Supplementary areas

This is the term given in the Horton (ILSAX) hydrological model to impervious areas that are not directly connected to the pipe system. It includes areas such as tennis courts and concrete paths that are surrounded by pervious (grassed) surfaces, so that water must pass over these and possibly infiltrate before reaching a point of entry to a pipe or channel.

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Supplementary areas are assumed to be evenly distributed next to the grassed area. If you have a situation where impervious areas are concentrated at the top of a sub-catchment, you should model this using two or more sub-catchments.

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