Hydrochorous dispersal through agricultural channels plays a role in structuring plant communities across agricultural landscapes. To date, research on seed retention in vegetated areas has mainly focused on vegetation types with simple architecture (often cylinders), which consequently do not represent real vegetation features. Here, we test the hypothesis that vegetation cover estimated at the water surface best explains floating seed retention in open channels. We therefore proposed an experiment to measure seed retention in a controlled environment across a large range of hydraulic conditions and vegetation architecture types. We used three types of artificial plants with contrasting morphotypes, and real seeds of Rumex crispus. Vegetation metrics were calculated on the basis of 3D plant models. We also tested the additivity of seed retention as a function of the length of vegetated area crossed by the seeds. We developed a semi‐empirical formula for predicting seed retention. The main results of the experiment show that (i) the seed retention rate reacts differently to changes in density according to species, (ii) vegetation cover at the free water surface, potentially in contact with seeds, is a generic predictor of floating seed retention whatever the nature of the vegetated cover, and (iii) 95% of seed retention was reached for a large range of surface vegetation ratios and length of vegetation cover. The proposed formula could be used by stakeholders (farmers and ecologists) to estimate the amount of vegetation needed in a channel to limit or enhance seed dispersal.

Core Ideas OMERE is a Mediterranean observatory of two farmed catchments in Tunisia and France. Water, contaminant, and erosion fluxes are monitored from plot to catchment scales. Soils and land use in relation with agriculture are surveyed. Scientific results concern elementary processes and integrated catchment functioning. Models were conceived for evaluation of land use and agricultural management scenarios. To account for the diversity of agricultural and ecosystem situations in hilly Mediterranean areas, the agro‐hydrological observatory OMERE (Observatoire Méditerranéen de l'Environnement Rural et de l'Eau) monitors two farmed catchments—one in northern Tunisia and the other in southern France. Mediterranean regions are typified by a highly variable climate, with an alternation of long droughts and intense storms, and by a strong heterogeneity of soil properties, due to a combination of climate, relief, parent materials, sparse vegetation, intense land use, man‐made infrastructure (ditches, terraces, etc.), and agricultural activities. In this context, OMERE aims to document the impacts of agricultural and land management on mass fluxes in Mediterranean farmed headwater catchments. The observation strategy is motivated by monitoring water, sediment, and contaminant fluxes and hydrologic and climatic variables at different spatial scales from cultivated plots and landscape elements to the catchment scale. These measurements have been performed at a fine time resolution over a long‐term scale and by surveying land use, agricultural practices, and soil surface characteristics. The long‐term observation strategy intends to support integrative multidisciplinary research for elucidating the conditions that improve soil and water management and delivery of ecosystem services in a Mediterranean rainfed cultivated context. The observatory has led to scientific insights regarding three scientific objectives: (i) to better understand the fluxes of water, erosion, and contaminants, especially pesticides, and of their natural and anthropogenic drivers on short‐ and long‐term scales; (ii) to analyze the aggregate effects of farming and land management on mass fluxes across scales, from plot to catchment or landscape scales; and (iii) to derive new scenarios for sustainable agricultural management and improved delivery of ecosystem services.