Build-Up Index (BUI)

The Build-Up Index (BUI) is a component of the FWI system that estimates the total amount of fuel available to sustain a fire. While the ISI predicts how fast a fire will spread, the BUI predicts how intense and deep-burning it will be.

Why it matters for fire weather

The BUI captures the slow-moving, cumulative side of fire danger — the progressive drying of fuel layers that occurs over days to months without significant precipitation.

High BUI values indicate conditions where:

• Fires burn deeper into organic soil layers, consuming more fuel
• More total energy is released, producing higher-intensity fires
• Fires are more persistent — smouldering combustion can continue for days after the fire front passes
• Deep organic layers participate in combustion, not just surface fuels

The BUI is especially relevant in ecosystems with deep organic soils — peatlands, mature forests with thick litter layers, and areas experiencing prolonged drought.

How it works

The BUI combines two fuel moisture codes (Van Wagner, 1987):

• DMC (Duff Moisture Code) — tracks moisture in loosely compacted organic layers at 5–10 cm depth. It responds over roughly 12 days and requires more than 1.5 mm of rain in 24 hours to begin recovery.
• DC (Drought Code) — tracks moisture in deep, compact organic layers at 10+ cm depth. It has a 52-day response time and requires more than 2.8 mm of rain to respond at all.

The BUI formula weights these two codes to reflect total fuel availability. When both DMC and DC are high, the BUI indicates that fuel layers from the surface through to deep organic matter are dry enough to participate in combustion.

Unlike the ISI, the BUI changes slowly. It takes days of rain or sustained dry weather to shift significantly. This makes it a measure of background fire potential rather than day-to-day fluctuation.

Key thresholds

BUI values reflect cumulative drought conditions. These approximate ranges are drawn from FWI system operational experience.

BUI	Physical significance
< 30	Low fuel availability. Fires remain primarily surface-level with moderate depth of burn.
30–60	Moderate fuel availability. The duff layer is drying and begins to participate in combustion. Fire persistence increases.
60–90	High fuel availability. Both intermediate and deeper fuel layers are dry. Fires burn with greater intensity and consume more fuel.
90–120	Very high fuel availability. Deep organic layers are dry. Fires are persistent and deep-burning.
> 120	Extreme fuel availability. Common in Mediterranean summers where the DC exceeds 400–600 (Dimitrakopoulos & Bemmerzouk, 2011). Fires consume all available organic material.

Note

In Mediterranean climates, BUI values that would be extreme in boreal forests are routine summer conditions. The DC component saturates at high values during the 3–5 month summer drought, keeping the BUI persistently elevated. This is one reason why regional calibration of FWI thresholds is essential.

How to read it in Wildflyer

BUI is computed automatically as part of the FWI system and displayed in the expert view alongside other FWI components. Because it changes slowly, the BUI is most useful for:

• Multi-day and seasonal context — how has cumulative drying progressed?
• Comparing to the ISI — a fire with high ISI (rapid spread) and high BUI (deep fuel availability) is more consequential than high ISI alone
• Post-rainfall assessment — has recent rain been sufficient to reduce the BUI, or was it too light to reach deeper fuel layers?

Sources

• Van Wagner, C.E. (1987). Development and structure of the Canadian Forest Fire Weather Index System. Forestry Technical Report 35, Canadian Forest Service.
• Dimitrakopoulos, A.P. & Bemmerzouk, A.M. (2011). Evaluation of the Canadian fire weather index system in an eastern Mediterranean environment. Meteorological Applications, 18(1): 83–93.