Open channels in agricultural landscapes were built over several centuries in order to drain excess run-off, avoid waterlogging and limit erosion in rain-fed agriculture, or to transport water in irrigated agriculture. Recently, it has been acknowledged that these open-channels supply other ecosystem services, as the limitation of weed spreading, or the mitigation of solute or particulate transfers towards surface or ground waters. These latter services are largely provided by the natural vegetation developing in the channels, although its presence could sometimes have negative impacts such as the loss of hydraulic conveyance capacity. Consequantly, we need to find trade-offs for management practices that can both optimize ecosystem services while maintaining a high level of hydraulic service for land drainage and irrigation.

In the recent field of eco-engineering, it has been shown that different plant species had specific capabilities and roles in these ecosystems, especially due to their morphotypes (i.e. plant architecture) and spatial organization (patches or homogeneous). Plants disperse over the agricultural landscapes mainly through seed vectors, the latter being transported either by fauna, gravity, water or wind. Seed dispersal contributes to the spread of invasive species, crop contamination by weeds or biodiversity conservation. Especially for hydrochorous species, i.e. plants whose seeds are dispersed by water flows, there is a complex link between the plant, the seeds and the water dynamics that needs to be disentangled. Indeed, these different plant patterns induce diverse effects on mean water velocity and turbulence fields, largely determining all types of solid transport. However, this potential of plant biodiversity for controlling these transport properties is for the moment largely unknown and under-exploited.

Therefore, we proposed an original project focusing on the functions of vegetation properties on hydraulics of open channels related to biotic transport. We concentrated on both heterogeneities of plant architecture, i.e. morphotypes with different ramification levels or leaf-to-stem ratios, and spatial heterogeneities of plant implantations, i.e. the spatial organization of the plant community.

Indeed, these two levels of organization are those relevant for ecosystem services management at the watershed scale. The specific functions that will be under study are the effect on water velocity fields and the subsequent effects on seed transport. The research questions are the following :

1) Can we characterize the effects of both interspecific and spatial heterogeneities of biodiversity on horizontal 2D flow fields in the channels ? What are the consequences for floating particles ?

2) Is it relevant to imagine management practices favoring specific morphotypes or spatial organization in key-locations of the channels, in order to maximize the bench of ecosystem services they provide ?