Drought is a period of prolonged, abnormally dry weather resulting in a water shortage. While drought does not provide the spark for a wildfire, it is a primary factor in their occurrence. Drought acts as a large-scale environmental pre-condition, transforming living and dead vegetation into readily available fuel. By drying out the landscape, drought creates the potential for large, intense fires, setting the stage for an ignition event to become a disaster.
How Drought Creates Flammable Conditions
Drought conditions fundamentally alter the vegetative landscape by significantly reducing the Fuel Moisture Content (FMC) in both living and dead plants. This moisture acts as a natural inhibitor to combustion. As the moisture content drops, the material requires far less energy to reach its ignition temperature.
Fine fuels, such as dry grasses and small twigs, react quickly to short-term dry spells or low daily humidity. These materials dry rapidly, making them the first to ignite and sustain the initial spread of a fire. Conversely, heavy fuels, including large logs and tree branches, require prolonged, deep-seated drought conditions to lose their internal moisture.
Extended drought depletes the water stored in the soil, preventing plants from absorbing necessary hydration. This lack of soil moisture stresses the plants, causing them to wilt, die, and add to the overall dead fuel load. When drought removes moisture, the energy required to start a fire is drastically lower, allowing a small spark to initiate a high-intensity blaze.
The Other Pillars of Wildfire: Weather and Ignition
While drought prepares the fuel, a wildfire requires two other essential components to ignite and spread: a source of ignition and immediate atmospheric weather conditions. These three elements—fuel, heat, and oxygen—are often described as the Fire Triangle. Fuel condition determines the fire’s potential intensity, but weather and ignition are the immediate catalysts.
Ignition sources provide the necessary heat to bring the dried fuel to its combustion point. Natural ignition primarily comes from lightning strikes, especially during dry thunderstorms that produce little rain. Human activity is responsible for starting a vast majority of wildfires, often through accidental causes like unattended campfires or sparks from equipment.
Immediate weather conditions dramatically influence the rate and direction of fire spread once ignition occurs. Wind is a powerful driver, supplying fresh oxygen and pushing flames forward, often carrying embers to start new spot fires. High ambient temperatures pre-heat surrounding fuels, accelerating the fire’s overall growth. Low relative humidity pulls remaining moisture out of the fuels, ensuring the fuel is as dry as possible before the fire arrives.
Assessing Drought Severity and Fire Risk
Scientists use specific metrics to quantify drought severity and translate it into actionable wildfire risk levels. The Keetch-Byram Drought Index (KBDI) is a common index that estimates the precipitation needed to return the top eight inches of soil to full saturation. Higher KBDI values, which can reach 800, indicate deeper, more intense drought that affects larger fuels and increases fire potential.
These drought indices are integrated into comprehensive forecasting tools like the National Fire Danger Rating System (NFDRS). The NFDRS uses daily inputs of weather and fuel moisture data to calculate various components of fire danger. A key output is the Energy Release Component (ERC), which reflects the total potential heat release per unit area at the flaming front of a fire.
The ERC measures the cumulative effects of drought, as it rises when both live and dead fuels dry out over time. Fire behavior models also account for the temporal scale of drying, differentiating between short-term dryness affecting fine fuels and long-term drought impacting 1000-hour time-lag fuels. These 1000-hour fuels are large-diameter wood that takes weeks to months to equilibrate with atmospheric moisture. By combining drought metrics with current weather, fire managers can issue accurate risk warnings and allocate resources effectively.