Buildup index


The Buildup index (\(BUI\)) is one of the two intermediate indices required for calculating the Canadian fire weather index (\(FWI\)\)). The \(BUI\) represents the total amount of fuel available to the spreading fire. It is a combination of the \(DMC\) and the \(DC\) (Van Wagner 1987).

The \(BUI\) is affected mainly by the \(DMC\). The \(DC\) only has a limited and variable weight in the equation (Van Wagner 1987).


For \(DMC\leqslant{0.4\cdot DC}\) the \(BUI\) is calculated as follows (Van Wagner and Pickett 1985):

\[BUI=0.8\cdot\dfrac{DMC\cdot{DC}}{DMC+0.4\cdot DC}, \mbox{ for DMC}\leqslant{0.4\cdot DC}\]

and for \(DMC>{0.4 \cdot DC}\) as follows:

\[BUI=DMC-\bigg(1-\dfrac{0.8\cdot DC}{DMC+0.4\cdot DC}\bigg)\cdot\big[0.92+(0.0114\cdot DMC)^{1.7}\big], \mbox{ for DMC}>{0.4\cdot DC}\]

where \(DMC\) is the Duff moisture code and \(DC\) the Drought code.


Original publication:
Van Wagner and Pickett (1985)
Van Wagner (1987)


Variable Description Unit
\(T\) air temperature °C
\(T_{dew}\) dew point temperature °C
\(H\) air humidity %
\(P\) rainfall mm
\(U\) windspeed m/s
\(w\) days since last rain
(or rain above threshold)
\(rr\) days with consecutive rain d
\(\Delta t\) time increment d
\(\Delta{e}\) vapor pressure deficit kPa
\(e_s\) saturation vapor pressure kPa
\(e_a\) actual vapor pressure kPa
\(p_{atm}\) atmospheric pressure kPa
\( PET\) potential evapotranspiration mm/d
\(r\) soil water reserve mm
\(r_s\) surface water reserve mm
\(EMC\) equilibrium moisture content %
\(DF\) drought factor -
\(N\) daylight hours hr
\(D\) weighted 24-hr average moisture condition hr
\(\omega\) sunset hour angle rad
\(\delta\) solar declination rad
\(\varphi\) latitude rad
\(Cc\) cloud cover Okta
\(J\) day of the year (1..365/366) -
\(I\) heat index -
\(R_n\) net radiation MJ⋅m-2⋅d-1
\(R_a\) daily extraterrestrial radiation MJ⋅m-2⋅d-1
\(R_s\) solar radiation MJ⋅m-2⋅d-1
\(R_{so}\) clear-sky solar radiation MJ⋅m-2⋅d-1
\(R_{ns}\) net shortwave radiation MJ⋅m-2⋅d-1
\(R_{nl}\) net longwave radiation MJ⋅m-2⋅d-1
\(\lambda\) latent heat of vaporization MJ/kg
\(z\) elevation m a.s.l.
\(d_r\) inverse relative distance Earth-Sun -
\(\alpha\) albedo or canopy reflection coefficient -
\(\Delta\) slope of the saturation vapor pressure curve kPa/°C
\(Cc\) cloud cover eights
\(ROS\) rate of spread m/h
\(RSF\) rate of spread factor -
\(WF\) wind factor -
\(WRF\) water reserve factor -
\(FH\) false relative humidity -
\(FAF\) fuel availability factor -
\(PC\) phenological coefficient -

Suffix Description
\(-\) mean / daily value
\(_{max}\) maximum value
\(_{min}\) minimum value
\(_{12}\) value at 12:00
\(_{13}\) value at 13:00
\(_{15}\) value at 15:00
\(_{m}\) montly value
\(_{y}\) yearly value
\(_{f/a}\) value at fuel-atmosphere interface
\(_{dur}\) duration
\(_{soil}\) value at soil level

Constant Description
\(e\) Euler's number
\(\gamma\)psychrometric constant
\(G_{SC}\)solar constant
\(\sigma\)Stefan-Bolzmann constant