Determining Fuel Moisture Content
Fuel Moisture Content - The amount of water present in fuel generally expressed as a percentage of the substance's weight when thoroughly dried at 100 degrees Celsius (C).
(CIFFC Glossary of Forest Fire Management Terms)
'For most fuels, no computational procedure yet developed is adequate to estimate their moisture content. Direct sampling of fuels is the best alternative. Regardless of the purpose of fuel moisture sampling, a standard procedure for collecting, processing, calculating and reporting the moisture content is necessary.' (Pollet and Brown 2007, p.1)
Forest fuel moisture content can be measured and/or estimated in a number of ways. The method implemented will be determined by the scope of the research project, equipment available and the fuel component(s) under study.
Forest Fuel Sampling Methodology
General Fuel Moisture Sampling Considerations
The goal of collecting fuel moisture content samples for fire behaviour research is to collect samples of 'the organic materials that influence the way a fire in those fuels will burn.' (Norum and Miller 1984) In this same publication the authors outline several standards for determining locations for sampling sites.
- Sampling sites should be representative of the range of conditions including elevation, position on slope, slope and aspect.
- Samples should be collected from the area to be burned.
- Use a line transect method across significant features of the burn area. A slope should be sampled from high point to low point with samples taken at intermediate points. Similarly, if there are extremes in moisture levels within the site, either extreme and intermediate points should be sampled.
- Fuel moisture sampling for prescribed burns should include sampling sites outside the prescribed burn area.
- Burn managers may need to develop contigency suppression plans in the case of anticipated extreme fire behaviour and the potential for escape fires.
Pollet and Brown (2007) have published detailed worksheets that can be used to determine the number of samples required to achieve valid representation. In the absence of these worksheets the following number of samples should be collected:
- Dead fuels - 3 to 5 samples of each size class of interest
- Live Fuels - 5 to 15 samples of each species of interest
Gibos (2010) offers additional general guidelines that can be applied to other fuel sampling projects.
- Flagging tape was used to mark the sampling areas to prevent trampling and compaction
- Samples were not collected from:
- Stronly sheltered areas
- Areas of atypical vegetation density
- Uncharacteristically open areas
- Depressions in the forest with higher moisture levesl
- Areas that had been compacted
With fine fuels being the primary fuel size studied by Gibos, each sampling event consisted of collecting three tins of needle litter and two tins of moss-tips from each location. Further specifics of Gibos' sampling methodology can be found in A.3.4 Collecting Samples pp. 138-140.
Countryman and Dean 1979 (pg. 9) describe essential procedures for marking and documentation of a sampling site.
- Mark the boundaries of the sampling site with durable fence posts.
- Mark the location of the sampling sites on a general map and provide access information to each site.
- Provide a written description for each sampling site including elevation, slope aspect and steepness, size of the area and predominant species. Give each sampling site an identifier and include pictures of the sampling site
Fuel Sampling Equipment
Because 'drying time is very sensitive to the compactness of the fuel sample', sample containers should be sized appropriately to the size of the fuel in order to prevent compressing the fuel sample. (Countryman and Dean 1979)
Countryman and Dean (1979) provides guidelines regarding the sampling containers.
- Samples should be dried in the same container that they have been collected in. Especially in live fuel, moisture can condense on the interior of the container and this moisture must be accounted for in the weighing and drying processes. Additionally, using the same container reduces the potential for material loss during transfer.
- Glass jars have the disadvantage of increased drying time required due to the large mass and low thermal conductivity of glass.
Pollet and Brown (2007) describe desirable characteristics of fuel moisture sampling containers. Sampling containers should:
- have tight-fitting lids
- be rust-proof
- be numbered
- be heat resistant
Recommended container types include plastic bottles with tight fitting lids, aluminum soil sampling cans and fuel sampling plastic bags with self-sealing zippered closures. Sampling containers and matching lids should be numbered with matching numbers in permanent marker or etching.
Additional fuel sampling equipment
Pollet and Brown (2007) suggest additional fuel sampling equipment including good quality pruning shears with two curved sharp blades, garden trowel, and an insulated plastic cooler with a handle. An electronic scale capable of accurately measuring to the nearest 0.1 gram is required to weigh the fuel samples in the field and after drying. This document provides a detailed equipment vendor list.
Fuel Moisture Content Measurement Tools
Most wildland fuel components can be measured using the oven drying method which will yield the most accurate results. However, oven drying is often impractical due to equipment and time requirements and other methodologies and tools will be more practical.
Oven drying method
Representative fuel samples are collected and stored in airtight containers for transport to the nearest drying oven. Oven drying yields a fuel moisture content reading (expressed as a percentage) calculated by comparing the weight of water in a fuel to the dry weight of the fuel. (Gravimetric fuel moisture content) This will be the most accurate measurement of fuel moisture but is time consuming and equipment may not be readily available at the sampling site.
Two general types of drying ovens are available - mechanical convection ovens and gravity convection ovens. Mechanical convection ovens are the preferred type since the forced circulation will minimize vertical and horizontal temperature gradients in the oven and fuel sample drying within the oven is more uniform. (Countryman and Dean 1979)
Another compact field deployable tool that uses oven drying principles is the Sigma Delta Technologies NeoSystems portable moisture meter which can measure fuel moisture in litter, duff and foliage. Operating instructions and the results of a study involving comparative analysis of the Neo Systems moisture meter and conventional cut and dry oven dry methodology can be found in the USDA United States Forest Service publication Herbaceous Measuring Device Comparison
Other Fuel Moisture Content Measurement Tools
Other methods are available for measuring fuel moisture content and these will be used when oven drying is not practical. Most other moisture content measuring tools use electrical conductivity or other principles to determine water content on a volume basis. (Volumetric moisture content)
DMM600 Duff Moisture meter - The DMM600 Duff Moisture meter made by Campbell Scientific, Inc. can be used to measure fuel moisture in the duff layer only.
The GE Timbermaster Protimeter is a conductivity moisture meter that can be used to measure moisture content in medium and heavy fuels.
Fine fuel moisture sticks - As part of the thesis study done by Gibos (2010), moisture content readings recorded by electronic fuel moisture sensors were compared with the fine fuel moisture content determined through destructive sampling. (Gibos 2010 p.80-84)
Sampling and Measuring Fuel Moisture Content in specific fuel components
Different fuel components within the wildland environment require appropriate fuel sampling techniques and moisture measurement tools. Examples of component based fuel sampling methodologies and fuel moisture content measurement tools are outlined and described in the following discussion and links.
Foliar Moisture Content
When sampling foliage, do not include new growth (less than 1 year old) since moisture content in the new growth will not be representative of the majority of foliage. Sample several trees at different heights and different aspects. Do not collect dead or diseased foliage, cones, or berriers. Fill sampling containers about 3/4 full. (Pollet and Brown 2007, p.13)
Norum and Miller (1984, p.24) provide a detailed process for collecting black spruce foliage and describe physical characteristics of black spruce foliage that will aid in distinguishing between new growth and old growth. 'The stem of 1-year-old growth is much lighter brown than the dark and slightly woody stem segment that grew the previous year.'
Over two growing seasons (1974-1975),Chrosciewicz (1986) sampled foliage from 4 different conifer species to plot the seasonal fluctuations of the foliar moisture content of each species. To minimize the impact of diurnal variation in foliar moisture content (Philpot 1965), all sampling of any given species was conducted during the same time frame (1200 - 1600) on the same afternoon. In contrast to sampling techniques implemented in Pollet and Brown (2007, p.13), the live branch samples were collected with a south facing exposure were cut from the lower third of the crown.(p. 158)
If burning is occurring during the spring, the spring dip in foliar moisture content may have an impact on fire behaviour and foliar samples should be collected to measure foliar moisture. If it is not possible to collect foliar samples, FMC can be estimated using the equations provided in the STX-3 report (page 20). Latitude, Longitude and Julian date are required inputs for these equations.
Estimates of foliar moisture content in North America forest conifers can be made by applying published foliar moisture content values. (Keyes 2006)
Oven drying will be the most appropriate and accurate methodology for measuring foliar moisture content.
Fine Fuel Moisture Content
Gibos (2010) outlines the fuel sampling methodology used for collecting fine fuels (needle litter and moss) in a research project studying the effect of slope and aspect on litter layer moisture content (page 138-140)
The sample processing methodology (oven drying) used for this project is described in the same document page 141-142
Oven drying will be the most appropriate and accurate tool for measuring moisture content in fine fuels. However, fine fuel moisture content can be estimated on-site using fuel moisture sticks.
Duff Moisture Content
'Moisture content of the duff has important influences on the depth of burning of the forest floor. It must be sampled carefully and selectively if the impacts and results of the fire are to be predicted well.' (Norum and Miller 1984, p.14) Basic guidelines for effective duff sampling are offereed in the publication titled Measuring Fuel Moisture Content in Alaska: Standard Methods and Procedures.
- Depth of the duff layer should be measured with the base of the moss and lichen or litter layer as a reference point.
- Since there may be variations in fuel moisture at different depths of the duff layer, several samples at distinct depths may need to be collected from the same spot.
- Sampling should be conducted in spots that are representative of the area. Avoid sampling from elevated mounds of duff unless these are a predominant feature of the duff layer in the area.
- Remove all parts of living plants, animal droppings, mineral soil and stones from the sample.
Duff layer fuel moisture content measurements can be achieved through oven drying or by using the DMM600 Duff Moisture meter.
Medium and Heavy Fuels Moisture Content
The Bureau of Land Management Fuel Moisture Sampling Guide (Pollet and Brown 2007) sets guidelines for the collection of medium and heavy fuel samples.
- When sampling medium and heavy fuels, choose samples from a log or branch that is down and dead and detached from the growth plant.
- Sample fuels should be resting on the ground not buried in the litter, duff or soil.
- Samples should be cut with a hand saw since chainsaws may leave an oil residue which could affect the dead fuel moisture results.
- Sample wafers should be cut less than 1/2" thick starting at least six inches from the end of the branch or log.
- Ensure that samples are sound with minimal decay and remove all lichen, loose or other debris.
- Samples should be collected in an appropriately sized sealsed plastic zip-lock bag and stored in a carrying case to be transported to the oven drying facility.
Zahn and Haase (2006) recommend using a chainsaw for collecting samples in the larger fuel classes. Evaluation results concluded that there were no differences in fuel moisture content in samples cut with a chainsaw and those cut with a handsaw.
Moisture content measurement in heavy fuels can be conducted by using the oven drying method or using the GE Protimeter Timbermaster. The Protimeter Timbermaster is a hand-held conductivity moisture meter that uses 1.3 cm. probes to measure moisture content in the fuel to a depth of about 1 cm. (Baxter 2009)
Countryman CM, Dean WA (1979) ‘Measuring Moisture Content in Living Chaparral: a field user's manual.’ USDA Forest Service, Pacific Southwest Forest and Range Experiment Station PSW-36. (Berkeley, CA) 27 pp.
Development and Structure of the CFFDRS. Information Report ST-X-3
Gibos, Kelsy 2010. Master of Science in Forestry, University of Toronto. Effect of Slope and Aspect on Litter Layer Moisture Content of Lodgepole Pine Stands in the Eastern Slopes of the Rocky Mountains of Alberta.
Norum RA, Miller M (1984) ‘Measuring Fuel Moisture Content in Alaska: Standard Methods and Procedures.’ USDA Forest Service, Pacific Northwest Forest and Range Experiment Station General Technical Report GTR-PNW-171 (Seattle, WA) 34 pp.
Philpot, C.W. 1965. Diurnal fluctuation in moisture content of ponderosa pine and whiteleaf manzanita leaves. U.s. Dep. Agric. For. Serv. Res. pap. INT-102.