Dead fuels represent a category of wildland fuels whose moisture content is controlled exclusively by environmental conditions. Accurate assessment of fuel moisture in dead fuels is critical since these fuels are typically involved in the start and initial spread of wildland fires. In the National Fire Danger Rating System (NFDRS), dead fuels are separated into four “time-lag” classes: 1-hour, 10-hour, 100-hour, and 1000-hour. The algorithms used operationally to calculate dead fuel moisture in the NFDRS are essentially the same ones developed in the 1970s. They use once-daily weather information (typically around 1400 local time) and require human intervention, every day, to enter a state-of-the-weather code, which triggers solar radiation estimates to estimate fuel temperature from the shelter temperature.

During the 1990s Ralph Nelson, formerly of the US Forest Service (USFS), developed a theoretical model for dead fuel moisture to take advantage of frequent weather observations from automated weather monitoring stations. The model accounts for the physics of moisture and heat transfer, and includes processes of rainfall and dewfall. The model as originally published (Nelson, 2000) was only for 10-hour dead fuels. Since 2000, however, Nelson developed fuel stick parameters to allow the model to be run for the three other size fuel classes. The resulting numerical model can be run with data at any time interval (e.g., 15-minute, hourly) and has four weather inputs: air temperature, relative humidity, solar radiation, and rainfall.

This paper compares the performance of the “Nelson model” for all four dead fuel classes against measured data during a 21-month period from Slapout, Oklahoma, in the Oklahoma panhandle. From April 1996 through December 1997, twice-daily weighings of Ponderosa pine fuel sticks representing all four fuel classes were made, from which fuel moisture was later calculated. The site was near an Oklahoma Mesonet weather station, which provided the input data for the Nelson model. During this period monthly average temperatures ranged from 32F to 80F (0C to 27C) and monthly precipitation from 0.01" to 6.84" (0.25 mm to 174 mm), so a wide range of weather conditions were encountered.

In conjunction with a USFS funded project, the Nelson model is being incorporated into the Oklahoma Fire Danger Model during 2003 to prototype the model in real-time data assimilation and forecast (National Weather Service's Eta model) environments. After the completion of this project, it is anticipated that the Nelson model will become a keystone element in the "Next Generation" NFDRS.