Hydrological regime and water budget of the Red River Delta (Northern Vietnam)

The Red River Delta (RRD) in Northern Vietnam represents a complex hydrological network of tributaries and distributaries that receive a large and seasonally fluctuating flow of water from the upper Red River basin and is also subjected to tidal influence. In this study, we attempted to assemble a database of discharge estimates within the RRD for 1996–2006 to elucidate the water circulation patterns in the system, enable quantification of major water fluxes and assess the water resources availability. Regular discharge measurements in the RRD are available for three upstream stations, while the other hydrological stations provide only water level records; however, the MIKE 11 model allowed overall calibration curves to be established, which enabled the conversion of available daily mean water level data into discharge values. Four gauging surveys were conducted under flood and dry season in 2007 and 2008 to experimentally validate these calibration curves.After the database was generated, a water balance was established for two years with contrasting climatic and hydrological characteristics. During the wet year (1996), the main branch of the Red River represented the largest input of freshwater to the sea (approximately 60%). Conversely, during the dry year (2006), the inputs were more evenly distributed among the three main fluvial branches. The total volume annually delivered to the sea from the RRD was approximately 140 and 100 km3 for 1996 and 2006, respectively. When the five sub-basins within the RRD were evaluated, it was shown that the water resources were far from evenly distributed within the area. In particular, the Bui sub-basin, which has the highest population density and the lowest water resources per unit area, is experiencing a critical situation in terms of pressure on water resources.