Unit 5: Fuel Moisture
Upon completion of this unit you will be expected to:
- Explain fuel moisture timelag and its value to fire managers.
- Name the six stages of vegetative development of living fuels (foliage), and give the average percent moisture content of each.
- Explain the relationships between relative humidity, wind, and moisture content of fine fuels and of large fuels.
- Explain how the amount and duration of precipitation affects moisture content of both fine fuels and large fuels.
- Describe how fuel moisture is determined for fuels in each of the four timelag categories for a fire area.
- Determine fuel moisture contents for dead 1-hour timelag fuels from fuel moisture tables, given necessary site data.
- Explain moisture of extinction, how it varies in natural fuel complexes, and how it affects fire ignition and spread.
The purpose of this unit is to help you make estimations of moisture content in various dead and live fuels, to identify those fuels which can burn, and to assess the chances of ignition from firebrands landing in new fuels or of fire spreading by preheating fuels ahead of a flaming front. You will also recognize that fuel moisture content is a very important input toward making fire behavior calculations and predictions.
Natural Fuels and Their Moisture Content
Fuel complexes vary greatly by areas or regions; with extremes from sparsely vegetated deserts, to rain forests with lush vegetation, to parched timber lands. If we view each as a potential fire environment, our immediate assessments must include fuel loadings and fuel moisture contents.
We would expect desert fuels to be dry for extended periods, but is there enough fuel to carry fire? The rain forest has abundant fuels which are generally too wet or too green to burn, but infrequently these areas do have fires. Extended summer drought periods occasionally make our timber lands extremely dry, sometimes to the point of being "explosive", should fires occur.
We can generalize at this point and say that when fuel moisture content is high, fires ignite and burn poorly, if at all; and when it is low, fires start easily, and spread and burn rapidly. These simple deductions might satisfy some fire managers, except that fuel moisture contents are frequently some place between the two extremes and fluctuate with changes in weather. During normal fire seasons, these same fire managers have experienced times when rapidly spreading fires suddenly stop, perhaps even go out, due to changes in their fuels and moisture contents. These fuels may have been on a different aspect, had a later curing date, or experienced a sudden change in relative humidity.
How does one measure fuel moisture content and then anticipate what changes will take place over time and space? First of all, fire managers have agreed upon a common description and unit of measure for fuel moisture content. This is the amount of water in a fuel, expressed as a percent of the oven dry weight of that fuel. If there were no moisture at all in the fuels, as if dried in an oven, the fuel moisture content would be zero percent. Fuels can be weighed before and after drying in an oven, and percent can be determined by dividing the difference between the original and dry weights by the dry weight. There are other, more practical ways of estimating fuel moisture percents in the field, and these will be discussed in this unit.
Fuel moisture content is the amount of water in a fuel expressed as a percent of the oven dry weight of that fuel.
Fire Danger is Mostly Dependent on Fuel Moisture
Fire managers are concerned with fire danger on a daily basis and make calculations of fuel moisture contents for fire danger-rating purposes. Some of the factors which are considered in the daily calculations are live to dead ratios of fuels; the stage of the growing cycle; the size classes of fuels; the daily weather elements; and the effects of accumulative weather on various fuel complexes.
Fire danger ratings have been designed for, and are usually adequate for general planning purposes, but do not provide enough data for fire behavior predictions about ongoing fires. In such cases, observed onsite data can help you refine estimates of fuel moisture percents. We will take you, step-by-step, through the process of estimating fuel moisture contents, given specific site data.
In estimating fuel moisture contents, you must remember that part of the fuel may be living vegetation, and part cured or dead vegetation. The two have different water retention mechanisms and different responses to the weather. Live vegetation has much higher moisture contents that fluctuate on a seasonal rather than a daily basis.
Living fuel includes both herbaceous plants and woody plant material. Herbaceous plants are either perennials, which sprout from the base, or annuals, which develop from seed each year. Herbaceous plants are relatively soft or succulent and do not develop woody, persistent tissue. The woody plant material that we are concerned with is small enough to be consumed in the flaming front of a fire. Mostly, this includes leaves, needles, and twigs. Herbaceous plants die each year, thus producing more dead, fine fuels. In grasses, perennials usually cure out later than the annuals; this is an important factor in assessing fire danger.
In living woody vegetation, the high fuel moistures are primarily in the foliage, and in new shoots or stems. In these fuels, the moisture content normally decreases as the growing season progresses, with lowest amounts occurring by late summer or autumn. Deciduous plants produce dead, fine fuels; whereas, most evergreen plants that retain their needles or leaves more than one season may have substantially reduced moisture contents.
Fuel Moisture Ranges in Natural Fuels
Fuel moisture ranges in natural fuels are:
- Living fuels, about 30 percent to over 300 percent.
- Dead fuels, about 2 percent to 30 percent.