Calibrating the Fine Fuel Moisture Code for grass ignition potential in Sumatra, Indonesia

Grass moisture and ignition studies were conducted in central Sumatra, Indonesia, to develop an indicator of grass ignition potential using the Fine Fuel Moisture Code (FFMC) of the Canadian Forest Fire Weather Index System. Moisture content of live and dead grass was measured at three sites every 6 days over an 8-month period. Grass curing was highly variable but averaged 37–39% and often exceeded 50% from April to mid-August. Grass fuel loads averaged 420–722 g/m2. There was a highly significant decrease in dead grass moisture content with increasing FFMC, decreasing grass height, and decreasing total grass biomass. The FFMC was the most influential factor, explaining 54–61% of the dead grass moisture content variation. Ignition tests were applied to live and dead grass samples with specific moisture contents. The ignition threshold of dead and live grass occurred at 35.4% and 27.8% moisture content, respectively. The dead grass ignition threshold corresponded to FFMC values of 81.0–83.3 at the three study sites. Of historical hot spots in South-east Asia, 86% occurred when the FFMC was ≥78, representing the lower 95% confidence interval of the dead grass ignition threshold. The FFMC was calibrated using experimental results for fire management applications.