This parameter assesses the water content of large logs and branches, from 3 to 8 inches of diameter. These fuels do not burn easily. However, if they do , an extreme heat will be generated, causing dangerous fire behavior conditions. The number 1000-Hour timelag, refers to the rate of change of its humidity. Given a particular atmospheric conditions (Relative Humidity and Air Temperature), there is an equilibrium moisture content, which will be reached if those conditions are mantained during an infinite time. The timelag, is the time that the fuel would need the moisture content to reach a value, between the current moisture content and the equilibrium moisture content. For, example, assume that the logs of the Figure have a moisture content of 20%.
If the current conditions are 20ºC and 70% of relative humidity, the quilibrium moisture content is:12.84 %. (See 1981 Fosberg, Rothermel & Andrews for details about the calculation of the equilibrium moisture content). After 1000 hours the moisture content would be:
As it can be seen, the fuel moisture tends to reach the equilibrium moisture content if the atmospheric conditions are mantained.
The Fuel Moisture content of the 1000-Hour timelag fuels is a very relevant parameter for the assessment of wildfire risk, for this reason is one for the paremeters which are continously monitored by the WFAS (Wildland Fire Assessment System).