A grass moisture model for the Canadian Forest Fire Danger Rating System

The Canadian Forest Fire Danger Rating System (CFFDRS) is used throughout Canada, and in a number of countries throughout the world, for estimating fire potential in wildland fuels. Estimates of fire danger generated from the system rely upon estimates of fuel moisture in three distinct moisture models from the Fire Weather Index (FWI) System. These standard fuel moisture models are representative of moisture in closed canopy jack pine or lodge pole pine stands. The applicability of these models to grasslands therefore has been questioned by some users of the system due to the exposure of grassland fuels and the very rapid fuel moisture time lags that have been observed in this type. Given that the CFFDRS is being adopted in, or adapted to, an increasing number of countries, a new faster reacting moisture code, associated with grassland, was developed to provide the system with representative moisture estimates over a wider range of fuel types. This new Grass Fuel Moisture Code (GFMC) retains the structure of the Fine Fuel Moisture Code (FFMC) in the FWI System, uses the same input weather observations, but also requires an estimate of solar radiation incident on the fuel. The model allows diurnal calculation of moisture, an important attribute given the fast reacting exposed fuels in grasslands. The development and structure of this new model is described and outputs of this new model, along with outputs from the existing FFMC model, are compared with field observations. Results show that the model tracks the diurnal variation in grassland moisture content more accurately than the existing model for diurnal calculation of the FFMC in the FWI System. Examples of the use of output from this new GFMC within the structure of the Canadian Forest Fire Danger Rating System are also presented along with procedures for calculating the GFMC when direct solar radiation measurements are not available.