Bend test for estimating moisture content of leaf litter and fine slash
Casals, P1; Vilà-Vilardell, L.1
1Forest Science and Technology Centre of Catalonia (CTFC)
Last updated: 22/09/2026
Recommended citation:
Casals P., Vilà-Vilardell, L. 2026. Bend test for estimating moisture content of leaf litter and fine slash. In: Pere Casals, Davide Ascoli, Lena Vilà-Vilardell, Imma Oliveras, Madoka Krick (eds.) 2026. Methods handbook for characterizing fire use behavior, assessing fire effects, and monitoring soil and vegetation responses at the stand level. FIRE-ADAPT project (number 101086416), deliverable D1.1
Indicator
Fuel flammability: Fuel moisture content
Brief introduction
When leaf litter or slash constitutes a major component of the forest floor, it is useful to have a simple field indicator of fuel moisture that allows comparisons among different days or forest conditions for a given fuel type.
One such method is the bend test, which relates the angle at which a fine fuel particle breaks to its fuel moisture content (FMC). In general, drier particles are more brittle and break at smaller angles, whereas moister particles can bend further before breaking.
Because this relationship may vary among fuel types and sites, the method should be calibrated and validated before widespread use, for example by comparing it through direct measurements of fuel moisture content.
Method specifications
Method type
Soft
Hard
Method timing
Before
fire
During
fire
After fire
(short-term)
After fire
(mid-term)
After fire
(long-term)
Field skills
Low
Medium
Hard
Lab skills
Low
Medium
Hard
Material cost
Low
Medium
Hard
Time cost
Low
Medium
Hard
Calculation expertise
Low
Medium
Hard
Equipment
- Hand gloves
Experimental design
The test involves collecting several particles from the forest floor (i.e. senesced needle or a fine branch) and slowly bending them into a loop until they break. The angle at which the loop breaks is correlated with the particle’s moisture content.
Methodology
- Collect several fuel particles from the forest floor, ensuring that samples are taken from different areas of the site. Particles collected should include senesced needles, hardwood leaves or fine branches.
- Slowly bend each particle into a loop by bringing the two ends together and gradually constrict the loop.
- Record the angle at which the particle breaks
Calculations
For needles:
- If the loop can be squeezed without breaking, the needles are likely too moist to sustain fire spread.
- If the needle breaks when the ends of the loop are approximately 2-3 cm apart, the moisture content is dry enough to carry a backfire.
- If the needle breaks into several pieces, it is very dry, and it may be advisable to reassess the burn objectives and consider the potential consequences of burning under such conditions.
For hardwood leaves (McCarthy 1927 in Wade 2013), bend the leaf at a right angle (90°):
- If the leaf crumbles into several pieces before reaching a 90º bend, the moisture content is low (<10%), suggesting the need to reassess the burn objectives.
- If the leaf breaks at approximately 90º, except along the veins, the moisture content is enough to carry a backfire (10-15%);
- If the leaf wrinkles or does not break cleanly when bent at 90º, the moisture content is likely greater than 20%, indicating that the fuel may be too moist to readily carry fire.
Relation with other methods
Data templates
Link to template (NA)
Further information
Banks, W.G., & Frayer, H.C. (1966) Rate of forest fire spread and resistance to control in the fuel types of the eastern region. U.S. Department of Agriculture, Forest Service, Fire Control Notes, vol 27. Washington, DC
Wade, D. (2013). Fuel moisture and prescribed burning. Southern Fire Exchange. SFE Fact Sheet 2013-5. 4 p. https://southernfireexchange.org/wp-content/uploads/2013-5.pdf