In general, the sub-catchments are established by dividing the main catchment into areas defined by tributaries and ridges. The link network connects these sub-catchments.

In the XPRafts context, sub-catchments are usually drawn to points where flows are required, changes in topography occur, retarding basins are proposed, or at other locations where hydrographs are needed. Links are then constructed to join the sub-catchments.

The runoff from a sub-catchment flows into the top of a link. The minimum number of sub-catchments is not important as the outflow from each single sub-catchment should be correct in itself.

The division of the catchment should reflect the individual analysis of homogeneous sub-catchments. The degree of urbanisation, slope, ground cover and type, etc, should be reasonably uniform within each sub-catchment.

If distinct changes in urbanisation or other characteristics occur within sub-catchments then further subdivision should be considered to define differences in catchment storages and flow times.

The figure shown in the Definition of Link page illustrates how links joining the tributaries to the main stream as having real lengths. They can also have zero length, i.e. a dummy link. The lag down such a link would be zero. Dummy links with dummy sub-catchment areas (e.g. 0 .01 ha) can be included to allow the generation of hydrographs at additional nodes in a network, particularly at junctions.