Micrometeorological measurements to assess fire fuel dryness

On a daily basis, the ratio of sensible heat flux density (H) to net radiation (Rn) minus soil heat flux density (G) increases as a surface becomes drier and, therefore, the ratio provides amethod to assess surface dryness. When a surface is wet, most of the available energy on the surface is partitioned to latent heat flux density (LE) and a dryness index defined as Fd = H/(Rn G) approaches zero. As the surface dries and more available energy is partitioned to H, the dryness index Fd approaches unity. Thus, if simple, cost effective methods to determine H, Rn, and G are available, then remote measurements can provide a direct method to evaluate fuel dryness at points within an area of interest.One advantage of the Fd method is that the dryness index integrates contributions of all combustion materials including aerial, surface and ground fuel. With recent advances in electronics, it is now possible to remotely monitor the variables needed to determine high frequency temperature data in remote locations and to estimate sensible heat flux density using the surface renewal (SR) method. The SR method uses fine-wire thermocouples to measure high frequency temperature. Temperature traces of high frequency data show ramp-like characteristics, and the statistical moments of the structure function are used to determine mean ramp characteristics during a sampling interval. Then H is computed using a conservation of energy equation. Field trials were conducted to determine if measurements of high frequency temperature and the SR technique are useful to determine Fd for Mediterranean shrublands. Data were collected in Sardinia, Italy. The Fd index data were compared with three slow response fire danger indices including the KBDI drought index, the drought factor (D) in the McArthur forest fire danger meter, and the fast response FFMC of the Canadian FWI. In addition, comparisons were made with the McArthur forest fire danger meter and a modified version of the KBDI using evapotranspiration estimated from the Hargreaves and Samani equation. It was concluded that estimating H using thermocouples and the SR method, in combination with Rn and G measurements or estimates, provides a low-cost method to calculate Fd as an indicator of weighted surface fuel, plant, and soil dryness. This method can potentially improve site specific information on fire fuel dryness without the need for travel and labor to visit remote sites for fuel dryness measurements.