XPRafts makes use of the Laurenson's Runoff Routing Module 1964 runoff routing procedure which was primarily aimed at rural catchments but modified by Aitken (1975) for use on urban catchments.

Laurenson's method was directed at single catchments, or more particularly, the derivation of a single hydrograph at the outlet of a catchment.

In the case of XPRafts, however, the sub-catchments are further divided into 10 subareas by constructing isochrones, lines of equal travel time from the sub-catchment boundary to the sub-catchment outlet.

The model storage delay parameters have been calibrated based on 10 isochronal areas, as indicated in Figure below, making up a sub-catchment. In many instances the division of a sub-catchment into 10 equal subareas provides very similar results.

This is particularly true in urban areas where isochrones vary with storm frequency and are often impossible to determine due to the complexity of the pipe and overflow network. As previously mentioned the current workbench version also allows for a single sub-area.

XPRafts uses Laurenson's method to derive separate sub-catchment outflow hydrographs.The hydrographs so derived are manipulated through the link network to the outlet of the total catchment via the conduit routing module.

The routing method is summarised and displayed in Figure below.