Hydropower Dams of the Mekong River Basin a Review of Their

Journal of Hydrology 568 (2019) 285–300 Contents lists available at ScienceDirect Journal of Hydrology journal homepage: www.elsevier.com/locate/jhydrol Review papers Hydropower dams of the Mekong River basin: A review of their hydrological impacts T ⁎ Jory S. Hechta,b, , Guillaume Lacombec, Mauricio E. Ariasd, Thanh Duc Dange,f,g, Thanapon Pimanh a Department of Civil and Environmental Engineering, Tufts University, Medford, MA, USA b Vermont EPSCoR, University of Vermont, Burlington, VT, USA c International Water Management Institute, Vientiane, Lao Democratic People’s Republic d Department of Civil and Environmental Engineering, University of South Florida, Tampa, FL, USA e Institute for Water and Environment Research, Thuy Loi University, Ho Chi Minh City, Viet Nam f Department of Civil and Natural Resources Engineering, University of Canterbury, Christchurch, New Zealand g Engineering Systems and Design Pillar, Singapore University of Technology and Design, Tampines, Singapore h Stockholm Environment Institute, Bangkok, Thailand ARTICLE INFO ABSTRACT This manuscript was handled by Marco Borga, Hydropower production is altering the Mekong River basin’s riverine ecosystems, which contain the world’s Editor-in-Chief, with the assistance of Baptiste largest inland fishery and provide food security and livelihoods to millions of people. The basin’s hydropower François, Associate Editor reservoir storage, which may rise from ∼2% of its mean annual flow in 2008 to ∼20% in 2025, is attenuating Keywords: seasonal flow variability downstream of many dams with integral powerhouses and large storage reservoirs. In Dams addition, tributary diversions for off-stream energy production are reducing downstream flows and augmenting Hydrological alteration them in recipient tributaries. To help manage tradeoffs between dam benefits (hydropower, irrigation, flood Hydropower control, domestic water supply, and navigation) and their consequences for livelihoods and ecosystems, we Mekong review observed and projected impacts on river flows along both the Mekong mainstream and its tributaries. We Reservoirs include the effects of diversions and inter-basin transfers, which prior reviews of flow alteration in the Mekong River basin management basin have largely neglected. We also discuss the extent to which concurrent changes in climate, water demand, and land use, may offset or exacerbate hydropower-induced flow alteration. Our major recommendations for assessing hydrological impacts in the Mekong and other basins undergoing rapid hydropower development include synchronizing and integrating observational and modeling studies, improving the accuracy of reservoir water balances, evaluating multi-objective reservoir operating rules, examining hydropeaking-induced flow alteration, conducting multi-dam safety assessments, evaluating flow indicators relevant to local ecosystems and livelihoods, and considering alternative energy sources and reservoir sedimentation in long-term projections. Finally, we strongly recommend that dam impact studies consider hydrological alteration in conjunction with fish passage barriers, geomorphic changes and other contemporaneous stressors. 1. Introduction most productive inland fishery (Baran and Myschowoda, 2009; Ziv et al., 2012), seasonally variable flows have sustained livelihoods, food In many river basins with emerging economies, rising energy de- security and ecosystem services for millennia (e.g. Fox and Wood, 2007; mands and campaigns to reduce fossil-fuel dependence have spurred Grumbine and Xu, 2011). Tradeoffs between dam benefits (e.g. hy- the rapid expansion of hydropower (e.g. Zarfl et al., 2015; Zhang et al., dropower production, flood control, irrigation, domestic water supply, 2017). Hydropower production, which could increase by over 70% in navigation) and their undesirable societal and ecological impacts (e.g. developing countries in the next few decades (Zarfl et al., 2015; IEA, community resettlement in low-fertility agricultural lands, declines in 2016), is threatening ecosystems in basins with some of the greatest fisheries, floodplain recession agriculture, and sediment and nutrient aquatic biodiversity, including the Amazon, Congo and Mekong transport, safety hazards posed by rapidly changing flows) are quite (Winemiller et al., 2016). In the Mekong basin, which has the world’s contentious and uncertain (e.g. ICEM, 2010; Intralawan et al., 2018). ⁎ Corresponding author at: Vermont EPSCoR, University of Vermont, Burlington, VT, USA. E-mail address: [email protected] (J.S. Hecht). https://doi.org/10.1016/j.jhydrol.2018.10.045 Received 17 May 2018; Received in revised form 25 August 2018; Accepted 18 October 2018 Available online 22 October 2018 0022-1694/ © 2018 Elsevier B.V. All rights reserved. J.S. Hecht et al. Journal of Hydrology 568 (2019) 285–300 For instance, estimates of hydropower benefits range from $6-32 billion whereas estimates of its potential damages to capture fisheries range from $2-13 billion (MRC, 2011; Intralawan et al., 2018). Improving these multi-sectoral and transboundary tradeoffs requires accurate characterizations of observed and projected hydrological changes. Indeed, hydrological changes in the Lower Mekong floodplain have been highly scrutinized due to its dense population and dependence on fisheries and flood recession agriculture, especially rice (e.g. Intralawan et al., 2018; Dang et al., 2018). Recent studies have synthesized re- gional hydrological alterations in the basin and their socioeconomic and ecological implications (Pokhrel et al., 2018). Lu et al. (2008) and Li et al. (2017a) have included reviews of prior studies on dam-induced hydrological alteration, while Fan et al. (2015) and Yu and Geheb (2017) have recently summarized observed hydrological impacts of Chinese hydropower dams in the Upper Mekong basin. Numerous recent studies (Grumbine et al., 2012; Johnston and Kummu, 2012; Pokhrel et al., 2018) have also advocated integrated modeling ap- proaches for assessing socioeconomic and ecological impacts of hydrological alteration. However, given the potential ecological and livelihood impacts of Mekong hydropower development, a more detailed review of hydrological alteration in different regions of the basin is warranted. Moreover, managing impacts from diversions and inter-basin transfers in tributary basins often requires mitigation strategies different from those suited for dams with large reservoirs that attenuate seasonal flow variability. Thus, this paper aims to synthesize and critically review existing knowledge regarding observed and projected hydrological alterations caused by hydropower dams on both the Mekong mainstream and tributaries. Section 2 provides an overview of the basin’s geo- graphy, outlines historical and projected trajectories of dam development, highlights some key impacts of hydrological alteration to ecosystems and livelihoods and briefly describes basin management efforts and challenges. Sections 3 and 4 detail the extent to which observed hydrological impacts and model-based projections have been assessed throughout the basin, respectively. Section 5 assesses the extent to which concurrent drivers of hydrological change may offset or exacerbate hydropower impacts. Section 6 summarizes recent advances in understanding the hydrological impacts of Mekong hydropower dams fi and identi es research gaps that could be addressed to further guide Fig. 1. The Mekong River basin: hydropower dams (MRC, 2015; WLE-Mekong, hydropower development in the Mekong and other basins worldwide. 2017) and mainstream hydrological stations analyzed in this study. 2. Background (2018a,b) for more detailed basin descriptions. 2.1. Basin overview 2.2. Hydropower dam development The Mekong is one of the world’s most prominent rivers. Its mean annual discharge of 14,500 m3/s (Wang et al., 2017) and length of While hydropower ambitions have existed for over half a century 4909 km (Liu et al., 2009) both rank tenth globally, while its drainage (Jacobs, 2002), most hydropower dams have been constructed in the area (795,000 km2) is the 25th largest (MRC, 2005). Its population was last decade (Fig. 2). In 2008, the Mekong was one of the least regulated approximately 70–75 million in 2005 (Ringler and Cai, 2006; Varis large river basins in the world, as its total active reservoir storage ca- et al., 2012) and could increase to 100–145 million by 2050 (Pech and pacity (8.6 km3) amounted to just 2% of its mean annual discharge Sunada, 2008; Varis et al., 2012). The Lower Mekong basin (LMB) (Kummu et al., 2010). Data from MRC (2015) suggests that the basin’s (Fig. 1) lies in the Southeast Asian countries of Lao PDR (25% of the active reservoir storage in 2025 (86.8 km3) is expected to be equal to basin area), Thailand (23%), Cambodia (20%), Vietnam (8%), and 19% of its mean annual discharge. This estimate falls within the Myanmar (3%). Meanwhile, the upstream portion of the basin (21%) in 17–23% range of earlier projections (Hoanh et al., 2010; Kummu et al., China is often known as the Upper Mekong Basin (UMB) or Lancang 2010, MRC, 2011). Note that the MRC databases used in these estimates Basin (MRC, 2005). have not contained numerous existing and planned dams in China, The LMB has a monsoonal climate with distinct wet and dry seasons. which according to the Greater Mekong Dam Database (GMDD) The wet season analyzed in most studies runs from June to November (https://wle-mekong.cgiar.org/maps/) from WLE-Mekong (2017), will while the dry season often lasts from December to

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