The open source R language contains packages, i.e. organized gatherings of functions, that enable to perform all steps necessary to undertake hydrological modelling (Slater et al., 2019). Especially, several hydrological models are proposed in R (Astagneau et al., 2021). In this webinar, we will give an overview of available R packages available for hydrological modelling and we will develop the possibilities of a specific package, airGR (Coron et al., 2017), which contains among other the GR4J rainfall-runoff model.