Developing a satisfactory procedure for determining the surface runoff of a catchment by routing rainfall excess through catchment storage. Earlier investigations lack generality. This a very general computational model. Model allows that rainfall intensity varies in both time and area. Catchment storage is both distributed and non-linear. Subsidiary investigations concerned storage routing of inflows through reaches having a delay time that is long in relation to time base of the inflow and separation of groundwater and interflow from surface runoff.