The Build-Up Index (BUI) is a numerical assessment of the total amount of fuel available, which combines the moisture index (IH) of the humus and the dryness index (IS). High values indicate fuels that are generally drier, leading to greater fuel consumption during a fire. Increased fuel consumption results in more intense fires and creates additional challenges for fire suppression.
The BUI estimates the total amount of fuel available to sustain a fire at any given time. It represents the accumulation of heat and dryness in vegetation and soils, allowing for the anticipation of a fire's ability to persist and grow. It is obtained by combining two other indices:
- the (moisture of the intermediate layers) / IH (Moisture Index of the humus)
- the (dryness of the deep layers) / IS (Dryness Index)
The BUI incorporates moderate to heavy fuels such as branches, dead wood, humus, or deep forest litter.It is primarily influenced by:
- air temperature
- precipitation (or prolonged absence thereof)
- soil moisture
It reflects a slow trend: it evolves over several days or weeks, unlike other more reactive indices like the FFMC.
Value < 40: low fuel availability, slow to moderate fireValue 40 – 90: moderately available fuel, sustained fireValue > 90: abundant fuel, intense and lasting fire
A high BUI indicates a persistent fire risk, capable of burning deeply and difficult to control, although the initial ignition depends on other factors.
- The BUI does not provide information on ease of ignition, but on the potential for fire development.
- It is particularly useful for assessing background dryness and the risk of a long-duration fire, especially in ecosystems rich in organic matter.
- It complements other indices in the FWI system, particularly during slow propagation, resurgence, or re-ignition phases.