More reliable estimation of runoff requires the development of a model to simulate rainfall disposition prior to runoff production on rural catchments. A mathematical model, based on the moisture storage capacities of the upper and subsoil zones of a catchment was developed and its general form was confirmed by comparison of predicted values with recorded data on initial abstraction from four pastoral catchments on texture-contrast soils with heavy clay subsoils at the Parwan Experimental Area, near Bacchus Marsh, Victoria. The modifications necessary for extending this model to ungauged areas were elaborated and changes in the elements of the model following pasture improvement were discussed.