Flooding duration in wet grasslands represents a crucial indicator for agricultural and biodiversity targets. However, flooding duration in wet grasslands is not an easy variable to observe by itself, all the more so because it is really heterogeneous in space, due to subtle topographic relief. Moreover, actual plant diversity, composition and community structure are the synthetic results of the hydro-meteorological history in the past. Wet grasslands are thus truly ungauged with respect to flooding duration, which has to be simulated in a robust manner both over the spatial territory and back over the past many years. A water-balance modelling with a daily time step, based on available meteorological data and on a precise topographic characterization, was proposed to assess the flooding duration at any location of a studied area situated along the west coast of France and over a period of six years. The simulated (calculated) flooding durations were used in the plant species distribution modelling based on 350 sampled vegetation quadrats. From field-based milestone observations of water level, the hydrological modelling was shown to predict well the annual cumulative flooding duration at any spot. A strong and consistent effect of flooding duration was evident for 29 species. Plant diversity appeared to reach a maximum at an intermediate flooding duration. The hydrology-vegetation modelling approach could thus lead to a robust and versatile tool to predict the consequences of changes in flooding regime on vegetation patterns.