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 regional 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 May. Typically, 75% have over 8.7 km3 of combined active and dead storage by 2025. of the river’s annual discharge of 460 km3 (MRC, 2005) passes through Recent dam status updates in the GMDD indicate that 64 of the 187 the delta between July and October, a period with extensive ﬂooding existing and proposed hydropower dams with an installed capacity of at that sustains many ecosystems and livelihoods (e.g. Kummu and least 15 MW in the entire Mekong basin had been commissioned by Sarkkula, 2008, Piman et al., 2013a). Meanwhile, 35% of its dry-season June 2017. Out of these 64 dams, 18 dams in China can generate up to ﬂow originates from China, making impacts of large UMB dams espe- 17,770 MW, over 91% of which is installed at the six large Lancang cially critical. See MRC (2005), Adamson et al. (2009), and MRC Cascade dams on the UMB mainstream. The other 46 LMB tributary

286 J.S. Hecht et al. Journal of Hydrology 568 (2019) 285–300

) 100 160 3

90 140 80 120 70 Nuozhadu 60 100 50 80 ĐƟǀĞ storagĞĐapacityofrĞsĞrǀoirs 40 60 Number of dams 30 Xiaowan 40 20 NumbĞrofdams 20

ve storage capacity of reservoirs (km of reservoirs capacity storage ve 10

ĐƟ 0 0

Fig. 2. Rapid increases in the number of dams and active reservoir storage capacity from 1960 to 2025 (MRC, 2015). Points: Number of dams. Shaded area: active storage capacity. (Dark grey: existing dams in 2017. Light grey: planned dams.) dams can produce up to 8650 MW. The combined installed capacity of (e.g. Poff et al., 2007) has also been observed in the Mekong basin all existing and proposed dams (> 15 MW) in the GMDD database ex- (Chea et al., 2016b). The Mekong River is also well known for its ceeds 65,000 MW, while other estimates range from nearly 59,000 MW abundance and diversity of migratory species, many of which travel (Dore et al., 2007) to 88,000 MW (Stone, 2011). These estimates in- hundreds of kilometers along free-flowing reaches to spawn (e.g. Hogan clude a controversial cascade of 11 proposed LMB mainstream dams et al., 2007). Some of these species are becoming endangered as the with a total installed capacity of 13,000 MW (WLE-Mekong, 2017). percentage of free-flowing reaches in the basin decreases. As in other Greater seasonal flow regulation from upstream hydropower reservoirs parts of the world (e.g. Anderson et al., 2018), Mekong dams have increases the feasibility of hydropower at downstream locations. substantially reduced river network connectivity (Ziv et al., 2012; Ou Moreover, if increased annual and dry-season flows projected under and Winemiller, 2016) and changed downstream flow variability vital many climate change scenarios (e.g. Hoang et al., 2016) exceed climate- to many fish species (Baran et al., 2011; Li et al., 2013). These physical driven increases in reservoir evaporation, both total and firm energy changes, in addition to changes in sediment and nutrient transport (e.g. production could increase. Such potential basin-wide changes in the Chea et al., 2016a; Kondolf et al., 2018) and overfishing pressure (e.g. hydropower generation capacity of the basin have not been modeled. Ngor et al., 2018a), are compromising fish biodiversity and production in the Mekong floodplains. Between 1990 and 2010, river connectivity declined from 93% to 77% and could drop as low as 10% (Grill et al., 2.3. Ecological implications of hydrological alteration 2014). Losses in connectivity have even caused run-of-river dams to disrupt fisheries in the Mekong basin, such as the Pak Mun Dam on the Dam-induced changes in streamflow affect many aspects of riverine Mun River in Thailand (e.g. Molle et al., 2010). While this study focuses ecosystems, including hydraulics (depth, velocity), temperature, along on flow alteration, further research is needed to untangle the effects of with nutrient and sediment transport (e.g. Power et al., 1995). Many passage barriers and flow alteration on Mekong fisheries and ecosys- studies around the world have shown that flow alteration often impacts tems. riverine species and ecosystems negatively (e.g. Poff and Zimmerman, While many ecological studies focused on flow-alteration impacts 2010). Ecological impacts of dam-induced flow alteration have been on specific species (e.g. Hogan et al., 2007; Sabo et al., 2017), some debated in the Mekong basin, including the loss of flood pulses, studies have assessed a wider range of riverine ecological communities floodplain habitat and wetlands (Ringler et al., 2004; Stone, 2010a; in the UMB (Kang et al., 2009; Li et al., 2013; Fan et al., 2015; Zhang MRC, 2017; Intralawan et al., 2018), as LMB residents depend on fish et al., 2018), and LMB tributaries (Ou and Winemiller, 2016; Ngor and other aquatic animals for 47–80% of their required protein intake, et al., 2018a,b). Although pronounced flow alteration has been asso- more than any other major basin in the world (Hortle, 2007). In par- ciated with severe ecological impacts (e.g. Baran et al., 2011), the op- ticular, the Tonle Sap Lake fishery enables Cambodia to have one of the timality of natural flow for some species, i.e. the natural flow paradigm world’s highest freshwater fish catch rates per capita (McIntyre et al., (Poff et al., 1997), has also been questioned. Most notably, Sabo et al. 2016). (2017) proposed that, if hydropower dams were collectively operated to Spatiotemporal variations in river and floodplain hydrology influ- generate year-round, dry-season flows with punctuated flood pulses, dai ence LMB fisheries. Several components of the flood pulse have been (bag-net) fisheries in the Tonle Sap River could maintain or even in- hypothesized to affect fish catches, including flood magnitude, dura- crease historical yields. As recently clarified (Halls and Moyle, 2018; tion, timing, and variability (Sabo et al., 2017; Welcomme, 1979). Holtgrieve et al., 2018), these “designer flows” are only one of several Overall, Halls et al. (2013) observed that larger annual floods tend to mitigation strategies. This strategy is only viable as long as there are no yield greater fish catches. Yet, this correlation is likely due to numerous major dams impeding fish migration in the Mekong floodplains and as hydrologically-driven processes, including fish population density and long as other environmental conditions critical for their habitat (e.g. reproduction (Halls and Welcomme, 2004), as well as floodplain pri- sediment delivery, primary production, and water quality) are intact. mary production and habitat (Chea et al., 2016b; Koponen et al., 2010). Other studies have also suggested that replicating annual flow varia- Numerous studies have investigated and reviewed dam impacts on bility through artificial flood releases may provide ecological benefits fisheries and diversity in fish species (e.g. Baran and Myschowoda, (Li and Zhao, 2016). Hydropower-induced changes in ecosystem pro- 2009; Kang et al., 2009; Dugan et al., 2010; Ziv et al., 2012; Pukinskis ductivity have also been investigated, as Arias et al. (2014a) suggested and Geheb, 2012; Zhang et al., 2018). The tendency for generalist that the annual average net primary productivity in open water and species to proliferate downstream of dams that regulate seasonal flow

287 J.S. Hecht et al. Journal of Hydrology 568 (2019) 285–300

floodplain areas of Tonle Sap Lake could decrease by 34% ( ± 4%) submission, we comment on its broader scope but not its findings. Prior under climate change and a 126-dam hydropower development sce- dam development scenarios have not considered possible increases in nario. Overall, a more detailed review of ecological impacts from dam hydropower potential from increased future precipitation, rapidly development and other concurrent drivers of changes, such as Castello lowering solar energy costs (Intralawan et al., 2018; Liu et al., 2018), and Macedo’s (2015) review on the Amazon basin, is warranted. growing concerns about high GHG emissions from some reservoirs (Räsänen et al., 2018), or possible overcapacity problems (World Bank, 2.4. Toward river basin management 2017). Hydropower integrated with other forms of renewable energy, including a floating solar power plant on the Lower Sesan 2 Reservoir in The Mekong River Commission (MRC), comprised of Thailand, Lao Cambodia, is also being evaluated (NHI, 2018). PDR, Cambodia and Vietnam, has facilitated many studies that inform transboundary basin management (e.g. MRC 2005, 2011, 2018a). China 3. Observed hydrological impacts and Myanmar also participate in many MRC activities as dialogue partners. Yet, relatively few transboundary streamflow regulations have This section surveys the extent of studies that have assessed the been implemented. The Mekong Agreement (MRC, 1995) the MRC actual hydrological impact of hydropower dams using observed flow countries signed articulates guidelines to limit excessive dry-season records and comparing pre- and post-dam periods in the Mekong basin, withdrawals, but these rules have never been specified (TFDD, 2018). including the conclusions inferable from these studies given short re- There are a limited number of binational cooperation efforts, such as cord lengths and other confounding factors. We partition this section the Vietnam-Cambodia agreement establishing minimum Sesan River into reviews of impacts observed in the UMB and LMB. UMB impacts flows (Ngo et al., 2018) and the recent fisheries management efforts encompass the effects that a cascade of dams within China has on between Lao PDR and Cambodia (MRC, 2018b). Often, dam operators Mekong mainstream flows in both the UMB and LMB. Meanwhile, the belonging to different companies make decisions without concern for review of flow alteration from LMB dams covers the observed hydro- downstream dams and overall basin impact (Piman et al., 2016; Schmitt logical effects of tributary dams on tributaries, the LMB mainstream and et al., 2018). (In the LMB, dams along tributaries frequently have dif- Lower Mekong floodplains. (See Section 4.3 for the projected effects of ferent owners (Jeuland et al., 2014), whereas HydroLancang owns all the LMB mainstream dams under construction.) major dams on the UMB mainstream except Dachaoshan (Yu and Geheb, 2017).) While the MRC has facilitated a transboundary flood 3.1. Impacts of upper Mekong dams on mainstream flow forecasting system for the mainstream (Pagano, 2014), the Mekong Agreement does not explicitly address potential transboundary impacts Hydrological effects of UMB dams have been scrutinized ever since along tributaries (Sithirith et al., 2016). The lack of tributary flood Manwan Dam (active storage capacity of 0.26 km3) was completed in forecasting and early warning systems exacerbated the consequences 1993 (e.g. Chapman and He, 1996; Lu and Siew, 2006). While some that Cambodian citizens residing along the Sekong River suffered from media reports have attributed LMB droughts to UMB dams (Campbell, the failure of an auxiliary ‘saddle’ dam of the Xe Pian – Xe Namnoy 2007; Stone, 2010b), most studies suggest that the smaller UMB project under construction in southern Lao PDR on July 23, 2018 mainstream dams (combined active storage capacity of 0.72 km3) built (Wallace and Leng, 2018). This breach unleashed a flash flood that has before Xiaowan and Nuozhadu (combined active storage capacity of left over 100 people dead or missing in both Lao PDR and Cambodia 32.1 km3) have either only minimally altered flow in the Mekong (Business Standard, 2018, URL: http://www.nationmultimedia.com/ mainstream (Campbell, 2007) or increased dry-season flows (Zhao detail/opinion/30357045). This catastrophe also prompted the gov- et al., 2012). Cochrane et al. (2014) detected statistically significant ernment of Lao PDR to suspend new dam projects and re-evaluate its increases in late dry-season flow (Feb-May monthly flows) between dam construction standards (Boyle, 2018). 1961 and 1990 and 1991–2010 (Fig. 3), although this study did not Widespread concerns about ecosystem and livelihood impacts of control for possible confounding factors, such as changes in precipita- seasonal flow regulation have largely not been considered in major tion and land use. Irregular releases from Manwan and Dachaoshan transboundary water resources agreements among Mekong countries Dams have also been associated with more variable daily and monthly (TFDD, 2018). While extreme floods threaten human safety, infra- flows during the dry season at Chiang Saen (Li and He, 2008; Lu et al., structure, and crops, the typical wet-season flood pulse benefits fish- 2014; Cochrane et al., 2014). Dry-season flow alteration associated eries and floodplain agriculture tremendously. The average annual with their construction has also been detected as far downstream as economic benefits of floods in LMB countries (US $8–10 billion) are Vientiane (He et al., 2006). more than hundred times greater than their average annual damages Meanwhile, the Xiaowan and Nuozhadu Dams, commissioned in (US $60–70 million) (MRC, 2010). In contrast, wet-season droughts, 2010 and 2014, respectively (and filled in 2008 and 2011, respec- often not considered as a hazard in tropical monsoon regions (Adamson tively), are altering streamflow seasonality in the Mekong mainstream and Bird, 2010), can impact agricultural and fisheries production sub- much more than earlier UMB dams did (Cochrane et al., 2014; Li et al., stantially. Hydropower dams that regulate seasonal flow substantially 2017a; Räsänen, 2017; Ji et al., 2018). Dry-season flows have increased may reduce the ecological and agricultural benefits of the annual flood dramatically as far downstream as Kratie, Cambodia (Li et al., 2017a), pulse, especially during drier years (e.g. Piman et al., 2013a). Hydro- including low flows in March-May 2014 that were 41–74% above pre- power dams may improve dry-season navigation downstream of dams, dam averages and 41–68% larger than simulated unregulated dis- but they may also impede passage further upstream and irregular re- charges (Räsänen, 2017). These increases fell within the range of earlier leases can be hazardous for small boats. These navigation tradeoffs have projections (Hoanh et al., 2010; Lauri et al., 2012; Räsänen et al., not been systematically examined. 2012). While hydropower development is not centrally coordinated, nu- The refilling of reservoirs during the beginning of the wet season merous basin-wide dam development scenarios have been devised may have delayed the date of the maximum daily flow by 22 days at (Kummu et al., 2010; MRC, 2011; Lauri et al., 2012; MRC, 2018a,b). both Chiang Saen and Stung Treng in 2010–2014 relative to their Some scenarios, such as the prominent ones in MRC (2011), consider 1960–1991 averages (Li et al., 2017a). However, this short post- ongoing changes in other sectors, such as water demand (agricultural, Xiaowan record limits the statistical strength of this conclusion, as does industrial and domestic), along with climate change. This allows for the filling of Nuozhadu (2011) during this post-impact period. Post- direct comparisons between different impacts. Recently, the MRC’s Xiaowan daily flow fluctuations are also greater (Cochrane et al., 2014; Council Study (MRC, 2018a,b) created a preliminary new scenario set. Li et al., 2017a; Mohammed et al., 2018b). Li et al. (2017a) also asso- However, since this study was still under review at the time of ciated a dramatic decrease in the duration of low-flow pulses (below the

288 J.S. Hecht et al. Journal of Hydrology 568 (2019) 285–300

Fig. 3. Change (%) in mean monthly water levels at key stations in the Lower Mekong basin between 1960–1991 and 1992–2013. (Data source: MR).

25th daily flow percentile), which are critical for LMB fisheries (Sabo increase the hydraulic head for power generation, numerous tributary et al., 2017), with irregular releases from Xiaowan Dam. Yet, while low- dams impound water for diversions to powerhouses at much lower flow pulses have shortened, irregular releases have also made extreme elevations, either further downstream along the same river or into ad- low flows even lower (Lu et al., 2014). Some measures have been made jacent tributary basins. Only two pumped-storage projects could be to mitigate negative consequences of downstream impacts within China found in the Mekong basin, both of which are located in Thailand (see Fan et al., 2015; Yu and Geheb, 2017) and strategies for reducing (Kraitud, 2017). impacts to flows entering the LMB mainstream and evaluate hydro- As expected, tributary dams with integral powerhouses and large power-ecological tradeoffs(Li et al., 2018) have also been explored. storage reservoirs have attenuated seasonal flow variability. In Lao However, the small Ganlanba re-regulation dam (0.12 km3, c.f. Kummu PDR, Lacombe et al. (2014a) examined daily flow records (available et al., 2010) is primarily being constructed to attenuate short-term since 1962) from several stations along the Nam Ngum River down- hydropeaking fluctuations, as restoring seasonal flow variability re- stream of a cascade of hydropower dams, the oldest of which, Nam quires far more storage. Ngum 1, was constructed in 1971. Without the dams, current irrigation needs would compete with environmental flow requirements during drier than normal years. In contrast, wet-season flows decreased by 3.2. Impacts of lower Mekong basin dams on mainstream and tributaries about 17% after Nam Ngum 1 Dam was commissioned. In other tributary basins where extensive flow alteration has been projected, such Relatively few streamflow stations downstream of tributary dams as the 3S basin, initial dams have only caused minor impacts because have pre- and post-dam flow records exceeding ten years (Lacombe they are largely run-of-river projects (Piman et al., 2013a). Tributary et al., 2014a; Lyon et al., 2017), which limits assessments of tributary studies have not examined flow alteration over interannual timescales, flow alteration. While numerous studies emphasize that dams attenuate despite numerous dams with storage capacities exceeding their annual flow seasonality (e.g. Adamson, 2001; Pokhrel et al., 2018), observed inflow volume (Bonnema and Hossain, 2017), and the interannual flow records indicate diverse hydrological impacts that depend, in part, persistence of hydroclimatic conditions in the region (Frappart et al., on the on-stream vs. off-stream location of hydropower turbines. To

289 J.S. Hecht et al. Journal of Hydrology 568 (2019) 285–300

Fig. 4. Diversions and inter-basin water transfers for hydropower production in the Nam Theun – Nam Kading basin, Lao PDR (adapted from Reis et al., 2015).

2018; Mohammed et al., 2018b). While diversions from Mekong tributaries have not been compre- Meanwhile, diversions have been associated with reduced flows hensively catalogued, other major tributary projects also have them, downstream of dams as well as flow augmentation in recipient basins including Houay Ho (Lao PDR) and Yali Falls (Vietnam). Diversions to (e.g. Baird et al., 2015; Chanudet et al., 2016). Changes in the Nam raise water levels in Nam Ngun 1 Reservoir have drastically reduced Theun/Nam Kading basin in Lao PDR (Fig. 4) showcase diverse forms of dry-season flows in the Nam Song River (Neua, 2007). Pronounced hydropower-induced flow alteration. The Nam Theun 2 (NT2) Dam fluctuations in flow downstream of diversions and in recipient basins (completed in 2010) (see Fig. 4), enables diversions from the Nam stemming from irregular electricity demands have been reported (e.g. Theun River into the Xe Bang Fai River, a separate Mekong tributary Chanudet et al., 2016), but have not been quantified. Extensive diver- (e.g. Stone, 2010a). Average diversions entering the Xe Bang Fai sions for irrigation in the Mun-Chi basin in northeastern Thailand may (220 m3/s) are nearly equal to the river’s natural mean annual flow explain the concurrence of the increasing precipitation and decreasing (266 m3/s at Mahaxai from 1989 to 2002) (Descloux et al., 2016). This flow trends that Ruiz-Barradas and Nigam (2018) detected at Pakse, Lao flow alteration affects the livelihoods of over 110,000 people PDR during the wet season, although more research is needed. As Kibler (International Rivers, 2018). Due to these diversions, downstream re- and Tullos (2013), who assessed recent hydropower diversions in leases from NT2 are just 2 m3/s, less than 1% of its mean annual inflow Southeast Asia’s Nu River basin, emphasized, these diversions can make of 238 m3/s, except during spills (Chanudet et al., 2016). Since NT2 reservoir storage capacity an inappropriate proxy for a dam’s hydro- heavily depletes flow in the Nam Theun River, releases from a hydrological impact. power reservoir on the Nam Gnouang River (NG), a Nam Theun tri- Next, many dam operators have employed profit-driven ‘hydro- butary, boost dry-season hydropower production at the Theun-Hinboun peaking’ strategies to meet energy demands that fluctuate substantially project (THXP) further downstream (Reis et al., 2015). This project can over daily (lower nighttime demands) and weekly (lower weekend divert up to 220 m3/s through a tunnel to a powerhouse that discharges demands) timescales (e.g. Wyatt and Baird, 2007; Reis et al., 2015; into the adjacent Nam Hinboun basin. In contrast, only 5 m3/s must be Trung et al., 2018). Consequently, increased flow fluctuations have released directly downstream (Reis et al., 2015), which has raised water been observed downstream of LMB tributary dams. Cochrane et al., temperatures and depleted dissolved oxygen (Warren, 1999). While (2014) even associated increased daily fluctuations in the Mekong post-NT2 streamflow changes are being monitored in the Nam Theun mainstream with hydropeaking in Thailand’s Chi-Mun tributary basin. and Xe Bang Fai basins (Descloux et al., 2016), rigorous assessments of Inadequate warnings about abrupt flow changes have compromised hydrological alteration have not been published. navigation and even led to drowning deaths (Lerner, 2003; Neua, 2007;

290 J.S. Hecht et al. Journal of Hydrology 568 (2019) 285–300

Soutthisombat et al., 2011). A few designated re-regulation dams, such upon simplified reservoir operation rules. To evaluate implications of as the small Sesan 4A dam (Meynell et al., 2014) and ones downstream operating rule assumptions, Räsänen (2017) simulated the hydrology of of the Theun-Hinboun and NT2 projects, aim to curtail flow alteration. the basin without UMB reservoirs and then applied a bias-correction However, these dams primarily modulate short-term fluctuations and procedure to remove systematic model error from these simulations cannot restore seasonal flow variability or attenuate flood hazards from before comparing them to observed discharges. Their observations of inter-basin transfers significantly. Only modeling studies have ad- flow alteration suggest reservoir operating rules could vary sub- dressed the extent to which downstream dams in cascades can also stantially from year to year, highlighting the importance of actual op- function as re-regulation dams (Jeuland et al., 2014; Piman et al., erating rules for modeling studies. While MRC (2011) and MRC (2018a) 2016). They show that cascade-optimal operations can mildly reduce scenarios have been configured to isolate dam impacts from other flow alteration (Jeuland et al., 2014) but more studies are needed since concurrent changes, they have yet to be used to make direct compar- impacts can vary considerably among reservoirs (Piman et al., 2016). isons with observed flow alteration. In addition, flow alteration during dam construction and reservoir We also examined the availability of hydrometeorological ob- filling has also impacted human safety, livelihoods and ecosystems in servations for simulating reservoir water balances. Although evapora- LMB tributaries, including the recent Xe Pian-Xe Namnoy auxiliary dam tion from many tributary reservoirs may substantially affect firm energy failure in southern Lao PDR, which has left over 100 people dead or production and exacerbate consequences of uncoordinated reservoir missing in Lao PDR and Cambodia (see Section 2.4). In Vietnam, the operations (Jeuland et al., 2014), estimates of open-water evaporation failure of a temporary diversion dam caused a large flood when the Yali are limited (Kummu et al., 2013). Coefficients relating pan and lake Falls Dam was constructed (Wyatt and Baird, 2007). Moreover, irre- evaporation are not widely available. Open-water evaporation esti- gular flow releases following its construction killed 39 people (Lerner, mates using only monthly temperature, solar radiation and sunshine 2003). Aquatic life in the Nam Lik River in Lao PDR was deeply dis- data (Morton, 1983), which have been comparable to estimates ob- turbed when dam construction completely halted downstream flow in tained using more data-intensive methods in tropical locations (dos Reis 2010 (Baran et al., 2011). Despite these myriad impacts, few studies and Dias, 1998; Vallet-Coulumb et al., 2001), could offer an initial have examined reservoir filling rates and other flow alterations before approach for reservoirs with negligible seepage losses. Finally, effects of dams are commissioned. (See Sections 4.1 and 4.2 for reviews of dam reservoir sedimentation on downstream releases have not been assessed safety studies.) in detail. Ultimately, assessments of hydrological change geared toward im- 3.3. Gaps in observational studies proving tradeoffs associated with dam development must evaluate flow indicators relevant for ecosystems and livelihoods. Li et al. (2017a) and Short tributary streamflow records, especially ones preceding dam Li et al. (2018) have examined changes in the magnitude and timing of construction, also make distinguishing dam-induced impacts from mainstream flow with the popular Indicators of Hydrologic Alteration natural hydroclimatic variability and other ongoing environmental (IHA), which aim to capture ecologically relevant dimensions of flow changes more difficult. While a comprehensive list of stations suitable alteration (Richter et al., 1996). Yet, applications of generalized fra- for assessing dam-induced flow alteration is unavailable, Ketelsen et al. meworks for characterizing site-specific or transferable flow-ecology (2014) offer a partial list of stations used to estimate 100-year floods at relationships, such as the Ecological Limits of Hydrologic Alteration dam sites, and Lacombe et al. (2014b) and Lyon et al. (2017) have (Poff et al. 2010), remain scant in the Mekong basin. While standard identified subsets of unregulated tributary stations for analyses of pre- flow-alteration metrics enable comparisons with other locations, in- dam conditions. Procedures for extending short records, filling inter- dicators tailored to Mekong livelihoods (e.g. Sabo et al., 2017; Zhang mittent records (Inomata and Fukami, 2008), or computing regional et al., 2018), such as dry-season days when riverbank gardens are ex- estimates of time series or flow statistics at ungauged locations using posed or, conversely, when small boat navigation is possible, could longer records in hydrologically similar regions could help estimate enrich tradeoff analyses. pre-dam flows at dam-impacted sites (Lacombe et al., 2014b; Ketelsen et al., 2014). Sub-daily streamflow and water level observations would 4. Projected hydrological impacts also improve assessments of hydropeaking impacts. Remotely sensed data are being increasingly used to overcome in This section focuses on studies of projected hydropower impacts situ data limitations (e.g. Liu et al., 2016; Ji et al., 2018; Wang et al., simulated using hydrological models without considering concurrent 2016). Recent satellite altimetry efforts have estimated daily water le- drivers of hydrological change. vels in the mainstream with sub-meter accuracy (Pham et al., 2018). Observations of changing water body surface areas and their relation- 4.1. Basin-wide studies ship with remotely sensed terrain and water surface elevations have been used to deduce reservoir operation patterns (Bonnema and Basin-wide modeling studies have concurred that increased re- Hossain, 2017) and changes in Tonle Sap Lake storage (Frappart et al., servoir storage would reduce wet-season flows and increase dry-season 2018). However, limitations for using satellite-based observations re- flows (e.g. Hoanh et al., 2010; Lauri et al., 2012; Piman et al., 2013a,b) main. For instance, correction factors have often been necessary for (Fig. 5). Using hydrological conditions from 1985 to 2000, Hoanh et al. incorporating satellite-based precipitation estimates into process-based (2010) investigated impacts of 6 UMB and 81 LMB dams (including 11 hydrological models (Liu et al., 2017; Mohammed et al., 2018a). mainstream dams), with a cumulative active storage capacity equal to To address the decision-making consequences of record length 16% of the basin’s annual flow at Kratie, Cambodia. Together, these limitations, some studies have applied hypothesis testing to indicate the prospective dams and other forms of water resources development likelihood that pre- and post-dam differences do not arise from sam- through 2030 would reduce wet-season flows by 8–17% and increase pling variability (Cochrane et al., 2014; Li et al., 2017a), although this dry-season flows by 30–60% relative to a baseline scenario with just 18 uncertainty has not been integrated into formal decision-making fra- LMB tributary dams. In a related study, Piman et al. (2013a) found that meworks (see Hecht, 2017). However, hypothesis testing cannot elim- six UMB dams and 82 LMB tributary dams (including 11 mainstream inate the possibility that other confounding factors explain differences dams) would increase dry-season flows at Kratie by 28% whereas wet- between two periods. In contrast, the effects of different dam config- season flows would drop by 9%. Under a long-range development urations can be compared directly through simulation models in which scenario through 2060 featuring 136 LMB and 6 UMB dams, wet-season the meteorological forcings and upstream watershed conditions are flows would decline by an additional 4% (-13% from baseline) whereas identical (e.g. Lauri et al., 2012). Yet, these comparisons often rely dry-season flows would increase by just 1% primarily because irrigation

291 J.S. Hecht et al. Journal of Hydrology 568 (2019) 285–300

Fig. 5. Baseline (1981–1992) and projected discharge from hydropower development at several gauging stations along the mainstream of the Mekong River. Station locations shown in Fig. 1. withdrawals would offset reservoir-induced increases. In contrast, Lauri sea level rise, flood control and navigation infrastructure, irrigation, et al. (2012) simulated greater dry-season increases (25–160%) and and land subsidence exacerbated by groundwater pumping (e.g. Le wet-season decreases (5–24%) at Kratie since they examined seasonal et al., 2007; Hoa et al., 2008; Västilä et al., 2010; Dang et al. 2016). flow changes strictly from a 126-dam cumulative development scenario Dang et al. (2018) found local water infrastructure development con- using discharge from 1982 to 1992. trolled wet-season levels in the upper Vietnamese delta while sea level Impacts of basin-wide hydropower development on flooding have rise and land subsidence increasingly influenced them near the coast. also been assessed. Using representative dry (1998), average (1997) Upstream dam impacts are expected to be greater during dry years and wet (2000) years, Piman et al. (2013a) also predicted that six UMB (Piman et al., 2013a; Dang et al., 2018). Emergency releases from UMB mainstream dams and 71 LMB tributary dams would decrease the dams to alleviate the LMB drought in 2016 (MRC and MWRC, 2016) river’s annually flooded area by 5.0% in a dry year, 6.6% in an average corroborated previous predictions that dry-season releases from UMB year, but only by 0.4% during a wet year. The extent to which this low hydropower dams could alleviate salinity and acidic groundwater wet-year reduction is driven by reservoir design constraints (storage constraints to dry-season agriculture in the delta (e.g. Hoa et al., 2008; and spillway capacity) or operating rules merits future research, espe- Hoanh et al., 2010; Piman et al., 2013a; Smajgl et al., 2015). However, cially considering projected increases in future floods (see Section 5.1). Piman et al. (2013a) projected that, by 2030, sea level rise will offset Numerous studies have projected impacts of basin-wide hydropower agricultural gains from dry-season releases made under normal hydro- development on the Lower Mekong floodplain, which begins down- power operations. Moreover, a drastic reduction in the delta’s sediment stream of Kratie in central Cambodia. These floodplains have a unique supply due to trapping in upstream reservoirs (e.g. Kummu et al., 2010; hydrology, as Tonle Sap Lake in Cambodia effectively serves as a nat- Kondolf et al., 2018) and, to a lesser extent, reduced cyclonic activity ural flood control reservoir for the Mekong River (see Uk et al. (2018) (Darby et al., 2016) are expected to accelerate land subsidence and for a recent review). In a typical year, the lake’s surface area expands aggravate coastal flooding. from a dry-season minimum of 2400 km2 to a wet-season maximum of 13,200 km2 (Kummu and Sarkkula, 2008). The hydraulic gradient be- 4.2. Upper Mekong dams impacts to mainstream flow tween the Mekong River and Tonle Sap Lake determines the flow direction between the two bodies of water, as Mekong flow enters the Potential hydrological changes from UMB dams have attracted lake via the Tonle Sap River during the wet season while the lake drains considerable attention, partly due to their transboundary implications to the Mekong Delta during the dry season. Since 54% of its annual (e.g. Chapman and He, 1996; Adamson, 2001; MRC, 2011; Lauri et al., surface water inflow originates from the Mekong mainstream on 2012; Räsänen et al., 2012; Wang et al., 2017; Mohammed et al., average (Kummu et al., 2013; Uk et al., 2018), the lake ecosystem and 2018b). Studies concur that UMB dams would attenuate seasonal flow its extremely productive fishery are sensitive to hydrological changes variability in the LMB mainstream. For instance, Räsänen et al. (2012) from upstream dam development (e.g. Lamberts, 2008; Arias et al., projected that six UMB dams would increase dry-season flow by 90% 2014a). Upstream hydropower development is expected to attenuate and decrease wet-season flow by 22% at Chiang Saen. Recent ob- inter-seasonal variations in lake area and exacerbate impacts to aquatic servations of dam-altered flows have corroborated these projections and amphibious species during years with unusually dry monsoons ( Cochrane et al., 2014; Li et al., 2017a; Räsänen et al., 2017), although (Piman et al., 2013a; Arias et al., 2014c). longer post-dam records are needed for stronger statistical conclusions. As one travels downstream the Mekong mainstream into Vietnam, it Overall, UMB flow regulation has been expected to diminish further becomes increasingly difficult to distinguish hydropower-induced hy- downstream because LMB tributaries have more annual rainfall and less drological alteration from numerous local drivers of change, including reservoir storage. Some studies have also predicted that the flood

292 J.S. Hecht et al. Journal of Hydrology 568 (2019) 285–300 season will commence later due to seasonal reservoir filling (e.g. Flow alteration in the 3S basin could also propagate downstream Adamson, 2001), which recent observations have corroborated into the Mekong mainstream and Lower Mekong floodplain. Arias et al. (Räsänen et al., 2017; Li et al., 2017a). Wang et al. (2017) also esti- (2014b) estimated that 42 new 3S basin dams could increase Tonle Sap mated that UMB reservoirs could reduce the frequency of floods ex- Lake’s annual minimum level by 25–35 cm, which could inhibit tree ceeding 0.5-year recurrence intervals by 72% at Chiang Saen but only germination and fish migration. Piman et al. (2013a) showed that re- by 6.0% at Stung Treng, Cambodia. Reductions in annual maximum ductions in the annual maximum water level are less certain and that floods were lower (29.5% at Chiang Saen and 2.6% at Stung Treng), the timing of the Tonle Sap River flow reversal may vary by a few weeks possibly due to current reservoir storage limitations. While quantifying in either direction, compared to the pre-development date (6–8 days design floods whose return periods far exceed record lengths is chal- earlier in a typical year). The earlier reversal during a typical year stems lenging, Liang et al. (2017) estimated that the Lancang Cascade dams from higher mainstream water levels at the end of the dry season. could withstand a 5000-year flood, but that Manwan and Jinghong Hydropower and irrigation reservoirs within the lake’s catchment might Dams would be overtopped during a 10,000-year event (assuming hy- further attenuate seasonal lake-level extremes upon which many eco- drological stationarity). systems and livelihoods depend. Potential dam impacts have also been simulated in other tributary 4.3. Lower Mekong basin dam impacts on mainstream and tributaries basins, including those under innovative single- and multi-reservoir operating rules. Reis et al. (2015) showed that maintaining low re- Numerous studies have evaluated the potential hydrological effects servoir levels in the Nam Gnouang Reservoir (see Fig. 4) would facil- of the 11 LMB mainstream dams (ICEM, 2010; MRC, 2011; VNMRE, itate recession agriculture while reducing average annual hydropower 2015; Trung et al., 2018). Although their reservoirs are expected to by only 8.1%, although it substantially increases hourly peak demand convert many reaches from lotic to lentic (lacustrine) habitat, seasonal shortfalls between April and June. Sioudom (2013) simulated increases flow alteration from them is expected to be minimal since they store no in flooding in the Nam Hinboun basin caused by diversions from the more than a few days of their mean annual inflow. However, alteration Nam Theun River. In the Nam Ngum basin, Jeuland et al. (2014) de- at sub-daily to daily timescales from hydropeaking is expected to be monstrated that multi-reservoir coordination could increase basin-wide substantial. For instance, Trung et al. (2018) projected that dry-season benefits from hydropower and irrigation by 3–12%. Relatively few flows at Kratie (31 km below the dam furthest downstream in the cas- basin-wide modeling studies have assessed sub-daily flow alteration cade) might fluctuate by 16,000 m3/s and water levels might oscillate from hydropeaking in LMB tributaries (see Reis et al., 2015). by 2 m. In addition, dam-induced channel incision is expected to The recent catastrophic failure of an auxiliary dam of the Xe Pian-Xe deepen the channel, which, in turn, could expedite the propagation of Namnoy project in Lao PDR illuminates the need for more dam safety flood waves to the delta by a few days. assessments in the LMB. Ketelsen et al. (2014) found that 40% of 67 While much of the debate surrounding the basin’s hydropower dam LMB dams have design peak inflows lower than their 100-year floods, development has focused on mainstream impacts, substantial flow al- and 11% do not even have spillways that can convey this discharge. teration has been projected in many tributaries. One focal point of Follow-up studies are needed, including ones that compare design tributary studies has been the Sekong, Sesan and Srepok (3S) rivers floods estimated with different distributional assumptions. While the (Wyatt and Baird, 2007; Piman et al., 2013a, Arias et al., 2014b; Wild safety of individual dams has been assessed (e.g. Tingsanchali and and Loucks, 2014; Räsänen et al., 2014; Piman et al., 2016; Ngo et al., Tammanee, 2012), the hazards of large floods on LMB tributary cas- 2018), the source of 16–23% of the Mekong’s annual flow (Adamson cades have not been investigated. More importantly, as Micovic et al. et al., 2009; Piman et al., 2013b; Trang et al., 2017). Piman et al. (2016) point out, dam failure risk assessments must not only consider (2013b) found that dry-season flows in this tri-national watershed (Lao design inflows but also antecedent reservoir storage, emergency oper- PDR, Cambodia, Vietnam) could increase by 63% and wet-season flows ating rules, debris blockage, gate failures, seismic hazards, among other may decline by 22% when considering 23 proposed hydropower pro- factors. In addition, few studies have examined the possible magnifi- jects aiming to maximize their own hydropower production. Moreover, cation of floods due to emergency releases made to avoid dam failures under a full development scenario consisting of 41 projects, Piman et al. (Vattenfall Power Consultant AB, 2008). (2013b) projected a dry-season increase of 95% and a wet-season decrease of 25%. Piman et al. (2016) also found impacts on annual daily 4.4. Gaps in studies of projected impacts flow extremes to be even greater, as annual daily maxima and minima could be reduced and increased by 36% and 168%, respectively. In Hydrological impacts are frequently simulated within different addition, they showed that existing and near-term projects in the 3S areas of the Mekong basin, but basin-wide projections, especially ones basin, many of which are run-of-river dams, offered a much greater considering other ongoing water resources developments, are less fre- energy return per unit of reservoir storage than larger long-term pro- quent, ostensibly due to the large modeling effort required. In contrast, jects. However, this study did not examine potential impacts of smaller many tributary projections do not employ sets of scenarios that isolate dams to fish passage, including in the Sekong River, one of the Me- effects of different drivers of change. More model comparison studies kong’s last free-flowing tributaries. are also needed. As our review of 3S basin projections demonstrates, Other 3S basin studies have examined impacts to just the Sesan differences in reservoir operating rules, input data, study area defini- River. Räsänen et al. (2014) found that assumed hydropower-max- tions, and other assumptions can inhibit these comparisons. As post- imizing rules at the 11 largest hydropower dams would increase dry- dam hydrological records become longer, more “difference in differ- season flows by 53% and reduce wet-season flows by 11%. However, ences” studies that compare simulated hydrological changes with ob- when dam operations in the Vietnamese portion of the Sesan River served ones should be carried out. basin were simulated using government-issued operating rules, dry- Currently, many modeling studies focus on dam impacts often season flows increased by just 30% whereas wet-season flows dropped without considering alternative operating rules, or design choices, in- by 15% (Ngo et al., 2018). When minimum-flow requirements at the cluding re-regulation dams. Moreover, many assumptions, such as hy- Cambodian border downstream of the dams were also considered, dry- dropower maximization, are made when operating rules are unavail- season flows rose by 40%. This also suggests that minimum-flow re- able. More efforts to optimize the design and operation of individual quirements account for productive uses as well as the environmental dams, cascades (see Bogardi and Duckstein, 1992; Schmitt et al., 2018), demand. While other modeling differences between these two studies or dam implementation sequences in basins (Ziv et al., 2012; Grill et al., prevent a direct comparison, these findings motivate further efforts to 2014; Jager et al., 2015) could reveal mitigation opportunities. Flood include actual operating rules in reservoir simulations. hazards due to dam failure, emergency spills, multi-reservoir operation,

293 J.S. Hecht et al. Journal of Hydrology 568 (2019) 285–300 reservoir-induced backwater effects (Rubin et al., 2015), and geo- hydropower and climate change. Wang et al. (2017) noted that UMB morphic changes also merit further research. reservoir storage could alleviate increased flooding along the upper While comparing the performance of individual models lies beyond LMB mainstream (down to Nong Khai, Thailand), where they predict the scope of this review (see Kite (2001) and Johnston and Kummu flood-control benefits of reservoir storage will outweigh climate-driven (2012) for earlier modeling reviews), our review revealed opportunities increases in flood magnitude through 2070. However, they expect in- to improve some contemporary modeling practices. First, while sum- creased precipitation to immediately outweigh the flood-control bene- mary metrics, such as Nash-SutcliffeEfficiency, often facilitate quick fits of reservoir storage further downstream, a finding that Hoang et al. model comparisons, we strongly encourage performance indicators re- (2018) echo. Hoanh et al. (2010) also stressed that reservoirs cannot levant for ecosystems and livelihoods for decision-making applications. mitigate projected climate-induced flood increases, as they would re- (Also, see Krause et al. (2005) for a cautionary note about NSE). duce the LMB’s seasonally flooded area by less than 1%. Lauri et al. Second, uncertainty analysis is especially critical given that short and (2012) reported that climate change could either exacerbate or offset sparse hydroclimatic records can constrain the calibration and valida- 10% reductions in wet-season flow from hydropower at Kratie from tion of hydrological models. While many recent studies are beginning to 2032 to 2042 (-21% to +4%). While many GCMs predict increased dry- examine the effects of parameter uncertainty on streamflow simulations season flows (e.g. Hoang et al., 2016), even projected decreases in them (e.g. Shrestha et al., 2013; Khoi and Thom, 2015; Phomsouvanh et al., are unlikely to fully offset much larger hydropower-induced increases 2016), one issue that has received less attention is the frequent at- (e.g. Hoang et al., 2018). tenuation of hydrological extremes in continuous simulation models Other studies examining joint hydropower-climate change impacts (e.g. Rossi et al., 2009; Wang et al., 2017; Mohammed et al., 2018b). have focused on specific tributary basins. In the 3S basin, Ngo et al. While numerous factors can contribute to this tendency, including a (2018) predicted that climate change would partially offset increases in paucity of high-elevation meteorological stations (Rossi et al., 2009), dry-season flows from Jan-May, offset decreases from Jun-Jul, while simulated streamflow time series have frequently omitted calibration changes from Aug-Dec vary by sub-basin. Using ten GCM runs from residuals. While the site-specific consequences of this practice depend Lauri et al. (2012), Arias et al. (2014a) estimated that hydropower on the correlation between calibration residuals and discharge ob- development may worsen climate change-induced habitat losses in servations, a large-sample study outside the Mekong basin (Farmer and Tonle Sap Lake, as expanded areas of open water (+32–38%), along Vogel, 2016) finds that re-introducing calibration residuals into simu- with irrigated and rain-fed rice (+11–21%), may drastically reduce lated time series can alleviate the attenuation of discharge variance in riparian forests (+13–67%) and other seasonally flooded habitat. In simulation models. Considering calibration residuals could be espe- other tributary basins, such as the Nam Ou in northern Lao PDR, un- cially critical given the impacts that hydrological extremes have on li- certain directions of change in precipitation (-27% to +41%) and an- velihoods, ecosystems and human safety in the basin. nual streamflow (-27% to +160%) (Shrestha et al., 2013) confound efforts to mitigate hydropower impacts. Few studies have examined 5. Effects of concurrent environmental changes whether climate-induced increases in streamflow could offset flow de- pletion downstream of diversions or exacerbate flooding in surcharged Many ongoing changes in climate, water use, and land cover are basins (Phomsouvanh et al., 2016). occurring as new dams are being constructed (e.g. Hoanh et al., 2010; Piman et al., 2013a,b; Hoang et al., 2018; MRC, 2018a,b). Building on 5.2. Water demand the recent review by Pokhrel et al. (2018), we evaluate the extent to which these concurrent drivers of change could exacerbate or offset Historically, the Mekong has been one of the least irrigated basins in hydrological changes stemming from hydropower production. In addi- monsoonal Asia (Barker and Molle, 2004; Ringler et al., 2004; tion, we briefly review the hydrological impacts of dam-induced Haddeland et al., 2006). Yet, higher dry-season flows from hydropower changes to sediment transport. can enable greater dry-season irrigation withdrawals. In turn, these larger withdrawals are expected to offset elevated dry-season flows 5.1. Climate change increasingly since the largest dams have already been commissioned (MRC, 2011). Lacombe et al. (2014) documented the complementarity Numerous studies (e.g. Kingston et al., 2011; Cook et al., 2012; of hydropower and irrigation in the Nam Ngum tributary basin in Lao Lauri et al., 2012; Thompson et al., 2013; Fan and He, 2015; Hoang PDR, which differs from some other basins in the world where they et al. 2016; Shrestha et al., 2016a; Wang et al., 2017) have evaluated directly compete with one another (Zeng and Cai, 2017). possible hydrological effects of future changes in precipitation and LMB irrigation may expand from 6.6 million ha in 2010 to 8.2–9.7 temperature. Hydrological models run with recent Climate Model In- million ha in 2030 (Hoanh et al., 2010; MRC, 2011), including a dry- tercomparison Project 5 (CMIP5) projections have suggested that season increase from 1.2 to 1.8 million ha (MRC, 2011). Together, these streamflow will increase during both the wet (8 out of 10 scenarios) and estimates suggest a wet-season increase of 1.0–2.5 million ha. (Note dry seasons (all 10 scenarios) at the basin scale, with the uncertainty that irrigated areas are difficult to estimate, partly due to the diverse stemming primarily from precipitation projections (Hoang et al., 2016). forms of irrigation in the basin (Ringler et al., 2004; Pokhrel et al., In the LMB, these increases in precipitation are consistent with ob- 2018)). MRC (2018a) is also developing new projections of irrigation servations from the last half century (Lacombe et al., 2013). However, withdrawals based on national development plans, global food demand these projected changes are not ubiquitous, as annual fl ows may de- and climate change. Some studies project that most irrigation expansion crease in some tributary basins (e.g. Shrestha et al., 2016b; Ngo et al., will take place in Lao PDR and Cambodia, where more land is available, 2018). Global climate models (GCMs) also consistently predict in- and both reservoir storage and export-driven production are growing creases in extreme floods (Hirabayashi et al., 2013; Hoang et al., 2016; rapidly (ICEM, 2010; Erban and Gorelick, 2016; Trung et al., 2018). Perera et al., 2017) while projected changes in mean annual floods are However, elevated dry-season flows in the mainstream could also be less consistent (e.g. Wang et al., 2017) and observations at some diverted to northeastern Thailand, where irrigation infrastructure al- mainstream stations indicate a decreasing trend (Delgado et al., 2010). ready exists (Molle and Floch, 2008). GCM projections of future droughts are also uncertain, as Kiem et al. Numerous long-term hydropower impact studies that also consider (2008) projected reduced streamflow drought severity due to greater future changes in irrigation (Hoanh et al., 2010; MRC, 2011; Hoang precipitation whereas Thilakaranthne and Sridhar (2017) forecasted et al., 2018) estimate that dry-season flow increases are smaller than that greater interannual variability would worsen droughts. the ones that only consider hydropower development (e.g. Lauri et al., GCMs have been essential for projecting combined impacts of 2012). While tools for assessing possible irrigation withdrawals

294 J.S. Hecht et al. Journal of Hydrology 568 (2019) 285–300 downstream of hydropower dams under different climate change sce- expand by 19–63% to compensate for a 40% loss in fish protein narios have been developed (e.g. Hoang et al., 2018), they do not (without considering livestock intensification or the expansion of consider reservoir operations made for irrigation purposes, including aquaculture or reservoir and rice paddy fisheries). However, they did reservoirs primarily dedicated to irrigation in northeastern Thailand. not spatially model this land conversion, which inhibits the effects of Few studies have examined decreases in irrigation opportunities greater livestock production from being assessed with a distributed downstream of diversions. Higher dry-season flows in recipient basins hydrological model. While possible increases in river withdrawals for have motivated irrigation projects with varying success (Baird et al., rice irrigation in response to greater dry-season flows have been ex- 2015). amined (see Section 5.2)), the effects of new rice paddies on surface Next, the extent to which wet-season irrigation could exacerbate runoff (e.g. Costa-Cabral et al., 2008; Homdee et al., 2011; Lacombe dam-induced reductions in wet-season flow has not received as much et al., 2014b) have not been reported in hydropower impact studies. attention even though more than 80% of all irrigated land was for wet- season use in 2010 (MRC, 2011). Warmer temperatures could further 5.4. Sediment impacts increase this demand (Hoanh et al., 2010; Mainuddin et al., 2012; Shrestha, 2017), which could exacerbate the wet-season droughts that Dam-induced flow alteration is not expected to be constant over compromise many livelihoods dependent on fisheries and floodplain time due to changes in sediment transport that will affect reservoir and agriculture. Hoang et al. (2018) simulated evapotranspiration from downstream river channel capacities. Projected rates of reservoir sedi- hydropower-enabled rice irrigation under different climate scenarios, mentation vary widely. Wild and Loucks (2014) predicted that only 5 but did not report the resulting increases in irrigation demand. While out of 41 3S basin reservoirs would lose more than 20% of their active domestic and industrial sectors consume much less water than agri- storage over the next century due to their large dead storage volumes. culture, a doubling of the water demand by 2030 could worsen dry- In contrast, Manwan Dam’s reservoir, located in the erosion-prone season droughts (e.g. Johnston et al., 2010), making flow regulation UMB, had already lost 16.7% of its active storage capacity (0.26 km3) from hydropower more valuable for water supply. and 75.9% of its dead storage capacity (0.66 km3) between 1993 and While hydropower development can provide extensive irrigation co- 2009 before Xiaowan Dam and erosion control projects were con- benefits during the dry season (Lacombe et al., 2014a), rising water structed upstream (Fan et al., 2015). levels upstream of new mainstream dams and channel migration below Basin-wide changes in sediment loads have also been examined, them may require as many as half of all existing and planned pumping including both the entrapment of sediment in reservoirs (Kummu et al., stations to be relocated, resized and retrofitted to deal with greater 2010; Wild and Loucks, 2014) as well as the effects of downstream daily flow fluctuations (ICEM, 2010). In contrast, Pokhrel et al. (2018) sediment starvation (e.g. Kondolf et al., 2014; Kondolf et al., 2018). Li suggest that higher dry-season flows may elevate groundwater levels, et al. (2017b) suggested that upstream dams have been a major driver thereby reducing pumping costs. Many riverbank gardens may become of the net erosion the delta has experienced since 2005. MRC mon- permanently inundated, including 54% on the Mekong mainstream itoring data indicate a dramatic reduction in suspended sediment loads (ICEM, 2010). Higher dry-season flows may also force farmers to grow following the construction of Xiaowan Dam, when the average sus- new varieties of rice (Fox and Wood, 2007). Increased in irrigation pended loads at the Chiang Saen station in northern Thailand decreased withdrawals necessary for producing food to replace declining river from 60 to 10 million tonnes per year (Mt/yr), an 83% reduction, at capture fisheries (see Orr et al., 2012) have not been assessed. Pakse in Lao PDR from 120 to 60 Mt/yr, (a 50% reduction), and at Kratie, Cambodia from 160 to 90 Mt/yr, a 43% reduction (Piman and 5.3. Land cover change Shrestha, 2017). These observed decreases are consistent with prior modeling results (Kummu et al., 2010, Kondolf et al., 2014). If all Numerous basin-wide studies (e.g. Kite, 2001; Kiem et al., 2008; planned Mekong basin dams are built, the sediment load entering the Costa-Cabral et al., 2008; MRC, 2018a,b) as well as ones in smaller delta may decrease from 51–69% (Kummu et al., 2010) to 96% tributary watersheds (e.g. Lacombe et al., 2010; Ty, 2011; Lacombe and (Kondolf et al., 2014). (See Wild et al. (2015) for options for routing Pierret, 2013; Lacombe et al., 2014b, 2016; Lyon et al., 2017; Shrestha sediment through reservoirs.). Overall, more information about the et al., 2017)offer some insight regarding the extent to which changes in interannual variability of sediment loads, trapping in multi-reservoir land cover may offset or exacerbate hydropower-induced fl ow altera- cascades (e.g. Kummu et al., 2010; Wild and Loucks, 2014), deposition tion. These impacts depend on many site-specific factors. For instance, in active storage zones, reservoir sediment compaction, and changes in converting a low-density forest to intensive agriculture may reduce upstream erosion (e.g. Shrestha et al., 2013) is needed. The effects of high flows (Shrestha et al., 2017), which could exacerbate the reduction changes in channel and floodplain storage capacity downstream of in wet-season flows from hydropower. However, deforestation has often dams (Kondolf et al., 2014; Rubin et al., 2015) on channel-floodplain been associated with increased runoff due to the lower evapo- connectivity (Kondolf et al., 2018) and on flood propagation (Trung transpiration of newly developed land cover (e.g. Lacombe et al., 2010; et al., 2018) are starting to be assessed, particularly along vulnerable Lacombe and Pierret, 2013). Lacombe et al. (2016) observed that af- alluvial and deltaic river reaches. Studies that examine both hydro- forestation from planting or from natural regeneration had opposing logical and geomorphic drivers of changing flood hazards are essential effects on annual streamflow since planting, especially for commercial for assessing changes in floodplain water levels (see Slater et al., 2015 timber, tends to reduce soil infiltration capacity whereas natural re- for an example outside of the Mekong basin). generation increases it. These site-specific nuances are important to consider when examining the cumulative effects of hydropower de- 6. Concluding remarks velopment and land-use changes on river flow variability. Finally, it should be noted that in some cases, land-use policies and soil man- Understanding hydropower-induced changes to the hydrology of the agement initiatives aim to improve long-term hydropower production Mekong basin is essential for anticipating and mitigating their negative by reducing reservoir sedimentation (see Arias et al. (2011) and impacts. Although many studies have analyzed hydrological impacts of Shrestha et al. (2017) for Mekong basin examples). Mekong hydropower dams, observed and projected changes to both Hydrological effects of changes in land cover stemming from eco- mainstream and tributaries flows have not been reviewed in detail. nomic development catalyzed by new dams, such as road construction While most dams with integral powerhouses are expected to regulate and mining, also merit consideration (e.g. Shrestha et al., 2016b; seasonal flow variability, diversions for off-stream power generation Hennig et al., 2016; Hoang and Tran, 2017; Pokhrel et al., 2018). Orr can drastically decrease downstream flows and augment flows in ad- et al. (2012) estimated that livestock production areas would need to jacent basins, a change which prior reviews have not captured.

295 J.S. Hecht et al. Journal of Hydrology 568 (2019) 285–300

Although LMB mainstream dams are often considered run-of-river op- new observations and/or projections become available, including reg- erations, they are expected to modify diurnal flow variability sub- ular assessments of flow alteration using stakeholder-relevant in- stantially, and their effects on flood propagation are underexplored. dicators will be essential for transboundary management. In addition, Importantly, these studies collectively indicate that other dam design future studies must carefully consider the extent to which flow altera- metrics, including release and diversion capacity, should complement tion versus other impacts of dams (e.g. connectivity, sediment trans- storage capacity in future assessments of the hydrological effects of port) and other ongoing environmental pressures (e.g. indiscriminate Mekong dams. In addition, flood hazards from dam failures, emergency fishing) jointly affect ecosystems and livelihoods in the basin. spills and inter-basin transfers also require further study, as does re- Finally, our review offers guidance for research in other basins servoir evaporation. where extensive hydropower development is possible. We especially We also assess whether concurrent changes in climate, water de- encourage the coordination of observational and modeling studies and mand, and land cover offset or exacerbate hydrological impacts of hy- integration of their results when assessing dam-induced flow alteration. dropower. While some uncertainty in the direction of hydrological re- In addition, studies should consider: (i) the uncertainty associated with sponses to climate change remains, modeling studies increasingly the limited length of available pre- and post-dam streamflow records, suggest that both wet- and dry-season flows will increase in many parts which makes it challenging to distinguish hydropower impacts from of the basin. Under this circumstance, on-stream hydropower devel- climate variability, (ii) hydrological change metrics relevant for local opment could be increasingly important for offsetting climate-change ecosystems and livelihoods (e.g. dry-season days when riverbank gar- impacts during the wet season but would exacerbate them during the dens are exposed or, conversely, when navigation is possible), (iii) dry season. Meanwhile, increased dry-season flows could enable irri- different timescales of hydrological alteration, including sub-daily ones, gation to meet growing food and export demands, as allowed by land (iv) different modes of hydropower production (on-stream vs. off- availability. However, fertile land flooded by new impoundments or stream), (v) impacts of inter-basin transfers, (vi) dam operations during temporarily submerged by higher river water levels may compromise floods and their influence on downstream flood hazards (including dam many agricultural livelihoods (e.g. riverbank gardens). The combined failure risks), (vii) concurrent environmental changes that might offset effects of increased wet-season irrigation and seasonal flow regulation or exacerbate hydropower-induced flow alteration, (viii) conducting could negatively impact livelihoods and ecosystems reliant upon the holistic assessments through integrated models featuring direct and annual flood pulse. Finally, joint impacts of these concurrent changes indirect pathways through which dams can alter streamflow, (ix) also depend on the on-site vs. off-site nature of hydropower production. scheduling and coordination of hydropower projects to minimize basin- For instance, greater precipitation could offset diversions in donor ba- wide impacts, especially in transboundary river basins or where hy- sins but aggravate flooding in recipient basins. Yet comparing hydro- dropower cascades have multiple dam operators and (x) assessing long- logical alteration from dams and other concurrent drivers of change is term hydrological effects of reservoir sedimentation. Addressing these insufficient for addressing transboundary and multi-sectoral tradeoffs gaps would facilitate the inter-sectoral and transboundary coordination of dam development. Rather, integrated models featuring indirect needed to improve tradeoffs between hydropower production and other pathways through which hydropower induces hydrological alteration, river management objectives. such as land-cover change near dams, are necessary to fully examine them (e.g. Ringler et al., 2004; Johnston and Kummu, 2012; Pokhrel Acknowledgements et al., 2018). In addition, while flow alteration often harms riverine ecosystems This research originated from an earlier review (Hecht and (Poff et al., 1997), ecological objectives could be integrated into dam Lacombe, 2014) published as part of the State of Knowledge Series on operations to improve livelihood-relevant ecological indicators beyond Mekong hydropower that the Consultative Group on International what hydropower operations conventionally yield (Sabo et al., 2017, Li Agricultural Research (CGIAR) Research’s Program on Water, Land and et al., 2018). Indeed, the uncertain ecological effects of hydropower Ecosystems (WLE) coordinated. This review was initiated during an dams further underscore the need for more quantitative relationships internship that the first author had with the International Water Man- between flow alteration and ecological responses. Such assessments agement Institute’s Southeast Asia Regional Office, which was funded must untangle other drivers of ecological change, such as connectivity by the National Science Foundation’s Water Diplomacy IGERT program and sediment concentrations, from flow alteration (e.g. McManamay at Tufts University (Grant 0966093) without any involvement in the et al., 2013; Knight et al., 2014; Hogan et al., 2018). Multi-reservoir direction of this manuscript. The authors’ current institutions provided operation rules that mitigate ecological and livelihood impacts (e.g. additional in-kind support. We would also like to thank the editors and Lacombe et al., 2014a; Jeuland et al., 2014; Reis et al., 2015) also merit reviewers for their useful comments that greatly improved this manu- further research, as do ecological effects of dam construction sequences. script. Finally, the authors do not have any individual or collective Large investments in water resources modeling (Johnston and conflicts of interest. Kummu, 2012) naturally spark a discussion regarding our current ability to characterize hydrological impacts of dams in the Mekong. References While some simulations have reproduced seasonal flow alteration well (Räsänen et al., 2017), simplified representations of reservoir operating Adamson, P.T., 2001. Hydrological perspectives of the Lower Mekong. Int. Water Power rules often make capturing shorter-term fluctuations more challenging. Dam Constr. 16–21 Available here: http://www.waterpowermagazine.com/features/ ff fl featurehydrological-perspectives-of-the-lower-mekong/. Preliminary e orts to examine changes in ow predictability (e.g. Adamson, P.T., Bird, J., 2010. The Mekong: a drought-prone tropical environment? Int. J. Mohammed et al., 2018b) must be expanded to address forecasting Water Resour. Dev. 26 (4), 579–594. needs for specific livelihood objectives. More attention should also be Adamson, P.T., Rutherfurd, I.D., Peel, M.C., Conlan, I.A., 2009. Chapter 4 – The hy- fl drology of the Mekong River. In: The Mekong. Academic Press, San Diego, pp. 53–76. paid to estimates of extreme events and sub-daily ow variation given https://doi.org/10.1016/B978-0-12-374026-7.00004-8. their impacts to ecosystems and livelihoods. Also, continued efforts to Anderson, E.P., Jenkins, C.N., Heilpern, S., Maldonado-Ocampo, J.A., Carvajal-Vallejos, compare the modeling implications of choosing different meteor- F., Encalada, A.C., Rivadeneira, J.F., Hidalgo, M., Cañas, C.M., Ortega, H., Salcedo, ological and other environmental datasets (e.g. soil, land cover) is es- N., Maldonado, M., Tedesco, P.A., 2018. Fragmentation of Andes-to-Amazon connectivity by hydropower dams. Sci. Adv. 4 (1). https://doi.org/10.11236/sciadv. sential. Studies that explore how uncertainty in input variables, and aao1642. parameters, and future conditions influence uncertainty in simulated Arias, M.E., Cochrane, T.A., Kummu, M., Lauri, H., Holtgrieve, G.W., Koponen, J., Piman, streamflow (e.g., Shrestha et al., 2016b) would help assess the robust- T., 2014a. Impacts of hydropower and climate change on drivers of ecological productivity of Southeast Asia’s most important wetland. Ecol. Model. 272, 252–263. ness of ecological and livelihood impact assessments. Protocols for Arias, M.E., Piman, T., Lauri, H., Cochrane, T.A., Kummu, M., 2014b. Dams on Mekong updating management plans based on previous modeling efforts when tributaries as significant contributors of hydrological alterations to the Tonle Sap

296 J.S. Hecht et al. Journal of Hydrology 568 (2019) 285–300

Floodplain in Cambodia. Hydrol. Earth Syst. Sci. 18, 5305–5315. https://doi.org/10. Fan, H., He, D., 2015. Temperature and precipitation variability and its effects on 5194/hess-18-5303-2014. streamflow in the upstream regions of the Lancang-Mekong and Nu-Salween rivers. J. Arias, M.E., Cochrane, T.A., Elliott, V., 2014c. Modelling future changes of habitat and Hydrometeorol. 216, 2248–2263. fauna in the Tonle Sap wetland of the Mekong. Environ. Cons. 41, 165–175. https:// Fan, H., He, D., Wang, H., 2015. Environmental consequences of damming the main- doi.org/10.1017/S0376892913000283. stream Lancang-Mekong River: a review. Earth – Sci. Rev. 146, 77–91. Arias, M.E., Cochrane, T.A., Lawrence, K., Killeen, T.J., Farrell, T.A., 2011. Paying the Farmer, W.H., Vogel, R.M., 2016. On the deterministic and stochastic use of hydrologic forest for electricity: a modelling framework to market forest conservation as pay- models. Water Resour. Res. https://doi.org/10.1002/2016WR019129. ment for ecosystem services benefiting hydropower generation. Environ. Conserv. 38, Fox, J., Wood, J.L., 2007. Dry-season flood-recession rice in the Mekong delta: two 473–484. thousand years of sustainable agriculture? Asian Perspec. 38 (1), 37–50. Baird, I.G., Shoemaker, B.P., Manorom, K., 2015. The people and their river, the World Frappart, F., Biancamaria, S., Normandin, C., Blarel, F., Bourrel, L., Aumont, M., Azemar, Bank and its dam: Revisiting the Xe Bang Fai River in Laos. Dev. Change 46 (5), P., Vu, P.-L., Le Toan, T., Lubac, B., Darrozes, J., 2018. Influence of recent climate 1080–1105. events on the surface water storage of the Tonle Sap Lake. Sci. Tot. Environ. 636, Baran, E., Myschowoda, C., 2009. Dams and fisheries in the Mekong basin. Aquat. 1520–1533. Ecosyst. Health 12 (3), 227–238. Grill, G., Ouellet Dallaire, C., Fluet Chouinard, E., Sindorf, N., Lehner, B., 2014. Baran, E., Larinier, M., Ziv, G., Marmulla, G., 2011. Review of the fish and fisheries as- Development of new indicators to evaluate river fragmentation and flow regulation at pects in the feasibility study of the environmental impact assessment of the proposed large scales: a case study for the Mekong River Basin. Ecol. Indic. 45, 148–159. Xayaburi Dam on the Mekong mainstream. Report prepared for the World Wildlife https://doi.org/10.1016/j.ecolind.2014.03.026. Fund Greater Mekong. Available here: http://assets.panda.org/downloads/wwf_ Grumbine, R.E., Dore, J., Xu, J.-C., 2012. Mekong hydropower: drivers of change and xayaburi_dam_review310311.pdf. governance challenges. Front. Ecol. Environ. 10 (2), 91–98. Barker, R., Molle, F., 2004. Evolution of irrigation in South and Southeast Asia. Grumbine, R.E., Xu, J.-C., 2011. Mekong hydropower development. Science 332 (6026), Comprehensive Assessment Research Report 5. Comprehensive Assessment 178–179. Secretariat, Colombo Available here: http://www.iwmi.cgiar.org/assessment/files/ Haddeland, I., Lettenmaier, D.P., Skaugen, T., 2006. Effects of irrigation on the water and pdf/publications/ResearchReports/CARR5.pdf. energy balances of the Colorado and Mekong basins. J. Hydrol. 324 (1–4), 210–223. Bogardi, J.J., Duckstein, L., 1992. Interactive multiobjective analysis embedding the Halls, A.S., Moyle, P.B., 2018. Comment on “Designing river flows to improve food se- decision maker’s implicit preference function. J. Amer. Water Resour. Assoc. 28 (1), curity futures in the Lower Mekong Basin” by J.L. Sabo, A. Ruhi, G.W. Holtgrieve, V. 75–88. Elliott, M.E, Arias, P.B. Ngor, T.A. Räsänen, N. So. Science 358 (1270). https://doi. Bonnema, M., Hossain, F., 2017. Inferring reservoir operating patterns across the Mekong org/10.1126/science.aao1053. eaao1053. Basin using only space observations. Water Resour. Res. 53. https://doi.org/10. Halls, A.S., Paxton, B.R., Hall, N., Hortle, K.G., So, N., Chheng, P., Peng, B.N., Boonsong, 1002/2016WR019978. S., 2013. Integrated Analysis of Data from MRC Fisheries Monitoring Programmes in Boyle, D., 2018. Laos suspends new dam projects following catastrophe. Voice Am the Lower Mekong basin (No. MRC Technical Paper No. 33). Mekong River Available here: https://www.voanews.com/a/laos-dam-project-on-pause/ Commission, Phnom Penh, Cambodia. 4518337.html. Halls, A.S., Welcomme, R.L., 2004. Dynamics of river fish populations in response to Campbell, I.C., 2007. Perceptions, data, and river management: Lessons from the Mekong hydrological conditions: a simulation study. River Res. Applic. 20, 985–1000. River. Water Resour. Res. 43, W02407. https://doi.org/10.1029/2006WR005130. He, D., Feng, Y., Gan, S., Magee, D.L., You, W.H., 2006. Transboundary hydrological Castello, L., Macedo, M., 2015. Large-scale degradation of Amazonian freshwater eco- effects of hydropower construction on the Lancang River. Chin. Sci. Bull. 51, 16–24. systems. Glob. Change Biol 22-990-1007 doi:10.1111.gcb.13173. Hecht, J.S., 2017. Making multi-stakeholder water resources decisions with limited Chapman, D., He, D.-M., 1996. Downstream Implications of China’s Dams on the Lancang streamflow information. PhD Dissertation. Department of Civil and Environmental Jiang (Upper Mekong) and their Potential Significance for Greater Regional Engineering, Tufts University. Cooperation. Australian National Mekong Resource Centre, Sydney. Hecht, J.S., Lacombe, G. 2014. The effects of hydropower dams on the hydrology of the Chanudet, V., Guédant, P., Rode, W., Godon, A., Guérin, F., Serça, D., Deshmukh, C., Mekong Basin. State of Knowledge Series 5. Vientiane, Lao PDR, CGIAR Research Descloux, S., 2016. Evolution of the physico-chemical water quality in the Nam Program on Water, Land and Ecosystems (WLE). Theun 2 Reservoir and downstream rivers for the 5 years after impoundment. Hennig, T., Wang, W., Magee, D., He, D., 2016. Yunnan’s fast-paced large hydropower Hydroecol. Appl. 19, 27–61. development: a powershed-based approach to critically assessing generation and Chea, R., Grenouillet, G., Lek, S., 2016a. Evidence of water quality degradation in Lower consumption paradigms. Water 8, 476. https://doi.org/10.3390/s8100476. Mekong basin revealed by self-organizing map. PLoS One. https://doi.org/10.1371/ Hirabayashi, Y., Mahendran, R., Koirala, S., Konoshima, L., Yamazaki, D., Watanabe, S., journal.pone.0145527. Kim, H., Kanae, S., 2013. Global flood risk under climate change. Nat. Clim. Change. Chea, R., Guo, C., Grenouillet, G., Lek, S., 2016b. Toward an ecological understanding of https://doi.org/10.1038/nclimate1911. a flood-pulse system lake in a tropical ecosystem: food web structure and ecosystem Hoa, L.T.V., Shigeko, H., Nhan, N.H., Cong, T.T., 2008. Infrastructure effects on floods in health. Ecol. Model. 323, 1–11. the Mekong River Delta in Vietnam. Hydrol. Proc. 22 (9), 1359–1372. Cochrane, T.A., Arias, M.E., Piman, T., 2014. Historical impact of water infrastructure on Hoang, H.T.M., Tran, K.D., 2017. Hydropower, people’s livelihood, and forest cover water levels of the Mekong River and the Tonle Sap system. Hydrol. Earth Syst. Sci. change: Transnational impact of Yali Fall Dams to Sesan community, Ouyadav dis- 18, 4529–4541. trict, Rattanakiri province, Cambodia. Int. J Southeast Asian Stud. Cook, B.I., Bell, A.R., Anchukaitis, K.J., Buckley, B.M., 2012. Snow cover and precipita- Hoang, L.P., Lauri, H., Kummu, M., Koponen, J., van Vliet, M.T.H., Supit, I., Leemans, R., tion impacts on dry season streamflow in the Lower Mekong Basin. J. Geophys. Res. Kabat, P., Ludwig, F., 2016. Mekong River flow and hydrological extremes under 117, D16116. https://doi.org/10.1029/2012JD017708. climate change. Hydrol. Earth Syst. Sci. 20, 3027–3041. https://doi.org/10.5194/ Costa-Cabral, M.C., Richey, J.E., Goteti, G., Lettenmaier, D.P., Feldkotter, C., Snidvongs, hess-20-3027-2016. A., 2008. Landscape structure and use, climate, and water movement in the Mekong Hoang, L.P., van Vliet, M.T.H., Kummu, M., Lauri, H., Koponen, J., Supit, I., Leemans, R., basin. Hydrol. Proc. 22 (12), 1731–1746. Kabat, P., Ludwig, F., 2018. The Mekong’s future flows under multiple drivers: How Dang, T.D., Cochrane, T.A., Arias, M.E., Van, P.D.T., 2018. Future hydrological alterations climate change, hydropower developments and irrigation expansions drive hydro- in the Mekong Delta under the impact of water resources development, land sub- logical changes. Sci. Tot. Environ. https://doi.org/10.1016/j.scitotenv.2018.08.160. sidence and sea level rise. J. Hydrol. Reg. Stud. 15, 119–133. Hoanh, C.T., Kittipong, J., Lacombe, G. and Srinetr, V., 2010. Impacts of Climate Change Dang, T.D., Cochrane, T.A., Arias, M.E., Van, P.D.T., de Vries, T.T., 2016. Hydrological and Development on Mekong Flow Regimes. First assessment – 2009. MRC Technical alterations from water infrastructure development in the Mekong floodplains. Paper No. 29. Vientiane, Mekong River Commission. Available here: http://www. Hydrol. Proc. 30, 3824–3838. https://doi.org/10.1002/hyp.10894. mrcmekong.org/assets/Publications/technical/tech-No29-impact-of-climate-change. Darby, S.E., Hackney, C.R., Leyland, J., Kummu, M., Lauri, H., Parson, D.R., Best, J.L., pdf. Nicholas, A.P., Aalto, R., 2016. Fluvial sediment supply to a mega-delta reduced by Hogan, Z., Baird, I.G., Radtke, R., Vander Zanden, M.J., 2007. Long distance migration shifting tropical-cyclone activity. Nature 539, 276–279. and marine habitation in the tropical Asian catfish, Pangasius krempfi. J. Fish Biol. Delgado, J.M., Apel, H., Merz, B., 2010. Flood trends and variability in the Mekong River. 71, 818–832. https://doi.org/10.1111/j.1095-8649.2007.01549.x. Hydrol. Earth Syst. Sci. 14, 407–418. Hogan, Z., Campbell, T., Weisberg, P.J., Null, S.E., Chandra, S., 2018. Mekong fish need Descloux, S., Guedant, P., Phommachanh, D., Luthi, R., 2016. Main features of the Nam more than design flows. Comment on “Designing river flows to improve food security Theun 2 hydroelectric project (Lao PDR) and the associated environmental mon- futures in the Lower Mekong Basin” by J.L. Sabo, A. Ruhi, G.W. Holtgrieve, V. Elliott, itoring programmes. Hydroecol. Appl. 19, 5–25. M.E, Arias, P.B. Ngor, T.A. Räsänen, N. So. Science 358 (1270). https://doi.org/10. Dore, J., Xiaogang, Y., Yuk-shing, K., 2007. China’s energy reforms and hydropower 1126/science.aao1053. eaao1053. expansion in Yunnan. In: Lebel, L., Dore, J., Daniel, R., Koma, Y.S. (Eds.), Holtgrieve, G.W., Arias, M.E., Ruhi, A., Elliott, V., Nam, S., Ngor, P.B., Räsänen, T.A., Democratizing Water Governance in the Mekong Region. Silkworm Books, Chiang Sabo, J.L., 2018. Response to comments on Designing river flows to improve food Mai, pp. 55–92 ISBN 9749511255. security futures in the Lower Mekong Basin” by J.L. Sabo, A. Ruhi, G.W. Holtgrieve, Dos Reis, R.J., Dias, N.L., 1998. Multi-season lake evaporation: energy budget estimates V. Elliott, M.E, Arias, P.B. Ngor, T.A. Räsänen, N. So. Science 358 (1270). https://doi. and CRLE model assessment with limited meteorological observations. J. Hydrol. org/10.1126/science.aao1053. eaao1053. 208, 135–147. Homdee, T., Pongput, K., Kanae, S., 2011. Impacts of land cover changes on hydrologic Dugan, P.J., Barlow, C., Agostinho, A., Baran, E., Cada, G.F., Chen, D., Cowx, I.G., responses: a case study of Chi River Basin, Thailand. J. Jpn. Soc. Civ. Eng. Ser. B1 67 Ferguson, J.W., Jutagate, T., Mallen-Cooper, M., Marmulla, G., Nestler, J., Petrere, I-31–I-36. M., Welcomme, R.L., Winemiller, K.O., 2010. Fish migration, dams, and loss of Hortle, K.G., 2007. Consumption and the Yield of Fish and Other Aquatic Animals from ecosystem services in the Mekong Basin. Ambio 39 (4), 344–348. the Lower Mekong Basin (No. MRC Technical Paper No. 16). Mekong River Erban, L., Gorelick, S., 2016. Closing the irrigation deficit in Cambodia: implications for Commission, Vientiane, Lao PDR. transboundary impacts on groundwater and Mekong River flow. J. Hydrol. 535, https://www.business-standard.com/article/news-ians/death-toll-reaches-36-in-laos- 85–92. dam-collapse-98-missing-118081300164_1.html.

297 J.S. Hecht et al. Journal of Hydrology 568 (2019) 285–300

ICEM (International Center for Environmental Management), 2010. Strategic Lacombe, G., Douangsavanh, S., Vogel, R.M., McCartney, M., Chemin, Y., Rebelo, L.-M., Environmental Assessment (SEA) of hydropower on the Mekong mainstream. Sotoukee, T., 2014b. Multivariate power-law models for streamflow prediction in the Vientiane, Mekong River Commission.at http://www.icem.com.au/documents/ Mekong Basin. J. Hydrol. Reg. Stud. 2, 35–48. envassessment/mrc_sea_hp/SEA_Final_Report_Oct_2010.pdf. Lacombe, G., Pierret, A., 2013. Hydrological impact of war-induced deforestation in the IEA (International Energy Agency), 2016. International Energy Outlook 2016 – with Mekong Basin. Ecohydrol. 6, 901–903. projections to 2040. Available here: https://www.eia.gov/outlooks/ieo/pdf/ Lacombe, G., Smakhtin, V., Hoanh, C.T., 2013. Wetting tendency in the Central Mekong 0484(2016).pdf. Basin consistent with climate-change-induced atmospheric disturbances already ob- Inomata, H., Fukami, K., 2008. Restoration of historical hydrological data of Tonle Sap served in East Asia. Theor. Appl. Climatol. 111 (1–2), 251–263. Lake and its surrounding areas. Hydrol. Proc. 22, 1337–1350. Lacombe, G., Pierret, A., Hoanh, C.T., Sengtaheuanghoung, O., Noble, A.D., 2010. International Rivers, 2018. Nam Theun 2 Dam. Available here: https://www. Conflict, migration and land-cover changes in Indochina: a hydrological assessment. internationalrivers.org/campaigns/nam-theun-2-dam. Ecohydrology 3 (4), 382–391. Intralawan, A., Wood, D., Frankel, R., Costanza, R., Kubiszewski, I., 2018. Tradeoff Lamberts, D., 2008. Little impact, much damage: the consequences of Mekong River flow analysis between electricity generation and ecosystem services in the Lower Mekong alterations for the Tonle Sap ecosystem. In: Kummu, M., Keskinen, M., Varis, O. basin. Ecosyst. Serv. 30, 27–35. (Eds.), Modern Myths of the Mekong: A Critical Review of Water and Development Jacobs, J.W., 2002. The Mekong River Commission: transboundary water resources Concepts, Principles and Policies. Helsinki, Water & Development Publications, planning and regional security. Geogr. J. 168 (4), 289–383. Helsinki University of Technology, pp. 3–18. Jager, H.I., Efroymson, R.A., Opperman, J.J., Kelly, M.R., 2015. Spatial design principles Lauri, H., de Moel, H., Ward, P.J., Räsänen, T.A., Keskinen, M., Kummu, M., 2012. Future for sustainable hydropower development in river basins. Renew. Sust. Energ. Rev. 45, changes in Mekong River hydrology: impact of climate change and reservoir opera- 808–816. tion on discharge. Hydrol. Earth Syst. Sci. 9 (2012), 4603–4619. Jeuland, M., Baker, J., Bartlett, R., Lacombe, G., 2014. The costs of uncoordinated in- Le, T.V.H., Nguyen, H.N., Wlanski, E., Cong, T.T., Shigeko, H., 2007. The combined im- frastructure management in multi-reservoir river basins. Environ. Res. Lett. 9 (10). pact on the flooding in Vietnam’s Mekong River delta of local man-made structures, https://doi.org/10.1088/1748-9326/9/10/105006. sea level rise, and dams upstream in the river catchment. Estuar. Coast. Shelf S. 71 Ji, X., Li, Y., Luo, X., He, D., 2018. Changes in the lake area of Tonle Sap: possible linkage (1–2), 110–116. to runoff alterations in the Lancang River? Remote Sens. 10, 866. https://doi.org/10. Lerner, M., 2003. Dangerous Waters: Violations of International Law and Hydropower 3390/rs100060866. Development along the Sesan River. Oxfam America, Phnom Penh, Cambodia. Johnston, R.M., Lacombe, G., Hoanh, C.T., Noble, A., Pavelic, P., Smakhtin, V., Li, D., Long, D., Zhao, J., Lu, H., Hong, Y., 2017a. Observed changes in flow regimes in the Suhardiman, D., Kam, S.P, Choo, P., 2010. Climate change, water and agriculture in Mekong River basin. J. Hydrol. 551, 217–232. the Greater Mekong Subregion. Colombo, Sri Lanka. pp. 52. IWMI Research Report Li, D., Wan, W., Zhao, J., 2018. Optimizing environmental flow operations based on 136. explicit quantification of IHA parameters. J. Hydrol. 563, 510–522. Johnston, R.M., Kummu, M., 2012. Water resources models in the Mekong basin: a re- Li, J., Dong, S., Peng, M., Yang, Z., Liu, S., Li, X., Zhao, C., 2013. Effects of damming on view. Water Resour. Manage. 26, 429–455. the biological integrity of fish assemblages in the middle Lancang-Mekong River Kang, B., He, D., Perrett, L., Wang, H., Hu, W., Deng, W., Wu, Y., 2009. Fish and fisheries basin. Ecol. Indic. 34, 94–102. in the Upper Mekong: current assessment of the fish community, threats and con- Li, J., Zhao, J., 2016. Hydropower-ecologic benefits trade-off analysis of cascade reservoir servation. Rev. Fish Biol. Fisheries 19, 465–480. operation. J. Hydroelec. Eng. 3592, 37–44 (in Chinese). Ketelsen, T., Parameswaran, A., Tilleard, S., Kenny, A., Mai, K.V., Nguyen, M.T., Taylor, Li, S., He, D., 2008. Water level response to hydropower development in the Upper L., 2014. Reservoir Flood Control Index: Assessing the Flood Risk Capacity for Control Mekong River. Ambio 37 (3), 170–176. in Mekong Hydro-electric Reservoirs. MK 12 Technical Note. Hanoi, Vietnam. Li, X., Liu, J.P., Saito, Y., Nguyen, V.L., 2017b. Recent evolution of the Mekong Delta and Khoi, D.N., Thom, V.T., 2015. Parameter uncertainty analysis for simulating streamflow the impacts of dams. Earth Sci. Rev. 175, 1–17. in a river catchment in Vietnam. Global Ecol. Conserv. 4, 538–548. Liang, Z., Huang, H., Cheng, L., Hu, Y., Yang, J., Tang, T., 2017. Safety assessment for Kibler, K.M., Tullos, D.D., 2013. Cumulative biophysical impact of small and large hy- dams of the cascade system of Lancang River in extreme situations. Stoch. Environ. dropower development in Nu River, China. Water Resour. Res. https://doi.org/10. Res. Risk Asst. 31 (9), 2459–2469. 1002/wrcr.20243. Liu, H., Krishna, V., Leung, J.L., Reindl, T., Zhao, L., 2018. Field experience and per- Kiem, A.S., Ishidaira, H., Hapuarachchi, H.P., Zhou, M.C., Hirabayashi, Y., Takeuchi, K., formance analysis of floating PV technologies in the tropics. Prog. Photovolt. Res. 2008. Future hydroclimatology of the Mekong River basin simulated using the high- Appl. 1–11. resolution Japan Meteorological Agency (JMA) AGCM. Hydrol. Proc. 22 (9), Liu, K.-T., Tseng, K.-H., Shum, C.K., Liu, C.-Y., Kuo, C.-Y., Liu, G., Jia, Y., Shang, K., 2016. 1382–1394. Assessment of the impact of reservoirs in the Upper Mekong River using satellite Kingston, D.G., Thompson, J.R., Kite, G., 2011. Uncertainty in climate change projections radar altimetry and remote sensing imageries. Remote Sens. 8, 367. of discharge for the Mekong River basin. Hydrol. Earth Syst. Sci. 15 (2011), Liu, S., Lu, P., Liu, D., Jin, P., Wang, W., 2009. Pinpointing source and measuring the 1459–1471. lengths of the principal rivers of the world. Int. J. Digit. Earth 2 (1), 80–87. https:// Kite, G., 2001. Modeling the Mekong: hydrological simulation for environmental impact doi.org/10.1080/17538940902746082. studies. J. Hydrol. 253 (1–4), 1–13. Liu, X., Liu, F.M., Wang, X.X., Li, X.D., Fan, Y.Y., Cai, S.X., Ao, T.Q., 2017. Combining Knight, R.R., Murphy, J.C., Wolfe, W.J., Saylor, C.F., Wales, A.K., 2014. Ecological limit rainfall data from rain gauges and TRMM in hydrological modelling of Laotian data- functions relating fish community response to hydrologic departures of the ecological sparse basins. Appl. Wat. Sci. 7, 1496–1497. flow regime in the Tennessee River basin, USA. Ecohydrology 7, 1262–1280. Lu, X.X., Li, S., Kummu, M., Padawangi, R., Wang, J.-J., 2014. Observed changes in the Kondolf, G.M., Rubin, Z.K., Minear, J.T., 2014. Dams on the Mekong: cumulative sedi- water flow at Chiang Saen in the lower Mekong: impacts of Chinese dams? Quatern. ment starvation. Water Resour. Res. 50, 5158–5169. https://doi.org/10.1002/ Int. https://doi.org/10.1016/j.quaint.2014.02.006. 2013WR014651. Lu, X.X., Li, S., Kummu, M., Padawangi, R., Wang, J.-J., 2008. Are the Chinese dams to be Kondolf, G.M., Schmitt, R.J.P., Carling, P., Darby, S., Arias, M., Bizzi, S., Castelletti, A., blamed for the lower water levels in the Lower Mekong? Modern Myths Mekong Cochrane, T.A., Gibson, S., Kummu, M., Oeurng, C., Rubin, Z., Wild, T., 2018. 39–51. Changing sediment budget of the Mekong: cumulative threats and management Lu, X.X., Siew, R.Y., 2006. Water discharge and sediment flux changes over the past strategies for a large river basin. Sci. Tot. Environ. 625, 114–134. decades in the Lower Mekong River: possible impacts of the Chinese dams. Hydrol. Koponen, J., Lamberts, D., Sarkkula, J., Inkala, A., Junk, W.J., Halls, A.S., Kshatriya, M., Earth Syst. Sci. 10, 181–195. 2010. Primary and Fish Production Report (Final Report), DMS – Detailed Modelling Lyon, S.W., King, K., Polpanich, O., Lacombe, G., 2017. Assessing hydrologic changes Support Project. MRC Information Knowledge Management Programme/Finnish across the Lower Mekong Basin. J. Hydrol. Reg. Stud. 12, 303–314. Environment Institute (SYKE)/EIA Centre of Finland Ltd. Mainuddin, M., Kirby, M., Hoanh, C.T., 2012. Water productivity responses and adap- Kraitud, N., 2017. Hydropower projects in Thailand. Available here: http://www. tation to climate change in the lower Mekong basin. Water Int. 37 (1), 53–74. mrcmekong.org/assets/Uploads/1.-Hydropower-plan-Thailand.pdf. McIntyre, P.B., Reidy Liermann, C.A., Revenga, C., 2016. Linking freshwater fishery Krause, P., Boyle, D.P., Bäse, F., 2005. Comparison of different efficiency criteria for management to global food security and biodiversity conservation. Proc. Natl. Acad. hydrological model assessment. Adv. Geosci. 5, 89–97. Sci. 113, 12880–12885. https://doi.org/10.1073/pnas.1521540113. Kummu, M., Lu, X.X., Wang, J.J., Varis, O., 2010. Basin-wide sediment trapping efficiency McManamay, R.A., Orth, D.J., Dolloff, C.A., Mathews, D.C., 2013. Application of the of emerging reservoirs along the Mekong. Geomorphology 119, 181–197. ELOHA framework to regulated rivers in the Upper Tennessee River basin: a case Kummu, M., Sarkkula, J., 2008. Impact of the Mekong River flow alteration on the Tonle study. Environ. Manage. https://doi.org/10.1007/s00267-013-0055-3. Sap flood pulse. Ambio 37 (3), 185–192. Meynell, P.-J., Carew-Reid, J., Räsänen, T., Tilleard, S., Ketelsen, T., 2014. Exploring Kummu, M., Tes, S., Yin, S., Adamson, P., Józsa, J., Koponen, J., Richey, J., Sarkkula, J., environmental flow regimes: Case study of the Lower Se San River. Project report: 2013. Water balance analysis for the Tonle-Sap Lake-floodplain system. Hydrol. Proc. Challenge Program on Water & Food Mekong Project MK3 “Optimizing the man- 28 (4), 1722–1733. agement of a cascade of reservoirs at the catchment level” ICEM – International Lacombe, G., Ribolzi, O., de Rouw, A., Pierret, A., Latsachak, K., Silvera, N., Pham Dinh, Centre for Environmental Management, Hanoi, Vietnam. R., Orange, D., Janeau, J.L., Soulileuth, B., Robain, H., Taccoen, A., Sengphaathith, Micovic, Z., Hartford, D.N.D., Schaefer, M.G., Barker, B.L., 2016. A non-traditional ap- P., Mouche, E., Sengtaheuanghoung, O., Tran Duc, T., Valentin, C., 2016. proach to the analysis of flood hazard for dams. Stoch. Environ. Res. Risk Asst. 30 (2), Contradictory hydrological impacts of afforestation in the humid tropics evidenced 559–581. by long-term monitoring and simulation modeling. Hydrol. Earth Syst. Sci. 20, Mohammed, I.N., Bolten, J.D., Srinavasan, R., Lakshmi, V., 2018a. Improved hydrological 2691–2704. decision support system for the Lower Mekong River basin using satellite-based earth Lacombe, G., Douangsavanh, S., Baker, J., Hoanh, C.T., Bartlett, R., Jeuland, M., observations. Remote Sens. 10, 885. https://doi.org/10.3390/rs10060885. Phongpachith, C., 2014a. Are hydropower and irrigation development complements Mohammed, I.N., Bolten, J.D., Srinavasan, R., Lakshmi, V., 2018b. Satellite observations or substitutes? The example of the Nam Ngum River in the Mekong Basin. Water Int. and modeling to understand the Lower Mekong River Basin streamflow variability. J. 39 (5), 649–670. Hydrol. 564, 559–573.

298 J.S. Hecht et al. Journal of Hydrology 568 (2019) 285–300

Molle, F., Floch, P., 2008. Megaprojects and social and environmental change: the case of Stromberg, J.C., 1997. The natural flow regime: a paradigm for river conservation the Thai “water grid”. Ambio 37 (3), 199–204. and restoration. BioScience 47 (11), 769–784. Molle, F., Wester, P., Hirsch, P., 2010. River basin closure: processes, implications and Poff, N.L., Olden, J.D., Merritt, D.M., Pepin, D.M., 2007. Homogenization of regional river responses. Agric. Water Manage. 97 (4), 569–577. dynamics by dams and global biodiversity implications. Proc. Nat. Acad. Sci. 104 Morton, F.I., 1983. Operational estimates of areal evapotranspiration and their sig- (14), 5722–5727. nificance to the science and practice of hydrology. J. Hydrol. 66, 1–76. Poff, N.L., Richter, B.D., Arthington, A.E., Bunn, S.E., Naiman, R.J., Kendy, E., Acreman, MRC (Mekong River Commission), 2018a. The study on the sustainable management and M., Apse, C., Bledsoe, B.P., Freeman, M.C., Henriksen, J., Jacobson, R.B., Kennen, development of the Mekong River basin, including impacts of mainstream hydro- J.G., Merritt, D.M., O’Keeffe, J.H., Olden, J.D., Rogers, K., Tharme, R.E., Warner, A., power projects. Available here: http://www.mrcmekong.org/highlights/the-study- 2010. The ecological limits of hydrological alteration (ELOHA): A new framework for on-sustainable-management-and-development-of-the-mekong-river-including- developing regional environmental flow standards. Freshw. Biol. 55 (1), 147–170. impacts-of-mainstream-hydropower-projects/. Poff, N.L., Zimmerman, J.K.H., 2010. Ecological responses to altered flow regimes: a MRC (Mekong River Commission), 2018b. Cambodia, Laos to formalise new bilateral literature review to inform the science and management of environmental flows. cooperation framework on fisheries management. Available here: http://www. Freshwater Biol. 55, 194–205. mrcmekong.org/news-and-events/news/cambodia-lao-pdr-to-formalise-a-new- Pokhrel, Y., Burbano, M., Roush, J., Kang, H., Sridhar, V., Hyndman, D.W., 2018. A re- bilateral-cooperation-framework-on-fisheries-management/. view of the integrated effects of changing climate, land use, and dams on Mekong MRC (Mekong River Commission), 2017. The Council Study: Study on the sustainable River hydrology. Water 10, 266. https://doi.org/10.3390/w10030266. management and development of the Mekong River, including impacts of main- Power, M.E., Sun, A., Parker, G., Dietrich, W.E., Wootton, J.T., 1995. Hydraulic food- stream hydropower projects. BioRA Final Technical Report Series. Volume 4: chain models. BioScience 45, 159–167. Assessment of Planned Development Scenarios. Vientiane, Lao PDR. 145 pp. Pukinskis, I.L., Geheb, K., 2012. State of Knowledge: The impacts of dams on the fisheries MRC (Mekong River Commission), 2015. Mekong dam database. of the Mekong. State of Knowledge Series 1. Vientiane, CGIAR Challenge Program on MRC (Mekong River Commission), 2011. Assessment of basin-wide development sce- Water and Food. Available here: https://cgspace.cgiar.org/bitstream/handle/10568/ narios – Main report. Vientiane, Mekong River Commission. Available here: http:// 34821/State%20of%20Knowledge%20Fisheries%20English.pdf?sequence=1. www.mrcmekong.org/assets/Publications/basin-reports/BDP-Assessment-of-Basin- Räsänen, T.A., 2017. Observed river discharge changes due to hydropower operations in wide-Dev-Scenarios-2011.pdf. the Upper Mekong basin. J. Hydrol. 545, 28–41. MRC (Mekong River Commission), 2010. State of the Basin Report 2010. Vientiane, Räsänen, T.A., Joffre, O.M., Someth, P., Thanh, C.T., Keskinen, M., Kummu, M., 2014. Mekong River Commission. Available here: http://www.mrcmekong.org/assets/ Model-based assessment of water, food, and energy trade-offs in a cascade of mul- Publications/basin-reports/MRC-SOB-report-2010full-report.pdf. tipurpose reservoirs: Case study of the Sesan Tributary of the Mekong River. J. Water MRC (Mekong River Commission), 2005. Overview of the Hydrology of the Mekong Resour. Plann. Manage. 141, 05014007. Basin. Vientiane, Mekong River Commission. Available: http://www.mekonginfo. Räsänen, T.A., Koponen, J., Lauri, H., Kummu, M., 2012. Downstream hydrological im- org/assets/midocs/0001968-inland-waters-overview-of-the-hydrology-of-the- pacts of hydropower development in the Upper Mekong Basin. Water Resour. mekong-basin.pdf. Manage. 26 (12), 3495–3513. MRC (Mekong River Commission), 1995. Agreement on the co-operation for the sus- Räsänen, T.A., Varis, O., Scherer, L., Kummu, M., 2018. Greenhouse gas emissions of tainable development of the Mekong River Basin. Available here: http://www. hydropower in the Mekong River Basin. Environ. Res. Lett. https://doi.org/10.1088/ mrcmekong.org/assets/Publications/policies/agreement-Apr95.pdf. 1748-9326/aaa817. MRC (Mekong River Commission) and MWRC (Ministry of Water Resources of the Reis, J., Culver, T.B., Lacombe, G., Senaratna Sellamuttu, S., 2015. Reservoir operation People's Republic of China), 2016. Joint Observation and Evaluation of the for recession agriculture in Mekong basin. J. Water Resour. Plann. Manage. https:// Emergency Water Supplement from China to the Mekong River. Mekong River doi.org/10.1061/(ASCE)WR.1943-5452.0000485. Commission, Vientiane, Lao PDR Available here: http://www.mrcmekong.org/as- Richter, B.D., Baumgartner, J.V., Powell, J., Braun, D.P., 1996. A method for assessing sets/Publications/Final-Report-of-JOE.pdf. hydrologic alteration within ecosystems. Conserv. Biol. 10 (4), 1163–1174. Natural Heritage Institute. 2018. Key findings of NHI’s Sambor Hydropower Dam Ringler, C., Cai, X., 2006. Valuing fisheries and wetlands using integrated economic-hy- Alternatives Assessment. Available here: http://n-h-i.org/wp-content/uploads/2018/ drologic modelling – Mekong River basin. J. Water Resour. Plann. Manage. 132 (6), 03/Cambodia_Key-Findings-of-Sambor-Assessment_updated-11-Mar-2018.pdf. 480–487. Neua, K.M., 2007. The continuing impacts of the ADB-funded Nam Song Diversion Ringler, C., von Braun, J., Rosegrant, M.W., 2004. Water policy analysis for the Mekong Project, Lao PDR. Unpublished field research report. Available here: http://www. River basin. Water Int. 29 (1), 30–42. internationalrivers.org/files/attached-files/nam_song_report_final.pdf. Rossi, C.G., Srinivasan, R., Jirayoot, K., Duc, T.L., Souvannabouth, P., Binh, N., Gassman, Ngo, L.A., Masih, L., Jiang, Y., Douven, W., 2018. Impact of reservoir operation and P.W., 2009. Hydrologic evaluation of the Lower Mekong River Basin with the soil and climate change on the hydrological regime of the Sesan and Srepok Rivers in the water assessment tool model. Int. Agr. Eng. J. 18, 1–13. Lower Mekong basin. Clim. Change. https://doi.org/10.1007/s10584-016-1875-y. Rubin, Z.K., Kondolf, G.M., Carling, P.A., 2015. Associated geomorphic impacts from Ngor, P.B., McCann, K.S., Grenouillet, G., So, N., McMeans, B.C., Fraser, E., Lek, S., Mekong basin dam construction. Int. J. River Basin Manage. 13 (1), 105–121. 2018a. Evidence of indiscriminate efishing ffects in one of the world’s largest inland Ruiz-Barradas, A., Nigam, S., 2018. Hydroclimate variability and change over the Mekong fisheries. Nat. Sci. Rep. https://doi.org/10.1038/s41598-018-27430-1. River basin: Modeling and predictability and policy implications. J. Hydrometeor. 19, Ngor, P.B., Legendre, P., Oberdoff, T., Lek, S., 2018b. Flow alterations by dams shaped 849–869. fish assemblage dynamics in the complex Mekong-3S river system. Ecol. Indic. 88, Sabo, J.L., Ruhi, A., Holtgrieve, G.W., Elliott, V., Arias, M.E., Ngor, P.B., Räsänen, T.A., 103–114. So, N., 2017. Designing river flows to improve food security futures in the Lower Orr, S., Pittock, J., Chapagain, A., Dumaresq, D., 2012. Dams on the Mekong River: lost Mekong Basin. Science 358 (1270), eaao1053. https://doi.org/10.1126/science. fish protein and the implications for land and water resources. Global Environ. aao1053. Chang. 22 (4), 925–932. Schmitt, R.J.P., Bizzi, S., Castelletti, A., Kondolf, G.M., 2018. Improved trade-offsof Ou, C., Winemiller, K.O., 2016. Seasonal hydrology shifts production sources supporting hydropower and sand connectivity by strategic dam planning in the Mekong. Nature fishes in rivers of the Lower Mekong Basin. Can. J. Fish. Aquat. Sci. 73 (9), Sust. 1, 96–104. 1342–1362. Shrestha, B., Babel, M.S., Maskey, S., van Griensven, A., Uhlenbrook, S., Green, A., Pagano, T.C., 2014. Evaluation of Mekong River Commission operational flood forecasts, Akkharath, L., 2013. Impact of climate change on sediment yield in the Mekong River 2000–2012. Hydrol. Earth Syst. Sci. 18, 2645–2656. basin: a case study of the Nam Ou basin, Lao PDR. Hydrol. Earth Syst. Sci. 17, 1–20. Pech, S., Sunada, K., 2008. Population growth and natural-resources pressures in the Shrestha, B., Cochrane, T.A., Caruso, B.S., Arias, M.E., 2017. Land use change uncertainty Mekong River Basin. Ambio 37 (3), 219–224. on streamflow and sediment projections in areas undergoing rapid development. Perera, E.D.P., Sayama, T., Magome, J., Hasegawa, A., Iwami, Y., 2017. RCP8.5-based Land Degrad. Dev. https://doi.org/10.1002/ldr.2831. future flood hazard analysis for the Lower Mekong River basin. Hydrol. 4, 55. https:// Shrestha, B., Cochrane, T.A., Caruso, B.S., Arias, M.E., Piman, T., 2016a. Uncertainty in doi.org/10.3390/hydrology4040055. flow and sediment projections due to future climate scenarios for the 3S Rivers in the Pham, H.T., Marshall, L., Johnson, F., Sharma, A., 2018. Deriving daily water levels from Mekong Basin. J. Hydrol. 540, 1088–1104. https://doi.org/10.1016/j.jhydrol.2016. satellite altimetry and land surface temperature for sparsely gauged catchments: a 07.019. case study for the Mekong River. Remote Sens. Environ. 212, 31–46. Shrestha, B., Maskey, S., Babel, M.S., van Griensven, A., Uhlenbrook, S., 2016b. Sediment Phomsouvanh, V., Phetpaseuth, V., Park, S.J., 2016. Study on climate change impacts on related impacts of climate change and reservoir development in the Lower Mekong hydrological response using a SWAT model in the Xe Bang Fai River Basin, Lao River Basin: a case study of the Nam Ou Basin, Lao PDR. Clim. Change 1–15. People’s Democratic Republic. J. Kor. Geog. Soc. 51 (6), 779–797. Shrestha, S., 2017. Quantifying the impact of climate change on crop yield and water Piman, T., Lennaerts, T., Southalack, P., 2013a. Assessment of hydrological changes in the footprint of rice in the Nam Oon Irrigation Project, Thailand. Sci. Tot. Environ. Lower Mekong Basin from basin-wide development scenarios. Hydrol. Proces. 27 599–600, 689–699. (15), 2115–2125. Sioudom, K., 2013. Basin profile for the Nam Theun/Nam Kading, Nam Hinboun and Xe Piman, T., Cochrane, T.A., Arias, M.E., Green, A., Dat, N.D., 2013b. Assessment of flow Bang Fai, Lao PDR. Project report: Challenge Program on Water & Food Mekong changes from hydropower development and operations in Sekong, Sesan and Srepok project MK3 “Optimizing the management of a cascade of reservoirs at the catchment Rivers of the Mekong Basin. J. Water Resour. Plann. Manage. 139 (6), 723–732. level.” In: ICEM – International Centre for Environmental Management, Hanoi Piman, T., Cochrane, T.A., Arias, M.E., 2016. Effect of proposed large dams on water Vietnam, 2014. flows and hydropower production in the Sekong, Sesan and Srepok Rivers of the Sithirith, M., Evers, J., Gupta, J., 2016. Damming the Mekong tributaries: water security Mekong Basin. Riv. Resear. Appl. 32, 2095–2108. https://doi.org/10.1002/rra.3045. and the MRC 1995 agreement. Water Policy 18 (6), 1420–1435. Piman, T., Shrestha, M., 2017. Case Study on Sediment in the Mekong River Basin: Slater, L.J., Singer, M.B., Kirchner, J.W., 2015. Hydrologic versus geomorphic drivers of Current State and Future Trends. UNESCO and Stockholm Environment Institute trends in flood hazard. Geophys. Res. Lett. https://doi.org/10.1002/2014GL062482. (SEI). Smajgl, A., Toan, T.Q., Nhan, D.K., Ward, J., Trung, N.H., Tri, L.Q., Tri, V.P.D., Vu, P.T., Poff, N.L., Allan, J.D., Bain, M.B., Karr, J.R., Prestegaard, K.L., Richter, B.D., Sparks, R.E., 2015. Responding to rising sea levels in the Mekong Delta. Nat. Clim. Change 5,

299 J.S. Hecht et al. Journal of Hydrology 568 (2019) 285–300

167–174. Wang, W., Lu, H., Leung, L.R., Li, H., Zhao, J., Tian, F., Yang, K., Sothea, K., 2017. Dam Soutthisombat, K., Sioudom, K., Rattana, P., 2011. Catchment profile of the Theun construction in Lancang-Mekong River Basin could mitigate future flood risk from Hinboun Expansion Project Nam-Theun01 and Nam-Theun02 (3T) Rivers in Lao PDR. warming-induced intensified rainfall. Geophys. Res. Lett. https://doi.org/10.1002/ Unpublished draft report. 2017GL075037. Stone, R., 2011. Mayhem on the Mekong. Science 333 (6044), 815–818. Wang, W., Lu, H., Yang, D., Sothea, K., Yang, J., Gao, B., Peng, X., Pang, Z., 2016. Stone, R., 2010a. Along with power, questions flow at Laos’s new dam. Science 328 Modelling hydrologic processes in the Mekong River basin using a distributed model (5977), 414–415. driven by satellite precipitation and rain gauge observations. PLoS One 11 (3), Stone, R., 2010b. Severe drought puts spotlight on Chinese dams. Science 327 (5971), e0152229. https://doi.org/10.1371/journal.pone.0152229. 1311. Warren, T.J., 1999. A monitoring study to assess the localized impacts created by the Nam Thilakaranthne, M., Sridhar, V., 2017. Characterization of future drought conditions in Theun-Hinboun Hydro-Scheme on fisheries and fish populations. Report prepared for the Lower Mekong River basin. Weather Clim. Extremes 17, 47–58. the Theun-Hinboun Power Company (THPC). Vientiane, THPC. Available here: Thompson, J.R., Green, A.J., Kingston, D.G., Gosling, S.N., 2013. Assessment of un- http://www.internationalrivers.org/files/attached-files/study_prepared_by_fisheries_ certainty in river flow projections for the Mekong River using multiple GCMs and specialist.pdf. hydrological models. J. Hydrol. 486 (2013), 1–30. Water, Land, and Ecosystems – Mekong, 2017. Mekong Hydropower Map and Portal. Tingsanchali, T., Tammanee, S., 2012. Assessment of hydrological safety of Mae Sruai Available here: https://wle-mekong.cgiar.org/maps/. Dam, Thailand. Procedia Engineer. 32, 1198–2004. Welcomme, R.L., 1979. Fisheries Ecology of Floodplain Rivers [Tropics]. Longman. Trang, N.T.T., Shrestha, S., Shrestha, M., Datta, A., Kawasaki, A., 2017. Evaluating the Wild, T.B., Loucks, D.P., 2014. Managing flow, sediment, and hydropower regimes in the impacts of climate and land-use change on the hydrology and nutrient yield in a Sre Pok, Se San, and Se Kong Rivers of the Mekong basin. Water Resour. Res. 50, transboundary river basin: a case study in the 3S River Basin (Sekong, Sesan, and 5141–5157. https://doi.org/10.1002/2014WR015457. Srepok). Sci. Total Environ. 576, 586–598. Wild, T.B., Loucks, G.P., Annandale, G.W., Kaini, P., 2015. Maintaining sediment flows Transboundary Freshwater Dispute Database, 2018. Available from the College of Ocean, through hydropower dams in the Mekong River basin. J. Water Resour. Plann. Earth, and Atmospheric Sciences at Oregon State University (COEAS). http://www. Manage. 142 (1), 05015004. transboundarywaters.orst.edu. Winemiller, K.O., McIntyre, P.B., Castello, L., Fluet-Chouinard, E., Giarrizzo, T., Nam, S., Trung, L.D., Duc, N.A., Nguyen, L.T., Thai, T.H., Khan, A., Rautenstrauch, K., Schmidt, C., Baird, I.G., Darwall, W., Lujan, N.K., Harrison, I., Stiassny, M.L.J., Silvano, R.A.M., 2018. Assessing cumulative impacts of the proposed Lower Mekong Basin Fitzgerald, D.B., Pelicice, F.M., Agostinho, A.A., Gomes, L.C., Albert, J.S., Baran, E., Hydropower Cascade on the Mekong River Floodplains and Delta – Overview of in- Petrere Jr., M., Zarfl, C., Mulligan, M., Sullivan, J.P., Arantes, C.C., Sousa, L.M., tegrated modeling methods and results. J. Hydrol. https://doi.org/10.1016/j.jhydrol. Koning, A.A., Hoeinghaus, D.J., Sabaj, M., Lundberg, J.G., Armbruster, J., Thieme, 2018.01.029. M.L., Petry, P., Zuanon, J., Torrente Vilara, G., Snoeks, J., Ou, C., Rainboth, W., Ty, T.V., 2011. A spatial impact assessment of human-induced intervention on hydro- Pavanelli, C.S., Akama, A., van Soesbergen, A., Sáenz, L., 2016. Balancing hydrological regimes: a case study in the upper Srepok River basin, Central Highlands of power and biodiversity in the Amazon, Congo, and Mekong. Science 351, 128–129. Vietnam. Int. J. River Basin Manage. 9 (2), 103–116. World Bank Group (World Bank), 2017. Systematic Country Diagnostic: Lao People’s Uk, S., Yoshimura, C., Siev, S., Try, S., Yang, H., Oeurng, C., Li, S., Hul, S., 2018. Tonle Democratic Republic. Available here: http://documents.worldbank.org. Sap Lake: Current status and important research directions for environmental man- Wyatt, A.B., Baird, I.G., 2007. Transboundary impact assessment in the Sesan River Basin: agement. Lakes Reserv. 1–13. The case of the Yali Falls Dam. Int. J. Water Resour. Dev. 23 (3), 427–442. Vallet-Coulomb, C., Legesse, D., Gasse, F., Travi, Y., Chernet, T., 2001. Lake evaporation Yu, X., Geheb, K., 2017. Hydropower environmental mitigation measures on the Lancang estimates in tropical Africa (Lake Ziway, Ethiopia,). J. Hydrol. 245, 1–18. River. State of Knowledge Series 8. Vientiane, Lao PDR, CGIAR Research Program on Varis, O., Kummu, M., Salmivaara, A., 2012. Ten major rivers in monsoon Asia-Pacific: an Water, Land and Ecosystems (WLE). assessment of vulnerability. Appl. Geog. 32, 441–454. Zarfl, C., Lumsdon, A., Berlekamp, J., Tydecks, L., Tockner, K., 2015. A global boom in Västilä, K., Kummu, M., Sangmanee, C., Chivanno, S., 2010. Modelling climate change hydropower dam construction. Aquat. Sci. 77, 161–170. impacts on the flood pulse in the Lower Mekong floodplains. J. Water Clim. Change 1 Zeng, R., Cai, X., 2017. Hydropower versus irrigation – an analysis of global patterns. (1), 67–86. Environ. Res. Lett. 12, 034006. Vattenfall Power Consultant AB in association with Ramboll Natura AB and Earth Systems Zhang, C., Ding, L., Ding, C., Chen, L., Sun, J., Jiang, X., 2018. Responses of species and Lao, 2008. Lao People’s Democratic Republic: Preparing the Cumulative Impact phylogenetic diversity of fish communities in the Lancang River to hydropower de- Assessment for the Nam Ngum 3 Hydropower Project. Stockholm. Available here: velopment and exotic invasions. Ecol. Indic. 90, 261–279. http://ifcext.ifc.org/ifcext/spiwebsite1.nsf/0/ Zhang, X., Li, H.-Y., Deng, Z.D., Ringler, C., Gao, Y., Hejazi, M.I., Leung, R., 2017. Impacts 3302CBDA8520406A8525795D007A0BDB/$File/Disclosure%20-%20Preparing of climate change, policy and Water-Energy-Food nexus on hydropower develop- %20for%20CIA%20TAC%20Report%20Feb%202008%20.pdf. ment. Renew. Energy. https://doi.org/10.1016/j.renene.2017.10.030. Viet Nam Ministry of Natural Resources and Environment (VMNRE), 2015. Study on the Zhao, Q., Liu, S., Deng, L., Dong, S., Yang, J., Wang, C., 2012. The effects of dam con- impacts of mainstream hydropower on the Mekong River. Report prepared by HDR, struction and precipitation variability on hydrologic alteration in the Lancang River Inc. and the Danish Hydrological Institute (DHI). Available here: https://mekongeye. Basin of southwest China. Stoch. Env. Res. Risk. Assess. 26 (7), 993–1011. com/wp-content/uploads/sites/2/2016/04/MDS-Final-Project-Report-Eng.pdf. Ziv, G., Baran, E., Nam, S., Rodriguez-Iturbe, I., Levin, S.A., 2012. Trading-offfish bio- Wallace, J., Leng, L., 2018. News of Laos dam failure didn’t reach them, but the water did. diversity, food security, and hydropower in the Mekong River Basin. Proc. Natl. Acad. New York Times, August 1, 2018. Available here: https://www.nytimes.com/2018/ Sci. 109, 5609–5614. https://doi.org/10.1073/pnas.1201423109. 08/01/world/asia/laos-cambodia-dam-flooding.html.

300