Nutrient (N, P, Si) transfers in the subtropical Red River system (China and Vietnam): Modelling and budget of nutrient sources and sinks

The Red River system, which has a population density that varies from 80 to over 1000 inhabitants km−2 in the different sectors of its watershed, is a typical example of a subtropical system experiencing high human pressure. A monthly survey of nutrient concentration was conducted at the outlet of the three main tributaries (Da, Thao and Lo) and at Hanoi on the main branch of the Red River in the upper part of the delta. Samples from headwater streams or irrigation channels draining homogeneous land use classes also were analysed. A GIS database describing the characteristics of the watershed in terms of morphology, climate, land use and population has been established. This served as the basis for implementing the Seneque/Riverstrahler model, which allowed calculation of nutrient transfers and transformations along the whole drainage network of the Red River system. The model was validated using the measured nutrient concentrations obtained during the survey and was used to establish an overall description and a comprehensive budget of nutrient transfer, retention and delivery for the three main sub-basins and the upper part of the delta upstream from Hanoi.