Prediction of diurnal change in 10-h fuel stick moisture content

Abstract

Several methods are available for estimating the moisture content of 10-h response time fuels in the U.S. National Fire Danger Rating System (NFDRS). These fuels are represented by an array of four 1.27 cm diameter ponderosa pine (Pinus ponderosa Dougl. ex Laws.) dowels weighing about 100 g when oven dry. The prediction model currently used in the NFDRS is driven by information from afternoon weather readings. To improve responsiveness of the predictions to weather change, a 10-h stick moisture content prediction model is developed that uses observations (air temperature and relative humidity, insolation, and rainfall amount) available from a remote automatic weather station (RAWS). Equations describing the transfer of heat and moisture at the surface and within a 10-h stick are derived and then solved numerically. Collection of field experimental data on weather, stick weight, and stick temperature to guide development of the model is briefly described, and predicted and observed mean moisture contents are compared. Additional 10-h stick moisture content data, collected independently, are used to test model predictions. Calculated values are sometimes outside the bounds of variability in moisture content determined from the data, suggesting the need for further tests. The model simulates diurnal change in moisture content and temperature of 10-h sticks but can be adapted to cylindrical wood sticks of any practical size.