This option allows users to select ten unequal subareas. The sum of these ten subareas must be equal to the total sub-catchment area. This option is usually only used on rural sub-catchments. Urban sub-catchments, on the other hand, utilise ten equal subareas for hydrograph generation, unless they have unusual definable shapes.

## Isochronal Areas

The ten subareas are provided as input data to the program. The procedure for computing the isochrones is based on the assumption that travel time for any element of area is proportional to:

tt µ L/S^0.5

where:

tt = travel time

L = length along a reach of the major flow path

S = average slope of the reach.

The summation is carried out for each selected point in the sub-catchment along the flow path to the outlet. Laurenson's (1964) procedure for estimating isochrones is summarised as follows:

- A large number of points uniformly distributed over a sub- catchment are marked on a contour map of the sub-catchment.
- For each point the distances between adjacent contours along the flow path to the outlet are tabulated.
- These individual distances are raised to the 1.5 power since time of flow through any reach is assumed proportional to: tt µ L/S^0.5

i.e. tt µ L^1.5/H^0.5

where H = contour interval

Since H is constant the time of flow is proportional to L^1.5. (A correction has to be made for the lowest reach since the outlet of the catchment is not, in general, on a contour. This correction involves multiplying the length of the lowes reach by (H/H1)0.5 where H1 is the fall through the lowest reach.) The lengths to the power of 1.5 are then summed for each point.

- The sums obtained in (c) are divided by the greatest sum to give relative travel times for all points.
- Isochrones are then drawn through the points of relative travel time to give lines of equal travel times to the outlet. These are designated as the 1, 2, 3 ..... 10 isochrones. The areas between adjacent isochrones are referred to as the subareas.

It is recommended, unless very large sub-catchments are being considered or flow paths and times through sub-catchments are particularly variable, that 10 equal subareas be considered to save data preparation. The model in fact, provides a default for this treatment if required.

Area 1 starts at the top of the sub-catchment. Area 10 is at the bottom (at the outlet) of the sub-catchment. Areas are measured in hectares, and the 10 areas should add up to the total sub-catchment area which is input separately.