This paper considers the task of establishing periodic linguis- tic summaries of the form "Regularly, the data take high values", enriched with an estimation of the period and a linguistic formulation. Within the framework of methods that address this task testing whether the dataset contains regularly spaced groups of high and low values with approx- imately constant size, it proposes a mathematical morphology (MM) approach based on watershed. It compares the proposed approach to other MM methods in an experimental study based on artificial data with different forms and noise types.