Potential table to answer the questions below.
Questions
What do you expect would happen if the 1-hour fuel moisture content dropped to around 12 percent? (Choose the best answer.)
The correct answer is c.
The likelihood of ignition will increase. Drop the 1-hour fuel moisture to the 11-14 percent range, and ignitability becomes "medium" with easy burning conditions. At 1-hour fuel moisture below 10 percent, the ignition hazard becomes high.
Based on the Severe Fire Potential table, what type of burning conditions should be anticipated when the 1-hour fuel moisture is below 7 percent? (Choose all that apply.)
The answers are all of the above.
1-hour fuel moisture values between 5 and 7 percent tend to result in dangerous burning conditions that can cause long distance spotting and ignite aerial fuels. 1-hour fuel moistures below 5 percent pose critical burning conditions with rapid spreading of spot fires and the probability of extreme fire behavior.
Making Your Own Assessment
Click the play button to view the video.
It is important for firefighters to observe conditions on the fireline, note what they find, and brief others in an attempt to find significant thresholds.
Fuel-moisture related factors that indicate a threshold include, among others:
- The timing of the pick up in fire behavior each day (e.g., first torching tree, change in burning of jackpots used to clear slash, first noted active fire spread)
- The timing of a fire behavior transition during the burning period and information about the nature of the transition (e.g., surface to crown fire)
- The fire environment where the fire behavior occurred (e.g., what fuels were burning, aspect, elevation, slope, shading, changes in temp/relative humidity/wind, changes in sky conditions)
After observing these factors, it may be possible to isolate some important weather thresholds, at least for similar fireline conditions.
Live Fuel Moisture
All living fuels have different physiological properties that dictate how they take in and store moisture from the environment, particularly from soil. These properties result in characteristic patterns of rising and falling fuel moistures over the course of a fire season and influence the fuel's overall flammability. Fuel moisture can also determine the temperature at which the combustible gases will ignite. Fuels with high live fuel moistures, such as from new foliage during spring green-up, might not burn at all. However, these same fuels can reach critical and explosive fire potentials during late summer or autumn.
Herbaceous Fuels
Living fuels include herbaceous plants, which are either annual (developing from seed each year) or perennial (sprouting from the root system each year). Herbaceous fuels are relatively soft and do not develop woody, persistent tissue. They have leaves and stems that usually die down to the soil level and eventually become dead fuels. They are typically most important to fire behavior when they cure and transition to dead fuels as the growing season progresses.
The amount of grasses that grow and time it takes to cure them usually vary from year to year, and some perennial grasses never cure. Grass color can sometimes, but not always, be used as an indicator of the stage of curing. The degree of curing is important, because the ratio of live-to-dead grass influences fire spread. Normally, at least one third of the grasses must be dead for the fuel to carry fire.
One particular grass, called cheatgrass or annual brome (Bromus tectorum), often determines the severity of fire seasons on range lands in the Great Basin area. This grass is present in almost every state and in most continents of the world.
Cheatgrass stands normally cure out by early summer to produce abundant fine fuels that can be “explosive” when fuel moisture contents are very low. Fuel moisture in the living stage is above 100 percent. In the curing or transition stage, the moisture content drops to between 30 and 100 percent, and in the dead stage, the moisture content is below 30 percent. It is not until this dead stage that cheatgrass will ordinarily carry fire, but there can be some variations.
Cheatgrass exhibits coloration that helps indicate its stage of development and probable moisture content range.
Questions
What is the likely moisture content range of the fuel shown in Image 1? (Choose the best answer.)
The correct answer is c.
Above 100 percent: The green coloring of the cheatgrass in the image indicates that it is in its living stage. Fuel moisture content is highest during this period.
What is the likely moisture content range of the fuel shown in Image 2? (Choose the best answer.)
The correct answer is b.
30-100 percent: The purple coloring of the cheatgrass in the image indicates that it is curing, or drying out. Fuel moisture contents in this stage are typically between 30 and 100 percent.
What is the likely moisture content range of the fuel shown in Image 3? (Choose the best answer.)
The correct answer is a.
Below 30 percent: The brownish coloring of the cheatgrass indicates that it has cured, or dried, fully. Moisture content is lowest in this period. The three images show how cheatgrass color changes with season, from green to purple and then to a straw color as its moisture content declines.
Perennials vs. Annuals
Perennial and annual herbaceous vegetation includes grasses and forbs, which can become primary contributors to fire problems in many areas of the country. Perennial grasses and forbs usually cure out later than annuals, and this difference should be noted when assessing fire potential.
The degree of curing of annual and perennial grasses strongly affects the fire potential.
Forbs include milkweed, clover, sunflower, and other non-grass plants and herbs. When forbs compose a significant portion of the herbaceous fuels, they tend to impede fire ignition and spread until they have cured fully.
Woody Fuels
Other living fuels include woody plant material. Woody fuels include live leaves, needles, and twigs that can ignite if the approaching fire produces enough heat or if they are dry enough to carry a fire themselves.
Evergreen vs. Deciduous
Evergreen plants have needles or leaves that persist year round, though they generally shed a portion of them each year. Evergreens appear green throughout the year, but their moisture content can vary widely as the growing season progresses. The fluctuations in moisture content reflect growth processes as well as drought effects.
Deciduous plants shed their leaves annually and produce dead, fine fuels.
Fuel Groups or Categories
Of the six fuel groups discussed in Unit 3, only the timber-litter and slash blowdown groups assume that live fuels are not burned in the flaming front.
"Grass" fuels include live herbaceous grasses and forbs. These fuels will not burn at all early in the season but transition into fuels that burn more easily as they cure into dead fuels. This curing can occur prematurely due to frost damage or drought stress.
"Shrub" fuels include live woody fuels that exhibit a wide range of fire behavior based on fuel moisture. These fuel moistures are affected by the shrubs' growth and development through the growing season, as well as by factors including frost damage, drought stress, and aging.
"Grass-shrub" fuels include both herbaceous and woody fuels whose fuel moistures respond to the growing season and stresses from frost or drought.
"Timber understory" fuels have woody fuels that resist or contribute to burning at the flaming front depending on the moisture levels, similar to "shrub" fuels.
Seasonal Variation
The moisture content of new growth will generally remain high as long as adequate moisture is available in the surrounding environment. Air temperature, humidity, soil moisture, soil temperature, and plant physiology all affect the availability of moisture.
During the summer and fall, moisture content decreases to low levels in the dormant season and remains that way until the growth cycle begins again in the spring.
This live fuel moisture cycle can fluctuate based on plant species and location, including elevation, aspect, and steepness of slope.
Live herbaceous fuel moisture levels change relatively slowly with time, usually over days or weeks. When plant growth begins in the spring, the moisture content of new plant material rises rapidly to a peak value that is often greater than 200 percent of its dry weight value. After reaching peak moisture content, the subsequent fall in moisture level is usually slower, eventually leading to curing.
In woody fuels, developing new growth follows a similar pattern with a rapid rise in fuel moisture as the shoots, leaves and needles expand in the spring. The moisture content in older foliage and twigs also increases, but the peak value will be less than that found in new growth. Each year, some or all of the foliage will cure and fall, usually based on the onset of autumn weather conditions. The moisture content of woody larger stems and trunks changes only nominally with season.
Questions
How might an early green-up in the spring affect the live fuel moisture of herbaceous grasses and forbs? (Choose the best answer.)
The correct answer is b.
The fuel moisture content of live fuels could be higher than they would normally be at this time of year because the emergence of new plant material begins earlier.
It's been a very hot dry summer in the area where you are working. What live fuel moisture content would you expect for woody fuels? (Choose the best answer.)
The correct answer is a.
The fuel moisture content would likely be below normal as the hot, dry air reduces the amount of moisture available to the fuels. The heat and lack of moisture put stress on the fuels, lowering the fuel moisture content.
Canopy or Crown Fuels
Crown fires are one of the most significant ways live fuels impact the fire environment. Crown fires involve the live crowns or canopies of trees and shrubs. These fires can only occur when there is sufficient surface fire in the dead fuels and adequately low live fuel moistures in the crown. These low moistures in crown fuels are a particular danger in the spring, when developing new growth can cause the fuel moisture of mature needles to fall to critical levels. This "spring dip" phenomenon, often seen in northern evergreen forests, temporarily increases the potential for torching and crown fire.
There is only one exception to the general rule that live fuel moistures change slowly over time. This exception occurs when the preheating is due to the fire itself.
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Crown fuels become preheated as fire consumes the surface fuels. Eventually, these drier fuels allow a crown fire to ignite.
The ability of live fuels to ignite and burn aggressively can change as the surface fire increases in intensity and provides adequate heating of the live fuels by convection and radiation.
Critical Values
Critical live fuel moisture is defined as the moisture content at which sustained, fast spreading, high intensity wildfires can occur. Critical live fuel moistures vary with plant type. For instance, gambel oak has a critical live fuel moisture of 130 percent, while sagebrush and conifers have critical live fuel moistures of 100%. Manzanita has the critical live fuel moisture of 80%, while chamise has a critical live fuel moisture of 60%.
Warning actions should be taken when live fuels begin to approach the critical live fuel moistures. Factors including wind speed, slope, and dead fuel moisture can all have contributing or confounding effects.
Assessing Live Fuel Moisture
Abnormal burning conditions can occur when live moisture contents decrease or the amount of dead fuels increases because of:
- long drought periods
- insect and disease damage
- early curing of annuals
- frost kill
- harvesting of timber or other vegetation
- blowdowns or ice storms
As a result of these factors, firefighters need to monitor live fuel conditions, assess fuel moistures, and observe how the fuels are burning.
Field Reference Table
The first way to estimate live fuel moistures is based on the time of year, as moisture varies with season. Refer to the table in Appendix B of the Fireline Handbook, which correlates stages of growth (right column) with average live fuel moisture (left column). The growth stages and average moisture contents can help determine fire potential. Be sure to observe local conditions to note how the actual live fuel moistures in your area might deviate from what's suggested in the table.
Question
Rank the fuels shown in the photos from highest fuel moisture to lowest fuel moisture content. Table 6 can be used to help you estimate fuel moisture values. (Choose the best answer.)
The correct answer is c.
Order c, a, b: Fuel moisture content is highest in new plant growth. Mature plants, such as the blooming flowers in image (a), have fuel moisture contents of around 100 percent. Once fuels begin coloring and entering dormancy, live fuel moisture contents are generally less than 50 percent.
Field Sampling Techniques
The second, and most accurate, method of determining live fuel moisture is to collect and analyze samples from your area of concern.
When collecting samples, both fresh new growth and perennial older growth should be chosen.
The samples should come from a range of elevations, aspects, and species. The site selected for sampling should represent the fuel model of concern. These fuels should be the main carrier of fire or the ones that represent most of the plant species in the area. Because moisture cycles of deciduous vegetation are very different from evergreen vegetation, it is often necessary to collect samples of both to get an accurate representation of the fuel moisture in an area.
Once collected, each sample should be weighed, dried, and weighed again. Fuel moisture can be calculated using the standard formula: (wet weight – dry weight)/dry weight).
National Fuel Moisture Database
The National Fuel Moisture Database (NFMD) provides sampled and measured fuel moisture information for live, as well as dead, fuels. The information in the database is routinely updated by fuels specialists who monitor, sample, and calculate fuel moisture data.
The National Fuel Moisture Database serves the following purposes:
- Provide a repository for sampled live- and dead-fuel moisture. Essentially, the database serves as a national archive of the fuels record.
- Provide a one-stop resource for people needing fuel moisture data, eliminating multiple data requests and exhaustive searches.
- Allow easy viewing of the data in formats consistent with other land management applications.
Fire Danger PocketCards
The Fire Danger PocketCard is a method of communicating fire danger to firefighters. The PocketCard information is based on fuel conditions, including live fuel and dead fuel moistures. The objective of the cards is to lead to greater awareness of fire danger and, subsequently, increased firefighter safety. The PocketCard provides a description of seasonal changes in fire danger in a local area. They are useful, therefore, to both local and out-of-area firefighters. Specific pocket cards can be obtained online at http://fam.nwcg.gov/fam-web/pocketcards/.
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The PocketCard has a very important day-to-day "pre-suppression" use. When the morning and afternoon weather is reported each day, the actual and predicted weather and other indices are announced. The firefighters can reference their cards and determine how the conditions fit in the range of possible values for the danger rating. This important information should be discussed at morning crew meetings, as well as at tailgate safety meetings.
Many PocketCards use the Energy Release Component (ERC) as the reference index. This number represents the potential "heat release" per unit area in the flaming zone, and can provide guidance for several important fire activities. It can also be considered a composite fuel moisture value as it reflects the contribution of all live and dead fuels to potential fire intensity. The ERC is a cumulative index, reflecting a "build-up" in conditions. As live fuels cure and dead fuels dry, the ERC values increase, providing a means for representing local drought severity.
Use the PocketCard provided for the Flathead North and Glacier National Park region to determine the fire danger based on the following information.
The date is July 29. Not considering local weather conditions, which category would best summarize an ERC of 45 and 1000-hour fuel moisture of 9 percent? (Choose the best answer.)
The correct answer is c.
An ERC of 45 would exceed the 90th percentile at all times of the season, and combined with the low 1000-hour fuel moisture, fire danger in this region would be extreme. The situation would be further complicated by weather variables, including wind speed over 10 mph, relative humidity less than 25 percent, and temperature over 85°F.
Summary
Fuel moisture content is the amount of water in a fuel, expressed as a percent of the oven dry weight. The percent is determined by dividing the difference between the wet and dry weights by the dry weight and multiplying by 100.
Relative humidity and wind strongly affects fuel moisture content. For fine fuels, the movement of water vapor from a higher to lower concentration allows the fuels to approach an equilibrium moisture content. Large fuels do not tend to have moisture contents near the equilibrium moisture content because of the time durations required.
Precipitation and soil moisture directly affect fuel moisture content, raising the fine dead fuel moisture more rapidly than any other factor. Large fuels react more slowly to precipitation, and their moisture content depends strongly on the duration of the precipitation.
Fuels require some time period to adjust to changes in moisture. The time needed for a fuel particle to lose 63 percent of the difference between its initial moisture content and its equilibrium moisture content is the timelag.
Dead fuels can be classified in four timelag categories. Fuels less than ¼ inch in diameter are considered 1-hour timelag fuels. 10-hour fuels have diameters between ¼ inch and 1 inch. 100-hour fuels have diameters between 1 inch and 3 inches, while fuels 3 to 8 inches in diameter are referred to as 1000-hour fuels.
Critical live fuel moisture is defined as the moisture content at which sustained, fast spreading, high intensity wildfires can occur. Critical live fuel moistures vary with plant type: they can be as low as 60% for chamise and up to 130 percent for gambel oak.
Live fuel moisture can be obtained through estimates based on time of year, or by collecting and drying samples.
Knowledge of fuel moisture content can help you anticipate fire behavior when working an incident, but you should always be aware that a number of factors can influence the burning conditions.
General descriptions of fuel moisture effects given in the Incident Response Pocket Guide tables can alert firefighters to possible hazards or changes in the wildland fire environment, but it is also important to carefully assess the fire environment on a specific fireline, including fuels, terrain, wind, temperature, and relative humidity.
Thank for completing S-290 Unit 10: Fuel Moisture. Please follow the links to take the quiz and share your feedback with us via the survey.