Highlights:
·
Human activities, such as dam
construction, significantly altered the flow regimes in the Mekong River,
particularly after the completion of two large dams, namely Xiaowan and
Nuozhadu in 2010 and 2014, respectively. Stream flow data from 1960 to 2014
obtained from five stations located along the Mekong mainstream are divided
into three periods, i.e., the pre-impact period (1960–1991), the transition
period (1992–2009), and the post-impact period (2010–2014).
·
The construction and operation of dams
clearly have significant impacts on low pulse duration. It is observed that
climate change dictated the changes in the annual stream flow during the
transition period 1992–2009 (82.28%), whereas human activities contributed more
in the post-impact period 2010–2014 (61.88%). The results of this study could
provide a reference for reservoir operation in the upstream regions considering
both ecological and economic benefits of such operations, as well as maximize
the interests of stakeholders in this region.
*
In
river ecosystems, the flow regime of runoff plays a significant role in many
fundamental ecological processes (Poff
and Zimmerman, 2010). Changes in flow regimes within the context of climate
change and human activities are significant to the hydrological community,
receiving considerable global attention. Climate change and human activities
have been considered as the two primary factors affecting flow regimes (Li et al.,
2006 ; Ma
et al., 2014). In some basins, human activities
are the main factors that alter flow regimes, particularly during the
construction and operation of large reservoirs (Poff
et al., 1997 ; Fan
et al., 2015). Climate change can also be the
dominant factor that alters flow regimes (Li et al., 2006), which can change the pattern of
precipitation and potential evaporation (Wang and Hejazi, 2011). Human activities, such as dam
construction and water withdrawal activities (i.e., irrigation, industry, and
municipal demands), directly change flow regimes (Ma et al., 2014), thereby changing river ecosystems
(Gippel,
2001; Poff et al., 1997; Richter et al., 2003 ; Richter et al., 2006). The impacts of the two factors are often analyzed
separately. However, the effects of climate change and human activities are
always combined, and their effects on some river basins are difficult to
identify. For example, climate change may cause changes in precipitation,
increasing the impacts caused by dams as more water would be regulated by
reservoirs during the long dry seasons (Lu et al., 2014).
·
The
Mekong River is the ninth largest river globally, and the Mekong River Basin
(MRB) harbors one of the most productive and diverse ecosystems in South Asia (Kuenzer
et al., 2013). Ecological issues in the MRB have been a major concern to
stakeholders, researchers, and other professionals globally. Over 70 dams spread
across six riparian countries make the management of this transboundary river
quite complicated and sensitive. Several studies focused on the hydrological
effects of hydropower dams on the Mekong River, particularly the dams in China.
·
Some
researchers have argued that construction of all the 78 tributary dams would
produce less energy and pose greater environmental risks as compared to having
six mainstream dams upstream of the Mekong River (Ziv
et al., 2012). Recently, Cochrane et al. (2014) analyzed the alteration in water
levels in six stream flow gauging stations along the mainstream of the Mekong
River from 1960 to 2010 and discussed the relationship between annual
fluctuations and active reservoir storage. However, the two largest reservoirs
built after 2010, Xiaowan and Nuozhadu, were not considered in the aforementioned
study; hence, the impacts of these reservoirs remain largely unknown
Fig.
1
Location
of Mekong River Basin and the 6 completed dams and 19 planned dams
along the
mainstream.
*
Downstream
of the LMB, several important ecological sites closely related to the flow
regimes of the river exist; among them, Lake Tonle Sap and the Mekong Delta are
the most famous. Tonle Sap, which is the largest lake in Southeast Asia
(covering an area of 8800 km2 on average) and connected with
the mainstream of the Mekong River at Phnom Penh, Cambodia, is regarded as the
“natural reservoir” of the MRB. The water flowing into Tonle Sap from the
Mekong River during the wet season (June to October) is nearly six times than
that during the dry season (November to May) (Cochrane et al., 2014). Tonle Sap is rich in aquatic
products and contains more than 300 species of freshwater fish. The downstream
area of Phnom Penh is called the Mekong Delta, the area of which is ∼55,000 km2
(4000 km2 in Vietnam; 11,000 km2 in Cambodia).
The Mekong Delta is subjected to frequent drought and flood events and
influenced by tidal and seawater intrusion, particularly during dry seasons (Zhang et al., 2001).
*
No
dam was constructed on the mainstream of the Mekong River until 1992. China has
built six hydropower dams on the mainstream of the Lancang River (i.e., the
UMB) since 1992 (as shown in Table 2). The two largest dams are Xiaowan
(14.56 km3 in total storage, completed in 2010) and Nuozhadu
(22.4 km3 in total storage, completed in 2014), contributing to
36% and 55% of the total storage capacity of all the existing reservoirs in the
basin, respectively. Although the streamflow regime has been altered because of
large-scale dams, the hydropower generation of the Mekong River has kept
increasing in recent years (MRC, 2010). In the subsequent decades, 19 dams
were planned to be built on the mainstream of the Mekong River (8 in China, 10
in Laos, and 1 in Cambodia), and over 14 large dams (>10 MW) would be
built in the tributaries. The primary objective of these dams is hydropower
generation, which is expected to yield considerable economic benefits for the
riparian countries. Moreover, the construction and operation of dams would
improve navigation conditions, increase irrigation opportunities, and enhance
flood control capacity (Lu et al., 2014). However, the construction and
operation of large dams would also affect the patterns of streamflow, resulting
in multiple changes in streamflow regimes, thereby causing a negative impact on
ecosystems. The daily flow-time series data obtained from the Mekong River
Commission (http://portal.mrcmekong.org/index) are used to explore these
comprehensive changes in the streamflow regimes in this study.
*
Our
large dams were constructed and completed after the year 2009, namely the
Jinghong, Xiaowan, Gongguoqiao, and Nuozhadu (largest) Dams, which were
completed in the years 2009, 2010, 2011, and 2014, respectively, with active
storage capacities of 1.23 km3, 14.56 km3,
3.16 km3, and 22.4 km3, respectively. The
completion of these dams significantly increased the RC values of Chiang Saen
from 30 mm to 230 mm, whereas the initial filling of these dams
reduced the annual streamflow in Chiang Saen located near the downstream region
(Fig. 5a). However, gauging stations located
farther downstream show a different trend because their distances from these
dams are considerable (Fig. 5). The impacts of the dams gradually
decreased, reflected by the fact that no clear trend is observed in Stung Treng
station. This is because only approximately 16% of the streamflow in this
station comes from the upstream dammed area in China, and the downstream
hydrologic processes dominated the flow regime. Given that the effects of the
reservoirs decrease from the upstream region to the downstream region, the
focus of this study is to analyze the most influenced upstream station (Chiang
Saen) and the least influenced downstream station (Stung Treng).
·
In
addition to reservoir construction and operation, other factors may also affect
the flow regimes (Cook et al., 2012)
*
The
flow regimes have changed to some degree after the construction and operation
of dams on the mainstream, particularly in the upstream, observed by several
gauging stations during the transition period (1992–2009) and post-impact
period (2010–2014). The mean flow and minimum flow increased in the dry
seasons, while the mean flow and maximum flow decreased in the wet seasons. The
low pulse duration significantly declined, and the number of fluctuations
increased. The changes in the post-impact period are more severe than in the
transition period.
·
The
most important negative impact of these changes is the destruction of fish
habitats (Ziv
et al., 2012). Similar to many tropical rivers globally, fish ecosystems in
the MRB are extremely fragile. Many important and unique fishes in the MRB are
seasonally migratory fishes, and natural flow regimes and upstream shoals and
rapids are very important for their breeding. These changes in flow regime also
influence other river in ecosystems.
*
However,
increased flow in the dry seasons can significantly benefit agricultural
irrigation in the downstream areas. Water resources in the upper reach of the
Mekong River are mostly used for power generation without water withdrawal. The
five countries along the LMB are agricultural countries; the Chi-Mun basin has
the largest irrigation system in the MRB. With growing population, water demand
for food and irrigation markedly increased. Barker and Molle (2004) estimated that only 3% of the
irrigated land in the basin was exploited in 2002. MRC (2011) reported that irrigated areas in the
LMB countries would increase from 6.6 million ha in 2010 to 9.7 million ha in
2030.
*
Moreover,
reduction in wet-season flow, but increase in dry-season flow due to the
operation of the upstream dams are beneficial for downstream flood and drought
management, navigation, and even the ecosystem in the Mekong Delta. For
instance, on March 15 and April 10 of 2016, because of the extreme drought
event in the Mekong Delta in Vietnam, the Chinese government released more than
2.7 billion m3 of stored water from the upstream Jinghong
Reservoir to the Mekong Delta for agriculture and ecosystem demands. The
discharge from the Jinghong Reservoir increased to 2000 m3/s,
approximately twice the mean annual discharge or more than three times the
natural discharge for the same period. This joint effort was a successful event
for transboundary river management (MRC, 2016).
*
Both
ecosystem and human demands in the MRB should be considered. Given that a large
number of dams in the upper and lower reaches of the Mekong River are to be
constructed in the near future, the impacts of human activities on streamflow
would likely intensify, posing great challenges for river management. Moreover,
given the uncertainty of climate change, collaborative and multi-objective
management of these reservoirs are the keys to address both ecosystem and human
demands; hence, environmental flow for ecosystem demands should be incorporated
into reservoir operation objectives in the MRB.
*
This
study used long-term daily streamflow observations in five gauging stations
along the mainstream of the Mekong River to examine changes and trends in the
IHA and eco-flow metrics (eco-surplus and eco-deficit). The results show that
damming in the upstream area led to a declining trend in the annual streamflow
in the upstream Chiang Saen gauging station, whereas no clear effect on the
downstream Stung Treng station was observed. The operation of the dams reduces
streamflow in the wet seasons, but increases the streamflow in the dry seasons,
resulting in unique seasonal variations in the streamflow based on the eco-flow
metrics at Chiang Saen during the period 2010–2014.
*
During
the transition period 1992–2009, climate change contributed to 82.29% of the
total streamflow change, whereas human activities contributed to 62% of the
total change during the post-impact period 2010–2015. It is shown that only 0.57%
of the land use in the MRB changed during 2000–2010, implying that the land-use
change was not a significant factor among human activities.
*
Both
positive and negative impacts of changes in the flow regime on the management
of water resources and river ecosystems are discussed. On one hand, changes in
the flow regime may influence fish habitats; on the other hand, reservoirs
highly benefit hydropower generation, irrigation, and other human needs. The
results of this study can serve as a basis for detecting changes in the flow
regime, which are valuable in transboundary cooperation and reservoir
operation, particularly for better consideration of environmental flow to
address both ecosystem and human needs in the future.
·
Characteristics of hydropower
reservoirs in Upper Mekong Basin.
Dams
|
Year Completed
|
Total storage
|
Hydropower capacity
|
Dam height
|
Dam type
|
(km3)
|
(MW)
|
||||
Manwan
|
1992
|
0.92
|
1500
|
126
|
Gravity dam
|
Dachaoshan
|
2003
|
0.93
|
1350
|
110
|
Gravity dam
|
Jinghong
|
2009
|
1.23
|
1500
|
118
|
Gravity dam
|
Xiaowan
|
2010
|
14.56
|
4200
|
300
|
Arch dam
|
Gongguoqiao
|
2011
|
3.16
|
900
|
105
|
Gravity dam
|
Nuozhadu
|
2014
|
22.4
|
5500
|
254
|
Gravity dam
|
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