We evaluate here on a large set of French catchments the impact of the rain gauge density and rain gauge network configuration on the uncertainty of several hydrological computations, based on the GR4J daily rainfall– runoff model [Perrin et al., 2003]. Four hydrological applications are considered: (i) daily runoff simulation, (ii) long-term average streamflow assessment, (iii) high-flow quantiles assessment, and (iv) low-flow quantiles assessment.