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Unit 5: Fuel Moisture

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Fuel Moisture Ranges in Natural Fuels

Note the approximate fuel moisture ranges in natural fuels: Living fuels--about 30 percent to over 300 percent. Dead fuels--about 2 percent to 30 percent. Why the big difference? Well, living cells have the capability of holding large amounts of water, while dead fuels have been found to be fiber saturated at approximately 30 percent and can absorb little more water.

Even though live fuel moisture content can be accurately determined by oven drying and weighing procedures, fire considerations are usually satisfied with a good estimate. The table featured below gives approximate moisture content percents for six stages of vegetative development. These stages and their average moisture contents are a contributing factor to determining fire propagation. Study this table since you will be required to know the six stages and their percent moisture contents.

Annual herbaceous vegetation, such as grasses, is a primary contributor to fire problems in many areas of the country. The amount of vegetation and the time of curing usually vary from year to year. One such species is Bromus tectorum, commonly called cheatgrass or annual brome. Although most common on dry areas of the West, it can be found in almost every state and is present in most continents of the world. Cheatgrass is a primary contributor to, and usually dictates, the severity of fire seasons on range lands in the Great Basin area of the United States. Here cheatgrass stands normally cure out by early summer to produce abundant, fine, flashy fuels, which are frequently termed "explosive" when fuel moistures are very low.

Moisture Content (percent) State of Vegetative Development
300 Fresh foliage, annuals developing, early in the growing cycle.
200 Maturing foliage, annuals developing with full turgor.
150 Maturing foliage midway in development cycle.
120 Foliage nearing maturity, new growth nearly complete.
90 Mature foliage, new growth complete and comparable to older perennial foliage.
60 Entering dormancy, coloration starting, some leaves may have dropped from stems.

Moisture Content Changes
Moisture content changes i Cheatgrass (annual).

This figure illustrates the annual growth period and moisture contents of cheatgrass during various stages of its short growth cycle. Coloration of the plant is an excellent indicator of its stage of development and probable moisture content range. As cheatgrass goes into its curing stage, it turns from green to purple; then, finally, it develops a straw color as it cures and its moisture content declines and fluctuates with changing weather factors.

The moisture contents in the three stages for cheatgrass would appear to be somewhat lower than those indicated in the above table comparing moisture content with stage of vegetative development. Actually there will be some variation by species, and the table above should be used as a general guide only.

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: http://ocw.usu.edu/Forest__Range__and_Wildlife_Sciences/Wildland_Fire_Management_and_Planning/Unit_5__Fuel_Moisture_2.html. This work is licensed under a Creative Commons License Creative Commons License