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Résumé :
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With the development of rice cultivation in the 50's, a great part of the Isle of Camargue, the central area of the Rhone river delta, was polderized. The Vaccares lagoon system (110 km2 , 65 km2 for the Vaccares lagoon itself) represents the main part of the 'Reserve Nationale de Camargue'. Without human intervention, it is hydraulically isolated from the Rhone river and the Mediterranean sea. It receives drainage water from precipitation and flooded ricefields from its relict catchment. Our hydrological study concerned the Fumemorte catchment (65 km2, with 20 km2 of ricefields). Irrigation for rice, in 1993 and 1994, was estimated at 56.2 and 52.5 mm3 respectively. There is great variability in water consumption from one sector to another: 1 400 mm to 5 500 mm. The drainage volume was 45 mm3 in 1993 and 53 mm3 in 1994, a wetter year. Annual fluxes of 400 tons of nitrogen (N) and 150 tons of phosphorus (P) entered the catchment. With a scenario of maximum transfer, only 5.5% of entering N and 4% of P were exported from the Fumemorte catchment to the Vaccares lagoon. Organic matter exported by the drainage network represents 5 to 6 times the quantity introduced with Rhone water. We attempted to quantify the processes of consumption, storage and loss which occur in the catchment. From preliminary results on the production of a seagrass bed in the Vaccares, we found that, at an annual scale, nutrient fluxes from the catchment seem minor compared to other sources (lagoon sediments, atmosphere).
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