Reservoir-Instantaneous-Unit-Hydrograph: the reservoirs effect on travel time distribution and peak discharge reduction

Several processes and mechanisms affect flood frequency curves at different spatial and temporal scales, such as the operation of reservoirs. In this work we present a new framework to estimate the peak discharge reduction for every possible spatial configurations of reservoirs, by introducing the novel concept of Reservoir-influenced Instantaneous Unit Hydrograph (RIUH). The RIUH represents the probability distribution of the travel time of an area, influenced (i.e. typically increased) by the presence of reservoirs. The results show the impact of single and multiple reservoirs on peak discharge reduction by varying their location and storage coefficients. Mainly, the reservoirs located in series reduce the peak discharge at the outlet; on the contrary, reservoirs located on the tributaries of the main river, which drain a smaller area, do not show a strong effect. This framework is exemplified for a real catchment (Central Italy) by investigating different configurations of reservoirs.