The simulation of flows at the outlet of a catchment has to be carried out for many engineering and water resources management applications, such as the design and the management of structures, the forecast of floods or low flows, and the detection of impacts. For these objectives, Cemagref began to develop, in the early 1980s, some hydrological models (from the Génie Rural GR) to make the link between the rainfall fallen on a catchment and its flow observed at the outlet [MIC 83]. Beyond their practical aspect, these models raised several major questions on how the transformation of the rainfall into streamflow should be represented at the catchment scale.