B is either directly input for each sub-catchment or estimated from Equation (8) which was derived by Aitken (1975). The value of B for each sub-area is assumed to equal the average value of B for the sub-catchment.

where:

B = mean value of coefficient B for sub-catchment
A = sub-catchment area (km2)
U = fraction of catchment that is urbanised. (Where U = 1.0, the catchment is fully urbanised and when U = 0.0, the catchment is completely rural)
Sc = main drainage slope of sub-catchment (%). (The longest path of the sub-catchment, starting at sub-catchment outlet running up the main channel then if necessary branching off at the furthest

This equation was initially derived from six urban catchments in Australia with the following ranges applying:

A varied from 0.8 km² to 56 km²
U varied from 0.0 to 1.00
Sc varied from 0.22% to 2.90%.

However over the last fourteen years a wide range of areas, slopes and urbanization outside these ranges have been tested with a high degree of success. See Sobinoff et al. (1983).

For gauged catchments, deduced B values, evaluated as the average value from recorded rainfall/runoff events, should be used in preference to generalized regression estimates.

As U in certain instances can be rather vague, data input in this respect has been amended to include a % impervious parameter for each sub-catchment in place of the U term.

The model interprets U in terms of %I based on the following ratios:

I%

U

0

0

30

0.7

50

1.0

100

2.0*

* For regression purpose only this value is extrapolated from the original data limited of 50% impervious area.