scholarly journals River Discharge and Water Level Changes in the Mekong River: Droughts in an Era of Mega-Dams

2021 ◽  
Author(s):  
XiXi Lu ◽  
Samuel Chua
Keyword(s):  
Water Level ◽  
Water Discharge ◽  
Dry Season ◽  
Water Levels ◽  
Mekong River ◽  
Wet Season ◽  
Discharge Data ◽  
Mekong Basin ◽  
Lower Mekong Basin ◽  
Dry Conditions

While 1992 marked the first major dam – Manwan – on the main stem of the Mekong River, the post-2010 era has seen the construction and operationalisation of mega dams such as Xiaowan (started operations in 2010) and Nuozhadu (started operations in 2014) that were much larger than any dams built before. The scale of these projects implies that their operations will likely have significant ecological and hydrological impacts from the Upper Mekong Basin to the Vietnamese Delta and beyond. Historical water level and water discharge data from 1960 to 2020 were analysed to examine the changes to streamflow conditions across three time periods: 1960-1991 (pre-dam), 1992-2009 (growth) and 2010-2020 (mega-dam). At Chiang Saen, the nearest station to the China border, monthly water discharge in the mega-dam period has increased by up to 98% during the dry season and decreased up as much as -35% during the wet season when compared to pre-dam records. Similarly, monthly water levels also rose by up to +1.16m during the dry season and dropped by up to -1.55m during the wet season. This pattern of hydrological alterations is observed further downstream to at least Stung Treng (Cambodia) in our study, showing that Mekong streamflow characteristics have shifted substantially in the post-2010 era. In light of such changes, the 2019-2020 drought – the most severe one in the recent history in the Lower Mekong Basin – was a consequent of constructed dams reducing the amount of water during the wet season. This reduction of water was exacerbated by the decreased monsoon precipitation in 2019. Concurrently, the untimely operationalisation of the newly opened Xayaburi dam in Laos coincided with the peak of the 2019-2020 drought and could have aggravated the dry conditions downstream. Thus, the mega-dam era (post-2010) may signal the start of a new normal of wet-season droughts.

scholarly journals Changing Land Use and Population Density Are Degrading Water Quality in the Lower Mekong Basin

Water ◽  
2021 ◽  
Vol 13 (14) ◽  
pp. 1948
Author(s):  
Flavia Tromboni ◽  
Thomas E. Dilts ◽  
Sarah E. Null ◽  
Sapana Lohani ◽  
Peng Bun Ngor ◽  
...  
Keyword(s):  
Water Quality ◽  
Land Use ◽  
Population Density ◽  
Local Population ◽  
Reference Conditions ◽  
Mekong River ◽  
Land Use Impacts ◽  
Wet Season ◽  
Mekong Basin ◽  
Lower Mekong Basin

Establishing reference conditions in rivers is important to understand environmental change and protect ecosystem integrity. Ranked third globally for fish biodiversity, the Mekong River has the world’s largest inland fishery providing livelihoods, food security, and protein to the local population. It is therefore of paramount importance to maintain the water quality and biotic integrity of this ecosystem. We analyzed land use impacts on water quality constituents (TSS, TN, TP, DO, NO3−, NH4+, PO43−) in the Lower Mekong Basin. We then used a best-model regression approach with anthropogenic land-use as independent variables and water quality parameters as the dependent variables, to define reference conditions in the absence of human activities (corresponding to the intercept value). From 2000–2017, the population and the percentage of crop, rice, and plantation land cover increased, while there was a decrease in upland forest and flooded forest. Agriculture, urbanization, and population density were associated with decreasing water quality health in the Lower Mekong Basin. In several sites, Thailand and Laos had higher TN, NO3−, and NH4+ concentrations compared to reference conditions, while Cambodia had higher TP values than reference conditions, showing water quality degradation. TSS was higher than reference conditions in the dry season in Cambodia, but was lower than reference values in the wet season in Thailand and Laos. This study shows how deforestation from agriculture conversion and increasing urbanization pressure causes water quality decline in the Lower Mekong Basin, and provides a first characterization of reference water quality conditions for the Lower Mekong River and its tributaries.

scholarly journals Water discharge and sediment flux changes in the Lower Mekong River

2005 ◽  
Vol 2 (6) ◽  
pp. 2287-2325 ◽  
Author(s):  
X. X. Lu ◽  
R. Y. Siew
Keyword(s):  
Water Discharge ◽  
Sediment Concentration ◽  
Dry Season ◽  
Water Levels ◽  
Sediment Flux ◽  
Mekong River ◽  
Water Level Fluctuations ◽  
Main Stream ◽  
Discharge Water ◽  
Ecological Changes

Abstract. The Lower Mekong River has witnessed extremely low water levels over the past few years. There is speculation that the changes are a consequence of the construction and operation of the Chinese cascade dams in the upper part of the Mekong main stream, the Lancang River. Dam construction on upper streams can produce a series of induced effects downstream, particularly in terms of water, sediment, channel and ecological changes. The infilling of the Manwan reservoir in 1992 caused water levels to fall to record lows in various parts of the Mekong River, and sediment concentration values decreased similarly. Analyses of discharge and sediment flux at various gauging stations on the Lower Mekong River have indicated a disruption in water discharge, water fluctuations and sediment transport downstream of the Manwan Dam, after its reservoir was infilled in 1992. Dry season flows showed a declining trend, and water level fluctuations in the dry season increased considerably in the post-dam (1993–2000) period. Monthly suspended sediment concentration (SSC) has also decreased significantly in several gauging stations in the post-dam period. The estimation of sediment flux is challenging since the measurements of SSC were sporadic. Our estimation based on the available data indicated that the areas along the upper-middle and lowermost reaches of the Mekong River have experienced a decline in sediment flux, possibly due to sedimentation in the Manwan Dam. However, the decrease is only statistically significant at Chiang Saen. Areas located in the mid-length of the river show less sensitivity to the operation of the Manwan Dam, as sediment fluxes have remained stable or even increased in the post-dam period.

scholarly journals Water discharge and sediment flux changes over the past decades in the Lower Mekong River: possible impacts of the Chinese dams

2006 ◽  
Vol 10 (2) ◽  
pp. 181-195 ◽  
Author(s):  
X. X. Lu ◽  
R. Y. Siew
Keyword(s):  
Water Discharge ◽  
Sediment Concentration ◽  
Dry Season ◽  
Water Levels ◽  
Sediment Flux ◽  
Mekong River ◽  
Water Level Fluctuations ◽  
Main Stream ◽  
The Past ◽  
Ecological Changes

Abstract. The Lower Mekong River has witnessed extremely low water levels over the past few years. There is speculation that the changes are a consequence of the construction and operation of the Chinese cascade dams in the upper part of the Mekong main stream, the Lancang River. Dam construction on upper streams can produce a series of induced effects downstream, particularly in terms of water, sediment, channel and ecological changes. Analyses of discharge and sediment flux at various gauging stations on the Lower Mekong River have indicated a disruption in water discharge, water fluctuations and sediment transport downstream of the first Chinese dam among the 8 cascades (i.e. the Manwan Dam), after its reservoir was infilled in 1992. Dry season flows showed a declining trend, and water level fluctuations in the dry season increased considerably in the post-dam (1993–2000) period. Monthly suspended sediment concentration (SSC) has also decreased significantly in several gauging stations in the post-dam period. The estimation of sediment flux is challenging since the measurements of SSC were sporadic. Our estimation based on the available data indicated that the areas along the upper-middle and lowermost reaches of the Mekong River have experienced a decline in sediment flux, possibly due to sedimentation in the Manwan Dam. However, the decrease is only statistically significant at the nearest gauging station below the Dam (i.e. Chiang Saen). Areas located in the mid-length of the river show less sensitivity to the operation of the Manwan Dam, as sediment fluxes have remained stable or even increased in the post-dam period.

scholarly journals Ranking of CMIP5 GCM Skills in Simulating Observed Precipitation over the Lower Mekong Basin, Using an Improved Score-Based Method

Water ◽  
2018 ◽  
Vol 10 (12) ◽  
pp. 1868 ◽  
Author(s):  
Yunfeng Ruan ◽  
Zhijun Yao ◽  
Rui Wang ◽  
Zhaofei Liu
Keyword(s):  
General Circulation ◽  
Coupled Model ◽  
General Circulation Models ◽  
Dry Season ◽  
Positive Trend ◽  
Wet Season ◽  
Mekong Basin ◽  
Good Ability ◽  
Lower Mekong Basin ◽  
Annual Time

This study assessed the performances of 34 Coupled Model Intercomparison Project Phase 5 (CMIP5) general circulation models (GCMs) in reproducing observed precipitation over the Lower Mekong Basin (LMB). Observations from gauge-based data of the Asian Precipitation-Highly Resolved Observational Data Integration Towards Evaluation of Water Resources (APHRODITE) precipitation data were obtained from 1975 to 2004. An improved score-based method was used to rank the performance of the GCMs in reproducing the observed precipitation over the LMB. The results revealed that most GCMs effectively reproduced precipitation patterns for the mean annual cycle, but they generally overestimated the observed precipitation. The GCMs showed good ability in reproducing the time series characteristics of precipitation for the annual period compared to those for the wet and dry seasons. Meanwhile, the GCMs obviously reproduced the spatial characteristics of precipitation for the dry season better than those for annual time and the wet season. More than 50% of the GCMs failed to reproduce the positive trend of the observed precipitation for the wet season and the dry season (approximately 52.9% and 64.7%, respectively), and approximately 44.1% of the GCMs failed to reproduce positive trend for annual time over the LMB. Furthermore, it was also revealed that there existed different robust criteria for assessing the GCMs’ performances at a seasonal scale, and using multiple criteria was superior to a single criterion in assessing the GCMs’ performances. Overall, the better-performed GCMs were obtained, which can provide useful information for future precipitation projection and policy-making over the LMB.

scholarly journals Seasonal hydrology shifts production sources supporting fishes in rivers of the Lower Mekong Basin

2016 ◽  
Vol 73 (9) ◽  
pp. 1342-1362 ◽  
Author(s):  
Chouly Ou ◽  
Kirk O. Winemiller
Keyword(s):  
Benthic Algae ◽  
Dry Season ◽  
Fish Biomass ◽  
Wet Season ◽  
Tissue Samples ◽  
Isotopic Signatures ◽  
Riparian Plants ◽  
Mekong Basin ◽  
Lower Mekong Basin ◽  
Three Rivers

Seasonal hydrology is assumed to be an important reason why the Lower Mekong Basin supports highly productive and biodiverse inland fisheries. We used C and N stable isotope ratios of tissue samples to estimate primary production sources supporting fish biomass in the Mekong and three large tributaries in Cambodia. We used a Bayesian mixing model to estimate relative contributions of four alternative production sources — seston, benthic algae, riparian grasses, and riparian macrophytes. There was little seasonal variation in isotopic signatures of riparian plants, but benthic algae and seston showed large seasonal shifts in carbon ratios. Seston and benthic algae were the most important production sources supporting fish biomass overall during the dry season, and riparian vegetation was the most important source during the wet season. Sources contributed differentially to biomass of trophic and habitat guilds, especially during the dry season. A dam on the upper Sesan River has changed hydrology, channel geomorphology, and other factors and, compared with the other three rivers, its fish biomass appears to derive from algae to a greater extent.

scholarly journals Historical impact of water infrastructure on water levels of the Mekong River and the Tonle Sap System

2014 ◽  
Vol 11 (4) ◽  
pp. 4403-4431 ◽  
Author(s):  
T. A. Cochrane ◽  
M. E. Arias ◽  
T. Piman
Keyword(s):  
Water Level ◽  
Climatic Factors ◽  
Water Levels ◽  
Mekong River ◽  
Water Level Fluctuations ◽  
Water Infrastructure ◽  
Infrastructure Development ◽  
Mekong Basin ◽  
Tonle Sap ◽  
Fall Rates

Abstract. The rapid rate of water infrastructure development in the Mekong basin is a cause for concern due to its potential impact on fisheries and downstream natural ecosystems. In this paper we analyse the historical water levels of the Mekong River and Tonle Sap system by comparing pre and post 1991 daily observations from six stations along the Mekong mainstream from Chiang Sean (northern Laos), to Stung Treng (Cambodia), and the Prek Kdam station on the Tonle Sap River. Observed alterations in water level patterns along the Mekong are linked to temporal and spatial trends in water infrastructure development from 1960 to 2010. We argue that variations in historical climatic factors are important, but they are not the main cause of observed changes in key hydrological indicators related to ecosystem productivity. Our analysis shows that the development of mainstream dams in the upper Mekong basin in the post-1991 period have resulted in a significant increase of 7 day minimum (+91.6%), fall rates (+42%), and the number of water level fluctuations (+75) observed in Chiang Sean. This effect diminishes downstream until it becomes negligible at Mukdahan (northeast Thailand), which represents a drainage area of over 50% of the total Mekong Basin. Further downstream at Pakse (southern Laos), alterations to the number of fluctuations and rise rate became strongly significant after 1991. The observed alterations slowly decrease downstream, but modified rise rates, fall rates, and dry season water levels were still quantifiable and significant as far as Prek Kdam. This paper provides the first set of evidence of hydrological alterations in the Mekong beyond the Chinese dam cascade in the upper Mekong. Given the evident alterations with no precedence at Pakse and downstream, post-1991 changes can also be directly attributed to water infrastructure development in the Chi and Mun basins of Thailand. A reduction of 23 and 11% in the water raising and fall rates respectively at Prek Kdam provides evidence of a diminished Tonle Sap flood pulse in the post-1991 period. Given the observed water level alterations from 1991 to 2010 as a result of water infrastructure development, we can extrapolate that future development in the mainstream and the key transboundary Srepok, Sesan and Sekong subbasins will have an even greater effect on the Tonle Sap flood regime, the lower Mekong floodplain, and the delta.

scholarly journals Historical impact of water infrastructure on water levels of the Mekong River and the Tonle Sap system

2014 ◽  
Vol 18 (11) ◽  
pp. 4529-4541 ◽  
Author(s):  
T. A. Cochrane ◽  
M. E. Arias ◽  
T. Piman
Keyword(s):  
Water Level ◽  
Climatic Factors ◽  
Water Levels ◽  
Mekong River ◽  
Water Level Fluctuations ◽  
Water Infrastructure ◽  
Infrastructure Development ◽  
Mekong Basin ◽  
Tonle Sap ◽  
Fall Rates

Abstract. The rapid rate of water infrastructure development in the Mekong Basin is a cause for concern due to its potential impact on fisheries and downstream natural ecosystems. In this paper, we analyze the historical water levels of the Mekong River and Tonle Sap system by comparing pre- and post-1991 daily observations from six stations along the Mekong mainstream from Chiang Saen (northern Thailand), to Stung Treng (Cambodia), and the Prek Kdam station on the Tonle Sap River. Observed alterations in water level patterns along the Mekong are linked to temporal and spatial trends in water infrastructure development from 1960 to 2010. We argue that variations in historical climatic factors are important, but they are not the main cause of observed changes in key hydrological indicators related to ecosystem productivity. Our analysis shows that the development of mainstream dams in the upper Mekong Basin in the post-1991 period may have resulted in a modest increase of 30-day minimum levels (+17%), but significant increases in fall rates (+42%) and the number of water level fluctuations (+75%) observed in Chiang Saen. This effect diminishes downstream until it becomes negligible at Mukdahan (northeast Thailand), which represents a drainage area of over 50% of the total Mekong Basin. Further downstream at Pakse (southern Laos), alterations to the number of fluctuations and rise rate became strongly significant after 1991. The observed alterations slowly decrease downstream, but modified rise rates, fall rates, and dry season water levels were still quantifiable and significant as far as Prek Kdam. This paper provides the first set of evidence of hydrological alterations in the Mekong beyond the Chinese dam cascade in the upper Mekong. Given the evident alterations at Pakse and downstream, post-1991 changes could also be directly attributed to water infrastructure development in the Chi and Mun basins of Thailand. A reduction of 23 and 11% in the water raising and falling rates respectively at Prek Kdam provides evidence of a diminished Tonle Sap flood pulse in the post-1991 period. Given the observed water level alterations from 1991 to 2010 as a result of water infrastructure development, we can extrapolate that future development in the mainstream and the key transboundary Srepok, Sesan, and Sekong sub-basins will have an even greater effect on the Tonle Sap flood regime, the lower Mekong floodplain, and the delta.

scholarly journals Water levels affect photosynthesis and nutrient use more than salinity in a scrub Red Mangrove forest of the southeastern Florida Everglades

2021 ◽  
Author(s):  
J. Aaron Hogan ◽  
Edward Castañeda-Moya ◽  
Lukas Lamb-Wotton ◽  
Christopher Baraloto
Keyword(s):  
Water Level ◽  
Mangrove Forest ◽  
Leaf Gas Exchange ◽  
Critical Role ◽  
Dry Season ◽  
Water Levels ◽  
Marginal Effect ◽  
Mangrove Forests ◽  
Wet Season ◽  
Porewater Salinity

AbstractPhotosynthesis is an essential process to mangrove forest carbon cycling, which plays a critical role in the global carbon cycle. We investigated how differences in mangrove island micro-elevation (i.e., habitat) affect tree physiology in a scrub mangrove forests of the southeastern Everglades. We measured leaf gas exchange rates of scrub Rhizophora mangle trees monthly during 2019, hypothesizing that CO2 assimilation (Anet) and stomatal conductance (gsw) would decline with increases in water level and salinity, with larger differences at mangrove islands edges than centers, where inundation and salt stress are greatest. Water levels varied between 0 and 60 cm, rising during the wet season (May-October) relative to the dry season (November-April). Porewater salinity ranged from 15 to 30 ppt, being higher at mangrove island edges compared to centers. Anet maximized at 15.1 µmol m-2 s-1, and gsw was typically <0.2 mol m-2 s-1, both of which were greater in the dry than the wet season and greater at mangrove island centers than edges. After accounting for season and habitat, water level had a positive effect on Anet in both seasons, but no effect on gsw. Similarly, porewater salinity had a slightly positive marginal effect on Anet but a negligible effect on gsw Our findings suggest that water levels drive variation in Anet more than salinity in Everglades scrub mangroves, while also constraining Anet more than gsw, and that the interaction between permanent flooding and habitat varies with season as physiological stress is alleviated at higher-elevation mangrove island center habitats in the dry season. Additionally, habitat heterogeneity leads to differences in nutrient and water acquisition and use between trees growing in island centers versus edges, creating distinct physiological controls on leaf physiology and photosynthesis which could ultimately affect carbon flux dynamics of scrub mangrove forests across the Everglades landscape.

scholarly journals Impacts of Mainstream Hydropower Dams on Fisheries and Agriculture in Lower Mekong Basin

2020 ◽  
Vol 12 (6) ◽  
pp. 2408 ◽  
Author(s):  
Yuichiro Yoshida ◽  
Han Soo Lee ◽  
Bui Huy Trung ◽  
Hoang-Dung Tran ◽  
Mahrjan Keshlav Lall ◽  
...  
Keyword(s):  
Developing Countries ◽  
Electricity Generation ◽  
Lao Pdr ◽  
Mekong River ◽  
Environmental Damage ◽  
Dam Construction ◽  
Mekong Basin ◽  
Hydropower Potential ◽  
Hydropower Dams ◽  
Lower Mekong Basin

The riverine ecosystems of the Mekong River Basin possess the world’s most productive inland fishery and provide highly productive food crops for millions of people annually. The development of hydropower potential in the Mekong River has long been of interest to governments in the region. Among the existing 64 dams, 46 dams have been built in the Lower Mekong Basin (LMB) to produce up to 8650 MW of electricity. Additionally, of the 123 proposed built hydropower dams, eleven hydropower plants have been nominated for the river mainstream and are expected to install a total of 13,000 MW in the LMB countries. However, serious concerns have intensified over the potential negative economic consequences, especially on fisheries and agriculture in Cambodia and Vietnam. To date, most of the concerns have concentrated on the impacts on hydrology, environment, livelihood, and diversity in the LMB attributed to hydropower development. This paper, however, discusses the fishery and agricultural sectors of the LMB and focuses on the downstream floodplains of Cambodia and Vietnam. The dam construction has caused greater losses of biodiversity and fisheries than climate change in the LMB. The reduction of 276,847 and 178,169 t of fish, 3.7% and 2.3% of rice, 21.0% and 10.0% of maize will contribute to a decrease of 3.7% and 0.3% of the GDP of Cambodia and Vietnam, respectively. Lao PDR may benefit the most revenue from electricity generation than the other country in the LMB, as most of the proposed dams are projected in the country. Cambodia burdens 3/4 of the reduction of total capture fishery destruction, whilst Lao PDR, Thailand, and Vietnam endure the remaining 1/3 losses. The tradeoff analyses reveal that losses of capture fisheries, sediment or nutrients, and social mitigation costs are greater than the benefits from electricity generation, improved irrigation, and flood control of the LMB region. The socioeconomic and environmental damage caused by hydropower dams in developing countries, including the Mekong, is greater than the early costs in North America and Europe. It is proposed that dam construction for hydropower in the Mekong River, as well as other rivers in developing countries, should be gradually removed and shifted toward solar, wind, and other renewable resources.

scholarly journals Seasonal Water Level Fluctuation and Concomitant Change of Nutrients Shift Microeukaryotic Communities in a Shallow Lake

Water ◽  
2020 ◽  
Vol 12 (9) ◽  
pp. 2317
Author(s):  
Yang Liu ◽  
Ze Ren ◽  
Xiaodong Qu ◽  
Min Zhang ◽  
Yang Yu ◽  
...  
Keyword(s):  
Water Level ◽  
Soluble Reactive Phosphorus ◽  
Dry Season ◽  
Water Level Fluctuation ◽  
Water Level Fluctuations ◽  
Rrna Gene ◽  
Level Fluctuation ◽  
Wet Season ◽  
Concomitant Change ◽  
Modular Structures

Seasonal water level fluctuations (WLFs) impose dramatic influences on lake ecosystems. The influences of WLFs have been well studied for many lake biotas but the microeukaryotic community remains one of the least-explored features. This study employed high-throughput 18S rRNA gene sequencing to investigate the spatiotemporal patterns of microeukaryotic communities in the dry and wet seasons with concomitant change of nutrients in Poyang Lake, which experiences huge seasonal WLFs. The results showed that the dry season and wet season had distinct microeukaryotic community compositions and structures. In the dry season, Ciliophora (13.86–40.98%) and Cryptomonas (3.69–18.64%) were the dominant taxa, and the relative abundance of these taxa were significant higher in the dry season than wet season. Ochrophyta (6.88–45.67%) and Chlorophyta (6.31–22.10%) was the dominant taxa of microeukaryotic communities in the wet season. The seasonal variation of microeukaryotic communities was strongly correlated to seasonal nutrient variations. Microeukaryotic communities responded significantly to dissolved organic carbon, total nitrogen, nitrate, and soluble reactive phosphorus in the dry season, and correlated to nitrate and total phosphorus in the wet season. The microeukaryotic community showed different modular structures in two seasons, and nutrient variations were the key factors influencing seasonal variations of the modular structures. Moreover, microeukaryotic community networks based on different seasons indicated that the microeukaryotic community co-occurrence patterns were not constant but varied largely associating with the nitrogen and phosphorus variations under the effects of WLFs. Our results are important for understanding how microeukaryotic communities respond to nutrient variation under seasonal water level fluctuation.

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