This table (Rothermel, 1983) suggests the fuel moisture associated with the phenology or stages of plant development through a year that includes dormancy.
Trends in live fuel moisture vary widely, but NFDRS and US Fire Behavior Prediction methods categorize them as herbaceous and woody fuel moistures. NFDRS models trend live fuel moisture according to these stages of plant development:
1978 NFDRS expects the user to identify a greenup date, after which live fuel moistures increase to maximum levels over a fixed number of days established by the climate class designation. After that, live fuel moistures transition trends follow 1000hr (and x1000h) trends until fully cured or freeze/frozen conditions are selected.
1988 NFDRS replaced the greenup and transition trends with user selected designations of Season and greenness level for herbaceous and woody fuels.
2016 NFDRS uses a weather based index of plant development, called the Growing Season Index (GSI), to automate the process. It identifies when greenup begins, how fast it progresses, the maximum live fuel moisture, transition curing, and when freeze/frozen dormant conditions occur.
The Growing Season Index (Jolly et al, 2005) is a simple metric of plant physiological limits to photosynthesis. It is highly correlated to the seasonal changes in both the amount and activity of plant canopies. It predicts the green-up and senescence of live fuels and the influence of water stress events on vegetation. GSI is calculated as a function of the three indicators of important weather factors that regulate plant functions. These indicators are combined into a single indicator that integrates the limiting effects of temperature, water and light deficiencies. More information can be found at https://www.wfas.net.
Example 2014 seasonal values of the GSI and Live Fuel Moistures. Watford North Dakota (from wfas.net)
Upper and lower limits of the indicator functions used to calculate the Growing Season.
Example values of the GSI, their interpretation and effect on NFDRS Live Fuel Moistures.
The data and processing of the GSI, and the dependent live fuel moistures, make gridded map depictions possible and automated processing a reality.
As shown in this graph (Burgan, 1979), herbaceous fuel moisture influences both the flammability of living herbaceous vegetation and the transfer of living herbaceous fuel loads from and to dead fine fuels. The dashed line with the herbaceous load trend shows the trend for dynamic fire behavior fuel models.
Though similar in trend to herbaceous fuel moisture content, woody fuel moisture content ranges with less extremes:
There is no fuel load transfer between live and dead fuels based on woody fuel moisture.
This graphic shows the agreement between the Growing Season Index (GSI) trend and measured woody fuel moisture for Nevada Sagebrush and California Chamise, two very important fire landscapes. Note also how they differ from the 1978 NFDRS woody fuel moisture trend that was based on the 1000hr fuel moisture trend.
Foliar Moisture Content is defined (in the BehavePlus Variable) help as the moisture content of the conifer needles in tree crowns. It is used along with surface fire intensity and crown base height as input to the crown fire initiation model (described in section 5). Further, it is generally measured using only mature conifer needles (at least one-year old).
In some cases, evergreen hardwoods and deciduous species with resinous leaves will carry crown fire. Estimates of foliar moisture should reflect flammability of these crown fuels.
BehavePlus allows a range of 30%-300% as with other live fuels, but WFDSS allows only a range of 70% to 130. Default value is typically 100%.
The example plot below, for Abies lasiocarpa or Subalpine Fir, compares moisture content for new and old foliage. (Agee, et al 2002).
As shown in this graph, there is a measurable “Spring Dip” in measured foliar moisture content of mature needles associated with the emergence of new growth, at least among northern conifers. (Hirsch, 1996 and Jolly et.al., 2014)
Fuel Moisture >