The long-distance dispersal of weeds through open water channels has been poorly studied despite its primary importance for understanding weed spread in agro-ecosystems and its negative impacts on crop production. We proposed here an experimental study focusing on the quantification of external factors affecting the seed dispersal kernel during an irrigation event in a vegetated channel. We chose Johnsongrass seeds, one the most widespread weeds in the world that affects production of high-quality hay in permanent grasslands. We used a photogrammetric approach combined to differential GPS surveys to finely estimate the external factors susceptible to affect seed dispersal, either biotic or abiotic. We used fluorescent powders to efficiently detect the seeds after a dispersal event using UV lights.