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

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Abstract

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.

Introduction

Recently, water availability has become an issue of global concern. The total freshwater resources available worldwide are estimated to be approximately 43,750 km3/year (FAO, 2003). However, the world’s water resources have decreased from 17,000 m3/capita/yr in 1950 to 7 000 m3/capita/yr in the 2000s as a result of a near doubling of the global population. Furthermore, these resources are not distributed evenly. For example, in Asia, only 3400 m3 of water are available/capita/year while 24,000 m3/capita/yr are available in North America. In this context, evaluating water resources and water budgets for a given region in relation to the demand of the local human population could be a key factor in optimizing water resources management and reducing environmental pollution.

In Southeast Asia, the highest population densities are found in delta areas that are subject to high freshwater flows from less populated inland rivers and to tidal influence from the sea (Le et al., 2007, Luu et al., 2008). For example, in Vietnam, the population is concentrated within two large deltas, the Red River Delta (RRD) in the north and the Mekong Delta in the south. The present study focuses on the RRD, which plays an important role in the agricultural, industrial and economic development of the country. This area provides a good example of a region that is experiencing rapid population growth, industrialisation and economic development, which is leading to an increase in resource consumption and environmental degradation (Garnier and Billen, 2002, Le, 2003).

The goal of this study was to prepare a complete database of water discharge at some key stations in the RRD for 1996–2006 and then use this information to elucidate the complex water circulation within the delta area and to calculate a synthetic spatialized water budget for two years with contrasting climatic and hydrological characteristics (1996 and 2006). The budget developed in this study will allow characterization of the availability of water in relation to the spatial distribution of the population in the RRD.

Estimating the water balance of a tidal hydrological system is a complex task due to the large variations that occur in time and space in its components (Cook et al., 1998, Pasternack and Hinnov, 2003). In this study, we assembled a database of rainfall, evapotranspiration and water discharge in the river network of the RRD for a period of 10 years using (i) official recorded data, (ii) direct field measurements and (iii) the results of hydrological modelling. Among the numerous hydrological stations in the RRD, only three (Son Tay and Hanoi on the Red River and Thuong Cat on the Duong River) provide daily discharge data, while the remainder only provide water level measurements. Because of the tidal character of many rivers, direct conversion of available water level data into discharge values is difficult and has been the subject of several field studies and model development. Here, we attempted to reconstruct the discharge at several key stations and to propose an overall view of the hydrology of the RRD.

Section snippets

General characteristics of the Red River Delta

The Red River Delta (RRD), which is located in the northern part of Vietnam, is a very complex hydrological network that has been heavily impacted by human activity. The area of the RRD is approximately 14,300 km2, entirely lying below three meters above sea level and much of it does not rise more than one meter above sea level. The RRD is limited landwards by the town of Son Tay in the north–west (150 km from the sea) and seawards by the coastline, which extends 360 km from Hai Phong province in

Upstream water discharge of the delta

Since 1956, the National Institute of Meteorology, Hydrology and Environment (MONRE–IMHE) has measured the daily discharge at three hydrological stations in the upstream portion of the RRD, Son Tay and Hanoi on the Red River and Thuong Cat on the upstream Duong River. For this study, we only utilized the data collected from 1996 to 2006 (Fig. 4).

The Son Tay station, located on the Red River at the entrance of the delta, supplies most of the water to the delta. Throughout the study period, we

Water budgets in the Red River Delta for 1996 and 2006

The data discussed above enabled establishment of a comprehensive water budget for the five above-defined sub-basins in the delta (Table 2). Therefore, we considered two contrasting years within the 11 year period, 1996, which was a rather wet year, and 2006, which was a rather dry year. In fact, 1998 and 2003 were also two extreme years in term of meteorological and hydrological conditions but we have chosen 1996 and 2006 for establishing the water budget in order to evaluate the change in

Acknowledgments

This work was supported by the programme Day River – ANR, the Vietnamese Academy of Science and Technology (VAST, Vietnam), the Centre National de Recherche Scientifique (CNRS, France), the Institute of Research for Development (IRD, France), and the Agence Universitaire de la Francophonie (AUF). The PhD thesis of Luu Thi Nguyet Minh was supported by a scholarship from the French Embassy in Vietnam and by a cooperation agreement between the University Pierre and Marie Curie (UPMC) and the

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