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:
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).
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.