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Hydrological Sciences Journal
ISSN: (Print) (Online) Journal homepage: https://www.tandfonline.com/loi/thsj20
Understanding the anthropogenic development
impacts on long-term flow regimes in a tropical
river basin, Central Vietnam
Binh Quang Nguyen, Sameh Kantoush, Doan Van Binh, Mohamed Saber,
Duong Ngoc Vo & Tetsuya Sumi
To cite this article: Binh Quang Nguyen, Sameh Kantoush, Doan Van Binh, Mohamed Saber,
Duong Ngoc Vo & Tetsuya Sumi (2023): Understanding the anthropogenic development impacts
on long-term flow regimes in a tropical river basin, Central Vietnam, Hydrological Sciences Journal,
DOI: 10.1080/02626667.2022.2153298
To link to this article: https://doi.org/10.1080/02626667.2022.2153298
Published online: 12 Jan 2023.
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Understanding the anthropogenic development impacts on long-term ow regimes
in a tropical river basin, Central Vietnam
Binh Quang Nguyen
a,b
, Sameh Kantoush
a
, Doan Van Binh
c
, Mohamed Saber
a
, Duong Ngoc Vo
b
and Tetsuya Sumi
a
a
Water Resource Center, Disaster Prevention Research Institute (DPRI), Kyoto University, Kyoto, Japan;
b
Faculty of Water Resources Engineering, The
University of Danang – University of Science and Technology, Danang, Vietnam;
c
Master Program of Water Technology, Reuse and Management,
Faculty of Engineering, Vietnamese German University, Binh Duong, Vietnam
ABSTRACT
The Vu Gia Thu Bon (VGTB) basin constitutes the primary water supply in Central Vietnam. While climate
change disturbs stream discharges and aects ood extremes, upstream dam development may intensify
or mitigate such impacts. Therefore, this study provides a quantitative evaluation of long-term alterations
in the ow regimes of the VGTB rivers from 1977 to 2020 resulting from the impacts of upstream
anthropogenic developments. The datasets are divided into two periods, pre-2000 (1977–2000) and
post-2000 (2001–2020), using dierent indices and analytical methods. The analyses show that since
2011, reservoir operations have reduced the maximum and high-ow discharges downstream in excess
of climate change and land-use eects. However, due to the impact of water transfer by the Dak Mi 4
hydropower dam from the Vu Gia River to the Thu Bon River through a diversion channel, the minimum
and low-ow discharges decreased in the pre-dam period and increased in the post-dam period.
ARTICLE HISTORY
Received 27 June 2022
Accepted 31 October 2022
EDITOR
A. Castellarin
ASSOCIATE EDITOR
E. Davies
KEYWORDS
Vu Gia Thu Bon basin;
hydropower dams; water
transfer; flow regime
1 Introduction
Typhoons are a severe natural hazard that affects river basins
worldwide. Stronger typhoons usually produce heavy rainfalls,
enormous wind speeds, higher waves, and storm surges
(Larson et al. 2014). The consequences of typhoons include
damage to infrastructure, agricultural production and river-
banks; coastal erosion; and the loss of human life. The Vu Gia
Thu Bon (VGTB) basin (Fig. 1(a)) is located in the central
coastal region of Vietnam, which is prone to a tropical mon-
soon climate (RETA 2011, Ribbe et al. 2017). According to the
Japan Meteorological Agency (JMA), the area is frequently
affected by typhoons and tropical depressions. In Vietnam,
the majority of typhoons (70%) occur in the central parts of
the country (Fig. 1(b)), according to the Vietnam General
Department of Meteorology and Hydrology. Typhoons, tropi-
cal depressions, and cold air have caused heavy rain, leading to
severe flooding. Wang et al. (2014)’s key findings indicate that
increased rainfall in Central Vietnam since the beginning of
the 20th century is associated with increased typhoons. Indeed,
it has been revealed that there is a strong correlation between
the increasing trend of stronger typhoons and the extension of
the typhoon season over the last decade. Consistently, the
maximum rainfall has been found to increase, along with the
higher frequency of typhoons on the south coast of Vietnam
(Tan and Thanh 2013). In 2020, four typhoons (i.e. Noul,
Linfa, Molave, and Vam Co) affected the VGTB river basin,
three of which had magnitudes higher than the long-term
average (Fig. 1(d)). Due to the impacts of typhoons, the flood
peak at the Ai Nghia and Giao Thuy hydrological stations
recorded in the downstream basin reached 12.84 m and
8.97 m, respectively, higher than those in 1999 (Fig. 1(a)).
According to the report of the Commanding Committee for
Disaster Prevention and Search and Rescue in Quang Nam
Province, as a consequence of typhoons in 2020, 28 people
died, 19 people were reported missing, and 200 people were
injured, and the typhoons resulted in total economic losses of
approximately $460 million. The increase in the frequency
(Fig. 1(d)) and intensity of typhoons may be a result of climate
change.
The VGTB basin ranks fourth in terms of hydropower
potential in Vietnam (ICEM 2008); many hydropower
plants have been built and are operating in the region.
Although hydropower is essential for the country, it has
significant adverse effects on river systems, such as flow
regime alterations, sediment reduction, flooding, drought,
water shortages, agricultural production decreases, and sal-
ine intrusion intensification (Dinh 2016, Laux et al. 2017,
Ribbe et al. 2017, Firoz et al. 2018). The Vu Gia River is
the primary source of water for Da Nang city and regional
agricultural activities. A large volume of water was diverted
from the Vu Gia River to the Thu Bon River (Fig. 1(a))
when the Dak Mi 4 hydropower plant began operation in
2011. This water diversion led to deficits in the water
supply for agriculture and drinking water and increases in
salinity intrusion in Da Nang city. Consequently, a polemic
controversy between Da Nang city and Quang Nam
Province (Fig. 1(a)) regarding the impact of hydropower
development has begun. Da Nang city blames Quang Nam
Province for a large-scale hydropower cascading system
that negatively affects downstream water resources in the
CONTACT Binh Quang Nguyen nqbinh@dut.udn.vn; nguyen.binh.27y@st.kyoto-u.ac.jp Water Resource Center, Disaster Prevention Research Institute (DPRI),
Kyoto University, Kyoto 611-0011, Japan
HYDROLOGICAL SCIENCES JOURNAL
https://doi.org/10.1080/02626667.2022.2153298
© 2023 IAHS
dry season (Nauditt et al. 2017). Therefore, quantification
of the cascading effects of hydropower dams developed in
Quang Nam Province and in the entire VGTB basin on the
flow regimes in Da Nang city is urgently needed to ease
this conflict.
A complex series of impounding reservoirs in the VGTB
basin have been built (18 reservoirs) or planned (42 reservoirs)
to make the best use of elevation differences and maximize the
power generated through water diversion without compensat-
ing for reductions in the water level and environmental flow.
Figure 1. (a) Map of the VGTB River basin, (b) trajectories of typhoons affecting the basin, (c) frequency of typhoons by month, and (d) number of typhoons by year
(Source: National Institute of Informatics, Japan. http://agora.ex.nii.ac.jp/) (Japan Meteorological Agency 2013).
2B. Q. NGUYEN ET AL.
The distribution of these dams is heavily concentrated on
the Vu Gia River, with 12 small and large reservoirs, of
which two diversion channels divert water to the Thu
Bon River. Firoz et al. (2018) highlighted the impacts of
eight upstream dams (six in the Vu Gia and two in the
Thu Bon) over a short operational period in 2013 on
drought risk and streamflow. That study concluded that
the Vu Gia River has a high risk of hydrological drought,
which impacts the water supplied for irrigation and
drinking water in Danang city, especially in the dry
season. In the rainy season, these eight dams reduced
the monthly average discharge by 30% (Firoz et al.
2018). In contrast, the monthly average flow discharge
increased in the Thu Bon River from 24 m
3
/s to 62 m
3
/s
in the dry period (Firoz et al. 2018). Additionally, pre-
vious studies analysed the impacts of a limited number of
dams following operation periods of two years or more.
However, the cumulative effects of these 18 dams, plus
additional dams built over an extended period ending in
2020, on flow regime alterations due to a changing cli-
mate remain unknown and constitute the root of this
research.
Nauditt et al. (2017) examined the individual impact of a
hydropower reservoir with a diversion channel from the drier
Vu Gia River to the wetter Thu Bon River from 1980 to 2013.
These diversion processes reduced the mean monthly dis-
charge in Vu Gia by 13–125 m
3
/s. An additional individual
assessment of the Song Tranh 2 reservoir in the Thu Bon River
was performed to evaluate the changes in the released flow
based on daily measurement data from 1996 to 2018 (Ha and
Coynelb 2019). The study revealed that the average flow after
the Song Tranh 2 reservoir opened decreased by 103 m
3
/s,
from 864 m
3
/s to 761 m
3
/s. Phuong et al. (2020) used the
classical/modified Mann-Kendall and innovative-Sen methods
to evaluate the trend of hydrometeorological factors in the
VGTB basin over 36 years, from 1979 to 2014. The authors
used a continuous data series from 1979 to 2014 to assess the
general trend, which did not accurately reflect the flow char-
acteristics within a reservoir. The authors used only the classi-
cal/modified Mann-Kendall and innovative-Sen methods,
which do not reflect all the flow regime behaviours of the
basin. These studies are limited in that they do not consider
the effects of all reservoirs and long-term flow alterations at all
stations. The study period is relatively short, and flow indica-
tors have not thoroughly evaluated the flow regime alterations.
Therefore, in the present study, we attempt to assess flow
regime alterations based on a more extended period (1977–
2020) and consider all dams and flow transfer impacts.
Understanding the long-term changes in the flow regime
in the VGTB basin is of vital importance for sustainable
management and water resource distribution in the coming
decades. However, the previous studies are limited in terms
of data range (up to 2014) and thus do not assess the cascad-
ing effects of all dams (many of which were built more
recently) on the flow regime alterations in the VGTB basin.
Accordingly, this study aims to quantify changes in the long-
term flow regimes in the VGTB basin due to climate change,
reservoir cascading, and construction of water diversion
structures. The contributions of this paper are (1) a
comprehensive evaluation of the long-term flow regime
alterations considering a comprehensive range of hydrologi-
cal alteration indicators and (2) an understanding of the
impacts of reservoir operations, climate change/variability,
and land-use change on the flow regimes. The results of our
research provide evidence-based data regarding historical
changes in the hydrology of the VGTB basin, providing
stakeholders with helpful information for river basin man-
agement and solutions for informing future preparedness and
sustainable development.
2 Materials and methodology
2.1 Study area
The VGTB river basin (Fig. 1(a)) is the major basin in the
Central Coast region, Vietnam, with an area of 10 350 km
2
.
The basin has a tropical monsoon climate and two distinct
seasons: dry summer (January–August) and heavy rain
(September–December). The average annual rainfall varies
significantly, from 2000 mm in the central and downstream
regions to more than 4000 mm in the southern mountai-
nous areas. There are seasonal differences, with 65% to
80% of the annual rainfall concentrated from September
to December (RETA 2011). In the eight months of the dry
season, rain accounts for only 20% to 35% of the annual
rainfall (Nauditt et al. 2017). The driest period usually falls
between February and April, with approximately 3% to 5%
of the total annual rainfall.
The discharge of the basin is divided into three different
seasons: low flow (January–April), transition flow (May–
August), and high flow (September–December). Due to the
difference in the rainfall distribution, the runoff in the VGTB
basin varies significantly between seasons. The flow in the
rainy period accounts for approximately 62.5% to 69.2% of
the total annual flow. Every year, the basin is frequently hit by
four to eight floods. The flood peaks usually occur in October
and November due to different weather patterns, such as
typhoons, tropical depressions, cold air, and northeast mon-
soons (Fig. 1(c)) (T. T. Vu et al. 2011). The frequency of
cyclones ranges from one to two per year (Fig. 1(d)).
Figure 1(a) shows 18 existing dams and diversion facil-
ities and the river gauging stations considered in this
study. There are six reservoirs on the Thu Bon River,
with a total storage volume of 1575 million m
3
(Fig. 2
(a)). There are 12 reservoirs in the Vu Gia basin, with a
total storage volume of 1335 million m
3
(Fig. 2(b)). Eight
hydropower plants affect the flow at the Thanh My and
Nong Son stations: Dak Mi 2, Dak Mi 3, Dak Mi 4, Dak
Mi 4B, and Dak Mi 4C in the Vu Gia basin and Song
Tranh 2, Song Tranh 3, and Song Tranh 4 in the Thu Bon
basin (Fig. 1(a)). The Dak Mi 4 plant in the Vu Gia basin
was constructed in 2007 and began operation in 2011. A
tunnel was built in the Dak Mi 4 hydropower plant to
divert water and sediment from the Vu Gia River to the
Thu Bon River (Fig. 1(a)). This diversion led to a signifi-
cant alteration of the basin’s flow regime which is attrib-
uted to hydrological, drought and saline intrusion during
the dry season in Da Nang city (Firoz et al. 2018).
HYDROLOGICAL SCIENCES JOURNAL 3
2.2 Data collection
In this study, rainfall data from 15 raingauges were collected
from the Mid-Central Regional Hydro-Meteorological Center
(Fig. 1(a)). Because of the sparse distribution of raingauges (15
gauges) in the basin, we interpolated the rainfall data from
those stations to make a spatial map using the kriging method
in ArcGIS 10.4 following the method of Duong and
Gourbesville (2014). The discharge and water level data at
the Thanh My and Nong Son stations (the only two stations
monitoring discharges in the basin) were collected from 1977
to 2020, and the dataset was treated for gaps and missing data
(Fig. 3). Data on the operation of the hydropower plants were
obtained from the Natural Disaster Prevention and Control
Department of Quang Nam Province (NDPAC). Hydraulic
infrastructure data were also obtained, from Decision 1865/
QD-TTg: Procedures for Operating Reservoir Systems in the
VGTB River Basin (Government of Vietnam 2019). Land use
in 2001, 2005, 2010, and 2020 was collected from the project
“Land Use and Climate Change Interaction in Central
Vietnam (LUCCi)” (www.lucci-vietnam.info) to assess the
effect of land-use changes on runoff.
2.3 Methodologies
The flow regime is analysed based on the following methods:
statistical trend tests (Mann-Kendall, Sen’s slope), indicators of
hydrologic alteration (IHA), the index of hydrological regime
alteration (FQ), and flow regime metrics. The flood character-
istic indices are analysed as a peak over threshold (POT) using
the generalized Pareto distribution (GPD). We want to inves-
tigate the cumulative impacts of all dams after construction
from 2001 to 2020 and prior to construction from 1977 to 2000
to be considered as climate change impacts. Therefore, the
research is divided into two periods: pre-2000 (1977–2000)
and post-2000 (2001–2020).
2.3.1 Statistical trend tests
The nonparametric Mann-Kendall test and the slope method
of Sen are used to evaluate the long-term discharge and rainfall
changes. The nonparametric Mann-Kendall test is commonly
employed to detect monotonic trends (increasing or decreas-
ing) in data collected over time. The Mann-Kendall test is used
to determine the tendency of long-term data (Kendall 1938,
Mann 1945), and the rate of change is estimated using the
slope method of Sen (Sen 1968).
2.3.2 Indicators of hydrologic alteration (IHA)
IHA is a software program developed by scientists at the
Nature Conservancy (Richter et al. 1996, 1998, 2003). IHA
provides valuable information for those trying to understand
the hydrological impacts of human activities and develop
environmental flow recommendations for water managers.
IHA analysis can help statistically describe how patterns have
changed for a particular river or lake due to abrupt impacts
2008 2010 2012 2014 2016 2018 2020
0
500
1000
1500
01(e
garotslatoT
6
m
3
)
Time (Year)
(a) Vu Gia basin
2008 2010 2012 2014 2016 2018 2020
0
500
1000
1500
2000
Time (Year)
(b) Thu Bon basin
Figure 2. Total storage of hydropower dams in the (a) Vu Gia basin and (b) Thu Bon basin (ICEM 2008, MOIT 2015).
Figure 3. Daily discharge and rainfall at the (a) Thanh My station and (b) Nong Son station. The black line is the discharge, and the blue (shaded) line is the rainfall.
4B. Q. NGUYEN ET AL.
such as dam construction or land and water use changes
(Opperman 2006). This method includes 32 hydrological indi-
cators, which are categorized into three large groups: (1) mag-
nitude, (2) timing, and (3) duration and frequency (Table 1).
2.3.3 Index of hydrological regime alteration (FQ)
The FQ, adopted from Alcayaga et al., evaluates changes in the
frequency and duration of high flow and low flow.
FQ %ð Þ ¼ NQpost
NQpre �100 100 (1)
where FQ %ð Þ indicates the frequency of change, and NQpre
and NQpost are the number of days when the flow is higher than
the high flow or lower than the low flow in the pre-2000 and
post-2000 periods, respectively.
2.3.4 Impact assessment of the flow regime metrics
Reservoir operation depends mainly on operational objectives
and hydrometeorological conditions and can lead to changes
in flow regimes (Zhang et al. 2018). To see changes over the
long term, flow metrics (magnitude, variability and frequency,
duration, timing, and rate of change) are used (Zhang et al.
2018, Van Binh et al. 2020). Twenty-three indicators in flow
mode metrics over different years are used to evaluate the
effects of reservoirs, including relative and absolute values
(Table 1). The equation for calculating these values is adopted
from Zhang et al. (2018) and Van Binh et al. (2020).
ADi¼Vi
post �
Vi
pre (2)
RDi¼ADi
�
Vi
pre �100%(3)
�
Vi
pre ¼PN
1Vi
pre
N(4)
where AD and RD are the absolute and relative deviations
of the i
th
metric, respectively; Vi
post and Vi
pre are the values
of the ith metric in the post-2000 and pre-2000 periods,
respectively; �
Vi
pre is the mean value of the ith metric in the
pre-2000 period; and N is the number of years in the pre-
2000 period. If the deviation is greater than 0, the flow
regime metrics are positively impacted. If it is less than 0,
then the flow regime metrics are negatively impacted.
When it is equal to 0, the flow regime metrics have no
impacts. The relative difference is divided into five grades
based on the percentiles adopted by Zhang et al. (2018):
slight (−15% ≤ RD < −5% or 5% < RD ≤ 15%), moderate
(30% ≤ RD < −15% or 15% < RD ≤ 30%), high (−45% ≤
RD < −30% or 30% < RD ≤ 45%), extreme (RD < −45% or
45% < RD), and no impact (−5% ≤ RD ≤ 5%).
2.3.5 Peak over threshold (POT) method
The POT method is used to analyse the flood frequency for the
Vu Gia and Thu Bon basins. The POTs are the flood peaks that
are more significant than a given threshold in each year. POT
modelling provides additional flexibility and a more compre-
hensive description of peak floods (Lang et al. 1999).
The POT method depends on two factors: independent
criteria and threshold selection. Therefore, how the threshold
is selected is important. The first step is the consideration of
the independence conditions, and the second step is the
threshold selection. The distribution of the POT series can be
determined by the GPD proposed by Pickands (1975).
3 Results
3.1 Alterations of the ow regime in the dry season
For the flow magnitude, the low-flow discharge of the Vu
Gia (one-day minimum, Q90, May–August) was mainly
positively impacted at slight to extreme grades from 2001
to 2011 and negatively affected at severe rates from 2012
to 2020 (Fig. 4(a)). The low-flow discharge of the Thu
Bon was positively and negatively impacted to a signifi-
cant degree before becoming positively impacted from
2012, except for 2020, in which it was negatively impacted
(Fig. 4(b)). The annual impact variation in the dry season
increased slightly in the Vu Gia but increased consider-
ably in the Thu Bon in the post-2000 period (Fig. 5, Table
2). The increased rates of minimum and low-flow dis-
charges were 1–21% for Thanh My and 20–57% for
Nong Son.
For the flow variability and frequency, the impacts of
the flow metrics were negative to positive in the Vu Gia
starting in 2011 (Fig. 4(a)). The most impacted areas in the
Thu Bon showed extremely negative grades (Fig. 4(b)). The
duration of the low-flow season in the post-2000 period
Table 1. Flow regime metrics used in the impact assessment of long-term alterations in the VGTB basin.
Group Regime characteristic Hydrologic metric Units
Magnitude Average flow Mean daily discharge of a year m
3
/s
Mean daily discharge in each month during the low-flow period m
3
/s
Mean daily discharge in each month during the transition-flow period m
3
/s
Mean daily discharge in each month during the high-flow period m
3
/s
Discharges in the 40th and 60th percentiles of the FDC: Q40 and Q60 m
3
/s
High flow Annual one-day maximum discharge m
3
/s
Extreme high-flow discharge Q10 (10th percentile of the FDC) m
3
/s
Low flow Annual one-day minimum discharge m
3
/s
Extreme low-flow discharge Q90 (90th percentile of the FDC) m
3
/s
Timing High flow Julian date of the one-day maximum discharge day
Low flow Julian date of the one-day minimum discharge day
Duration and frequency High flow Index of hydrological regime alteration in high flow: FQ-high flow %
Low flow Index of hydrological regime alteration in low flow: FQ-low flow %
HYDROLOGICAL SCIENCES JOURNAL 5
Figure 4. Heatmap quantifying the alteration of the flow regime metrics in the Vu Gia and Thu Bon basins. Blue indicates a positive impact, and red indicates a negative
impact.
1/14
1/28
2/11
2/25
3/11
3/25
4/8
4/22
5/6
5/20
6/3
6/17
7/1
7/15
7/29
8/12
8/26
9/9
9/23
10/7
10/21
11/4
11/18
12/2
12/16
12/30
0
200
400
600
High - flow
Low – flow
m(egrahcsiD
3
/s)
Date
Pre-2000
Post-2000
Transition – flow
(a) Vu Gia basin
1/14
1/28
2/11
2/25
3/11
3/25
4/8
4/22
5/6
5/20
6/3
6/17
7/1
7/15
7/29
8/12
8/26
9/9
9/23
10/7
10/21
11/4
11/18
12/2
12/16
12/30
0
500
1000
1500
High - flow
Low – flow
m(egrahcsiD
3
/s)
Date
Pre-2000
Post-2000
Transition – flow
(b) Thu Bon basin
Figure 5. Differences in the discharge hydrographs between the two periods in the transition-flow, low-flow, and high-flow seasons in (a) the Vu Gia basin and (b) the
Thu Bon basin. The blue line is the discharge in the pre-2000 period, and the red line is the discharge in the post-2000 period.
6B. Q. NGUYEN ET AL.
increased by 25% in the Vu Gia and decreased by 60% in
the Thu Bon (Table 2).
For the flow timing, the Julian date of the minimum dis-
charge was delayed from 172 to 191 days (19 days) in the post-
2000 period in the Vu Gia. The appearance of the minimum
release in the Thu Bon occurred six days earlier, from 225 to
219 days (Table 2).
There was a distinguishable deviation in the annual
impact variations in the basins in the dry season. The
low-flow discharge began changing in approximately 2011;
the change became obvious in 2012. The red colour
(extreme negative) extended from 2012 to 2020 for the
Vu Gia. In contrast, the blue colour (extreme positive)
was common from 2012 to 2019 for the Thu Bon. These
changes were consistent with the operating years of the
Dak Mi 4 dam in the Vu Gia basin and the Song Tranh
2 dam in the Thu Bon basin. In contrast, the regulation of
Dak Mi 4 decreased the mean daily inflow annually in the
Vu Gia. As a result, the regulation of the dam increased the
expected flow variability and low-flow frequency.
3.2 Alterations of the ow regime in the rainy season
The results from the POT method (Fig. 6 (a) and (b)) served as
a guideline for analysing the flood frequency and identifying
peak floods. The thresholds from the POT method for the Vu
Gia and Thu Bon basins were 850 and 2400 m
3
/s, respectively.
This method allowed us to characterize the statistical distribu-
tion of shorter record lengths post- and pre-dam. In the Vu
Gia, based on the Mann-Kendall test, the discharge increased
statistically from 1977 to 2000 and decreased statistically from
2001 to 2020. In the Thu Bon, the trends in the two periods
increased. In contrast to Firoz et al. (2018), we found that the
longer period had a statistically significant frequency. Figure 7
illustrates the results of the flood frequency distribution for the
Vu Gia and Thu Bon basins based on the GPD. The frequency
of flood flows in the post-2000 period was smaller than that in
the pre-2000 period in the Vu Gia (Fig. 7(a)). Water received
from the Vu Gia combined with reservoir operations led to a
higher flood frequency in the Thu Bon (Fig. 7(b)).
Due to anthropogenic intervention in the post-2000 per-
iod, the maximum discharges in the Vu Gia decreased by
6%, and those from the Thu Bon increased by 21% (Table 2).
The magnitude of the high-flow discharges (one-day max-
imum, Q10, September–December) in the Vu Gia were
positively impacted from 2001 to 2011 before becoming
negatively impacted with small to severe rates from 2012
to 2020 (Fig. 4(a)). For the Bon, high-flow discharges were
positively and negatively impacted and clearly negatively
impacted in 2019 (Fig. 4(b)). The high-flow discharge in
both basins increased during the four months of the rainy
season, except for November in the Thu Bon. The largest
increases in the Vu Gia and Nong Son were 44% and 57%,
respectively.
The high-flow frequency (FQ-high flow) in the Vu Gia
changed from a positive to a negative impact grade starting in
2011, whereas it was mostly a positive grade in the Thu Bon (Fig. 4
(a) and (b)). The duration of the high-flow discharge decreased in
the Vu Gia by 33% and did not change in the Thu Bon.
For the flow timing, the patterns of discharge hydrographs
in the Vu Gia and Thu Bon were also altered (Fig. 5 (a) and
((b)): the peak discharge occurred 10 and 11 days later in the
Table 2. Results of the IHA analysis to determine discharge alterations at the Thanh My and Nong Son stations.
Indicator Units
Thanh My station Nong Son station
Pre-2000 Post-2000 Deviation magnitude (%) Pre-2000 Post-2000 Deviation magnitude (%)
January m
3
/s 92 100 8 (9) 194 234 40 (20)
February m
3
/s 59 59 1 (1) 116 148 32 (27)
March m
3
/s 43 47 4 (9) 76 119 43 (57)
April m
3
/s 34 38 5 (13) 59 91 32 (53)
May m
3
/s 40 41 2 (4) 80 130 50 (62)
June m
3
/s 38 41 3 (7) 76 97 21 (27)
July m
3
/s 39 40 1 (2) 61 72 11 (19)
August m
3
/s 39 42 3 (7) 56 96 40 (72)
September m
3
/s 54 62 7 (14) 88 139 50 (57)
October m
3
/s 126 181 55 (44) 297 322 25 (8)
November m
3
/s 222 224 2 (1) 606 570 −37 (−6)
December m
3
/s 179 200 22 (12) 427 482 55 (13)
Annual m
3
/s 80 90 10 (12) 178 208 30 (17)
One-day minimum m
3
/s 22 23 1 (4) 30 40 10 (34)
Three-day minimum m
3
/s 24 24 1 (3) 31 43 12 (40)
Seven-day minimum m
3
/s 25 26 1 (4) 33 47 14 (41)
30-day minimum m
3
/s 30 33 4 (12) 40 62 22 (55)
90-day minimum m
3
/s 36 43 8 (21) 62 86 24 (40)
One-day maximum m
3
/s 1995 1875 −120 (−6) 4635 5625 990 (21)
Three-day maximum m
3
/s 1180 1219 39 (3) 3408 4527 1119 (33)
Seven-day maximum m
3
/s 816 843 27 (3) 2144 2904 760 (35)
30-day maximum m
3
/s 497 438 −59 (−12) 1247 1286 39 (3)
90-day maximum m
3
/s 278 293 15 (5) 748 801 53 (7)
Date of minimum Day 172 191 19 (11) 225 219 −6 (−3)
Date of maximum Day 301 311 10 (3) 304 315 11 (4)
Low pulse count Number 13.0 6.5 −7 (−50) 10.0 8.5 −2 (−15)
Low pulse duration Day 4 5 1 (27) 5 2 −3(−56)
High pulse count Number 7.5 7.5 0 (0) 7.5 10.0 3 (33)
High pulse duration Day 3 2 −1 (−33) 3 3 0 (0)
Rise rate Number 6.8 6.6 0 (−3) 15.4 20.0 5 (30)
Fall rate Number −4.2 −4.2 0 (−2) −8.0 −16.1 −8 (102)
HYDROLOGICAL SCIENCES JOURNAL 7
post-2000 period than in the pre-2000 period, respectively
(Table 2).
3.3 Alterations of the average ow
The results of the Sen slope test show that the mean annual
discharge in the Vu Gia (p = .012) and Thu Bon (p = .027)
basins significantly increased in the pre-2000 period by
0.14 m
3
/s/year and 0.25 m
3
/s/year, respectively. However, dur-
ing the post-2000 period, the annual discharge slightly
decreased in the Vu Gia (by 0.21 m
3
/s/year) and slightly
increased in the Thu Bon (by 0.1 m
3
/s/year) (Fig. 8).
1975 1980 1985 1990 1995 2000 2005 2010 2015 2020
0
1000
2000
3000
4000
5000
1975 1980 1985 1990 1995 2000 2005 2010 2015 2020
0
2000
4000
6000
8000
10000
1975 1980 1985 1990 1995 2000 2005 2010 2015 2020
0
1000
2000
3000
4000
5000
1975 1980 1985 1990 1995 2000 2005 2010 2015 2020
0
2000
4000
6000
8000
10000
(d)
(c)
(b)
(a)
m(egrahcsiD
3
/s)
Observation Threshold
Vu Gia basin
850
2400
Thu Bon basin
Peak over threshold (POT)
m(egrahcsiD
3
/s)
Date Date
Figure 6. Threshold and POT samples selected in (a, c) the Vu Gia basin and (b, d) the Thu Bon basin. The black line is the observed discharge from 1977 to 2020, the red
(horizontal) line is the threshold of the POT, and the point is the peak over the threshold.
Figure 7. GPD of the POT in the pre-2000 and post-2000 periods for the (a) Vu Gia basin and (b) Thu Bon basin.
8B. Q. NGUYEN ET AL.
The flow alteration in the dry and rainy seasons led to
changes in the average flow in both basins. For the magnitude
of the flow metrics, the average flow discharge in the Vu Gia
(mean, Q40, Q60) was positively impacted from 2001 to 2011
and negatively impacted with extreme grades from 2012 to
2020 (Fig. 4(a)). The average flow discharge in the Thu Bon
was mostly positively impacted (Fig. 4(b)). In the post-2000
period, the average annual flows in the Vu Gia and Thu Bon
increased by 12% (from 80 to 90 m
3
/s) and 17% (from 178 to
208 m
3
/s), respectively (Table 2).
1975 1980 1985 1990 1995 2000 2005 2010 2015 2020
0
100
200
300
400
1975 1980 1985 1990 1995 2000 2005 2010 2015 2020
0
200
400
600
800
(b) Thu Bon basin
Discharge (m
3
/s)
m(egrahcsiD
3
/s)
Year
Pre - 2000
P = 0.01224, S = 3.265
Post - 2000
P = 0.2058, S = - 4.18082
(a) Vu Gia basin
0
5000
10000
15000
Rainfall (mm)
Year
Pre - 2000
P = 0.02727, S = 5.979
Post - 2000
P = 0.6732, S = 2.054746
0
5000
10000
15000
20000
Rainfall (mm)
Figure 8. Long-term annual discharge and rainfall in the (a) Vu Gia basin and (b) Thu Bon basin. The black and red (shaded) lines are the discharge, and the blue bars
are the rainfall.
Figure 9. Temporal and spatial variations in the rainfall in the VGTB basin. (a) Mean annual rainfall in the pre-2000 period, (b) mean annual rainfall in the post-2000
period, and (c) mean annual rainfall changes in the post-2000 period relative to the pre-2000 period. (d–g) Long-term monthly means of the dry season, rainy season,
annual rainfall, and one-day maximum values from 1979 to 2020.
HYDROLOGICAL SCIENCES JOURNAL 9
3.4 Temporal and spatial variability in the relationships
between rainfall and discharge
The impact of climate variability and the generated upstream
inflow and operation of dam reservoirs are causing changes in
time and space. Therefore, we wanted to investigate the climate
change/variability drivers of flow alterations. Figure 9(a) and (b)
show spatiotemporal variations in the rainfall in the VGTB basin.
The most considerable changes in rainfall in the pre-2000 and
post-2000 periods occurred mainly in the mountainous areas;
smaller changes in rainfall occurred in the plains (Fig. 9(c)). The
Mann-Kendall test of the rainfall showed no statistically signifi-
cant trends in the dry season, rainy season, annual rainfall, and
one-day maximum values in the VGTB basin. However, a slight
increase was estimated from 1979 to 2020 (Fig. 9(d–g)). A com-
parison of the rainfall in the post-2000 period with that in the pre-
2000 period shows that the rainfall increased slightly in the rainy
season and annually and rose sharply in the dry season, by 4.94%,
11.65%, and 26.84%, respectively (Table 3).
In this paper, we mainly compare the pre-2000 and post-2000
periods. Therefore, we discussed these two periods first; then, we
elaborated by further discussing post-2010 (2011–2020) to pro-
vide more evidence of dam impacts. Figure 10(b) shows that the
correlations between the cumulative runoff and cumulative
rainfall in the Thu Bon were linear in the three periods.
However, in the Vu Gia, the curves were linear from 1979 to
2010 and changed suddenly starting in 2011 (Fig. 10(a)). This
result shows that water transfer via the Dak Mi 4 plant reduced
the flow on the Vu Gia River. Water stress on the Vu Gia
resulted from the diversion of water at Dak Mi 4 to the Thu Bon.
The total rainfall in the dry season in the Vu Gia basin
during the post-2010 period was 1090 mm, 47.4% higher than
that in the pre-2000 period and 5.7% lower than that in the
post-2000 period (Fig. 11(a)). The dry flow was 36.1% and
46.8% lower than those in the pre-2000 and post-2000 periods,
respectively. The mean annual flow in the post-2010 period
was 82 m
3
/s, 33.9% and 40.4% lower than those in the pre-2000
and post-2000 periods, respectively (Fig. 11(a)). In the Thu
Bon basin, the total rainfall values in the dry seasons of the pre-
2000, post-2000, and post-2010 periods were approximately
1021, 1204, and 1193 mm, respectively. Once it began to
receive flow from the Vu Gia River, the flow of the Thu Bon
in the post-2010 period was higher than those in the pre-2000
and post-2000 periods by 59.5% and 42.7%, respectively (Fig.
11(b)). Currently, Da Nang city often lacks water for domestic
and agricultural production in the dry season. In addition,
saltwater intrusion is more severe. Da Nang city and Quang
Nam Province often place stress on water sources from the
VGTB basin. Da Nang city requires Quang Nam to compen-
sate for the water transferred by the Dak Mi 4.
3.5 The impact of hydropower and diversion on the
alteration ow regime
3.5.1 The impact of hydropower and diversion on flood
control
The most critical flood control issues for the downstream
part of the VGTB basin are the reduction in the peak flood
and the duration of the high-water level (Nguyen 2020).
Table 3. Changes in the dry season, rainy season, and annual rainfall values in the pre-2000 and post-2000 periods.
Time
Pre-2000 Post-2000
Rainfall (mm) Rainfall (mm) Change (%)
Dry season Mean 831 1054 + 26.84
One-day maximum 132 90 − 31.85
Rainy season Mean 1881 1974 + 4.94
One-day maximum 301 389 + 29.35
Annual 2712 3028 + 11.65
0 50000 100000 150000
0
50000
100000
150000
0 50000 100000 150000
0
100000
200000
300000
400000
Post-2000 (2001-2010)
y = 1.21x + 17610
R = 0.998
Pre-2000 (1979-2000)
y = 1.556x - 4842.3
R = 0.996
01(QevitalumuC m )
Cumulative P (mm)
(a) Vu Gia basin
Post-2010 (2011-2020)
y = 0.648x + 70856
R = 0.987
Post-2000 (2001-2010)
y = 2.566x - 2410.9
R = 0.999
Post-2010 (2011-2020)
y = 2.74x - 22278
R = 0.999
Cumulative P (mm)
(b) Thu Bon basin
Pre-2000 (1979-2000)
y = 2.54x - 3321.2
R = 0.999
Figure 10. Curves of the cumulative rainfall and discharge in the pre-2000 (1979–2000), post-2000 (2001–2010), and post-2010 (2011–2020) periods for the (a) Vu Gia
basin and (b) Thu Bon basin.
10 B. Q. NGUYEN ET AL.
The flood storage capacity influences the decrease in the
peak flood downstream. It is important to forecast the flow
to the reservoir to maintain a suitable storage capacity.
Currently, there are few raingauges in the basin, which
makes it challenging to accurately forecast the flow to the
reservoirs. The large flood of 2017 is a typical example; the
inaccurate flow forecast of the Dak Mi 4 reservoir did not
lead to a cut-off flood peak, which resulted in flooding
downstream (Fig. 12). The Song Tranh 2 reservoir had
better forecasts and operation.
1 2 3 4 5 6 7 8 9 10 11 12
0
100
200
300
400
1 2 3 4 5 6 7 8 9 10 11 12
0
200
400
600
800
1000
1 2 3 4 5 6 7 8 9 10 11 12
0
200
400
600
800
)mm
(
n
o
i
ta
t
i
picerP
Pre-2000 Post-2000 Post-2010
(a) Vu Gia basin
1 2 3 4 5 6 7 8 9 10 11 12
0
200
400
600
800
1000
(b) Thu Bon basin
m(egrahcsiD
3
/s)
Month
Pre-2000 Post-2000 Post-2010
Month
Figure 11. Monthly average rainfall and discharge of the Vu Gia and Thu Bon basins. The black line plots the pre-2000 (1979–2000) period, the blue line plots the post-
2000 (2001–2010) period, and the red line plots the post-2010 (2011–2020) period.
Figure 12. (a) River network and reservoirs upstream of the VGTB basin. (b) Inflow, outflow and diversion of the Dak Mi 4 reservoir. (c) Inflow and outflow of the Song
Tranh 2 reservoir in the 2017 flood.
HYDROLOGICAL SCIENCES JOURNAL 11
3.5.2 The impact of water diversion structures
According to the research results of Firoz et al. (2018), the
intensity and frequency of droughts in the entire VGTB
basin mainly depend on upstream hydropower operation
and water transfer from the Vu Gia basin to the Thu Bon
basin by the Dak Mi 4 plant. The average annual amount
of water transferred by the Dak Mi 4 plant is approxi-
mately 1.08 × 10
9
m
3
(average 34.17 m
3
/s, 26.7% of the
Vu Gia River’s flow). Moreover, with high energy demand
in the dry season, some of the water needed for the mini-
mum release of the Vu Gia River was used for power
generation and discharge to the Thu Bon River. Part of
the sediment along the flow is transferred to the Thu Bon
basin, which leads to an imbalance of the natural state in
the Vu Gia basin. This imbalance affects the sediment and
morphology downstream.
IHA was used to analyse the periods before and after the
building of the Dak Mi 4 reservoir at Thanh My: pre-2010
(1977–2010) and post-2010 (2011–2020). The flow regime
alterations were considerably reduced during the dry and
rainy seasons (Fig. 13). The minimum and low-flow release
decreased by 53.5–76.3%. The maximum discharges signif-
icantly decreased during the post-2010 period by 37.5%.
Similarly, the high-flow clearance decreased by 57.3–67.4%.
As a result, the date of the minimum discharge increased
from 180 to 193 days.
4 Discussion
The results of the analysis show that the flow regime changes in
the dry and rainy seasons in the two basins (Figs 4–8, Table 2).
The annual discharge in the Vu Gia basin decreased from 2001
to 2020 (especially from 2011 to 2020), although the correspond-
ing rainfall increased (Figs 9–11). Therefore, we argue that these
alterations in discharge are mainly due to reservoir operations
and water transfer. However, another factor is land use/cover
(LULC), which also has a significant impact on watershed
hydrology. Therefore, we investigated whether the impacts of
land-use changes act as drivers of flow alterations.
4.1 Drivers of ow alterations from land-use change
To understand the hydrological regime of the basin, it is
necessary to assess the relationship between the watershed
hydrological processes and LULC change (Meiyappan and
Jain 2012). LULC changes may significantly affect the
watershed hydrological regime and surface runoff of the
basin (Jia et al. 2007). To clarify the effects of LULC change
on the runoff in the VGTB basin, LULC maps from 2001, 2005,
2010, and 2020 were used to analyse the river section upstream
of the Thanh My and Nong Son stations. The analysis shows
that the land cover in the upstream area is dominated by
forests, followed by mixed agricultural land, built-up areas,
Figure 13. Changes in the extreme and monthly discharges before the Dak Mi 4 reservoir opened (pre-2010) and after the Dak Mi 4 reservoir opened (post-2010) at the
Thanh My station from the IHA results.
Table 4. Statistics of LULC change in the VGTB basin from 2001 to 2020, from upstream to Thanh My station in the Vu Gia basin and Nong Son station in the Thu Bon
basin.
No. Land-use types
Year 2001
Change in 2005 compared to
2001
Change in 2010 compared to
2001
Change in 2020 compared to
2001
Area (km
2
) Percentage (%) Area (km
2
) Percentage (%) Area (km
2
) Percentage (%) Area (km
2
) Percentage (%)
1 Forest 2710.98 51.89 −31.70 −1.17 100.69 3.71 59.16 2.18
2 Mixed agricultural land 2404.10 46.02 30.89 1.28 −112.92 −4.70 −78.44 −3.26
3 Built-up area 33.99 0.65 0.48 1.42 8.32 24.47 15.37 45.22
4 Water 74.91 1.43 0.32 0.43 3.91 5.22 3.91 5.22
12 B. Q. NGUYEN ET AL.
and water (Table 4). The forest area increased by 2.18% from
2001 to 2020. The other LULCs changed minimally over time.
This result is similar to the research of Nauditt et al. (2017).
Therefore, LULC changes cannot explain the changes in flow
in the VGTB basin, and thus the flow regime alterations are
likely related to the operation of the reservoirs.
4.2 Challenges of water resource management in the
downstream VGTB basin
The impacts of reservoir operation are particularly pro-
nounced for the Vu Gia basin. Therefore, the Vu Gia basin
is the most vulnerable. The Vu Gia basin mainly supplies
water for Da Nang city and sizeable agricultural irrigation
systems in Quang Nam Province (Fig. 1(a)). The flow and
water level downstream during the dry season depend on the
operation of the reservoirs. The flow from the reservoirs also
maintains water levels to supply water for domestic use and
agricultural production and to reduce salinity. The changes in
flow impact the cropping pattern downstream, which is
highly dependent on the water during the dry season. The
reduction in flow during the rainy season is expected to
reduce sediment and nutrient transport and possibly affect
aquatic habitats (Pitlick and Wilcock 2001). The change in
water quality due to sediment imbalance and the loss of
habitats have potentially created long-term impacts for com-
munities in the VGTB River basin.
Hydroelectric reservoirs upstream have retained significant
amounts of coarse sand, gravel, and suspended sediment
instead of transporting them downstream. This sediment
reduction may aggravate erosion downstream from the dam.
In addition, sand mining in the middle and downstream areas
has removed large deposits of sediments from the riverbed.
Finally, the changes in deposition have led to bed incision,
which then decreases the water level. Bed incision has affected
drinking water and agricultural production and increased salt-
water intrusion. These issues have been detected in the
Mekong and Red rivers (Kondolf et al. 2014, D. V. Vu et al.
2014, Nhan and Cao 2019, Van Binh et al. 2021). We anticipate
that similar consequences are highly likely to occur in the
VGTB basin. In recent years, saltwater intrusion hazard/risk
has increased in Vu Gia River and strongly impairs socio-
economic factors in Danang city, especially agricultural pro-
duction and drinking water supply (Viet 2014, Nga et al. 2020).
5 Conclusions and outlook
We evaluated the long-term discharge changes in the VGTB
river basin over 44 years (1977 to 2020) through a detailed
analysis of runoff and related factors such as rainfall, land use,
reservoir operation, and water diversion. We found that reser-
voir operation and water transfer by the Dak Mi 4 plant are the
main reasons for flow alterations in the Vu Gia and Thu Bon
rivers.
Based on the indicators analysed, the flow regime in the
post-2000 period changed compared to that in the pre-2000
period. The Vu Gia basin changed more than the Thu Bon
basin. Since 2011, reservoir operations have reduced the max-
imum and high-flow discharges downstream, exceeding the
climate change effect. However, in the dry season, due to the
impact of water transfer, the minimum, low-flow release
increased in the Thu Bon basin and decreased in the Vu Gia
basin. Reducing the flow downstream of the Vu Gia River
during the dry season leads to a decrease in the water level,
affecting the operations of pumping stations supplying domes-
tic and agricultural water in Da Nang city and parts of Quang
Nam Province. In addition, due to the decreased flow down-
stream, the salinity condition in Da Nang has become more
severe in recent years. Salinity penetrates farther inland and at
higher levels, seriously affecting the water supply.
Reservoirs have helped to regulate flow and reduce flooding
in downstream areas. However, there are still some floods with
low regulatory efficiency. The cause is indicated by the few
raingauging stations upstream. Therefore, maximizing the
positive efficiency of reservoirs and improving the flow fore-
casting of reservoirs by constructing more rainfall gauging
stations is necessary.
We also note that in the entire VGTB basin, there are only
two stations that measure upstream streamflow. The down-
stream tributaries include many hydropower plants. This leads
to difficulty in fully investigating the streamflow and impact of
reservoirs. Therefore, in our future work we will study the
entire basin using a hydrological model.
Funding
This work was funded by APN “Asia-Pacific Network for Global Change
Research” under project reference number CRRP2020-09MYKantoush
(Funder ID: https://doi.org/10.13039/100005536). Acknowledgment for
the Research Unit for Realization of Sustainable Society (RURSS) at Kyoto
University.
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