The Horton (ILSAX) loss and routing models were taken from the programs from which ILSAX was developed, particularly ILLUDAS and ILLUDAS-SA. All use a loss model involving depression storages and the Horton infiltration model for pervious areas, and the time-area method as a routing model to convert rainfall hyetographs to runoff hydrographs.

Terstriep and Stall, the developers of ILLUDAS, created a loss procedure for pervious areas using the U.S. Soil Conservation Service (Natural Resources Conservation Services) soil classifications. Malcolm Watson, developer of ILLUDAS-SA developed an intertwined loss and routing model that adapted the time-area method to allow the water running from one strip of catchment onto another to be infiltrated.

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