Buildup Index (Canadian Fire Weather Index System)

The Buildup Index answers a plain question: how much fuel is available to burn? It is built from two of the moisture codes of the Canadian Forest Fire Weather Index System. The duff moisture code ($P$ in the formula) follows the loose organic layer of moderate depth. The drought code ($D$) follows the deep compact layers and the large logs. Both codes are 0 for soaked ground and grow as the layers dry, a duff code of 100 or more being dry and a drought code of 600 or more a deep drought. The result $U$ has no units: it is zero for soaked ground and runs to a hundred or more in a summer drought.

Which expression applies depends on the two codes. When the duff is the smaller part, $P \le 0.4\,D$, the codes combine like resistances in parallel: $U = \frac{0.8\,P\,D}{P + 0.4\,D}$, with the drought code weighted down by the 0.4 and the whole scaled by 0.8. A parallel combination stays small if either part is small, so a very low duff code keeps the index low even when the drought code is high. When $P > 0.4\,D$ the combination is corrected so that it does not run past the duff's own value, and the second expression takes over: $U = P - \left(1 - \frac{0.8\,D}{P + 0.4\,D}\right)\left(0.92 + (0.0114\,P)^{1.7}\right)$.

The fire weather stations table of the Canadian Wildland Fire Information System, as of 22 Oct 2026, shows both branches. Easton Newman Field has a duff code of 40.8 and a drought code of 666.4, on the parallel branch, and a Buildup Index of 70.8. Albany, with 96.2 and 462.9, is also on the parallel branch and comes to 126.6. Oxford, with 11.6 and 24.5, has the duff code above 0.4 times the drought code, so the corrected branch applies, and its index of 11.5 sits just under the duff code itself. Run backwards, an index of 70.8 with a drought code of 666.4 returns the duff code of 40.8.

The structure is Van Wagner's, from his 1987 description of the Canadian Forest Fire Weather Index System, and Wang, Anderson and Suddaby (2015) publish updated source code for calculating the indices. The index is a rating, not a physical quantity, and it is only as good as the moisture codes put into it. It measures how much there is to burn, not how fast a fire would spread: that is the initial spread index, and the two are combined into the fire weather index, the intensity rating.