Rainfall-runoff models sometimes have to deal with hydrological systems for which the water balance cannot be closed without taking into account the relation between surface and groundwater. In this study we intend to clarify in which kind of situations surface hydrologists should worry about these relations. We also conduct a preliminary study about how underground exchanges could be integrated into a conceptual (reservoir-type) lumped rainfall-runoff model. We examine the different options which modellers use to close the water balance, and we study the likelihood of each solution. We show that both from the hydrological likelihood as well as from the modelling efficiency point of view, it is better to use explicitly a groundwater loss representation. Commonly used correcting factors applied to the climatic input data (rainfall and evapotranspiration) must be avoided in rainfall-runoff models, as they may lead to obviously unrealistic parameter values and yield a similarly unrealistic fluxes distribution.