1. Unit 10 Fuel Moisture

2. Unit 10 Objectives
Define critical live fuel moisture and the thresholds for various fuel types.
Identify three methods for obtaining live fuel moisture.
Describe the relationships between relative humidity, wind, and moisture content of fine and large fuels.

3. Unit 10 Objectives
Explain how the amount and duration of precipitation and soil moisture affect moisture content of fine and large fuels.
Define the fuel moisture timelag concept and its value to firefighters and fire managers.
Describe how fuel moisture is determined for dead fuels in each of the four timelag categories.

4. Unit 10 Objectives
Define moisture of extinction, how it varies in natural fuel complexes, and how it affects wildland fire ignition and spread.
Determine fuel moisture content for fine dead 1-hour timelag fuels from fuel moisture tables during daylight conditions.

5. When fuel moisture is high, fires ignite and burn poorly; when it is low, fires start easily, and spread and burn rapidly.
Fuel moisture will fluctuate with changes in weather, seasons, and topography.
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6. Fuel Moisture
Fuel moisture content is the amount of water in a fuel expressed as a percent of the oven dry weight of that fuel.
Wet Weight - Dry Weight
Dry Weight

7. Types of Fuel Moisture LIVE fuel moisture: DEAD fuel moisture:
Found in living plants
Ranges from 30% to 300%
Varies over space, species and seasons
DEAD fuel moisture:
Found in dead plants, forest litter, slash, etc.
Ranges from 2% to 30%
Can change quickly over time and space

8. Fuel Moisture Affects Combustion
Before a fuel can burn, the moisture in it must be converted to vapor through the heat process.
The greater the moisture content, the higher the heat temperatures required to dry the fuel.
The presence of moist fuel can affect the rate and direction that a wildland fire spreads.

9. Fuel moisture is one of the seven wildland environmental factors which must be continuously monitored for safety reasons.

10. Critical Live Fuel Moisture and the Thresholds for Various Fuel Types

11. Different physiological properties Different chemical contents
Herbaceous
Annuals
Perennials
Live Fuels
Coniferous
Woody
Deciduous

12. Variations in Live Fuels
Change seasonally
Differences based on species and fuel model
Vary in relation to aspect, elevation
Big picture influences:
Drought, frost kill, insects, diseases

13. Annual Trend of Jack Pine Foliar Moisture
300
280
260
240
220
200
180
160
140
120
100 80
Moisture Content (percent)
New Foliage
Old Foliage
March
Oct
Month of Year

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15. Moisture Content of Cheatgrass
250
Coloration
200
150
green
Moisture content (percent)
100
purple 50 30
straw
Spring Summer
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16. Fuel Models with Live Fuels
Herbaceous
FM Timber with grass understory
Woody
FM Chaparral 6 ft.
FM Brush 2 ft.
FM Southern rough
FM Timber with litter understory

17. Fuel Model 2
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18. Fuel Model 4
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19. Fuel Model 5
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20. Fuel Model 7
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21. Fuel Model 10

22. Rate of Spread Based on Live Fuel Moisture

23. Crown Fires Crown fires are live-fuels fires.
Can include large trees, but also the canopies of shrubs (oak brush and manzanita).
Need sufficiently low live fuel moisture to be a significant component to fire behavior.
Need adequate dead fuels to initiate a crown fire.

24. Critical Live Fuel Moisture
The moisture content at which sustained, fast spreading, high intensity wildfires occur.
Gambel Oak 130%
Sagebrush 100%
Conifers 100%
Manzanita 80%
Chamise 60%

25. Three Methods for Obtaining Live Fuel Moisture

26. Live Fuel Moisture Content Stage of Vegetative Development Moisture Content (Fireline Handbook, Appendix B)
Fresh foliage, annuals developing, early in growing cycle. Leaf-out, bud-break, early June.
300%
Maturing foliage, still developing with full turgor
Late June.
200%
Mature foliage, new growth complete and comparable to older perennial foliage. July to early August.
100%
Entering dormancy, coloration starting, some leaves may have dropped from stem. Mid-August - September.
50%
<30%
Completely cured. Treat as dead fuel.

27. Live Fuel Sampling Collect relative sample from your area of concern.
Be sure to sample the fresh new growth of the year as well as the perennial older growth.
Once collected, weigh it, dry it, weigh it again. Use formula.

28. On-line Data Sources

29. Relationships Between Relative Humidity, Wind, and Moisture Content of Fine and Large Fuels

30. Moisture Balance
Water moves from higher concentrations to lower concentrations

31. Moisture Exchange in Wildland Fuels
Humidity
Evaporation
Dew
Precipitation
Evaporation
Ground moisture
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32. Equilibrium Moisture Content
Occurs when there is no net loss or gain of moisture between fuels and the surrounding air.
Moisture content attained when subjected to a constant temperature and humidity condition for an indefinite length of time.

33. Environmental Factors Influencing Fuel Moisture
ELEVATION
REALTIVE
HUMIDITY
FUEL
MOISTURE
PRECIPITATION
WIND
INDIRECT INFLUENCES
DIRECT INFLUENCES
TEMPERATURE
SHADED
UNSHADED
SOLAR
RADIATION
SLOPE
ASPECT
CANOPY
CLOUDS
NIGHT
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34. Shaded vs. Unshaded Effects on Fuel Moisture
Air Temperature
85o
85o
Fuel Moisture 8%
110o 3%
160o
Surface Temperature
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35. Aspect Affects Fuel Moisture15 10 5
East slope
North slope
South slope
Moisture content (percent)
West slope
Time of day

36. Elevation Affects Fuel Moisture (Daytime)
Temperature RH Fuel Moisture
69°
73°
76°
80°
83°
87°
39%
35%
31%
27%
25%
22% 8% 7% 6% 5% 4%
6000 feet
5000 feet
4000 feet
3000 feet
2000 feet
1000 feet
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37. Slope Percent Affects Solar Heating
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38. Wind Affects Fuel Moisture
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39. Wind Affects Fuel Moisture
Wind can accelerate the rate at which a fuel reaches its moisture equilibrium.
During calm air conditions, the air next to the fuels tends to become saturated with water vapor, decreasing the evaporation rate of moisture from the fuel.
Wind removes this saturated air, continually replacing it with drier air and thus speeding up the evaporation process.

40. How the Amount and Duration of Precipitation and Soil Moisture Affect Moisture Content of Fine and Large Fuels

41. Fine Dead Fuels Fine, dead fuels react very rapidly to precipitation.
Additional rainfall has little effect on the fuels.
Rainfall can be responsible for wetting the soils in contact with fuels.

42. Duration of Precipitation and Fuel Moisture
1-hour
10-hour
100-hour
Fuel Moisture Content
(percent)
Hours of continuous precipitation
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43. The Fuel Moisture Timelag Concept and its Value to Firefighters and Fire Managers

44. Timelag is the time it takes dead fuel to reach 63% of the difference between its current moisture content and that it would reach at EMC (maintained at steady environmental conditions).
Timelag is an indication of the rate fuel gains or loses moisture due to changes in its environment.
It is expressed as the rate (hours) at which the fuel type approaches the moisture equilibrium of its surrounding atmosphere.

45. Timelag and Fuel Size Relationships
100 60 40 20 10 4 2 1
About
40 days
Timelag (days)
Branchwood diameter (inches)

46. Reaction Time of Fuels to Wetting and Drying
12” log
1/2” sticks
Moisture content
Day Last
before day of
Days after
Precipitation
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47. A fuel complex of homogeneous fuel is never seen.
A pure grass stand comes closest to being a homogeneous fuel.

48. Dead Fuel Timelag Categories
1-hour timelag fuels: 0 to 1/4 diameter
10-hour timelag fuels: 1/4” to 1” diameter
100-hour timelag fuels: 1” to 3” diameter
1000-hour timelag fuels: 3” to 8” diameter

53. Daily Relationship of Relative Humidity to Fine Dead Fuel Moisture
RH & moisture
percent
Moisture content
12PM 6AM NOON 6PM PM

54. How Fuel Moisture is Determined for Dead Fuels in Each of the Four Timelag Categories

55. Determining Dead Fuel Moisture
1-hour
Calculate from fuel moisture charts
NFDRS outputs
Measure - drying oven and scales
10-hour
Fuel moisture sticks
NFDRS outputs

56. Determining Dead Fuel Moisture
100-hour
NFDRS outputs
Measure - drying oven and scales
Calibrated moisture meter
1000-hour
NFDRS outputs

57. Moisture of Extinction: How it Varies in Natural Fuel Complexes and How it Affects Wildland Fire Ignition and Spread

58. Moisture of Extinction
Fuel moisture content at which a fire will not spread or spreads only sporadically and in a non-predictable manner.
Dependent upon:
Fuel loading
Size and shape
Arrangement
Chemical content
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59. Moisture of Extinction
Fuel Model 1 - Short grass %
Fuel Model 2 - Timber %
Fuel Model 3 - Tall grass %
Fuel Model 4 - Chaparral %
Fuel Model 5 - Brush %
Fuel Model 6 - Dormant brush %
Fuel Model 7 - Southern Rough %

60. Moisture of Extinction
Fuel Model 8 - Closed timber litter %
Fuel Model 9 - Hardwood litter %
Fuel Model 10 - Timber … %
Fuel Model 11 - Light slash %
Fuel Model 12 - Medium slash %
Fuel Model 13 - Heavy slash %

61. Determining Fuel Moisture Content in Fine Dead Fuels (1-hour timelag)
Dry Bulb Temperature
Relative Humidity
Reference Fuel
Moisture (RFM) +
Month
Shaded or Unshaded
Time of Day
Site Location
Aspect
Slope
Fuel Moisture Correction Value (FMC) =
Adjusted Fine Dead
Fuel Moisture (FDFM)
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62. Fine Dead Fuel Moisture Worksheet
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63. Table 2 Reference Fuel Moisture (daytime 0800-1959) Input Line 6
Go to Tables 3, 4, or 5 for corrections.
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64. Table 3 Fine Dead Fuel Moisture Content Corrections (daytime ) May, June, July Input Line 13
Note: B = 1,000 to 2,000 feet below site
L = + 1,000 feet of site location
A = 1,000 to 2,000 feet above site
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65. Table 4 Fine Dead Fuel Moisture Content Corrections (daytime ) Feb.-Apr., Aug.-Oct. Input Line 13
Note: B = 1,000 to 2,000 feet below site
L = + 1,000 feet of site location
A = 1,000 to 2,000 feet above site
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66. Table 5 Fine Dead Fuel Moisture Content Corrections (daytime ) November, December, January Input Line 13
Note: B = 1,000 to 2,000 feet below site
L = + 1,000 feet of site location
A = 1,000 to 2,000 feet above site
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67. Temperature & Humidity Selection
2,000 feet
Above Site (A)
New T°/RH%
Prediction
1,000 feet
Site Location (L)
T°/RH% Prediction
No adjustment needed
0 feet
1,000 feet
Below Site (B)
New T°/RH%
Prediction
2,000 feet

68. REFERENCE FUEL MOISTURE DAY (0800-1959)
Table 2
REFERENCE FUEL MOISTURE DAY ( )
Input Line 6
Example: Dry Bulb temp: 85o
RFM: 3
Relative Humidity: 22
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69. Fine Dead Fuel Moisture Worksheet85 22 3 60 62 8
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70. Table 4 – Fine Dead Fuel Moisture
Content Corrections, Day ( )
February, March, April, August, September, October
Example:
Month: August 20
S/U: Shaded
Time: 1200
Elevation: Mid-slope
Aspect: East
Slope: --
FMC%: 4%
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71. Fine Dead Fuel Moisture Worksheet
Aug
May
1200
1400 E S -- 20 4
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72. REFERENCE FUEL MOISTURE DAY (0800-1959)
Table 2
REFERENCE FUEL MOISTURE DAY ( )
Input Line 6
Example: Dry Bulb temp: 92o
RFM: 2
Relative Humidity: 16
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73. Table 5 – Fine Dead Fuel Moisture
Content Corrections, Day ( )
November, December, January
Example:
Month: November
S/U: Unshaded
Time: 1500
Elevation: 1500 Above
Aspect: West
Slope: 40%
FMC%: 3%
B = 1000 to 2000 feet below site location
L = +/ feet of site location
A = 1000 to 2000 feet above site location
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74. Solution to Question 3.a. 5 3a 92 --- 16 2 Nov 1500 W 40 3
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75. Solution to Question 3.b. 8 3b 75 --- 28 4 Oct 1700 N 20 4
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76. Solution to Question 3.b. 4 5 6 B L A 80 80 80 20 20 20 3 3 3 Aug Aug
1600
1600
1600 N N N 20 20 20 1 2 3 4 5 6
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77. Severe Fire Behavior Potential Related to Relative Humidity and Fuel Moisture Content
(For Western US only)
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78. Review Unit 10 Objectives
Define critical live fuel moisture and the thresholds for various fuel types.
Identify three methods for obtaining live fuel moisture.
Describe the relationships between relative humidity, wind, and moisture content of fine and large fuels.

79. Review Unit 10 Objectives
Explain how the amount and duration of precipitation and soil moisture affect moisture content of fine and large fuels.
Define the fuel moisture timelag concept and its value to firefighters and fire managers.
Describe how fuel moisture is determined for dead fuels in each of the four timelag categories.

80. Review Unit 10 Objectives
Define moisture of extinction, how it varies in natural fuel complexes, and how it affects wildland fire ignition and spread.
Determine fuel moisture content for fine dead 1-hour timelag fuels from fuel moisture tables during daylight conditions.