The data presented here and within
supporting literature provide strong evidence of
changing water resources in the Mekong River
Delta in general and in the Hau River mouth
in particular. Due to the combination of factors
related to climate change and human water
extraction activities, the Mekong River Delta
is currently facing many challenges, which will
only worsen in the future. Increasing salinity
intrusion, varying water flow regimes, and
general reduction of freshwater resources
Delta. More serious yet may be the rising sea
level and subsequent inundation of much of the
Delta. However, there are still many unknowns
regarding these effects. The establishment of a
water quality monitoring network for the Delta
is an urgent need, and standardization of
protocols is required to establish a database
for sharing information. Further, the Mekong
Delta Plan has been developed to provide
feasible action plans to help deal with the
variation of water sources and climate change.
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Journal of Fisheries science and Technology Special issue - 2015
50 • NHA TRANG UNIVERSITY
WATER RESOURCE VARIATION IN THE HAU RIVER MOUTH
Le Anh Tuan1, Hoang Thi Thuy2,Vo Van Ngoan3
ABSTRACT
The Hau River mouth is the largest estuary of the Mekong River Delta and is strongly influenced by both
discharge from the upstream Mekong River and the semi-diurnal tides of the East Sea. This aquatic ecosystem
is the interface between upstream freshwater flow and saltwater tidal flow, creating unique aquatic habitats
with significant socio-economic importance. The area is densely populated, with many people highly reliant
on aquatic ecosystem productivity and related aquaculture cultivation. This study examines changes to water
resources in the Hau River mouth based on hydrological variation analysis. There is evidence that, within
the last two decades, precipitation is decreasing in the early season but increasing in magnitude towards the
end of the wet season. Average water level within the Hau River estuary has increased at about 0.77 cm/year
with rising sea level, with concurrent increases in salinity intrusion. This is resulting in serious water supply
limitations and negative impacts on cultivation. Further, the underground water table is declining at a rate of
0.39 m/year, increasing the threat of land subsidence in the region. These factors combine to pose great threats
to Mekong River Delta in general and the Hau River mouth area in particular, where water resources face
challenges due to climate change and sea level rise, expansion of cultivated areas and water trans-boundary
related problems.
Keywords: Hau River mouth, Mekong River Delta; trend analysis; water resources variation
1 Corresponding author. Research Institute for Climate change – Can Tho University,
Tel: +84.913.619.499; Fax: +84 .7103.838.474; E-mail: latuan@ctu.edu.vn
2 Faculty of Natural Resources and Environment - Nong Lam University of HCM City
3 Office for Climate change – DONRE Ben Tre province
I. INTRODUCTION
The Mekong River Delta, located in
southern Vietnam, is an area of young geologic
origin formed by alluvial deposits from the
Mekong River that drains much of Southeast
Asia. The Hau River, or so-called Bassac
River or Hậu Giang in Vietnamese, is one of two
major branches of the Mekong River that
divides in Phnom Penh, Cambodia, before
crossing the border of Vietnam and continuing
to branch into smaller tributaries prior to
discharging in the East Sea (Figure 1). Together,
the Hau River in the south and the Tien River
in the north form the important waterway and
water-use corridor of the Lower Mekong River
Delta Region. The Hau River flows through
seven provinces, including An Giang, Can Tho,
Vinh Long, Tra Vinh and Soc Trang. Two
provinces in the Hau River mouth, Soc Trang
and Tra Vinh, have the highest poverty rate
in Vietnam (Vuong, 2011). The Mekong River
Delta is an essential part of daily life to the
people of this region, who use the river for their
daily drinking, irrigation, domestic use, fish
production, and transportation. Conversely,
the Mekong River Delta can be impacted by
human activities, which threaten the sustainable
development of the region (Trieu et al., 2007).
Journal of Fisheries science and Technology Special issue - 2015
NHA TRANG UNIVERSITY • 51
The Hau River has a 225-km length in
Vietnam and accounts for 5.17% the total
length of the main river. The Hau River’s width
is about 60 – 300 m when upon entering
Vietnam, and widens gradually as it flows to
the sea. Channel braiding within the Delta
forms many elongated islands, the largest of
which is Cu Lao Dung with the area of 249.4
km². At the river mouth in Cu Lao Dung district
of Soc Trang province, the average river width
is approximately 2 km, and the largest distance
between two riverbanks is up to 18 km. The
average depth of Hau River is 10 – 20 m, and
the maximum depth is over 40 m, although
depth tends to decrease closer to the sea due
to deposition of sedimentation. The land
surrounding the Hau River mouth (9.33°-
10.12° N, 105.71°-106.42° E), including Tran
De, Cu Lao Dung and Vinh Chau districts, is
very low with mean land surface elevation of
about 1.0 m above mean sea level. The Hau
River has the greatest water discharge of all
rivers in Vietnam. In October, the Hau River
can drain about 90% of the peak floodwater
of the Mekong River, and the total annual flow
can reach nearly of 215 billion m3.
The Hau River is currently facing many
challenges, including changes in hydrological
flow characteristics, declining water quality,
lowering water table and associated land
subsidence, riverbank and coastal erosion,
narrowing of natural lowlands during the
urbanization processes, expansion of agriculture
and fisheries production activities, and effects
of climate change, such as sea level rise and
further saltwater intrusion. During the dry
season, saltwater intrusion can reach more
than 60 km inland, leaving about 2.1 million
hectares of coastal area in the Mekong Delta
affected by saline waters (Tuan et al., 2007;
White, 2002). Increased salinity changes the
chemical environment, causing substantial
Fig. 1. Location of the Hau River mouth and data-survey points
Journal of Fisheries science and Technology Special issue - 2015
52 • NHA TRANG UNIVERSITY
changes in species composition of the local
ecological communities. Lack of adequate
surface freshwater resources, combined with
increasing demand for water for agricultural
production and domestic utilities, has resulted in
increased reliance on groundwater extraction.
This trend has aggravated saline intrusion
into the coastal aquifers and increased land
subsidence in this region (Hagenvoort and
Tri, 2013; Erban et al., 2014; Esther et al.,
2015). In some areas, high salinity water has
contaminated near surface aquifers, to the
point at which it is unfit for consumption.
Further, current and planned hydropower
dams in the mainstream and tributaries of the
Mekong River threaten to reduce sediment
deposition to the Delta’s floodplain areas,
exacerbating the land subsidence effect (Cook
and Tu, 2013). Sea level rise and land subsidence,
combined with rainy season storms and wind
driven waves, will pose a major threat to
local populations in the decades to come
(Rohkohl, 2014). In the current study, the objective
is to survey the variation in water resources at
the Hau River mouth area, including rainfall,
surface flow and groundwater sources.
II. RESEARCH APPROACHES AND
METHODOLOGY
This study used data collected from An
Giang, Can Tho and Soc Trang provincial
hydro-meteorological stations, and data
downloaded from the website of Mekong River
Commission (MRC) to detect temporal trends
in water resources characteristics. Trend lines
were fit using Microsoft Excel®. This study also
used the regional climate model, PRECIS, for
downscaling coarse scale Global Circulation
Models to derive climate change scenarios for
the Mekong River Delta (Jones et al., 2004).
PRECIS is a regional climate model developed
by the Hadley Centre for Climate Prediction and
Research. It can be used as a downscaling tool
that adds fine scale (high resolution) information
to large-scale projections of a Global Circulation
Model (Tuan and Supparkorn, 2011).
III. RESULTS AND DISCUSSION
1. Variation of weather patterns
Generally, there are two distinct seasons
in the Mekong River Delta: rainy season (from
May to October) and dry season (the rest of
the year). In the coastal areas, the starting
dates for rainy seasons may come one-to-two
weeks earlier than in the inland areas. Annual
average rainfall in the Delta historically
ranges from about 1,400 to 2,200 mm, with
more than 90% the rainfall occurring during the
rainy season. In the Hau River mouth area in
Soc Trang Province, average annual rainfall
is commonly higher than the Delta, ranging
from 1,660 to 2,230 mm. However, in last three
decades there is evidence that rainfall has
become more variable, with higher rainfall
during the wet season and lower rainfall during
the dry season. Also, timing of the onset of
rainy season and its duration has become more
variable, with significant effects on rainfall
amount. The annual rainfall totals in 1998 and
1999 were well above historical averages, up to
over 2,750 mm/year; however, in 1990, 1991,
1992, 2004, and 2006, rainfall totals were near
historical lows at less than 1,600 mm/year
(Figure 2). PRECIS regional climate modeling
predicts that, by the 2030s, Soc Trang province
will experience a decrease in annual precipitation
of 10-20% and the starting time of median rainy
seasons may be two weeks later compared
to the 1980s-1990s (Figure 3). During the dry
season, the model predicts a significant in-
crease in air temperature, approximately 0.6
°C in the duration of 1990 - 2013, a trend which
has already become apparent over the past 25
years (Figure 4).
Journal of Fisheries science and Technology Special issue - 2015
NHA TRANG UNIVERSITY • 53
2. Variation of river flow characteristics
The hydrological interaction of outgoing river
flow and incoming tides in the Hau River estuary
is extremely complex and is influenced by
many factors, including topography, seasonal
currents, wind directions, water temperature,
and constructed human activities. However,
rising sea level is likely to greatly impact this
area over the coming decades. Water level
within the Hau River estuary has already begun
to rise considerably, with mean river level
increasing at about 0.77 cm/year since 1985
(Figure 5). Given that most of the surrounding
land is only 1.0 – 2.0 meters above mean sea
level, this rise in water level threatens millions of
Vietnamese people in the Mekong River Delta.
Fig. 2. Variations and trendline of annual rainfall amounts in Soc Trang (1985 – 2012)
Fig. 3. Accumulated rainfall projection in Soc Trang under PRECIS model (Tuan, 2009)
Fig. 4. Variations and trend line of 5-year average temperature in Soc Trang (1990 – 2013)
Fig. 5. Variations and trend line of water level amplitudes (Max – Min) in Dai Ngai with standard
error bars in each calculated point (1985 – 2008)
Journal of Fisheries science and Technology Special issue - 2015
54 • NHA TRANG UNIVERSITY
3. Variations of seasonal salinity intrusion
The lower Hau River experiences severe
salt intrusion annually, usually reaching peak
salinity in March/April (Figure 6) corresponding
to the end time of the dry season and the
lowest flows in the Mekong River system. In
April 2010, the maximum salt intrusion into the
Hau River reached 40 km (4 g/L) to nearly 70
km (1 g/L) inland from the river mouth. As sea
levels rise and flows decrease, this annual salt
intrusion will reach further inland at higher salt
concentrations, with significant consequences
to the aquatic community and the human
communities within this region.
Fig. 6. Variations of maximum monthly salinity recorded in Dai Ngai (2002 - 2014)
4. Variation of groundwater level and land
subsidence risk
Reduced precipitation combined with
increased reliance on groundwater resources
is depleting shallow aquifers in the Mekong
Delta. For example, the water table in Soc
Trang City dropped more than 5 m from 1996
– 2012, with an average rate of decline of 0.39
m/year (Figure 7). By June 2013, the water
table at this site had reached -10.17 m. The
similar declines in the Pleistocene aquifer layer
were reported at sites in and around Tran De,
a coastal district of Soc Trang province in the
Hau River mouth (Figure 8).
Fig. 7. Variations and trend line of underground water table monitored in Soc Trang in the periods
of Dec. 1996 – Dec.2012 (Data source: NAWAPI, 2012)
Fig. 8. Variations of groundwater level (Aquifer Pleistocene middle – above) monitored
in the periods of 2001 – 2012 in in Tran De district (data 2004 is unavailable)
Journal of Fisheries science and Technology Special issue - 2015
NHA TRANG UNIVERSITY • 55
Code and Coordinator (X,Y) of
monitoring points: QST10A (629300, 1053050);
QST12B (614450, 1053900); QST13 (612100,
1051100); G8 (618500, 1051540); G25
(611160, 1056000) (Data source: Soc Trang
DONRE)
7. Changing the water environment
Unpublished water quality data collected in
2006 by the Soc Trang Department of Natural
Resources and Environment (DONRE) in the
My Thanh River, Vinh Chau District, Soc Trang
Province showed light organic matter
pollution with high suspended solid and total
iron contents (Table 1). In addition, suspended
solids, chlorine and coliform counts exceeded
the Vietnamese Standard in 2006. In the Hau
River mouth, the interface between fresh and
salt water varies with the tidal fluctuations
(Duc, 2008), but may serve as an effective
“nutrient trap” in which many minor chemical
contaminants become concentrated.
Table 1. Surface-water quality of My Thanh estuary, Vinh Chau District
No Parameter Unit
Results
2002 2003 2004 2005 2006
1 pH - 7.54 8.00 7.31 7.40 6.99
2 Turbidity NTU 88 304 108 19.40 622
3 DO mg/L 5.38 5.30 1.22 2.42 3.13
4 BOD5
20 mg/L ND ND 6.00 8.00 16
5 COD mg/L ND 18.10 27.60 26.00 20
6 SS mg/L 121 - - 631 539
7 NO3
- mg/L 0.40 0.60 5.20 30 6.4
8 Fe mg/L 1.29 0.02 - 0.268 23.5
9 NO2
- mg/L - - - - 3.0
10 NH4
+ mg/L - - - - 0.1
Notes: ND: Non-detection (-): No data (Data source: Soc Trang DONRE, 2006, unpublished)
IV. CONCLUSION
The data presented here and within
supporting literature provide strong evidence of
changing water resources in the Mekong River
Delta in general and in the Hau River mouth
in particular. Due to the combination of factors
related to climate change and human water
extraction activities, the Mekong River Delta
is currently facing many challenges, which will
only worsen in the future. Increasing salinity
intrusion, varying water flow regimes, and
general reduction of freshwater resources
threaten human populations in the Mekong
Delta. More serious yet may be the rising sea
level and subsequent inundation of much of the
Delta. However, there are still many unknowns
regarding these effects. The establishment of a
water quality monitoring network for the Delta
is an urgent need, and standardization of
protocols is required to establish a database
for sharing information. Further, the Mekong
Delta Plan has been developed to provide
feasible action plans to help deal with the
variation of water sources and climate change.
Journal of Fisheries science and Technology Special issue - 2015
56 • NHA TRANG UNIVERSITY
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