DROUGHT FACTOR MODELLING

A key component of the MacArthur Forest Fire Danger Meter is the modelling of the dryness of the fuel. This is expressed by the Drought Factor, which ranges from 0 to 10. If this is multiplied by 10 and called a percent, it gives the percentage of fine fuel that would be removed by a fire under the current conditions.

The DF is based on recent rainfall and on the Byram-Keetch Drought Index. The BKDI is the number of mm of rain needed to saturate the soil, and ranges up to a maximum of 200mm.

On any non-rainy day the heat of the sun increases the BKDI, by amount that depends on:

The calculations are based on an assumption that the terrain is level. If the terrain is not level then there will be differing levels of solar radiation, and thus differing drying rates.

It is possible to calculate the level of radiation, in Megajoules per square meter for the day. This depends on:



From month-to-month this varies considerably, but if we accept that this variation is primarily reflected in the daily maximum temperature, then the drying rate on any day for a site can be corrected by dividing the radiation level by the equivalent level on flat ground. This gives ratios that range from 0 for steep south-facing slopes at the Winter Solstice to over 1.8 for steep, north-facing slopes at the Summer Solstice.

We can then state that the daily change in BKDI should be calculated by the standard equation, and then multiplied by the appropriate value from the tables below. If no rainfall figures are available for the site, then use the best local figures, and a month-by month correction should work as a first approximation (e.g. if Canberra Airport's BKDI goes up by 15mm in August, then a 20 degree slope on the north face of Black Mountain probably went up by 15*1.5 = 22.5mm).

North-facing slopes

Slope Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun
0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0
10 1.3 1.3 1.2 1.1 1.0 1.0 1.0 1.0 1.1 1.1 1.2 1.3
20 1.6 1.5 1.3 1.1 1.0 1.0 1.0 1.0 1.1 1.2 1.4 1.6
30 1.8 1.6 1.4 1.2 1.0 0.9 0.9 1.0 1.1 1.3 1.5 1.8

Northwest/Northeast-facing slopes

Slope Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun
0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0
10 1.2 1.2 1.1 1.1 1.0 1.0 1.0 1.0 1.0 1.1 1.2 1.2
20 1.4 1.3 1.2 1.1 1.0 1.0 1.0 1.0 1.1 1.2 1.3 1.4
30 1.5 1.4 1.2 1.1 1.0 0.9 0.9 0.9 1.0 1.2 1.3 1.5

West/East-facing slopes

Slope Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun
0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0
10 0.8 0.8 0.9 0.9 1.0 1.0 1.0 1.0 0.9 0.9 0.8 0.8
20 0.5 0.6 0.7 0.8 0.9 0.9 0.9 0.9 0.8 0.7 0.6 0.5
30 0.3 0.4 0.5 0.7 0.8 0.8 0.8 0.8 0.7 0.6 0.4 0.3

South-facing slopes

Slope Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun
0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0
10 0.7 0.7 0.8 0.9 0.9 1.0 1.0 0.9 0.9 0.8 0.8 0.7
20 0.3 0.4 0.6 0.7 0.8 0.9 0.9 0.9 0.8 0.6 0.5 0.3
30 0.0 0.1 0.3 0.6 0.7 0.8 0.8 0.8 0.6 0.4 0.2 0.0

These data are derived from Applied Enivronmetric's Meteorological Tables, and are for latitude 35 degrees, 30 minutes South (roughly that of Tharwa).

This page is courtesy of Rick McRae.