The evaluation of diffuse impacts far downstream of man-made reservoirs is complex but useful for water resources management and impact studies. The influence of reservoirs is usually modeled using a spatially distributed approach. To date lumped approaches have not been used to address this problem, probably because the lumped mode is seen as a major obstacle to accounting for water works that are at specific locations within the watershed. Here we propose a way to explicitly account for artificial storages in a lumped hydrological model. The method was tested on a large set of 46 watersheds (in France, the United States and Brazil) influenced by 26 reservoirs representing various types of management objectives. The results indicate that the method developed can effectively account for reservoirs despite the lumped modeling approach, with significant improvements in the simulation of low flows, but more limited for high flows.