Unit Hydrograph

Abstract On the scale of a hydrologic event, a river basin schematically has two influences. The first is the yield of rainfall transformed into effective rainfall (in terms of runoff production), and the second is the transfer of runoff flows generated all over the basin territory to the unique outlet. The concept assumes that all transfer processes of the whole basin can be synthesized in a single linear function, called the unit hydrograph of the basin considered. It is at the root of the idea of the transfer function and is a kind of functional signature of the basin. This means that a characteristic function can be identified for a given basin. It is assumed that this function underlies any rainfall–runoff event and synthesizes all relevant hydrologic processes and geographic influences on the temporal scale of the event and the spatial scale fixed by the basin territory. Furthermore this means that the unit hydrograph should really be the hydrograph at the outlet, following an average impulse unit effective rainfall input, and that the response of the basin to any actual effective rainfall chronicle can be simulated through the sum of successive elementary affinities of the unit hydrograph. Such a holistic purpose necessarily faces problems of simplifying the natural complexity and of identifying the unit hydrograph itself.