The regionalisation issue is a real challenge for operational hydrology and has direct implications for the prediction on ungauged basins. Indeed, the first step of the development of a priori parameters estimation is to find the correlations between calibrated parameters and physical catchment descriptors. Our research investigates the relationships between the GR4J rainfall-runoff model parameters and catchment vegetation characteristics over a large sample of 221 French catchments. Besides, we also aim at improving the correlations between vegetation catchment characteristics and model parameters by refining the structure of the model. First, the links between GR4J calibrated parameters and catchment vegetation-type are investigated. Then, we try to improve these relations by introducing a description believed more physically sound in order to take into account vegetation-types, hence following the so-called downward approach. Results show that the GR4J model parameters cannot be obtained directly from vegetation characteristics. Moreover, the situation is not improved when using a more physically based approach to model evapotranspiration. We believe that these results, obtained over a large sample of catchment sample, are quite informative and are supported by previous findings (see for example Merz and Bloschl, 2004).