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Unit 10 Fuel Moisture

Unit 10 Fuel Moisture. 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.

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Unit 10 Fuel Moisture

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  1. Unit 10Fuel 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. 10-5-S290-EP

  6. Wet Weight - Dry Weight Dry Weight 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.

  7. Types of Fuel Moisture • LIVE 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 Annuals Herbaceous 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 New Foliage Moisture Content (percent) Old Foliage March Oct Month of Year

  14. 10-14-S290-EP

  15. Moisture Content of Cheatgrass 250 Coloration 200 green 150 Moisture content (percent) 100 purple 50 30 straw 0 Spring Summer 10-15-S290-EP

  16. Fuel Models with Live Fuels • Herbaceous • FM 2 Timber with grass understory • Woody • FM 4 Chaparral 6 ft. • FM 5 Brush 2 ft. • FM 7 Southern rough • FM 10 Timber with litter understory

  17. Fuel Model 2 10-17-S290-EP

  18. Fuel Model 4 10-18-S290-EP

  19. Fuel Model 5 10-19-S290-EP

  20. Fuel Model 7 10-20-S290-EP

  21. FuelModel10

  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 ContentStage of Vegetative Development Moisture Content(Fireline Handbook, Appendix B) Fresh foliage, annuals developing, early in growing cycle. Leaf-out, bud-break, early June. 300% 200% Maturing foliage, still developing with full turgor Late June. 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 10-31-S290-EP

  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. WIND NIGHT ELEVATION CLOUDS CANOPY TEMPERATURE FUEL MOISTURE SHADED REALTIVE HUMIDITY ASPECT SLOPE UNSHADED SOLAR RADIATION PRECIPITATION INDIRECT INFLUENCES DIRECT INFLUENCES Environmental Factors Influencing Fuel Moisture 10-33-S290-EP

  34. Shaded vs. Unshaded Effects on Fuel Moisture Air Temperature 85o 85o Fuel Moisture 8% 110o 3% 160o Surface Temperature 10-34-S290-EP

  35. Aspect Affects Fuel Moisture 15 10 5 0 East slope North slope South slope Moisture content (percent) West slope 0 0600 1200 1800 Time of day

  36. Elevation Affects Fuel Moisture (Daytime) Elevation Temperature RH Fuel Moisture 39% 35% 31% 27% 25% 22% 8% 7% 6% 5% 5% 4% 69° 73° 76° 80° 83° 87° 6000 feet 5000 feet 4000 feet 3000 feet 2000 feet 1000 feet 10-36-S290-EP

  37. Slope Percent Affects Solar Heating 10-37-S290-EP

  38. Wind Affects Fuel Moisture 10-38-S290-EP

  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 ofFine 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 10-42-S290-EP

  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. About 40 days Timelag and Fuel Size Relationships 100 60 40 20 10 4 2 1 Timelag (days) 1.4 2 4 6 8 10 Branchwood diameter (inches)

  46. Reaction Time of Fuels to Wetting and Drying 12” log 1/2” sticks Moisture content Day Last before day of 1 2 3 4 5 6 Days after Precipitation 10-46-S290-EP

  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

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