An analysis of the dynamic component of the geomorphologic instantaneous unit hydrograph

A geomorphologic instantaneous unit hydrograph (IUH) consists of two components, one relevant to the geomorphology and the other to the hydraulic aspect describing the movement of a drop of water along a stream. Different formulations of the geomorphologic IUH are reviewed, and a contrast is drawn between the geomorphologic and hydraulic components of the geomorphologic IUH (GIUH) proposed by Rodriguez-Iturbe and Valdes (Water Resour. Res., 15(6); 1409-1420, 1979) and those of a width function based IUH (WFIUH). In this paper a comparison has been carried out of the original GIUH and a WFIUH which allows the effects of different geomorphologic and hydraulic components to be identified. The comparison, which is based on four sub-basins of the River Tyne, UK, clearly shows that the GIUH velocity parameter lacks physical interpretation, in contrast to the hydraulic parameters of the WFIUH, which are seen to be physically consistent. For practical application of the GIUH, an equation is then proposed to estimate the velocity parameter through the basin concentration time, the Horton length ratio and the length of the stream of the highest order of the channel network.