Fig.
1.
Map of (A)
the large dams (height > 15 m) in the Mekong River Basin and
the (B) existing, under construction and planned hydropower projects in the
Upper Mekong Basin (UMB). The hydrological stations for discharge analyses are
shown with blue triangles (A). (For interpretation of the references to colour
in this figure legend, the reader is referred to the web version of this
article.)
Highlights:
· *
The Upper Mekong Basin is undergoing
extensive hydropower development and its largest dams have recently become
operational. In this study we assess the discharge changes using observed river
discharge data and a distributed hydrological model over the period of
1960–2014. Our findings indicate that the hydropower operations have
considerably modified the river discharges since 2011 and the largest changes
were observed in 2014. According to observed and simulated discharges, the most
notable changes occurred in northern Thailand (Chiang Saen) in March-May 2014
when the discharge increased by 121–187% and in July-August 2014 when the
discharge decreased by 32–46% compared to average discharges. The respective
changes in Cambodia (Kratie) were 41–74% increase in March-May 2014 and 0–6%
decrease in July-August 2014 discharges.
·
* The key driver for the ecological
productivity of the Mekong River is the annual flood pulse resulting from the
seasonal monsoon climate (Holtgrieve
et al., 2013; Junk
et al., 1989; Lamberts,
2008 ;
MRC,
2010).
The Mekong’s flood pulse is characterised by distinct low flow season in
December-May and a high flow season in June-November (MRC,
2005).
The flood pulse sustains base functions of ecological productivity by
transporting vast amounts of sediments and nutrients and inundating extensive
floodplains, and also by providing a diversity of ecological habitats (Junk
et al., 1989 ; Junk
et al., 2006), such as the Tonle Sap Lake in
Cambodia (Arias
et al., 2014; Arias
et al., 2012 ; Holtgrieve
et al., 2013). However, the ongoing hydropower development is
likely to considerably affect the flow regimes and the annual flood pulse. The
amplitude of the annual flood pulse is expected to be reduced (Lauri
et al., 2012 ; Piman
et al., 2013) and thus affect the sediment and nutrient transport,
flood extent and related ecological habitats (Arias
et al., 2014; Arias
et al., 2012 ; Holtgrieve
et al., 2013). Moreover, the dams and reservoirs trap large
quantities of sediments and nutrients (Kondolf
et al., 2014; Kummu
et al., 2010 ; Maavara
et al., 2015), further decreasing the sediment load after observed
falling trend in it due to lower tropical-cyclone activity (Darby
et al., 2016). Moreover, dams are reported to
severely prevent fish migration in the Mekong (Baran
and Myschowoda, 2009 ; Ziv
et al., 2012).
· *
The largest hydropower projects in the
Mekong are built into the Upper Mekong Basin (UMB), where the river is also
known as the Lancang River. The UMB has currently seven completed large dams
and twenty more under construction or planned (Fig.
1
and Table
1)
(see also Hennig
et al., 2013). The hydropower development in the UMB
started by construction of Manwan dam, which became operational in 1993 and was
fully completed in 1995. The most recent development is the completion of
Nuozhadu dam in 2014, which is the largest hydropower project in the whole
Mekong Basin (Fig.
1
and Table
1).
The hydropower development in the UMB is driven by increasing energy demand in
the province of Yunnan and in the eastern parts of China and the need to reduce
emissions from energy production (Chang
et al., 2010; Chen
et al., 2010 ; Hennig
et al., 2013). The construction of large scale hydropower in the
UMB has raised considerable concerns regarding the impacts on the downstream
countries (e.g. Kattelus
et al., 2015 ; Kuenzer
et al., 2013).
·
* The future impacts of dams in the UMB on
dowstream river discharge are estimated at least in three studies (Hoanh
et
al., 2010; Lauri
et al.,
2012 ;
Räsänen
et
al., 2012). These studies estimate discharge changes caused by
Manwan, Dachaoshan, Gonguoqia, Jinghong, Xiaowan, Nuozhadu hydropower projects
(Fig.
1
and Table
1)
at Chiang Saen using model-based approaches. These studies estimate that at
Chiang Saen, the dry season discharge (Dec-May) is predicted to increase by
60–90% and the wet season discharge (Jun-Nov) to decrease by 17–22% (Hoanh
et
al., 2010; Lauri
et al.,
2012 ;
Räsänen
et
al., 2012). These changes are a result of storing of water into
reservoirs during the wet season and releases during the dry season. The
model-based estimates further show that the UMB hydropower operations are
subject to increase (decrease) the dry (wet) season discharge variability, and
that the discharge impacts are observable as far downstream as Kratie in
Cambodia (see location at Fig.
1)
(Räsänen
et al., 2012). Piman
et al. (2013) also provide estimates of hydrological
changes from basin-wide development scenarios, but they do not report changes
at Chiang Saen and specifically for hydropower development in the UMB. These
model simulations were performed with pre-2011 datasets and thus are not
validated against observed river discharge changes. In addition to above
analyses, the cumulative impacts of Manwan dam and climate have been studied (Zhao
et al., 2013). The Manwan dam is found to compensate
for climate variability.
·
*
The discharge data were obtained from
Mekong River Commission Secretariat’s quality assured database and the analysis
period was limited by the data availability from Chiang Saen station. We used
discharge data instead of water level data as the river geomorphology may have
changed in time and the rating curves have been updated in several occasions.
We did not have information of all rating curves and therefore we analysed the
data for possible inconsistencies between different periods with different
rating curves and did not find problems. We used the discharge data as such. We
chose these three hydrological stations for following reasons: (i) availability
of the data is continuous and long enough; (ii) Chiang Saen is the most upstream
station in the LMB and it captures well the hydropower operations along the
UMB; and (iii) Nakhon Phanom and Kratie show how the discharge anomalies
propagate along the Mekong River. Downstream of Kratie the discharge regime
becomes smoother and less variable as the discharge condition is dominated by
hydraulic features in addition to hydrologic condition.
· *
For estimating the impacts of hydropower
operations in the UMB on the river discharges at Nakhon Phanom and Kratie, the
impacts of the dams in the LMB needed to be excluded. This was done by
simulating the hydrology of the whole Mekong Basin and by estimating both
natural and regulated discharges at Nakhon Phanom and Kratie. We first
simulated the natural (non-regulated) discharge over the period 1998–2014
(scenario C1; see Table
2),
which represented situation without any hydropower operations in the whole
Mekong Basin. This was done with the distributed hydrological model VMod,
similarly as for the UMB and Chiang Saen for scenario B1. Then we included the
observed impacts of hydropower operations in the UMB to the simulation by using
the observed discharge at Chiang Saen (1998–2014, discharge scenario A1) as an
input to the model at that location, resulting to scenario C2. The hydrological
model simulation thus allowed us to estimate how the observed impacts of
hydropower operations at Chiang Saen propagate downstream to Nakhon Phanom and
Kratie, while simulating the natural, climate driven hydrology in the LMB.
·
The comparison of simulated natural
discharge (Scenario C1) with the simulated discharge with impacts of hydropower
operation in the UMB (C2) over the period of 1998–2014 reveals how the
discharge anomalies observed at Chiang Saen propagate downstream. During the
years of 2013 and 2014 the discharge changes observed at Chiang Saen caused
considerable discharge changes at Nakhon Phanom and Kratie (Figs. 4 and 5,
and Table
3).
The discharge impacts are stronger at upstream station Nakhon Phanom than at
Kratie.
· *
The general nature of changes in
simulated discharge were similar at Nakhon Phanom and Kratie as the observed
changes at Chiang Saen: dry season discharges increased and wet season
discharges decreased. The largest discharge changes were found at Nakhon Phanom
and Kratie in March-May 2014 when the simulations suggest 88–118% and 41–68%
increase in monthly average discharges, respectively, compared to simulated
natural non-regulated discharge (Table
3).
During the wet season, the largest changes were found at Nakhon Phanom and
Kratie in July-September 2014 when the monthly average discharges decreased
3–11% and 0–6%, respectively.
* We
identified eight record high and low discharge events in the observed discharge
at Chiang Saen during the period of 2010–2014, when compared to period of
1960–1990, i.e. before hydropower development in the UMB (Section 3.1). The model simulations allowed us to separate the
climate induced impacts from anthropogenic ones (Section 3.1). According to these simulations, the record events
were not caused by climate or weather events, but resulted from hydropower
operations in the UMB (Fig. 3). The record high discharges
occurred in March-April 2011, February-April 2013 and in January-April 2014,
while the record low discharges occurred in February-March 2010, June-July
2012, October 2013, July 2014 and in August 2014 (Fig. 2). The basin-wide simulations, in
turn, revealed that the identified record high and low discharges propagated
downstream the Mekong causing discharge anomalies as far as Kratie in Cambodia
(Section 3.3, Table 3). Thus, our results support the
literature (e.g. Räsänen et al., 2012) that the hydropower operations in
the UMB have caused considerable changes in the discharge regime of the Mekong
River.
*
In
addition, the results suggest that the hydropower operation in UMB are only
partially responsible for the observed discharge changes in Kratie. For
example, during the dry season of 2014 record high discharge was observed in
Kratie (Fig. 2C), but according to our simulations
the hydropower operations in UMB could explain only half of the observed
discharge increase (Table 3). The LMB has currently 45 large
operational dams (CGIAR WLE, 2015 ; MRC,
2015) (Fig. 1) and it is possible that they have
contributed to the increase in dry season discharge in Kratie in 2014.
· *
The impacts of hydropower operations in
the UMB on river discharges at Kratie are estimated earlier only by Räsänen
et al. (2012) and the current research is the first
one to estimate those on the base of observed discharge data. Our estimates
(scenario C) suggest smaller discharge change in February and larger discharge
changes in April-June than the estimation by Räsänen
et al. (2012) as shown in Fig. 6C.
Otherwise, the estimates suggest similar pattern of change. The differences
between our estimate and estimate in Räsänen
et al. (2012) are likely a result, at least
partially, of the comparison method. We compared our estimate of discharge
change during one year against the average discharge change of seven years of
simulated hydropower operations in Räsänen
et al. (2012). Altogether, the comparison at Chiang
Saen and Kratie shows that the observed river discharge impacts of hydropower
operations in the UMB were as expected, but they suggest that the discharge
impacts may vary from year to year, depending on variations in hydropower
operations and climate and weather conditions.
· *
Hydrological consequences: The
discovered discharge impacts of hydropower in the UMB reflect the hydropower
operations: the reservoirs store water during the wet season and release it
during the dry season thus increase dry season flows and reduce the wet season
flows. These changes resulted in dampening of the Mekong’s annual flood pulse,
particularly in the upper parts of the LMB. The decreased amplitude of the
flood pulse is expected to reduce the sediment and nutrient transport and
negatively affect the aquatic habitats that depend on large seasonal water
level fluctuation (see e.g. Lamberts,
2008).
The increased dry season flows are also expected to change the habitat
conditions in river channels through increased flow velocity and water depth
and decreased sunlight penetration (Bunn
and Arthington, 2002).
The results also show large and rapid
water level variations during the dry season, particularly after the completion
of the largest dams in the UMB (see e.g. Fig.
2).
This is expected to result in increased instability in the river geomorphology
(Brandt,
2000)
and in aquatic (Bunn
and Arthington, 2002) and riparian habitats (Nilsson
and Berggren, 2000).
The major concern is that the flood
pulse and discharge changes threaten the Mekong’s ecological productivity,
which is the basis for livelihood, income and food security for millions people
(Friend
et al., 2009 ; MRC,
2010). For example, the Mekong River is one of the world’s most productive
inland fisheries and fishing is one of the most important livelihoods in the
region. In food security context, fish and aquatic animals are in key role as
they provide 47–80% of consumed animal protein in the LMB countries (Hortle,
2007). In addition, the changes in annual flood pulse and short-term water
level variations affect the agricultural activities in flood plains and
riverbanks. The changes in annual flood pulse may also bring new opportunities
for the LMB countries. The increased dry season discharge increases the water
availability in the dry season, which may attract further hydropower
development, provide water for irrigation, and improve navigation possibilities
(ICEM,
2010).
However, the results also showed that the dry season discharge may vary
considerably due to hydropower operations, as predicted also by Räsänen
et al. (2012), and this may cause challenges for
downstream development. Altogether, the results reinforce the need for
developing upstream-downstream cooperation in terms of water resources management
and development.
·
* Future research directions: Three future
research directions are identified.
First,
there is a need to monitor and further estimate the future impacts of
hydropower development in the UMB. This analysis was based on time period (year
2014) when there were only seven operational hydropower projects in UMB and the
largest hydropower projects had only recently become fully operational (Table
1).
The total regulating capacity of the seven existing hydropower projects is over
23 km3 corresponding to 27% of the annual discharge at Chiang
Saen, 10% at Nakhon Phanom and 6% at Kratie. The future plans include 20 more
dams and they are likely to aggravate the already observed downstream impacts
as the regulating capacity in the UMB will increase in the future.
Secondly,
the variations in climate affect the hydropower operations and their cumulative
impacts are less studied. For example, during the dry season of 2010 the Mekong
River experienced exceptionally low water levels (Stone,
2010)
and our findings suggest that the low water levels, at least partially,
resulted from hydropower operations. Räsänen
et al. (2012) also suggested that climate variability
would affect hydropower operations and together they may increase variability
in river discharges, particularly the inter-annual variability of seasonal
discharges (see also Wang
et al., 2006). The climate change is also expected
to affect the hydrology in the Mekong River, but how it will affect the
discharge regimes is uncertain (Hoang
et al., 2016; Kingston
et al., 2011 ; Lauri
et al., 2012). The cumulative impacts of climate and hydropower operations
are likely to become more important in the future as the water resources are
developing in all countries of the Mekong River and the water is projected to
become an increasingly competed resource.
Thirdly,
the scientific empirical evidence on the effects of river flow alteration on
the aquatic ecology has remained limited in the Mekong River. Some research is
conducted, for example in the Tonle Sap lake-floodplain system (Arias
et al., 2013), but the empirical research in river
channels is scarce, not published or non-existing. The empirical research would
be highly important as it enables the development of case specific and
generalised assessment methodologies for assessing the impacts of future
hydropower development and thus provides guidance for developing hydropower
into more sustainable direction. For example, even small changes in hydropower
operations can be beneficial for aquatic ecosystems (Poff
and Schmidt, 2016).
* Our
findings indicate that since the year 2011, the hydropower operations in the
Upper Mekong Basin have resulted in considerable discharge changes throughout
the Mekong River, as far as in Cambodia. General changes were characterised by
increase in dry season discharges, and decrease in wet season discharges, and
large and rapid discharge fluctuations in the dry season. Largest changes were
observed in 2014 after the completion of Nuozhadu dam, the largest hydropower
project in the whole Mekong Basin. The earlier model-based predictions on the
discharge changes in the scientific literature are in line with the discharge
changes presented in this paper, although the observed changes in 2014 are
partly larger than the earlier predictions suggest. The findings further show
that the hydropower operations in the Upper Mekong Basin can explain only
partially observed river discharge changes in Cambodia (Kratie), which suggests
that river discharges are affected also by dam operations in the Lower Mekong
Basin. The impacts of hydropower development in the UMB on downstream discharge
are expected to vary from year to year depending on the water availability and
hydropower operations. Further, they are expected to increase in the future as
more dams are planned to be built.
*
The
observed river discharge changes may have major implications to downstream
countries and people. The major concern is that the flow changes affect
negatively the ecological productivity of the river system and thus affect the
water related livelihoods and economic activities in the downstream countries.
The upstream hydropower development may also increase downstream water
availability, which in turn can provide favourable opportunities for irrigation
and make the downstream hydropower development more attractive. However, the
utilisation of river flows in downstream countries may face challenges due to
unpredictable upstream flow regulations. The future challenge is how to
maintain the ecological functions of the river and share the benefits and losses
of hydropower development equitably. Addressing the negative impacts is urgent
and it requires strong transboundary cooperation.
* Acknowledgements:
We would like to thank the Mekong River Commission Secretariat for providing
the observed discharge data and the dam database for the Mekong River Basin. We
would also like to thank Kim Geheb at WLE Greater Mekong (CGIAR Research
Program on Water, Land and Ecosystems) for providing their dam database for the
Mekong River Basin. TAR received funding from Maa- ja
vesitekniikan tuki ry. and MK from Academy of
Finland project SCART (grant no. 267463) and Emil
Aaltonen Foundation project ‘eat-less-water’.
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