Drought Code (DC) represents the moisture content of deep, compact organic layers (approximately 10 to 20 cm below the surface) and serves as an indicator of seasonal drought effects on deep fuels. DC is a unitless, open-ended index where higher values correspond to drier conditions.
DC is the closest Canadian equivalent to 1250HR Moisture in the US system. Like 1250HR, it tracks the slow-drying component of the fuel moisture profile: the deep organic layers that take weeks to months to fully dry or rewet. These deep fuels do not influence day-to-day fire spread as directly as surface fuels, but they are critical for sustaining fire, enabling deep smoldering combustion, and affecting mop-up difficulty and holdover fire risk.
DC values typically start near 0 in spring following snowmelt and accumulate over the fire season. Values range from low (fuels have adequate moisture from recent precipitation) to extreme (conditions consistent with the most severe historical fire seasons).
Unlike the Fine Fuel Moisture Code (FFMC) or Initial Spread Index (ISI), which respond to daily weather changes, DC changes slowly and is not reset by short-duration rainfall. Only sustained, heavy precipitation meaningfully reduces DC values during the fire season.
DC is one of the three fuel moisture codes in the Fire Weather Index (FWI) system, alongside FFMC and the Duff Moisture Code (DMC). DC feeds into the Buildup Index (BUI), which in turn contributes to the overall FWI rating. Persistent high DC values late in the season often signal elevated risk of holdover fires and difficult mop-up conditions even after active fire behavior subsides.