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# Hydropower dams of the Mekong River basin: A review of their hydrological impacts

Jory S Hecht, Guillaume Lacombe, Mauricio E Arias, Thanh Duc Dang, Thanapon Piman

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Jory S Hecht, Guillaume Lacombe, Mauricio E Arias, Thanh Duc Dang, Thanapon Piman. Hydropower dams of the Mekong River basin: A review of their hydrological impacts. Journal of Hydrology, 2019, 568, pp.285-300. 10.1016/j.jhydrol.2018.10.045. hal-04973305

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Citation:
Hecht JS, Lacombe G, Arias ME, Dang TD, Piman T. Hydropower dams of the Mekong River basin: a review of their hydrological impacts. Journal of Hydrology. 10.1016/j.jhydrol.2018.10.045

Hydropower dams of the Mekong River basin: a
review of their hydrological impacts

16 Jory S. Hecht¹,²*, Guillaume Lacombe³, Mauricio E. Arias⁴, Thanh Duc Dang⁵,⁶,⁷, Thanapon Piman⁸

18 ¹Department of Civil and Environmental Engineering, Tufts University, Medford, MA, USA

19 ²Vermont EPSCoR, University of Vermont, Burlington, VT, USA

20 ³International Water Management Institute, Southeast Asia Regional Office, Vientiane, Lao PDR

21 ⁴Department of Civil and Environmental Engineering, University of South Florida, Tampa, FL, USA

22 ⁵Institute for Water and Environment Research, Thuy Loi University, Ho Chi Minh City, Vietnam

23 ⁶Department of Civil and Natural Resources Engineering, University of Canterbury, Christchurch, New Zealand

24 ⁷Engineering Systems and Design Pillar, Singapore University of Technology and Design, Tampines, Singapore.

25 ⁸Stockholm Environment Institute, Bangkok, Thailand

26 *Corresponding author
27 Abstract

Hydropower production is altering the Mekong River basin's riverine ecosystems, which contain the world's largest inland fishery and provide food security and livelihoods to millions of people. The basin's hydropower reservoir storage, which may rise from ~2% of its mean annual flow in 2008 to ~20% in 2025, is attenuating seasonal flow variability downstream of many dams with integral powerhouses and large storage reservoirs. In addition, some tributary diversions for off-stream energy production are reducing downstream flows and are augmenting them in recipient tributaries.

To help manage tradeoffs between dam benefits (hydropower, irrigation, flood control, domestic water supply, and navigation) and their consequences for livelihoods and ecosystems, we review observed and projected impacts on river flows along both the Mekong mainstream and its tributaries. We include the effects of diversions and inter-basin transfers, which prior reviews of flow alteration in the Mekong 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

In many river basins with emerging economies, rising energy demands and campaigns to reduce fossil-fuel dependence have spurred the rapid expansion of hydropower (Zarfl et al., 2015; Zhang et al., 2017). Hydropower production, which could increase by over 70% in developing countries in the next few decades (Zarfl et al., 2015; IEA, 2016), is threatening ecosystems in basins with some of the greatest aquatic biodiversity, including the Amazon, Congo and Mekong (Winemiller et al., 2016). In the Mekong basin, which has the world's most productive inland fishery (Baran and Myschowoda, 2009; Ziv et al., 2012), seasonally variable flows have sustained livelihoods, food security and ecosystem services for millennia (e.g. Fox and Wood, 2007; Grumbine and Xu, 2011). Tradeoffs between dam benefits (e.g. hydropower production, flood control, irrigation, water supply, navigation) and their undesirable societal and ecological impacts (e.g. community resettlement in low-fertility agricultural lands; declines in fisheries, floodplain recession agriculture, sediment and nutrient transport; safety hazards posed by rapidly changing flows) are quite contentious and uncertain (e.g. ICEM, 2010; Intralawan et al., 2018). For instance, estimates of hydropower benefits range from $6-32B whereas its potential consequences to capture fisheries range from $2-13B (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 observed and projected hydrological alterations in the basin and their socioeconomic and ecological implications (Pokhrel et al., 2018). Lu et al. (2008) and Li et al. (2017) have
73 included reviews of prior studies on dam-induced hydrological alteration, while Fan *et al.* (2015) 74 and Yu and Geheb (2017) have recently synthesized observed hydrological impacts of UMB 75 hydropower dams. Numerous recent studies (Grumbine *et al.*, 2012; Johnston and Kummu, 76 2012; Pokhrel *et al.*, 2018) have also advocated integrated modeling for assessing socioeconomic 77 and ecological impacts of hydrological alteration.

78 However, given the potential impacts of Mekong hydropower development, a more 79 detailed review of hydrological alteration in different regions of the basin is warranted. 80 Moreover, managing ecological and livelihoods impacts from diversions and inter-basin transfers 81 in tributary basins often requires different mitigation strategies than ones suited for dams with 82 large reservoirs that attenuate seasonal flow variability. Thus, the main objectives of this paper 83 are to synthesize and critically review existing knowledge regarding observed and projected 84 hydrological alterations caused by hydropower dams on the Mekong mainstream and tributaries. 85 Section 2 provides an overview of the basin, outlines historical and projected trajectories of dam 86 development, highlights some key impacts of hydrological alteration to ecosystems and 87 livelihoods and briefly describes basin management efforts and challenges. Sections 3 and 4 88 detail the extent to which observed and projected hydrological impacts have been assessed 89 throughout the basin, respectively. Section 5 assesses the extent to which concurrent drivers of 90 hydrological change may offset or exacerbate hydropower impacts. Section 6 summarizes recent 91 advances and identifies research gaps that could be addressed to further guide hydropower 92 development in the Mekong and other basins worldwide. 93
## 2. Background

### 2.1 Basin overview

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 4,909 km (Liu et al., 2009) both rank tenth globally, while its drainage area (795,000 km²) is the 25th largest (MRC, 2005). Its population was approximately 70-75 million in 2005 (Ringler and Cai, 2006; Varis et al., 2012) and could increase to 100-145 million by 2050 (Pech and Sunada, 2008; Varis et al., 2012). The Lower Mekong basin (LMB) (Figure 1) lies in the Southeast Asian countries of Lao PDR (25% of the basin area), Thailand (23%), Cambodia (20%), Vietnam (8%), and Myanmar (3%). Meanwhile, the upstream portion of the basin (21%) in China is often known as the Upper Mekong Basin (UMB) or Lancang Basin (MRC, 2005).
<figure><img src="image_2.png" /><figcaption>Figure 1: Hydropower dams in the Mekong basin and Mekong mainstream hydrological stations analyzed in this study</figcaption></figure>
The LMB has a monsoonal climate with a wet season often considered to run from May to October and a dry season lasting from November to April. The flood season analyzed in most studies runs from June to November due to runoff lags in the populous, flood-prone Lower Mekong floodplain and the timing of the seasonal flow reversal between the Mekong mainstream and Tonle Sap Lake. Typically, 75% of the river's annual discharge passes through the delta between July and October, a period with extensive flooding that sustains many ecosystems and livelihoods (e.g. Kummu and Sarkkula, 2008, Piman et al., 2013a). Meanwhile, 35% of its dry-season flow originates from China, making impacts of large UMB dams especially critical. See MRC (2005), Adamson (2009), and MRC (2018) for more detailed basin descriptions.

## 2.2 Hydropower dam development

While hydropower ambitions have existed for over half a century (Jacobs, 2002), most hydropower dams have been constructed in the last decade (Figure 2). In 2008, the Mekong was one of the least regulated large river basins in the world, as its total active reservoir storage capacity (8.6 km³) amounted to just 2% of its annual discharge (Kummu et al., 2010). However, recent estimates (MRC, 2015); Mekong-WLE, 2017) show that its active reservoir storage (including flood storage) in 2025 will equal approximately 19% of its mean annual flow. This estimate falls within the 17-23% range of earlier projections (Hoanh et al., 2010; Kummu et al., 2010, MRC, 2011). According to the Greater Mekong Dam Database (GMDD) (https://wle-mekong.cgiar.org/maps/), 64 of the 187 existing and proposed hydropower dams with an installed capacity of at least 15 MW had been commissioned by June 2017. Out of these 64 dams, 18 dams in China can generate up to 17,770 MW, over 91% of which is installed at the six large Lancang Cascade dams on the UMB mainstream. The other 46 LMB tributary dams can produce up to 8,650 MW. The combined installed capacity of all existing and proposed dams (>
131 15 MW) in the GMDD database exceeds 65,000 MW, while other estimates range from nearly
132 59,000 MW (Dore et al., 2007) to 88,000 MW (Stone, 2011). These estimates include a
133 controversial cascade of 11 proposed LMB mainstream dams with a total installed capacity of
134 13,000 MW (GMDD, 2017). Greater seasonal flow regulation from upstream hydropower
135 reservoirs increases the feasibility of hydropower at downstream locations. Moreover, if
136 increased annual and dry-season flows projected under many climate change scenarios (e.g.
137 Hoang et al., 2016) exceed climate-driven increases in reservoir evaporation, both total and firm
138 energy production could increase. Such potential basin-wide changes in hydropower generation
139 have not been modeled.

<figure><img src="image_3.png" /><figcaption>**Figure 2:** Rapid increases in the number of dams and active reservoir storage capacity from 1960 to 2025 (MRC, 2015 and GMDD, 2017). The point and area graphs show the number of dams and the corresponding active storage capacity of their reservoirs, respectively. Dark grey: existing dams. Light grey: planned dams</figcaption></figure>
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**2.3 Ecological implications of hydrological alteration**

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Many ecological impacts of dam-induced flow alteration have been debated (Ringler *et al.*, 2004; Stone, 2010a; Intralawan *et al.*, 2018), as LMB residents depend on fish and other aquatic animals for 47-80% of their protein, more than any major basin in the world (Hortle, 2007). Cambodia also has the world's highest freshwater fish catch rate per capita (McIntyre *et al.*, 2016). Spatiotemporal variations in river and floodplain hydrology influence LMB fisheries.

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Several components of the flood pulse have been hypothesized to affect fish catches, including flood magnitude, duration, timing, and variability (Sabo *et al.*, 2017; Welcomme, 1979). Overall, Halls *et al.* (2013) observed that larger annual floods tend to yield greater fish catches. Yet, this correlation is likely due to numerous hydrologically-driven processes, including fish population density and reproduction (Halls and Welcomme, 2004), as well as floodplain primary production and habitat (Chea *et al.*, 2016; Koponen *et al.*, 2010).

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Numerous studies have investigated dam impacts to fisheries and other riverine species (e.g. Baran and Myschowoda, 2009; Kang *et al.*, 2009; Dugan *et al.*, 2010; Pukinskis and Geheb, 2012; Zhang *et al.*, 2018). As in other parts of the world (Poff *et al.*, 2007), Mekong dams have substantially reduced fish production and diversity stemming from losses in river network connectivity (Ziv *et al.*, 2012; Ou and Winemiller, 2016) and changes in downstream flow (Baran *et al.*, 2011; Li *et al.*, 2013). Many migratory species travel hundreds of kilometers along free-flowing reaches to spawn (e.g. Hogan *et al.*, 2007), some of which are becoming endangered as the percentage of free-flowing reaches in the basin decreases. Between 1990 and 2010, river connectivity declined from 93% to 77% and could drop as low as 10% (Grill *et al.*, 2014). Even run-of-river dams have substantially disrupted fisheries in the Mekong basin, such as the Pak Mun Dam on the Mun River in Thailand (e.g. Molle *et al.*, 2010). Indeed, further

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research is needed to untangle the effects of passage barriers and flow alteration on Mekong
fisheries and ecosystems. However, this study focuses on flow alteration.

While many ecological studies focus on flow-alteration impacts on specific fisheries (e.g.
Hogan et al., 2007; Sabo et al., 2017), some studies have assessed a wider range of riverine
ecological communities in the UMB (Kang et al., 2009; Li et al., 2013; Fan et al., 2015; Zhang
et al., 2018), and LMB tributaries (Ou and Winemiller, 2016). Yet, while pronounced flow
alteration has been associated with severe ecological impacts (e.g. Baran et al., 2011), the
optimality of natural flow for some species, i.e. the natural flow paradigm (Poff et al., 1997), has
also been questioned. Most notably, Sabo et al. (2017) proposed that, if hydropower dams were
collectively operated to generate year-round dry-season flows with punctuated flood pulses,
LMB fisheries could maintain or even increase historical yields. Li and Zhao (2016) recommend
operating rules with artificial flood releases. Hydropower-induced changes in indicators of
ecosystem productivity have also been investigated, as Arias et al. (2014a) suggested that the
annual average net primary productivity in open water and floodplain areas of Tonle Sap Lake
could decrease by approximately 34%. Overall, a more detailed review of ecological impacts
from dam development and other concurrent drivers of changes, such as Castello and Macedo's
(2015) review on the Amazon basin, is warranted.

2.4 Toward river basin management

The Mekong River Commission (MRC), comprised of Thailand, Lao PDR, Cambodia
and Vietnam, has funded and facilitated many research projects that inform transboundary basin
management. China and Myanmar also participate in many MRC activities as dialogue partners.
Yet, relatively few transboundary streamflow regulations have been implemented. The Mekong
Agreement (1995) the MRC countries signed articulates guidelines to limit excessive dry-season
withdrawals, but these rules have never been specified (TFDD, 2018). Binational treaties also exist, such as the Vietnam-Cambodia agreement establishing minimum Sesan River flows (Ngo et al., 2017). While a few international agreements constrain dam operation, LMB dam operators, often belonging to different companies, tend to make decisions without concern for downstream dams and overall basin impact (Jeuland et al., 2014; Piman et al., 2016). In contrast, HydroLancang owns all major UMB dams except Dachaoshan (Yu and Geheb, 2017).

Widespread concerns about ecosystem and livelihood impacts of seasonal flow regulation have not been considered in major water resources agreements (TFDD, 2018). While extreme floods threaten human safety, infrastructure, and crops, the typical wet-season flood pulse benefits fisheries and floodplain agriculture tremendously. The average annual economic benefits of floods in LMB countries (US $8-10 billion) are more than hundred times greater than their average annual damages (US $60-70 million) (MRC, 2010). In contrast, wet-season droughts, often not considered a hazard in tropical monsoon regions (Adamson and Bird, 2010), can impact agricultural and fisheries production substantially. Hydropower dams may improve dry-season navigation downstream of dams, but they may also impede passage further upstream and irregular releases can be hazardous for smaller boats. These navigation tradeoffs have not been systematically examined.

While hydropower development is not centrally coordinated, numerous basin-wide dam development scenarios have been devised (Kummu et al., 2010; MRC, 2011; Lauri et al., 2012; MRC, 2018). Some scenarios, such as the prominent ones in MRC (2011), consider ongoing changes in other sectors, such as water demand (agricultural, industrial and domestic). This allows for direct comparisons between different impacts. Recently, the MRC's Council Study (MRC, 2018) created a preliminary new scenario set. However, since this study was still under
review at the time of submission, we comment on its broader scope but not its findings. Prior
dam development scenarios have not considered possible increases in hydropower potential from
increased future precipitation, rapidly lowering solar energy costs (Intralawan et al., 2018),
growing concerns about high GHG emissions from some reservoirs (Räsänen et al., 2018), or
possible overcapacity problems (World Bank, 2017). Hydropower integrated with other forms of
renewable energy, including a floating solar power plant on the Lower Sesan 2 Reservoir in
Cambodia, is also being evaluated (NHI, 2018).

221

222 **3. Observed hydrological impacts**

This section surveys the extent of studies that have compared pre- and post-dam flows in
the Mekong basin, including the conclusions inferable from these studies given short record
lengths and other confounding factors.

226 **3.1 Impacts of Upper Mekong dams on mainstream flow**

Hydrological effects of UMB dams have been scrutinized ever since Manwan Dam
(active storage capacity of 0.26 km³) was completed in 1993 (e.g. Chapman and He, 1996; Lu
and Siew, 2006). While some media reports have attributed LMB droughts to UMB dams
(Campbell, 2007; Stone, 2010b), most studies suggest that the operation of the smaller UMB
mainstream dams (combined active storage capacity of 0.72 km³) built before Xiaowan and
Nuozhadu (combined active storage capacity of 32.1 km³) have either only minimally altered
flow in the Mekong mainstream (Campbell, 2007) or increased dry-season flows (Zhao *et al.*,
2012). Cochrane *et al.* (2014) detected statistically significant increases in late dry-season flow
(Feb-May monthly flows) between 1961-1990 and 1991-2010 (Figure 3), although this study did
not control for possible confounding factors, such as changes in precipitation and land use.
237 Irregular releases from Manwan and Dachaoshan Dams have also been associated with more
238 variable daily and monthly flows during the dry season at Chiang Saen (Li and He, 2008; Lu et
239 al., 2014; Cochrane et al., 2014). Dry-season flow alteration associated with their construction
240 has also been detected as far downstream as Vientiane (He et al., 2006).

241 Meanwhile, the Xiaowan and Nuozhadu Dams, commissioned in 2010 and 2014,
242 respectively, are altering streamflow seasonality in the Mekong mainstream much more
243 (Cochrane et al., 2014; Li et al., 2017; Räsänen et al., 2017). Dry-season flows have increased
244 dramatically as far downstream as Kratie, Cambodia (Li et al., 2017), including low flows in
245 March-May 2014 that were 41-74% above pre-dam averages and 41-68% larger than simulated
246 unregulated discharges (Räsänen et al., 2017). These increases fell within the range of earlier
247 projections (Hoanh et al., 2010; Lauri et al., 2012; Räsänen et al. 2012). Releases to increase
248 reservoir storage for wet-season inflows may have delayed the date of the maximum daily flow
249 by 22 days at both Chiang Saen and Stung Treng in 2010-2014 relative to their 1960-1991
250 averages (Li et al., 2017). However, this short post-Xiaowan record limits the statistical strength
251 of this conclusion, as does the filling of Xiaowan (2010) and Nuozhadu (2014) during this post-
252 impact period. Post-Xiaowan daily flow fluctuations are also greater (Cochrane et al., 2014; Li et
253 al., 2017). Li et al. (2017) also associated a dramatic decrease in the duration of low-flow pulses
254 (below the 25th daily flow percentile), which are critical for LMB fisheries (Sabo et al., 2017),
255 with irregular releases from Xiaowan Dam. Yet, while low-flow pulses have shortened, irregular
256 releases have made extreme low flows even lower (Lu et al., 2014). China has planned some
257 measures to mitigate downstream impacts (see Fan et al., 2015; Yu and Geheb, 2017), including
258 attenuating hydropeaking fluctuations with the Ganlanba re-regulation dam (3.13 Mm³) further
259 downstream.
<figure><img src="image_4.png" /><figcaption>Figure 3: Change (%) in mean monthly water levels at key stations in the lower Mekong Basin between 1960-1991 and 1992-2013. (Data source: MRC).</figcaption></figure>
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**3.2 Impacts of Lower Mekong Basin dams on mainstream and tributaries**

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Relatively few streamflow stations downstream of tributary dams have pre- and post-dam flow records exceeding ten years (Lacombe *et al.*, 2014b; Lyon *et al.*, 2017), which limits assessments of tributary flow alteration. While numerous studies emphasize that dams attenuate flow seasonality (e.g. Adamson, 2001; Pokhrel *et al.*, 2018), observed flow records indicate diverse hydrological impacts that depend, in part, on the on-stream vs. off-stream siting of hydropower turbines. To increase the hydraulic head for power generation, numerous tributary dams impound water for diversions to powerhouses at much lower elevations, either further downstream along the same river or into adjacent tributary basins. Only two pumped-storage projects, located in Thailand (Kraitud, 2017), could be found in the Mekong basin.

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As expected, tributary dams with integral powerhouses and large storage reservoirs have attenuated seasonal flow variability. Lacombe *et al.* (2014a) examined daily flow records (available since 1962) measured at several stations along the Nam Ngum River downstream of a cascade of hydropower dams, the oldest of which, Nam 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 about 17% after Nam Ngum 1 Dam was commissioned. In other tributary basins where extensive flow alteration has been projected, such as the 3S basin, initial dams have only caused minor impacts because they are largely run-of-river projects (Piman *et al.*, 2013a). Tributary studies have not examined flow alteration over interannual timescales, despite numerous dams with storage capacities exceeding their annual inflow volume (Bonnema and Hossain, 2017).

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Meanwhile, diversions have been associated with reduced flows downstream of dams as well as flow augmentation in recipient basins (e.g. Baird *et al.*, 2015; Chanudet *et al.*, 2016).
Changes in the Nam Theun/Nam Kading basin in Lao PDR (Figure 4) showcase diverse forms of hydropower-induced flow alteration. The Theun-Hinboun Dam (THXP) enables the diversion of up to 220 m³/s through a tunnel to a powerhouse that discharges into the adjacent Nam Hinboun basin. In contrast, only 5 m³/s must be released directly downstream (Reis et al., 2015), which has raised water temperatures and depleted dissolved oxygen (Warren, 1999). Meanwhile, the Nam Theun 2 (NT2) Dam located further upstream (completed in 2010), enables diversions from the Nam Theun River into the Xe Bang Fai River, a separate Mekong tributary (e.g. Stone, 2010a). Downstream releases from NT2 are just 2 m³/s, less than 1% of its mean annual inflow of 238 m³/s, except during spills (Chanudet et al., 2016). Average diversions entering the Xe Bang Fai (220 m³/s) are nearly equal to the river's natural mean annual flow (266 m³/s at Mahaxai from 1989-2002) (Descloux et al., 2016). This flow alteration affects the livelihoods of over 110,000 people (International Rivers, 2018). While post-NT2 streamflow changes are being monitored in the Nam Theun and Xe Bang Fai basins (Descloux et al., 2016), rigorous assessments of hydrological alteration have not been published. Finally, since NT2 heavily depletes flow in the Nam Theun River, releases from a hydropower reservoir on the Nam Grouang River (NG), a Nam Theun tributary, sustain dry-season hydropower production at the Theun-Hinboun plant further downstream (Reis et al., 2015).
<figure><img src="image_5.png" /><figcaption>Figure 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)</figcaption></figure>

While Mekong diversions have not been comprehensively catalogued, other major tributary projects also have them, including Houay Ho (Lao PDR) and Yali Falls (Vietnam).
Diversions to raise water levels in Nam Ngun 1 Reservoir have drastically reduced dry-season flows in the Nam Song River (Neua, 2007). Irregular electricity demands causing pronounced fluctuations in flow downstream of diversions and in recipient basins have been reported (e.g. Chanudet et al., 2016), but have not been quantified (see Kibler and Alipour, 2017). Kibler and Tullos (2013), who assessed recent hydropower diversions in Southeast Asia's Nu River basin, emphasized that they can make reservoir storage capacity an inappropriate proxy for a dam's hydrological impact.

Next, many dam operators have employed profit-driven 'hydropeaking' strategies to meet energy demands that fluctuate substantially over daily (lower nighttime demands) and weekly (lower weekend demands) timescales (e.g. Wyatt and Baird, 2007; Reis et al., 2015; Trung et al., 2018). Consequently, increased flow fluctuations have been observed downstream of LMB tributary dams. Cochrane et al., (2014) even associated increased daily fluctuations in the Mekong mainstream with hydropeaking in Thailand's Chi-Mun tributary basin. Inadequate warnings about abrupt flow changes have compromised navigation and even led to drowning deaths (Lerner, 2003; Neua, 2007; Southisombat et al., 2011). A few designated re-regulation dams, such as the Sesan 4A dam (Meynell et al., 2014) and ones downstream of the Theun-Hinboun and NT2 projects, aim to curtail flow alteration. However, these dams primarily modulate short-term fluctuations and do not have the storage capacity to reduce seasonal flow alteration or flood hazards from inter-basin transfers significantly. Only modeling studies have addressed the extent to which downstream dams in cascades can also function as re-regulation dams (Jeuland et al., 2014; Piman et al., 2016). They show that cascade-optimal operations can mildly reduce flow alteration (Jeuland et al., 2014) but more studies are needed since impacts can vary considerably among reservoirs (Piman et al., 2016).
In addition, flow alteration during dam construction and reservoir filling has also impacted livelihoods and ecosystems in LMB tributaries and caused safety hazards. In Vietnam, the failure of a temporary diversion dam caused a large flood when the Yali Falls Dam was constructed (Wyatt and Baird, 2007). Moreover, irregular flow releases following its construction killed 39 people (Lerner, 2003). Aquatic life in the Nam Lik River in Lao PDR was deeply disturbed when dam construction completely halted downstream flow in 2010 (Baran et al., 2011). Despite these impacts, more detailed reservoir filling observations are unavailable.

## 3.3 Gaps in observational studies

Short tributary streamflow records, especially ones preceding dam construction, also make distinguishing dam-induced impacts from natural hydroclimatic variability and other ongoing environmental changes more difficult. Procedures for extending short records, filling intermittent records (Inomata et al., 2008), or computing regional estimates of time series or flow statistics at ungauged locations using longer records in hydrologically similar regions could help estimate pre-dam flows at dam-impacted sites (Lacombe et al., 2014b; Ketelsen et al., 2014). Satellite observations have also produced plausible estimates of reservoir residence time and seasonal flow alteration (Bonnema and Hossain, 2017, Liu et al., 2017), and can estimate daily water levels in the mainstream with sub-meter accuracy (Pham et al., 2018). While Lacombe et al. (2014b) and Lyon et al. (2017) have identified subsets of unregulated tributary stations, a comprehensive subset of dam-impacted ones with minimum pre- and post-dam record lengths is not available. Accessible sub-daily data (only daily data are typically available) would improve hydropeaking assessments.

To address record length limitations, some studies have applied hypothesis testing to indicate the likelihood that pre- and post-dam differences do not arise from sampling variability
(Cochrane et al., 2014; Li et al., 2017), although this uncertainty has not been integrated into formal decision-making frameworks (see Hecht, 2017). However, hypothesis testing cannot eliminate the possibility that other confounding factors explain differences between two periods, whereas hydrological simulations using the same meteorological and watershed inputs albeit different dam configurations can assess dam impacts directly. Räsänen et al. (2017) have applied a similar experimental design to assess UMB dams. However, they chose to compare bias-adjusted simulations to observations instead of simulating reservoir operations, which their observations of flow alteration suggest could vary substantially from year to year. While MRC (2011) and MRC (2018) scenarios have been configured to isolate dam impacts from other concurrent changes, they have not been designed for direct comparisons with observed alteration.

We also examined the availability of hydrometeorological observations for simulating reservoir water balances. Although evaporation from many tributary reservoirs may substantially affect firm energy production and exacerbate consequences of uncoordinated reservoir operations (Jeuland et al., 2014), estimates of open-water evaporation are limited (Kummu et al., 2013). Coefficients relating pan and lake evaporation are not widely available. Open-water evaporation estimates using only monthly temperature, solar radiation and sunshine data (Morton, 1983), which have been comparable to estimates obtained using more data-intensive methods in tropical locations (dos Reis and Dias, 1998; Vallet-Coulumb et al., 2001), could offer an initial approach for reservoirs with negligible seepage losses. Finally, effects of reservoir sedimentation on downstream releases have not been assessed in detail.

Ultimately, for hydrological change assessments to improve tradeoffs associated with dam development, they must examine flow indicators relevant for ecosystems and livelihoods. Li et al. (2017) have examined changes in the magnitude and timing of mainstream flow with the
popular Indicators of Hydrologic Alteration (IHA), which aim to capture ecologically relevant dimensions of flow alteration (Richter et al., 1996). Applications of generalized frameworks for characterizing site-specific or transferable flow-ecology relationships, such as the Ecological Limits of Hydrologic Alteration (Poff et al. 2010), are scant. While standard flow-alteration metrics enable comparisons with other locations, indicators tailored to Mekong livelihoods (e.g. Sabo et al., 2017; Zhang et al., 2018), such as dry-season days when riverbank gardens are exposed or, conversely, when navigation is possible, could enrich tradeoff analyses.

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## 4. Projected hydrological impacts

This section focuses on studies of projected hydropower impacts simulated using hydrological models without considering concurrent drivers of hydrological change.

### 4.1 Basin-wide studies

Basin-wide modeling studies have concurred that increased reservoir storage would reduce wet-season flows and increase dry-season flows (e.g. Hoanh et al., 2010; Lauri et al., 2012; Piman et al., 2013) (Figure 5). Using hydrological conditions from 1985-2000, Hoanh et al. (2010) investigated impacts of 6 UMB and 81 LMB dams (including 11 mainstream dams), with a cumulative active storage capacity equal to 16% of the basin's annual flow at Kratie, Cambodia. Together, these prospective dams and other forms of water resources development through 2030 would reduce wet-season flows by 8-17% and increase dry-season flows by 30-60% relative to a baseline scenario with just 18 LMB tributary dams. In a related study, Piman et al. (2013a) found that six UMB dams and 82 LMB tributary dams (including 11 mainstream dams) would increase dry-season flows at Kratie by 28% whereas wet-season flows would drop by 9%. Under a long-range development scenario through 2060 featuring 136 LMB and 6 UMB
dams, wet-season flows would decline by an additional 4% (-13% from baseline) whereas dry-season flows would increase by just 1% primarily due to offsetting irrigation withdrawals. In contrast, Lauri *et al.* (2012) simulate greater dry-season increases (25-160%) and wet-season decreases (5-24%) at Kratie since they examined seasonal flow changes strictly from a 126-dam cumulative development scenario using discharge from 1982-1992. While Dang *et al.* (2018) recently examined hydrological impacts of basin-wide hydropower development in the delta, no peer-reviewed studies since Lauri *et al.* (2012) have examined them throughout the LMB.

Finally, impacts of basin-wide hydropower development on flooding have also been assessed. Using representative dry (1998), normal (1997) and wet (2000) years, Piman *et al.* (2013a) also predicted that six UMB mainstream dams and 71 LMB tributary dams would decrease the river's annually flooded area by 6.6% by 2030 in normal years, but only by 1% during wet years.

<figure><img src="image_6.png" /></figure>

<figure><img src="image_7.png" /></figure>

<figure><img src="image_8.png" /></figure>

<figure><img src="image_9.png" /></figure>
**Figure 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 Figure 1.

## Projected effects on the Lower Mekong floodplain

Numerous studies have projected impacts of basin-wide hydropower development on the Lower Mekong floodplain, which begins downstream of Kratie in central Cambodia. These floodplains have a unique hydrology, as Tonle Sap Lake in Cambodia effectively serves as a natural flood control reservoir for the Mekong River. In a typical year, the lake's surface area expands from a dry-season minimum of 2,400 km² to a wet-season maximum of 13,200 km² (Kummu and Sarkkula, 2008). The hydraulic gradient between the Mekong River and Tonle Sap Lake determines the flow between the two bodies of water, as Mekong flow enters the lake via the Tonle Sap River during the wet season while the lake drains to the Mekong delta during the dry season. Since 54% of its annual inflow originates from the Mekong mainstream on average (Kummu et al., 2013), the lake ecosystem and its extremely productive fishery are sensitive to hydrological changes from (e.g. Lamberts, 2008; Arias et al., 2014a). Upstream hydropower development is expected to attenuate inter-seasonal variations in lake area and exacerbate impacts to aquatic and amphibious species during years with unusually dry monsoons (Piman et al., 2013a; Arias et al., 2014c).

As one travels downstream into Vietnam, it becomes increasingly difficult to distinguish hydropower-induced hydrological alteration from numerous local drivers of change, including sea level rise, flood control and navigation infrastructure, irrigation, and land subsidence exacerbated by groundwater pumping (e.g. Le et al., 2007; Hoa et al., 2008; Västilä et al., 2010; Dang et al. 2016). Dang et al. (2018) found local water infrastructure development controlled
437 wet-season levels in the upper Vietnamese delta while sea level rise and land subsidence
438 increasingly influenced them near the coast. Upstream dam impacts are expected to be greater
439 during dry years (Piman et al., 2013a; Dang et al., 2018). Emergency releases from UMB dams
440 to alleviate the LMB drought in 2016 (MRC and MWRC, 2016) corroborated previous
441 predictions (e.g. Hoa et al., 2008; Hoanh et al., 2010; Piman et al., 2013a; Smajgl et al., 2015)
442 that dry-season releases from UMB hydropower dams could alleviate salinity and acidic
443 groundwater constraints to dry-season agriculture in the delta. However, Piman et al. (2013a)
444 projected that, by 2030, sea level rise will offset agricultural gains from dry-season releases
445 made under normal hydropower operations. Moreover, a drastic reduction in sediment supply
446 due to trapping in upstream reservoirs (e.g. Kummu et al., 2010; Kondolf et al., 2018) and, to a
447 lesser extent, reduced cyclonic activity (Darby et al., 2016) is expected to aggravate flooding.

448 **4.2 Upper Mekong dams on mainstream flow**

449 Potential hydrological changes from UMB dams have attracted considerable attention,
450 partly due to their transboundary implications (e.g. Chapman and He, 1996; Adamson et al.,
451 2001; MRC, 2011; Lauri et al., 2012; Räsänen et al., 2012; Wang et al., 2017). Studies concur
452 that UMB dams would attenuate seasonal flow variability in the LMB mainstream. For instance,
453 Räsänen et al. (2012) projected that six UMB dams would increase dry-season flow by 90% and
454 decrease wet-season flow by 22% at Chiang Saen. Recent observations of dam-altered flows
455 have corroborated these projections (Cochrane et al., 2014; Li et al., 2017; Räsänen et al., 2017),
456 although longer post-dam records are needed for stronger statistical conclusions. Overall, UMB
457 flow regulation has been expected to diminish further downstream because LMB tributaries have
458 higher annual rainfall and less reservoir storage. Some studies have also predicted that the flood
459 season will commence later due to reservoir filling (e.g. Adamson et al., 2001), which recent
observations have also suggested (Räsänen et al., 2017; Li et al., 2017). Wang et al. (2017) also estimated that UMB reservoirs could reduce the frequency of floods exceeding 0.5-year recurrence intervals by 72% at Chiang Saen but only by 6.0% at Stung Treng, Cambodia. Reductions in annual maximum floods were lower (29.5% at Chiang Saen and 2.6% at Stung Treng), possibly due to reservoir storage limitations.

## 4.3 Lower Mekong basin dams on mainstream and tributaries

Numerous studies have evaluated the potential hydrological effects of the 11 LMB mainstream dams (ICEM, 2010; MRC, 2011; VNMRE, 2015; Trung et al., 2018). Although their reservoirs are expected to convert many reaches from lotic to lentic (lacustrine) habitat, seasonal flow alteration from them is expected to be minimal since they store no more than a few days of their mean annual inflow. However, alteration at sub-daily to daily timescales from hydropeaking is expected to be substantial. For instance, Trung et al. (2018) projected that dry-season flows at Kratie (31 km below the dam furthest downstream in the cascade) might fluctuate by 16,000 m³/s and water levels might oscillate by 2 m. In addition, geomorphic changes from damming are expected to deepen the channel, which could expedite the propagation of flood waves to the delta by a few days.

While much of the basin's dam polemic has focused on mainstream impacts, substantial flow alteration has been projected in many tributaries. One focal point of tributary studies has been the Sekong, Sesan and Srepok (3S) rivers (Wyatt and Baird, 2007; Piman et al., 2013a, Arias et al., 2014b; Wild and Loucks, 2014; Räsänen et al., 2014; Piman et al., 2016; Ngo et al., 2017), the source of 16-23% of the Mekong's annual flow (Adamson et al., 2009; Piman et al., 2013b; Trang et al., 2017). Piman et al. (2013b) found that dry-season flows in this tri-national watershed (Lao PDR, Cambodia, Vietnam) could increase by 63% and wet-season flows may
decline by 22% when considering 23 proposed hydropower projects aiming to maximize their own hydropower production. Moreover, under a full development scenario consisting of 41 projects, Piman et al. (2013b) projected a dry-season increase of 95% and a wet season decrease of 25%, while Piman et al. (2016) obtained similar results. They also found impacts on annual daily flow extremes to be even greater, as annual daily maxima and minima could be reduced (increased) by 36% and 168%, respectively. However, their hydropower-maximizing operating rules did not consider irregular releases associated with hydropeaking. In addition, they showed that existing and near-term projects offered a much greater energy return per unit of reservoir storage than larger long-term projects.

Other 3S basin studies have examined impacts to just the Sesan River. Räsänen et al. (2014) found that assumed hydropower-maximizing rules at the 11 largest hydropower dams would increase dry-season flows by 53% and reduce wet-season flows by 11%. However, when dam operations in the Vietnamese portion of the Sesan River basin were simulated using government-issued operating rules, dry-season flows increased by just 30% whereas wet-season flows dropped by 15% (Ngo et al., 2017). When minimum-flow requirements at the Cambodian border downstream of the dams were also considered, dry-season flows rose by 40%. It also suggests that minimum-flow requirements are designed to allocate increased dry-season flows for productive uses as well as preserve natural dry-season flows. While other modeling differences between these two studies prevent a direct comparison, these findings motivate further efforts to include actual operating rules in reservoir simulations.

Flow alteration in the 3S basin could also propagate downstream into the Mekong mainstream and Lower Mekong floodplain. Arias et al. (2014b) estimated that 42 new 3S basin dams could increase the lake's annual minimum level by 25-35 cm, which could inhibit tree
germination and fish migration. Piman *et al.* (2013a) showed that reductions in the annual maximum water level are less certain and that the Tonle Sap River flow reversal may vary by a few weeks in either direction of the pre-development date (6-8 days earlier in a typical year). The earlier reversal during a typical year stems from higher mainstream water levels at the end of the dry season. Hydropower and irrigation reservoirs within the lake’s catchment might further attenuate seasonal lake-level extremes upon which many ecosystems and livelihoods depend.

Potential dam impacts have also been simulated in other tributary basins, including innovative single- and multi-reservoir operating rules. Reis *et al.* (2015) showed that maintaining low reservoir levels in the Nam Grouang Reservoir (see Figure 4) would facilitate recession agriculture while reducing average annual hydropower by only 8.1%, although it substantially increases hourly peak demand shortfalls between April and June. Sioudom (2013) simulated increases in flooding in the Nam Hinboun basin caused by diversions from the Nam Theun River. In the Nam Ngum basin, Jeuland *et al.* (2014) demonstrated that multi-reservoir coordination could increase basin-wide benefits from hydropower and irrigation by 3-12%. Relatively few basin-wide modeling studies have assessed sub-daily flow alteration from hydropeaking in LMB tributaries (Reis *et al.*, 2015).

Finally, the ability of dams to convey large floods downstream has also been investigated, but merits further assessment. Ketelsen *et al.* (2014) found that 40% of 67 LMB dams have design peak inflows lower than their 100-year floods, and 11% do not even have spillways that can convey this discharge. Emergency dam releases can also magnify floods (Vattenfall Power Consultant AB, 2008), although these impacts have not been studied systematically.
528 **4.4 Sediment impacts**

529 Next, dam-induced flow alteration is not expected to be constant over time due to
530 changes in sediment transport that will affect reservoir and channel capacities. Projected rates of
531 reservoir sedimentation vary widely. Wild and Loucks (2014) predicted that only 5 out of 41 3S
532 basin reservoirs would lose more than 20% of their active storage over the next century due to
533 their large dead storage volumes. In contrast, Manwan Dam's reservoir, located in the erosion-
534 prone UMB, had already lost 16.7% of its active storage capacity (0.26 km³) and 75.9% of its
535 dead storage capacity (0.66 km³) between 1993 and 2009 before Xiaowan Dam and erosion
536 control projects were constructed upstream (Fan et al., 2015).

537 Basin-wide changes in sediment loads have also been examined (e.g. Kondolf et al.,
538 2018). MRC monitoring data indicate a dramatic reduction in suspended sediment loads
539 following the construction of Xiaowan Dam, when the average suspended loads at the Chiang
540 Saen station in northern Thailand decreased from 60 to 10 million tonnes per year (Mt/yr), an
541 83% reduction, at Pakse in Lao PDR from 120 to 60 Mt/yr, (a 50% reduction), and at Kratie,
542 Cambodia from 160 to 90 Mt/yr, a 43% reduction (Piman and Shrestha, 2017). These decreases
543 are consistent with prior modeling results (Kummu et al., 2010, Kondolf et al., 2014). If all
544 planned Mekong basin dams are built, the sediment load entering the delta may decrease by 51-
545 69% (Kummu et al., 2010) to 96% (Kondolf et al., 2014). (See Wild et al. (2015) for options for
546 routing sediment through reservoirs.) Overall, more information about the interannual variability
547 of sediment loads, trapping in multi-reservoir cascades (e.g. Kummu et al., 2010; Wild et al.,
548 2014), deposition in active storage zones, reservoir sediment compaction, and changes in
549 upstream erosion (e.g. Shrestha et al., 2013) is needed. Effects of changes in channel and
floodplain storage capacity downstream of dams (e.g. Rubin *et al.*, 2015) on flood propagation are starting to be assessed (Trung *et al.*, 2018).

## 4.5 Gaps in studies of projected impacts

Hydrological impacts are frequently simulated within different areas of the Mekong basin, but basin-wide projections, including ones considering other ongoing water resources developments, are less frequent, ostensibly due to the large modeling effort required. In contrast, many tributary projections do not employ sets of scenarios that isolate effects of different drivers of change. More model comparison studies are also needed. As our review of 3S basin projections demonstrates, differences in reservoir operating rules, input data, study area definitions, and other assumptions can inhibit these comparisons.

Currently, many modeling studies focus on dam impacts without considering alternative operating rules, or design choices, including re-regulation dams. Moreover, many assumptions, such as hydropower maximization, are made when operating rules are unavailable. More efforts to optimize the design and operation of individual dams, cascades (see Bogardi and Duckstein, 1992; Schmitt *et al.*, 2018), or dam implementation sequences in basins (Ziv *et al.*, 2012; Grill *et al.*, 2014; Jager *et al.*, 2015) could reveal mitigation opportunities. Flood hazards due to dam failure, emergency spills, multi-reservoir operation, reservoir-induced backwater effects (Rubin *et al.*, 2015), and geomorphic changes merit further research. Finally, while scrutinizing contemporary hydrological modeling practices in Mekong basin studies lies beyond this review (see Kite (2001) and Johnston and Kummu (2012) for reviews of earlier modeling studies), one must recognize that short observational records, along with sparse meteorological station networks (e.g. Lacombe *et al.*, 2014b; Räsänen *et al.*, 2014; Lyon *et al.*, 2017), constrain the calibration and validation of models used in projections.
573

574

**5. Effects of concurrent environmental changes**

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578

Many ongoing changes in climate, water use, and land cover are occurring as new dams are being constructed. In this section, we build on the recent review by Pokhrel *et al.* (2018) by evaluating the extent to which these concurrent drivers of change could exacerbate or offset hydrological changes stemming from hydropower production.

579

**5.1 Climate change**

580

581

582

583

584

585

586

587

588

589

590

591

592

593

594

595

Numerous studies (e.g. Kingston *et al.*, 2011; Cook *et al.*, 2012; Lauri *et al.*, 2012; Thompson *et al.*, 2013; Fan and He, 2015; Hoang *et al.* 2016; Shrestha *et al.*, 2016; Wang *et al.*, 2017) have evaluated possible hydrological effects of future changes in precipitation and temperature. Hydrological models run with recent Climate Model Intercomparison Project 5 (CMIP5) projections have suggested that streamflow will increase during both the wet (8 out of 10 scenarios) and dry seasons (all 10 scenarios) at the basin scale, with the uncertainty stemming primarily from precipitation projections (Hoang *et al.*, 2016). In the LMB, these increases in precipitation are consistent with observations from the last half century (Lacombe *et al.*, 2013). However, these projected changes are not ubiquitous, as annual flows may decrease in some tributary basins (e.g. Shrestha *et al.*, 2016; Ngo *et al.*, 2017). Global climate models (GCMs) also consistently predict increases in extreme floods (Hirabayashi *et al.*, 2013; Hoang *et al.*, 2016; Perera *et al.*, 2017) while projected changes in mean annual floods are less consistent (e.g. Wang *et al.*, 2017) and observations at some mainstream stations indicate a decreasing trend (Delgado *et al.*, 2010). GCM projections of future droughts are also uncertain, as Kiem *et al.* (2008) projected reduced streamflow drought severity due to greater precipitation whereas Thilakaranthe *et al.* (2017) forecasted that greater interannual variability would worsen droughts.
GCMs have been essential for projecting combined impacts of hydropower and climate change. Wang *et al.* (2017) noted that UMB reservoir storage could alleviate increased flooding along the upper LMB mainstream (down to Nong Khai, Thailand), where they predict flood-control benefits of reservoir storage will outweigh climate-driven increases in flood magnitude through 2070. However, they expect increased precipitation to immediately outweigh the flood-control benefits of reservoir storage further downstream. Hoanh *et al.* (2010) also stressed that reservoirs cannot mitigate projected climate-induced flood increases, as they would reduce the LMB's seasonally flooded area by less than 1%. Lauri *et al.* (2012) reported that climate change could either exacerbate or offset 10% reductions in wet-season flow from hydropower at Kratie from 2032-2042 (-21-4%). While many GCMs predict increased dry-season flows (e.g. Hoang *et al.*, 2016), even the decreases that some models project are unlikely to fully offset large hydropower-induced increases.

Other studies examining joint hydropower-climate change impacts have focused on specific tributary basins. In the 3S basin, Ngo *et al.* (2017) predicted that climate change would partially offset increases in dry-season flows from Jan-May, offset decreases in Jun-Jul, while changes from Aug-Dec vary by sub-basin. Using ten GCM runs from Lauri *et al.* (2012), Arias *et al.* (2014a) estimated that hydropower development may worsen climate change-induced habitat losses to Tonle Sap Lake, as expanded areas of open water (32-38%), along with irrigated and rain-fed rice (11-21%), may drastically reduce riparian forests (13-67%) and other seasonally flooded habitat. In other tributary basins, such as the Nam Ou in northern Lao PDR, uncertain directions of change in precipitation (-27-41%) and annual streamflow (-27-160%) (Shrestha *et al.*, 2013) confound efforts to mitigate hydropower impacts. Few studies have examined whether
618 climate-induced increases in streamflow could offset flow depletion downstream of diversions
619 and exacerbate flooding in surcharged basins.

620 **5.2 Water demand**

621 Historically, the Mekong has been one of the least irrigated basins in monsoonal Asia
622 (Barker and Molle, 2004; Ringler et al., 2004; Haddeland et al., 2006). Yet, higher dry-season
623 flows from hydropower are enabling greater dry-season irrigation withdrawals. In turn, these
624 larger withdrawals are expected to offset elevated dry-season flows increasingly since the largest
625 dams have already been commissioned (MRC, 2011). Indeed, the complementarity of
626 hydropower and irrigation in the Mekong basin (e.g. Lacombe et al., 2014a) differs from some
627 other basins where they directly compete with one another (Zeng and Cai, 2017).

628 LMB irrigation may expand from 6.6 million ha in 2010 to 8.2-9.7 million ha in 2030
629 (Hoanh et al., 2010; MRC, 2011). This includes a dry-season increase from 1.2 to 1.8 million ha
630 (MRC, 2011), which, considering these estimates of total irrigation expansion, suggest a wet-
631 season increase of 1.0-2.5 million ha. (Note that irrigated areas are difficult to estimate, partly
632 due to the diverse forms of irrigation in the basin (Ringler et al., 2004; Pokhrel et al., 2018)).
633 MRC (2018) is also developing new projections of water withdrawals for irrigation. Some
634 studies project that most irrigation expansion will take place in Lao PDR and Cambodia, where
635 more land is available, and both reservoir storage and export-driven production are growing
636 rapidly (ICEM, 2010; Erban and Gorelick, 2016; Trung et al., 2018). However, elevated dry-
637 season flows in the mainstream could also be diverted to northeastern Thailand, where irrigation
638 infrastructure already exists (Molle and Floch, 2008). Numerous long-term hydropower impact
639 studies (Hoanh et al., 2010; MRC, 2011) that include future changes in irrigation estimate dry-
640 season flow increases smaller than the ones that only consider hydropower development (Lauri
et al., 2012). Few studies have examined decreases in irrigation opportunities downstream of diversions while higher dry-season flows in recipient basins have spawned irrigation projects (Baird et al., 2015)

Next, the extent to which wet-season irrigation could exacerbate dam-induced reductions in wet-season flow has not received as much attention even though less than 20% of all irrigated land was for dry-season use in 2010 (MRC, 2011). Warmer temperatures could further increase this demand (Hoanh et al., 2010; Mainuddin et al., 2012). While domestic and industrial sectors consume much less water than agriculture, a doubling of the water demand by 2030 could worsen dry-season droughts (e.g. Johnston et al., 2010), making flow regulation from hydropower more valuable for water supply.

While hydropower development can provide extensive irrigation co-benefits during the dry season (Lacombe et al., 2014a), rising water levels upstream of new mainstream dams and channel migration below them may require as many as half of all existing and planned pumping stations to be relocated, resized and retrofitted to deal with greater daily flow fluctuations (ICEM, 2010). (In contrast, Pokhrel et al. (2018) suggest that higher dry-season flows may elevate groundwater levels, thereby reducing pumping costs). Many riverbank gardens may become permanently inundated, including 54% on the Mekong mainstream (ICEM, 2010). Higher dry-season flows may also force farmers to grow new varieties of rice (Fox and Wood, 2007). Increased withdrawals for producing food to replace declining capture fisheries (see Orr et al., 2012) have not been assessed.

## 5.3 Land cover change

While few studies directly compare the hydrological impacts of land cover and hydropower, numerous basin-wide studies (e.g. Kite, 2001; Kiem et al., 2008; Costa-Cabral et
al., 2008; MRC 2018) as well as ones in smaller tributary watersheds (e.g. Lacombe et al., 2010; Ty, 2012; Lacombe and Pierret, 2013; Lacombe et al., 2014b, 2016; Lyon et al., 2017; Shrestha et al., 2017) offer some insight regarding the extent to which changes in land cover may offset or exacerbate hydropower-induced flow alteration. These impacts depend on many site-specific factors. For instance, when low-density forest is converted to intensive agriculture, deforestation will exacerbate decreases in high flows from hydropower (Shrestha et al., 2017). However, deforestation has often been associated with increased runoff due to the lower evapotranspiration of newly developed land cover (e.g. Lacombe et al., 2010; Lacombe and Pierret, 2013). Lacombe et al. (2016) observed that afforestation from planting and natural regeneration had opposing effects on annual streamflow since planting, especially for commercial timber, tends to reduce soil infiltration capacity whereas natural regeneration increases it. These caveats also underscore the importance of site-specific studies for evaluating forest preservation policies upstream of hydropower reservoirs (Arias et al., 2011).

Hydrological effects of changes in land cover stemming from economic development catalyzed by new dams, such as road construction and mining, also merit consideration (e.g. Shrestha et al., 2016; Hennig et al., 2016; Hoang and Tran, 2017; Pokhrel et al., 2018). Orr et al. (2012) estimated that livestock production areas would need to expand by 19-63% to compensate for a 40% loss in fish protein (without livestock intensification or expansion of aquaculture or reservoir fisheries). However, they did not spatially model this land conversion, which inhibits it from being assessed with a distributed hydrological model. While increased withdrawals for rice irrigation have been examined (e.g. Hoanh et al., 2010; Piman et al., 2013a), the effects of rice paddies on surface runoff (e.g. Costa-Cabral et al., 2008; Homdee et al., 2011; Lacombe et al., 2014b), have not been incorporated into hydropower impact studies.
**6. Concluding remarks**

689 Understanding hydropower-induced changes to the hydrology of the Mekong basin is
690 essential for anticipating and mitigating their negative impacts. Although many studies have
691 analyzed its hydrological impacts, observed and projected changes to both mainstream and
692 tributaries flows have not been reviewed in sufficient detail. While most dams with integral
693 powerhouses are expected to regulate seasonal flow variability, diversions for off-stream power
694 generation can drastically decrease downstream flows and augment flows in adjacent basins, a
695 change which prior reviews have not captured. Meanwhile, LMB mainstream dams are expected
696 to affect primarily diurnal flow variability, though their effects on flood propagation are
697 underexplored. Flood hazards from dam failures, emergency spills and inter-basin transfers also
698 require further study, as does reservoir evaporation.

699 We also assess whether concurrent changes in climate, water demand, and land cover
700 offset or exacerbate hydrological impacts of hydropower. While some uncertainty in the
701 direction of hydrological responses to climate change remains, modeling studies increasingly
702 suggest that both wet- and dry-season flows will increase in many parts of the basin. Under this
703 circumstance, on-stream hydropower development could play an increasingly important role in
704 offsetting climate-change impacts during the wet season but would exacerbate them during the
705 dry season. Meanwhile, increased dry-season flows could enable irrigation to meet growing food
706 and export demands, as allowed by land availability. However, fertile land flooded by new
707 impoundments or temporarily submerged by higher river water levels (e.g. riverbank gardens)
708 may compromise many agricultural livelihoods. The joint impacts of these concurrent changes
709 and hydropower also depends on the on-site vs. off-site nature of hydropower production. For
instance, greater precipitation could offset impacts of diversions in donor basins while aggravating flood hazards in recipient basins.

However, comparing hydrological alteration from dams and other concurrent drivers of change is insufficient for addressing transboundary and multi-sectoral tradeoffs of dam development. Rather, integrated models featuring indirect pathways through which hydropower induces hydrological alteration, such as land-cover change near dams, are necessary to fully examine them (e.g. Ringler et al., 2004; Pokhrel et al., 2018). Future investments that increase evapotranspiration, such as food crops or timber, should carefully consider possible decreases in streamflow and, consequently, hydropower production. The effects of deforestation-driven increases in sediment yields on reservoir operations should also be assessed.

In addition, while flow alteration often harms riverine ecosystems (Poff et al., 1997), it could also improve some livelihood-relevant ecological indicators (Sabo et al., 2017). This uncertainty further underscores the need for more quantitative relationships between flow alteration and ecological responses. Such assessments must untangle other drivers of ecological change, such as connectivity and sediment concentrations, from flow alteration (e.g. McManamay et al., 2013; Knight et al., 2014; Hogan et al., 2018). Multi-reservoir operation rules that mitigate or even improve ecological and livelihood impacts (e.g. Lacombe et al., 2014a; Jeuland et al., 2014; Reis et al., 2015) also merit further research, as do ecological effects of dam construction sequences.

Finally, our review offers guidance for research in other basins where extensive hydropower development is possible. We especially encourage the coordination of observational and modeling studies when assessing dam-induced flow alteration. In addition, studies should consider: (i) uncertainty associated with the length of pre- and post-dam streamflow records, (ii)
733 hydrological change metrics relevant for local ecosystems and livelihoods, (iii) different timescales of hydrological alteration, including sub-daily ones, (iv) different modes of hydropower production (on-stream vs. off-stream), (v) impacts of inter-basin transfers, (vi) dam operations during floods, (vii) concurrent environmental changes that might offset or exacerbate hydropower-induced ones, (viii) direct and indirect pathways through which dams can alter streamflow, (ix) scheduling and coordination of hydropower projects to minimize basin-wide impacts, and (x) long-term effects of reservoir sedimentation. Addressing these issues could improve tradeoffs between hydropower production and other river management objectives.

741 Acknowledgements

742 This research originated from an earlier review (Hecht and Lacombe, 2014) published as part of the State of Knowledge Series on Mekong hydropower that the Consultative Group on International Agricultural Research (CGIAR) Research's Program on Water, Land and Ecosystems (WLE) coordinated. This review was initiated during an internship that the first author had with the International Water Management Institute's Southeast Asia Regional Office, which was funded by the National Science Foundation's Water Diplomacy IGERT program at Tufts University (Grant 0966093) without any involvement in the direction of this manuscript. The authors' current institutions provided additional in-kind support. Finally, the authors do not have any individual or collective conflicts of interest.

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