Elsevier

Journal of Hydrology

Volume 545, February 2017, Pages 28-41
Journal of Hydrology

Research papers
Observed river discharge changes due to hydropower operations in the Upper Mekong Basin

https://doi.org/10.1016/j.jhydrol.2016.12.023Get rights and content

Highlights

  • Large hydropower dams have become operational in the Upper Mekong Basin.

  • The downstream impacts were estimated using recent observed discharge data.

  • Hydropower operations increased (decreased) dry (wet) season discharges.

  • The discharge impacts were observable as far downstream as in Cambodia.

  • Discharge changes were similar or larger than existing model predictions suggest.

Abstract

The Upper Mekong Basin is undergoing extensive hydropower development and its largest dams have recently become operational. Hydropower is built to improve the regional energy supply, but at the same time, it has considerable transboundary impacts on downstream discharge regime and further on aquatic ecosystems, riparian livelihoods and food security. Despite the transboundary significance of the impacts, there is no public information on the hydropower operations or on the already observed downstream discharge impacts since the completion of the largest dams. Therefore, 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 earlier model-based predictions of the discharge changes are well in line with the observed changes, although observed changes are partly larger. The discharge impacts are expected to vary from year to year depending on hydropower operations. Altogether, the results highlight the need for strong transboundary cooperation for managing the downstream impacts.

Introduction

The increasing energy demand and the pursuit for economic growth in the Mekong Region (Fig. 1) has led to extensive hydropower development in the Mekong River and its tributaries (Grumbine et al. 2012). Currently there are over 58 large dams (height > 15 m) in the basin and there are plans to build 101 more (CGIAR WLE, 2015, MRC, 2015). While this development improves the regional energy supply, it has considerable negative effects, particularly on the ecological productivity of the Mekong (see e.g. Arias et al., 2014, Baran and Myschowoda, 2009, Halls and Kshatriya, 2009, Ziv et al., 2012). The ecological productivity of the Mekong is the basis for livelihoods and food security of a large proportion of over 70 million people in the basin (MRC, 2010, Friend et al., 2009).

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).

A major limitation has been the lack of public information on the hydropower operations in the UMB including the expected downstream impacts and the planned operations such as filling of the reservoirs and water releases. This has motivated several studies, including ours, to assess the observed downstream river discharge impacts. The river discharge impacts are estimated at least in four studies (Cochrane et al., 2014, Daming et al., 2006, Lu et al., 2014, Lu and Siew, 2006). Daming et al. (2006) analyse discharge changes during the construction phases and report changes in daily discharge stages that are limited to river section upstream Vientiane. The three later studies analyse river discharge changes at Chiang Saen in Northern Thailand (see location in Fig. 1) by comparing observed water level and discharge data from pre- and post dam construction periods. They report in post-dam period of 1992–2010 a reduction in minimum discharges (1-, 3-, and 7-day minimum) during the dry and wet seasons (Lu et al., 2014), increase in 30-day minimum water levels (+17%), increase in water level fall rates of consecutive days (+42%) and increase in the water level fluctuations (+75%), compared to pre-dam period of 1960–1992 period (Cochrane et al., 2014, Lu and Siew, 2006). In addition, increase in the monthly discharges of July (+15%) and decrease in monthly discharges of August (−9%) in the post-dam period compared to the pre-dam period is reported (Lu et al., 2014). These observation-based studies focus on pre-2011 period and do not therefore include the effects of the largest dams in the UMB, Xiaowan (completed in 2012) and Nuozhadu (2014). Moreover, these studies do not assess the impacts of the hydropower operations in the UMB on river discharges further downstream the Mekong River. A review of broader already occurred environmental impacts of hydropower development in the UMB is given by Fan et al. (2015).

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 aim of this paper is to provide the first analysis of the observed river discharge impacts of dams in the UMB since the completion of the Xiaowan and Nuozhadu hydropower projects. Our work extends the current knowledge by comparing the estimated river discharge changes due to hydropower operations to observed ones. We achieved our aim by analysing daily observed river discharge data over the period of 1960–2014 and by simulating river discharges with a distributed hydrological model. The analyses of this paper together with the past research provide a strong quantified evidence of the impacts of dams in the UMB on downstream discharge regime. The understanding of these transboundary river discharge impacts is highly important due to their significance for the aquatic resources of the downstream countries in the Lower Mekong Basin (LMB).

Section snippets

Methods

Given that there is no publicly available data on the reservoir operations and water releases from the hydropower reservoirs in the UMB, the impacts of hydropower dams on downstream river discharges had to be estimated using observed daily river discharge data downstream the reservoirs and a distributed hydrological model. First, we analysed recent observed discharge changes at Chiang Saen, Nakhon Phanom and Kratie (see locations in Fig. 1) by comparing observed daily discharge data from the

Observed river discharge changes at Chiang Saen, Nakhon Phanom and Kratie

The analysis of observed daily river discharges at Chiang Saen, Nakhon Phanom and Kratie (Scenario A2) shows considerable discharge changes during the years of 2010–2014 compared to historical discharges of 1960–1990 (A1), as shown in Fig. 2. These discharge anomalies are analysed in the following with focus on new record high and low discharge events.

2010: In February-March record low discharges were observed at Chiang Saen (Fig. 2A) and low discharges at Nakhon Phanom (Fig. 2B) and Kratie (Fig. 2

Discussion

In this article we provide detailed results of the downstream discharge anomalies since the closure of the largest hydropower dams in the Mekong River, located in Yunnan, China. By combining long-term observed discharge data with hydrological model simulations, we were able to distinguish the likely impacts of hydropower operations on discharge.

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

Conclusions

In this paper our aim was to estimate the downstream river discharge impacts caused by hydropower operations in the Upper Mekong Basin by analysing observed river discharges over the period of 1960–2014 and by simulating river discharges with a distributed hydrological model. The research was motivated by the recent completion of the largest hydropower projects in the Upper Mekong Basin, and by the lack of publicly available data on the hydropower and reservoir operations and their observed

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

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