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Future changes in Mekong River hydrology: impact of climate change and reservoir operation on discharge
Abstract and Figures
The transboundary Mekong River is facing two on-going changes that are estimated to significantly impact its hydrology and the characteristics of its exceptional flood pulse. The rapid economic development of the riparian countries has led to massive plans for hydropower construction, and the projected climate change is expected to alter the monsoon patterns and increase temperature in the basin. The aim of this study is to assess the cumulative impact of these factors on the hydrology of the Mekong within next 20–30 yr. We downscaled output of five General Circulation Models (GCMs) that were found to perform well in the Mekong region. For the simulation of reservoir operation, we used an optimisation approach to estimate the operation of multiple reservoirs, including both existing and planned hydropower reservoirs. For hydrological assessment, we used a distributed hydrological model, VMod, with a grid resolution of 5 km × 5 km. In terms of climate change's impact to hydrology, we found a high variation in the discharge results depending on which of the GCMs is used as input. The simulated change in discharge at Kratie (Cambodia) between the baseline (1982–1992) and projected time period (2032–2042) ranges from −11% to +15% for the wet season and −10% to +13% for the dry season. Our analysis also shows that the changes in discharge due to planned reservoir operations are clearly larger than those simulated due to climate change: 25–160% higher dry season flows and 5–24% lower flood peaks in Kratie. The projected cumulative impacts follow rather closely the reservoir operation impacts, with an envelope around them induced by the different GCMs. Our results thus indicate that within the coming 20–30 yr, the operation of planned hydropower reservoirs is likely to have a larger impact on the Mekong hydrograph than the impacts of climate change, particularly during the dry season. On the other hand, climate change will increase the uncertainty of the estimated hydropower impacts. Consequently, both dam planners and dam operators should pay better attention to the cumulative impacts of climate change and reservoir operation to the aquatic ecosystems, including the multibillion-dollar Mekong fisheries.
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... Runoff is a basic element of the hydrological cycle, which causes the change of river water regimes. Rainfall contributes to 80-90% of runoff in the LMRB, which is the major factor influencing flood occurrence (Delgado et al., 2012;Lauri et al., 2012;Wang et al., 2022). ...
... °C and by 1.2-8.6%, respectively, for the years 2032-2042 compared to the baseline of 1982-1992 (Lauri et al., 2012). Compared to the baseline of 1971-2000, the daily average temperature during 2036-2065 was predicted to increase by 2.4 °C and 1.9 °C from Representative Concentration Pathway (RCP) 8.5 and RCP4.5 ensembles under CMIP5 climate projections, respectively; while annual precipitation from the two ensembles increased by −3 to 5% (RCP8.5) ...
... The temperature increase tends to be greater in the southern and northern parts of the basin, whereas the patterns for annual precipitation varied with GCMs and emission scenarios but increases were more likely in the Lancang River basin (Hoang et al., 2016;Lauri et al., 2012). Patterns for precipitation changes were different even under the same emission scenario: e.g., the largest increase was in the middle basin for three GCMs, while in the northernmost and southern parts for the remaining GCMs (Lauri et al., 2012). ...
Droughts and floods are the main threats to the Lancang-Mekong River Basin (LMRB) . Drought mainly occurs during the dry season, especially in March and April, in the LMRB. The “dry gets drier” paradigm performs well in the LMRB, specifically in the Mekong Delta. Further, flood frequency and magnitude, which are determined by heavy rain, are also increasing in the LMRB. Droughts and floods show obvious seasonal and regional characteristics in the LMRB . The LMRB is a well-known rainstorm-flood basin. Floods in the LMRB are mainly caused by heavy rain. The LMRB is dominated by regional floods, and basin-wide floods rarely occur. From upstream to downstream, the flood peak and flood volume have shown increasing trends. Meanwhile, moving further downstream, the flood season ends later. In the upstream areas, floods are mainly concentrated in the period from July to October, with the highest probability of floods occurring in August. For the downstream areas, the flood season is from August to October. Climate change is one of the major factors affecting the LMRB’s droughts and floods . Global warming is an indisputable fact. Under global warming, extreme hydrological events show a tendency to increase. Climate models have suggested a future potential for increased flood frequency, magnitude, and inundation in the LMRB by 10–140%, 5–44% and 19–43%, respectively. Although the severity and duration of droughts are also increasing, the differences in drought indicators projected by different climate models are significant. Hydropower development was another major factor affecting droughts and floods in the LMRB . Large-scale hydropower development has drastically changed streamflow characteristics since 2009, causing increased dry season flow (+150%) and decreased wet season flow (−25%), as well as reduced flood magnitude (−2.3 to −29.7%) and frequency (−8.2 to −74.1%). Large-scale reservoirs will have a profound impact on hydrological characteristics, droughts and floods, agriculture, fisheries, energy supply, and environmental protection in the LMRB. Coupling climate models and hydrological models is the main way to study the impact of climate change and reservoir operation in the LMRB . Climate change indirectly affects hydrological characteristics by affecting meteorological parameters, while reservoirs can directly change the propagation from meteorological extreme events to hydrological extreme events by releasing/storing water in different situations. Hydrological models are the link connecting and quantifying the coupled effects of climate change and reservoirs. More studies are needed to develop a comprehensive understanding of the future impacts of climate change and reservoir operation on extreme events in the LMRB, as well as adaptation and mitigation measures.
... The construction and operation of reservoirs in river basins have a significant impact on river flow, sediment transport, channel morphology, and riverine ecosystems 2-7 . Some scholars, after evaluating the impact of dam construction on downstream water conditions, have pointed out that the duration and magnitude of river flow and peak flow change after the construction of dams [8][9][10][11][12][13][14] . After the construction of the Garrison Dam on the upper Missouri River in the United States, the peak flow decreased from an extreme value of 10,279 m 3 /s upstream of the dam to 4390 m 3 /s 12 . ...
... After the construction of the Garrison Dam on the upper Missouri River in the United States, the peak flow decreased from an extreme value of 10,279 m 3 /s upstream of the dam to 4390 m 3 /s 12 . In the Mekong River, reservoirs cause an increase in average monthly flow by 25-160% during the non-flood season (December-May) and a decrease by 3-53% during the flood season (June-October) 13 . Some studies 14-18 evaluating the impact of dam construction on hydrological conditions and sediment transport have found that the construction of dams has led to significant changes in sediment flux in many of the world's major rivers, such as the Mississippi River 19 in the United States, the Nile River 20 in Egypt, the Yangtze River 21,22 , and the Yellow River 23-25 in China. ...
The construction of large reservoirs has modified the process of water and sediment transport downstream, resulting in changes in the morphology of the river cross-section. Changes in water and sand transport and cross-sectional morphology are reflected in the rating curve at the cross-section. This study analyzed the variations in the rating curve at the Huayuankou (HYK) section and their influencing factors, and conducted water level predictions based on this relationship. The findings revealed that while the annual mean water level has shown a declining tendency over the past 20 years, the annual mean discharge has shown a constant pattern. The rating curve at this stretch narrowed from a rope-loop type curve in its natural condition to a more stable single curve as a result of the construction of the dam upstream of the HYK section. The effect of pre-flood section morphology and the water–sediment process on the scattering degree of the rating curve is inverse; increasing roughness and hydraulic radius decreases scattering degree, while increasing sand content and sand transport rate increases scattering degree. Using the measured data from 2020 as an example, the feasibility of predicting cross-sectional water levels using the rating curve was verified. The prediction results were accurate when the flow was between 1000 and 2800 m³/s; However, when the flow was between 2800 and 4000 m³/s, the forecast results were typically slightly lower than the measured values. Overall, the method demonstrates good predictive accuracy. Insight from the method can be used to predict water levels to better inform decision making about water resources management, and flood emergency response in the lower Yellow River.
... Previous studies have suggested that these declines may variously be attributed to: (i) reductions in ood magnitudes on the Mekong River due to climate change [16][17][18][19] ; (ii) reductions in ood season water levels on the Mekong River due to ow regulation by upstream dams 15,[20][21][22] , and urban development (projects for irrigation, ood control, domestic water supply, and navigation) in the Mekong basin 11,16,20 and; (iii) infrastructure development on the Tonle Sap oodplain (road improvements, embankments), σ σ σ σ σ σ which has obstructed water ow from the Mekong River to the Tonle Sap oodplain and prevented it from reaching the lake 9 . However, rainfall and ow measurements do not wholly support the rst two points. ...
The Tonle Sap Lake (TSL), a vital component of the Mekong River, is renowned as one of the world’s most productive lake-wetland systems. The lake’s high productivity is intimately related to an annual flood pulse that is driven by Mekong River flood waters forcing a unique flow reversal along the Tonle Sap River into the lake. During the dry season the floodwaters are returned to the Mekong River, sustaining vital freshwater fluxes to the downstream delta, inhabited by 23 million people. Recent observations have revealed notable changes in the timing and duration of the reverse flow into the TSL, resulting in associated reductions in lake inundation extents. Previous work has identified changes in flow regimes as a possible cause of the observed decline of the reverse flow. In contrast, here we show how riverbed lowering along the mainstem of the Mekong River – driven by accelerating channel bed sand mining and trapping of sediments through upstream hydropower damming – of 3.06 m (σ= 2.03 m), has resulted in a reduction of the water flux into the TSL by up to 47% from 1998 to 2018. We additionally show that projected future (to the year 2038) riverbed lowering, resulting from ongoing sandmining, of up to 5.92 m (σ) = 2.84 m), would result in a further decline of water flux into the TSL of ~ 69% relative to the bathymetry condition in 1998. These ongoing reductions are reducing the maximum extent of seasonally flooded areas by ~ 40% around the lake, presenting a critical threat to its biological productivity and the entire functioning of the TSL flood pulse system. Additionally, these changes in the reverse flow would increase, by around 26 billion m³, the flow that would be transmitted downstream into the Mekong delta during the monsoon season, potentially contributing to increased flood risk downstream as well as reducing dry season ‘return’ water fluxes to the delta by 59%, presenting risks of accelerated saltwater intrusion and reduced agricultural productivity within the delta. Taken together our modelling results show the importance of sediment and river bed levels to the sustainability of the TSL flood pulse and that its future function will be significantly diminished if current levels of sediment extraction from the Mekong system continue.
... NASA, (2021) suggest that under scenario SSP5-8.5, the VMD could experience a mean sea level rise (SLR) of 1 m by the end of the century. This level of sea level rise could contributing to the threat of 75 greater flood magnitudes for the region (Västilä et al., 2010;Lauri et al., 2012;Hoang et al., 2016;MONRE, 2016;Minderhoud et al., 2019). According to Minderhoud et al., (2019), approximately 51% of the VMD plain would be submerged if the mean sea level rose by 0.8 m. ...
The Ca Mau Peninsula plays a critical role in the agricultural and aquacultural productivity of the Vietnam Mekong Delta (VMD), central to regional food security and the population’s economic and social welfare. Unfortunately, this region has also historically been a hotspot for natural disasters, particularly from flooding, which is initiated by seasonal river flux upstream and heightened sea levels downstream, but also exacerbated by global climate change (e.g., increased rainfall and sea-level rise, tropical storm surges) and human activities (e.g. river bed lowering, land subsidence). The potential risks associated with rising inundation levels is important information for the future sustainability of the region and its ability to adapt to both current and forthcoming changes. The research around the influence of such drivers on future flood risk, in the Ca Mau Peninsula, is incomplete, primarily due to the absence of a quantitative coastal inundation map corresponding to future compounded scenarios. In this study, we therefore evaluate flooding dynamics in the Ca Mau peninsula using a fully calibrated 1D model, to represent a range of anthropogenic and climate change compound scenarios. Our findings indicate that factors such as increased high-flows upstream, alterations in the riverbed of the main Mekong channel, and occurrences of storm surges effecting the mainstream Mekong River, are unlikely to significantly affect inundation dynamics in this region. However, land subsidence, rising sea levels, and their combined effects emerge as the primary drivers behind the escalation of inundation events in the Ca Mau peninsula, both in terms of their extent and intensity, in the foreseeable future. These results serve as vital groundwork for strategic development and investment as well as for emergency decision-making and flood management planning, providing essential insights for shaping development policies and devising investment strategies related to infrastructure systems in an area which is rapidly developing.
... Dam infrastructures have been recognized to alter the flow regime of the Mekong (Binh et al., 2020b;Lauri et al., 2012;Lu et al., 2014;Li et al., 2017a;Räsänen et al., 2017). Binh et al. (2020b) analyzed the long-term discharge along the entire Mekong from Chiang Saen to the VMD. ...
In the Mekong Basin, rice plants have low yields because of degraded soils, fresh water is becoming scarce, and rice cultivation consumes a lot of water. Beijing-directed rules to govern the river and its plan to build cascade dams upstream have weakened the Mekong River. The Mekong River Commission was established in 1957, but for over 50 years, the Commission still has only Laos, Cambodia, Thailand, and Vietnam as members. To confront all these troubling trends and challenges and the sustainability of the region, riparian countries in the lower Mekong areas have sought partnerships with support from the USA, Japan, Australia, and Korea. Many water development projects in the Mekong Basin have been researched and implemented by bilateral, transboundary, regional, and international collaborations. Although the collaboration is still not holistic as expected, increasing regional and international collaboration projects conducted recently have provided the bright potential for integrated sustainable water resources development and management in the Mekong Basin. A synthetic review of previous water development projects within the Mekong Basin in this chapter introduces an overview of opportunities and challenges and implies lessons for further water development projects in the future among riparian countries in the Mekong Basin and other transboundary river basins in the world.
... Many studies have highlighted that rice yields increased during the first 20 years of this new infrastructure and intensified regulation of the hydrological regime of delta (Sakamoto et al 2009;Lauri et al., 2012;Tri, 2012;Chen et al., 2012;Tran et al., 2018). The amount of land cultivating triple-rice farming increased, while the land area cultivated for double-rice crops decreased (Sakamotung et al., 2009). ...
Sinking and shrinking, the Vietnamese Mekong Delta is a materialization of dynamic river flows, sediment flows, and coastline processes. Past policy aspirations and extensive water infrastructures have shaped the delta into one the most significant food producing landscapes in Southeast Asia. Yet, these changes have also created new environmental risks by transforming the hydrological system. Research has produced a growing and increasingly diverse empirical literature on the delta's environmental context, without necessarily providing water resource managers, policymakers and practitioners with the information needed to galvanize more resilient development. This focus review presents a detailed overview of the recent scientific findings, exploring how the management of water resources is changing, as well as their inter‐relationship with land use, policy, socio‐economic transitions, and global environmental crises. Compound and systemic risks to the delta include climate change, hydrometeorological hazards, upstream developments and an unsustainable development trajectory. We outline scientific knowledge gaps, as well as the pressing need for sharable analysis‐ready data and innovations. Finally, we provide recommended future research avenues for multiscale actions toward a sustainable and resilient delta future.
This article is categorized under: Human Water > Water Governance
Science of Water > Water Extremes
Science of Water > Water and Environmental Change
... Construction and operation of reservoirs in the LMR basin has a significant impact on river discharge (Hecht et al., 2019). In addition, several drought events have occurred in recent years across the LMR basin (Keovilignavong et al., 2021;Kondolf et al., 2014;Lauri et al., 2012). It is important to monitor changes in reservoir water storage and, hence, quantify the impact of reservoir operation on the redistribution of surface water resources and the mitigation of droughts. ...
The Lancang-Mekong River (LMR) is an important transboundary river in Southeast Asia shared by China, Myanmar, Laos,Thailand, Cambodia, and Vietnam from upstream to downstream. Construction and operation of dams in the LMR basin has profoundly affected its natural streamflow regime. It is therefore important to monitor changes in reservoir water storage and to quantify the impact of reservoir operation on the redistribution of
surface water resources over this basin. Given the difficulty in obtaining in-situ measurements of reservoirs on the LMR, we integrated multisource remote sensing, including satellite altimetry and optical and synthetic aperture radar (SAR) images, to generate weekly water levels and water storages of nine largest reservoirs on the main stem of the LMR from 2017 to 2021. Specifically, partial surface water extent (SWE) of reservoirs was extracted from Sentinel-1 SAR images and digital elevation models (DEMs), using Random Forest algorithms trained by partial SWE derived from Sentinel-2 optical images, showing an overall accuracy higher than 95%. Based on the partial SWE and water level estimates from ICESat-2 and Global Ecosystem Dynamics Investigation (GEDI, International Space Station-based) data, the relationships between water levels and partial SWE were derived to convert partial SWE into water level time series. Furthermore, water storage time series of the nine reservoirs were obtained from water level time series and hypsometric functions derived from SRTM DEMs that were corrected by ICESat-2 data to remove systematic errors. For the Xiaowan Reservoir on the Lancang River, there is close agreement between remote sensing-derived water levels and in-situ water levels in terms of a normalized RMSE lower than 5%. Results indicate that multisource remote sensing has large potential for high-temporal-resolution monitoring of reservoir water levels and water storage. This could more precisely evaluate impacts of cascade reservoirs on the streamflow of the LMR and facilitate drought and flood mitigation for riverine countries.
Despite its energy benefits, hydropower dam development often causes ecological damages and social disruption, including downstream livelihood impacts, and biodiversity loss. Current methods for analyzing changes in downstream inundation extent due to dam operation typically rely on historical ground or satellite observations, or on coupled hydrological‐hydrodynamic modeling. However, while the former fails to isolate hydropower impacts from climate variations, the latter suffers from extensive input data requirements and high computational burden. This study proposes a novel hybrid framework integrating satellite data‐driven Forecasting Inundation Extents using REOF (Rotated Empirical Orthogonal Function) analysis (FIER), and the process‐based Hydrological Predictions for the Environment (HYPE) model incorporating the Integrated Reservoir Operation Scheme (IROS). The framework enables the isolated assessment of long‐term hydropower impacts on downstream inundation dynamics with computational efficiency and reduced ground data requirements, making it suitable for poorly gauged regions. Applying FIER‐HYPE‐IROS to the Lower Mekong River basin (LMB), a region significantly affected by dam proliferation impacting fisheries and agriculture, we found that dam operations decreased decadal‐average wet season water levels by up to 5% and increased dry season levels by up to 11%. Wet season inundation occurrence decreased by 11 days and the inundated area by 6%, while dry season inundation occurrence extended by 6 days and the surface water area increased by 40%. Although the current framework does not explicitly assess the downstream hydrological modifications, it offers a cost‐effective alternative for evaluating upstream alterations on inundation dynamics, such as dam operations, particularly in poorly gauged regions.
Natural fluctuations in river flow are central to the ecosystem productivity of basins, yet significant alterations in daily flows pose threats to the integrity of the hydrological, ecological, and agricultural systems. In the dammed Mekong River, the attribution of these large daily flow changes to upstream regions remains mechanistically unexamined, a factor blamed on challenges in estimating the time required for large daily shifts in upstream river flow to impact the downstream regions. Here, we address this by integrating a newly developed sub-basin modeling framework that incorporates 3D hydrodynamic, response time, and hydrological models. This integration allows us to estimate the time required between two hydrological stations and to distinguish the contribution of sub-basins and upstream regions to large daily river flow alterations. Findings revealed a power correlation between river discharge and the required time to reach downstream stations. Significant fluctuations in the river's daily flow were evident before the advent of the era of human activities, i.e., before 1992. This phenomenon persisted throughout subsequent periods, including the growth period from 1992 to 2009 and the mega-dam period spanning from 2010 to 2020, with minimal variation in the frequency of events. Sub-basins were found to significantly contribute to mainstream discharge- a contribution which led to a significant contribution of sub-basins into mainstream daily large river flow shifts. The outcomes and model derived from the sub-basin approach hold significant potential for managing river fluctuations and have broader applicability beyond the specific basin studied.
In 1971, scientists from Mahidol University in Thailand and the Smithsonian Institution in the USA formed a research team to study a new species of Schistosoma in the Mekong River in Thailand and Laos. The studies, completed during 1971–1973, prior to the construction of any dams or restrictions to the natural flow regime of the Mekong River, provide a unique description of the natural ecological state of the river that can serve as a baseline for current research. The natural transmission of Schistosoma japonicum, Mekong Strain, was first reported on Khong Island, Laos in 1973 using sentinel mice. The first detailed description of the habitat ecology of the snail vector Neotricula aperta was done on-site in 1971 simultaneously with that research and is unique in providing the only description of the river shoreline habitat before any dams were built and any alteration of the natural flow regime was in place. Aggregating current information in a Place-Based Conceptual Model (PBCM) as an organizing template, along with current habitat models that combine ecological data with e-flows, can be developed and used as a tool to predict suitable habitats for snails. The natural flow regime of the Mekong River prior to any impoundments is described with current updates on the potential impacts of climate change and dams with flow-related snail habitat characteristics, including sediment drift and water quality. The application of the PBCM to describe and compare descriptive information on current and potential future N. aperta/S. mekongi habitat is discussed.
The Mekong River in Southeast Asia thanks its regular annual flood to the southwest monsoon. At longer time scales, the monsoon is a spatially and temporally variable circulation, with different annual to millennial variation for different regions. In this paper, the Indian and the Western Pacific components of the monsoon were analyzed to draw a light on the interannual flood variability of the Mekong River.
The focus is on the variance of flood season flows at 8 stations on the Mekong River, as well as on well-known climate indexes that reflect the dynamics of the monsoon circulation and ocean temperature anomalies. An effort was made to identify the temporal resolution that contains most of the interannual variability of both flood regime of the Mekong and monsoon intensity.
We found a close connection between the Western Pacific monsoon and the discharge in Kratie and other stations in the Southern Mekong region. In the frequency domain, the interannual to decadal variance of the Mekong discharge closely follows that of the Western Pacific monsoon. More importantly, the well-known regime shift of 1976 in the North Pacific is detectable in the frequency domain for flood discharge and monsoon intensity. This suggests a relationship between Pacific sea surface temperature and monsoon variance, which is a good predictor for flood variance. This dependence influences the probability of occurrence of floods in the Mekong Delta.
The Mekong River in Southeast Asia thanks its regular annual flood to the southwest monsoon. At longer time scales, the monsoon is a spatially and temporally variable circulation, with different annual to millennial variation for different regions. In this paper, the Indian and the Western Pacific component of the monsoon were analyzed to draw a light on the interannual flood variability of the Mekong River.
The focus is on the variance of flood season flows at 8 stations on the Mekong River, as well as on well-known climate indexes that reflect the dynamics of the monsoon circulation and ocean temperature anomalies. An effort was made to identify the temporal resolution that contains most of the interannual variability of both flood regime of the Mekong and monsoon intensity.
We found a close connection between the Western Pacific monsoon and the discharge in Kratie and other stations in the Southern Mekong region. In the frequency domain, the interannual to decadal variance of the Mekong discharge closely follows that of the Western Pacific monsoon. More importantly, the well-known regime shift of 1976 in the North Pacific is detectable in the frequency space for flood discharge and monsoon intensity. This suggests a relationship between Pacific sea surface temperature and monsoon variance, which is a good predictor for flood variance. This dependence influences the probability of occurrence of floods in the Mekong Delta.
Increasing complexity and multidisciplinarity of water management has resulted in the development of broader approaches such as Integrated Water Resources Management (IWRM). This paper discusses the IWRM and particularly its social and participatory dimensions based on the practical experience gained from the socio-economic analysis within a modelling project in Cambodia's Tonle Sap Lake. It is argued that water-related socio-economic analysis can significantly contribute to water modelling and impact analysis work because it helps to link modelling with the most relevant social and economic issues. This way modelling is also better able to answer to the needs of integrated water resources management.
Despite the importance of surface water in Manitoba, Canada, there has been relatively little investigation of the impact of climate change on the hydrology of the province. This study examines streamflow characteristics in northern Manitoba basins (Taylor and Burntwood River basins) under climate scenarios generated by global climate models (GCM) from 2 agencies (the UK Hadley Centre and the Canadian Centre for Climate Modelling and Analysis). The hydrological model SLURP (Semi-distributed Land Use-based Runoff Processes) was run with perturbed meteorological data based on the GCM simulations under 2 greenhouse gas emission scenarios (A2 and B2) for 2 future periods (2041-2070 and 2071-2099). A method to incorporate the changed variability in future temperature and precipitation to the climate scenarios was also tested. All scenarios project wetter and warmer climates, and simulation results show that the mean annual runoff is likely to increase in every scenario. The largest seasonal increase occurs in late spring and the least in winter and summer. The daily flow exceeded by 80 % of the total records is also projected to substantially increase, resulting in much fewer days with extreme low flow compared to the control period. Results indicate that the combination of a hydrological model with simple GCM-based scenarios can produce results comparable to those obtained from regional climate modelling. Explicit incorporation of changed variability in climate scenarios results in higher runoff than consideration of only changes in means.