To find and understand how different factors can attenuate the surface runoffs is an important and actual challenge in the context of the present-day severe ooding led by the heavy rainfalls which start appearing more often lately due to the global climate changes. Here, we introduce a mathematical model that takes into account the presence of vegetation on the soil surface and the water-soil and water-plant interactions. We qualitatively and quantitatively estimate the role of the plants in the continuum soil-plant-atmosphere by comparing measured results of di erent tests found in the literature with the one given by our model.