Résumé
Based on the use of the Seneque/Riverstrahler model, a prospective scenario of water quality has been developed based on expected demographic, land use and agricultural production changes in the Red River delta in northern Vietnam in order to evaluate the resulting changes in water pollution and nutrient loading delivered to the coastal zone at the horizon 2050. The Vietnamese population is predicted to reach 112 million in 2050 with 57% that live in urban area. The amount of wastewater expressed in kg of suspended matter, N and P per inhabitant per day is estimated to decrease by about 10% in 2050 compared with current values. On the other hand, the wastewater discharge should increase twice, i.e. to 200 l/inhabitant/day. Besides, if the productivity of agriculture in the Red River Delta is to be nearly doubled in the future, this would likely result in a N surplus of about 150 kgN/ha/yr on the agricultural soils. Assuming the same leached fraction as currently observed, this would imply a mean leaching rate of 15 kgN/ha/yr, i.e. 3 times the present level. By applying the above scenario to the Red River delta, our simulation shows that nutrient elements such as ammonium, phosphorus and silica seem to remain at the same concentration value. However, the concentration of these last elements will be largely on the dependence of the water volume due to the flood. Indeed, the delta floodplain improves largely the biogeochemical processes involved in the nutrient recycling. Our modelling results highlight that the Red River Delta system is already at its maximum of the nutrient recycling capacity. So an increase of discharge from urban wastewaters and from agricultural intensification will lead immediately and definitely to an increase of the polluted waters in the hydrological system of the Nhue and Day Rivers, in spite of efforts in urban wastewater treatments.