Determining Fuel Moisture

Fuel moisture content can be determined in several ways, from cheap and approximate to expensive and precise. Methods discussed here by no means include all field or laboratory methods, but do include: titrimetry (volumetric analysis), gravimetry, moisture indicator sticks, solvent distillation, statistical correlations, formulas and graphs, ocular methods, and remote sensing.

Titrimetry

Titrimetry or volumetric water content measures percent water content of a known volume of fuel. This analysis is performed by:

• Measuring the dimensions of the fuel volume (the length, width, and height of a fuel volume, i.e., 1 m³) of interest. This known volume is collected and weighed as a “wet” weight.

• The wet sample is then sealed and transported to a drying oven and dried (usually between 70 and 100^oC) to a constant weight.

• The “dry” sample is weighed and subtracted from the “wet” field weight.

• This difference is divided by the original wet weight and the resulting value is the volumetric water content (%).

The advantages of this method are its ability to integrate different fuels for a better estimate of a stand or site fuel moisture and its low cost (Pyne et al. 1996).

Gravimetry

Gravimetric fuel moisture content measures the percent water content of a known weight of fuel. Sampling is very similar to that used for volumetric water content, except that the original volume of fuel does not need to be measured:

• Collecting field fuels and weighing their “wet” weight.

• The wet sample is then sealed and transported to a drying oven and dried (usually between 70 and 100^oC) to a constant weight.

• The “dry” sample is weighed and subtracted from the “wet” field weight.

• This difference is divided by the original wet weight and the resulting value is gravimetric fuel moisture content (%).

The advantages of gravimetric analysis are its ease of sampling (1-, 10-, 100-, and 100-hour fuels can be “grabbed” and weighed without measuring a fuel’s dimensions), low cost, and subjectivity of fuels sampled (Pyne et al. 1996). It is often used to measure moisture content for specific classes of fuels: grass, forbs, woody debris, etc...

Moisture indicator sticks

Moisture indicator sticks are another method of determining fuel moisture. The sticks are 0.625 cm (¼”) diameter x 50 cm (20 in.) long wooden (pine sapwood) dowels that mimic water absorption of 10-hour timelag fuels. Moisture sticks are placed on support brackets 25 cm (10 in.) above a fuelbed and allowed to equilibrate for some time prior to observation. Measurements of moisture are made on a standard scale and applied as a site average. Moisture sticks have the advantage of being site specific and quickly measured.

Tabular, Statistical correlations, and graphs

Tabular or statistical correlation and graphical methods are a popular method of fuel moisture determination. Many tables have been generated that integrate relative humidity, air temperature, and wind speed to yield fine fuel (1-hour timelag fuel) moisture content. These measures are easily acquired (through field, belt weather kit, or weather station measurements) and tables are quick. This method has utility for fine fuels and for occasions when quick determination of fuel moisture is critical. For example, see the following tables to predict litter moisture.

• (Fine fuel moisture content of dead grass (1-hour timelag))

• (Moisture content of pine needle litter)

• (Average moisture of hardwood litter under an upland oak stand in Eastern Tennessee)

• (Total litter moisture content from duration of (last) rain and days since (last) rain data)

Ocular methods

Ocular methods of determining fuel moisture are another method of fuel moisture determination. Ocular methods include observation of vegetation color, either directly or by using remote sensing imagery (Pyne et al. 1996). Experienced burners in the Southeast can manually bend, twist, and break litter fuels (1-hour timelags) to estimate relative fuel moisture contents. These methods can be inexpensive (even analysis of remote sensing data is inexpensive over large landscapes), but require extensive experience and can lack utility in fire management situations.

Solvent distillation

Solvent distillation is another method of determining fuel moisture content. Distillation consists of heating or boiling a fuel sample of known weight or volume. The volatilized gases and vapor are captured, cooled, and collected. The weight or volume of the collected liquid is subtracted from the original weight or volume, and then divided by the original weight or volume. The resulting value is the distillation fuel water content. This method is precise, but is labor and equipment intensive.

Instrumental techniques

Several fuel moisture determining instruments are used in southeastern fuels. Some instruments, developed for lumber analysis, use electric resistance probes to determine woody moisture content. Other techniques use small hand-held heating chambers that vaporize water of fine and small woody fuels. Both of these instrumental techniques have cost and applicability constraints, but there is great potential for hand held, low-cost, precise field fuel moisture devices.

Remote sensing

Recently, remotely sensing has been used to characterize fuel moisture. The Wildland Fire Assessment System produces daily maps of live fuel moisture estimates. The moisture image will be most useful in shrub communities where users have measured live fuels (ground truthed) and developed correlations between measured and image moisture values.