Nitrogen and phosphorus exportation in the Garonne Basin (France)

Abstract

Nitrogen balance was determined in two basins of the southwest of France: the Garonne (52,000 km²) and one of its tributaries, the Girou (520 km²). Phosphorus balance was estimated only in the Girou watershed. Monthly water samples were collected during the period 1971 to 1981. The nitrogen balance showed that there was a good relationship between the exportation rate (ratio between river exportations and fertilizer inputs) and the mean annual discharge in the two watersheds. During a humid hydrological year, the exportation rate reached 40-50%. Phosphorus balance in the Girou watershed showed that the exportation rate is lower than 1%. The contribution of the different phosphorus sources to the annual streamflux was estimated as 7% for biogeochemical processes, 32% for non-point-pollution due to fertilizer application and 61% for point-pollution due to sewage effluent.