Comparing Measured Duff Moisture with a Water Budget Model and the Duff and Drought Codes of the Canadian Fire Weather Index

Abstract

Fuel moisture plays an important role in predicting wildfire spread rates, fuel consumption, and heat output. The purpose of this study was to find how much we can simplify an F and H layer moisture model by comparing an empirical-phenomenalistic drying model with a mechanistic water budget that included all of the major water fluxes. Traditionally fuel moisture has been calculated using phenomenalistic exponential drying rate models, which use standard meteorological station variables (temperature, precipitation, relative humidity, day length, and others). First, we report on comparisons between field-measured F and H soil layer moisture and moisture estimates based on the Duff Moisture Code and Drought Code of the Canadian Forest Fire Weather Index (FWI) in Pinus contorta and Picea engelmannii forests during dry and wet years. Next, an F and H layer water budget is used to understand possible reasons for the differences between the Duff Moisture Code, the Drought Code, and the moisture measured in the field. For the pine forest, the Duff Moisture Code was a good estimate of the F layer moisture during and shortly after precipitation events in both the dry and wet years but underestimated moisture when duff was drying. The pine Drought Code underestimated the H layer moisture in the dry year and overestimated it in the wet year. For the spruce forest, in the dry year the Duff Moisture Code overestimated the moisture during and after precipitation and underestimated it in dry periods. However, in the wet year, the code overestimated moisture most of the time. The spruce Drought Code underestimated the moisture content in both the wet and dry years. Results from the water budget model suggest that the difference in the F layer moisture between the field measurements and both the Drought Code and Duff Moisture Code is due to the lack of coupling of water flow between the F and H layers in the codes. In particular, the diurnal water movement from the H to F layer during the drying part of the season is integral to the water budget. To improve predictions based on the fuel moisture codes, coupled water and heat budgets along with the hydrologic properties of the F and H layers should be incorporated into the codes to enable more accurate prediction of duff moisture and calibration for different types of duff. FOR. SCI. ❚❚(❚):000–000.