Carbon Emission from Cascade Reservoirs: Spatial Heterogeneity and Mechanisms.

Nitrous oxide emissions from cascade hydropower reservoirs in the upper Mekong River

Spatial patterns of diffusive greenhouse gas emissions from cascade hydropower reservoirs

The neglect of the change in inundation area leads to overestimation of carbon emission in cascade reservoirs

Effects of reservoir cascades on diversity, distribution, and abundance of fish assemblages in three Neotropical basins.

Hydrodynamic impact on trace metals in sediments in the cascade reservoirs, North China.

The construction and operation of reservoirs disrupt the natural flow regime of rivers, reducing flow velocities and creating prolonged anaerobic conditions, particularly in steep-gradient, deeply incised river channels. These conditions facilitate microbial decomposition of organic matter—originating from terrestrial plants and soils—leading to greenhouse gas emissions, such as carbon dioxide (CO₂) and methane (CH₄). Cascade reservoir systems, composed of multiple reservoirs connected in an upstream-downstream configuration, introduce further complexities due to the interactions between upstream discharges and downstream reservoirs. These interactions influence water temperature, flow disturbance, and material transport, among other factors.The study employed the CE-QUAL-W2 model to developed a coupled hydrodynamic and water quality model for analyzing carbon transmitting among the five cascade reservoirs along the Wujiang River in Southwest China (Figure 1). This two-dimensional model, uses x-z plane layered grids to simulate water flow, temperature, and carbon cycling dynamics under specific power station intake location scenarios. By simulating these scenarios, we assessed how changes in power station intake elevations influence the carbon balance of individual and cascade reservoir systems.Figure 1 The Wujiang River basin and the spatial distribution of five cascade reservoirsThe results indicate that for individual reservoirs such as WJD Reservoir, raising the intake elevation of the power station enhances surface water disturbance, which enhanced CO₂ diffusion across the water-air interface near the dam (Figure 2). However, this adjustment significantly reduces carbon release to downstream areas, thereby increasing the reservoir’s overall carbon retention capacity.Figure 2 Average CO2 diffusion fluxes across the water-air interface in the WJD Reservoir under different scenarios (scenario A-G represent progressively higher intake elevations at the WJD power station.When the intake elevation of upstream DF Reservoir was raising, its carbon retention capacity improved. However, the warmer discharged water inhibits vertical carbon sedimentation in downstream reservoirs. This led to the accumulation of Total Inorganic Carbon (TIC) and Total Organic Carbon (TOC) in shallow water layers of downstream reservoirs (Figure 3).Figure 3 Vertical distribution of TOC (left) and TIC (right) at the WJD reservoir dam under different intake elevations of upstream hydropower stations (Scenarios I–V represent progressively higher intake elevations).Consequently, carbon transport to downstream reservoirs increased, reducing the total carbon sink capacity of the cascade reservoir system. The findings highlight a trade-off between local and system-wide carbon retention in cascade reservoirs. While elevating intake locations at individual reservoirs can improve carbon retention locally, the downstream impacts—such as reduced vertical carbon sedimentation and increased carbon transport—diminish the overall carbon storage efficiency of the cascade reservoirs system. Future reservoir management strategies should consider these complex interactions to balance energy production with environmental sustainability.

Dam construction is widespread, changing the hydrological and biogeochemical conditions and thereby the bacterial communities in the rivers of the earth. To date, knowledge is lacking about bacterial communities in cascade reservoirs. Here, we investigated the bacterial communities and potential functions of nine cascade hydropower reservoirs in 1290 km of the upper Mekong River (Lancang River in China). Along the reservoir cascade, the water temperature, rather than the presence of dams, was the main cause for the geographical patterns of bacterial community composition. Within a reservoir, significant spatial differences in sediment bacterial communities were observed between the tail, middle, and head of a reservoir. The differences in sediment properties resulted by flow velocity‐sieved sedimentation from the tail to head of a reservoir caused the spatial variation in sediment bacteria communities, forming potential hotspots for biogeochemical cycling in the middle of the reservoir. In contrast, unlike in deep lakes and deep single reservoirs, the bacterioplankton community composition had no distinctly layered features in the deep cascade reservoirs, because density‐induced underwater currents and convection resulting from hydropower production reduced the vertical hydro‐environmental gradients. This study provides a novel perspective on the processes affecting the distribution and function of bacterial communities in river‐reservoir cascades, and is a first step toward forecasting the consequences of microbially mediated biogeochemical cycling in existing and future reservoirs worldwide.

The evolution of dam-break floods in downstream and cascade reservoirs is an extremely complicated process. Based on the three-dimensional Navier–Stokes equation and smoothed particle hydrodynamics theory, the complicated dam-break flood flow was simulated for such cases. The evolution of the dam-break flood in the downstream reservoir and cascade reservoirs and its impact on the downstream dam were obtained, and the results were found to agree well with experimental results. According to force analysis, the dam-break flood effect can be divided into a high-speed impact area and a pressure-transmission area. The former can cause damage due to instantaneous impact on the dam and the latter can cause damage arising from oscillation fatigue. These conclusions have value in water resources and hydropower engineering for preventing the outburst of a cascade reservoir group.

A multiscale attribution framework for separating the effects of cascade and individual reservoirs on runoff

Currently, CO2 emissions from cascade hydropower reservoirs are not well understood. In this study, we investigated the seasonal carbon dioxide partial pressure (pCO2) and related environmental factors in 4 cascading reservoirs (Hong Jia Du, Dong Feng, Suo Feng Ying, and Wu Jiang Du) of Wujiang River, southwest China. The results showed that pCO2 in the surface water of these reservoirs had obvious spatiotemporal changes and generally decreased from the riverine zone to the lacustrine zone in each reservoir. In summer, pCO2 was highest downstream of the dam because of stratification and deep water discharge for hydropower generation, whereas pCO2 was much lower in the surface water of the lacustrine zone because of carbon removal by photosynthesis. When water temperature was low, however, pCO2 was higher in the surface water of the lacustrine zone because of respiration and organic decomposition. Among these reservoirs, only Suo Feng Ying had CO2 emissions higher than the average value of natural lakes. In addition, CO2 emission flux showed an exponentially negative relationship with hydraulic retention time of reservoirs, based on this work and other reports of reservoirs in the Yangtze River basin.

This study assesses the seasonal regulation of river discharge by hydropower dam-induced cascade reservoirs in the Lancang River and its effect on downstream freshwater and estuarine saltwater intrusion. There are eight main reservoirs in the Lancang River, with a total regulation capacity of 25.67 billion m3, which regulates river discharge by conserving water in the flood season and releasing water in the dry season. River discharge during the dry season from 1960 to 2009 accounted for 21% of the annual discharge before the cascade reservoirs were constructed and increased to 33% from 2010 to 2015 after the cascade reservoirs were constructed at the Jinghong hydrological station, which is the lowermost station in the Lancang River. During the 2016 extreme drought in the lower Mekong River basin, the river discharge increased by 550, 367, 1283, 969, and 524 m3/s in January, February, March, April, and May, respectively, regulated by the cascade reservoirs at the Jinghong hydrological station. Considering runoff, tides, wind, and continental shelf currents, a high-resolution three-dimensional numerical model was used to simulate the effect of regulation of river discharge by the cascade reservoirs in the Lancang River on the saltwater intrusion in the Mekong River Delta (MRD). The simulation results show that the seasonal regulation of river discharge by the cascade reservoirs in the Lancang River weakens estuarine saltwater intrusion during the dry season, especially in the sand bar areas, which is much more significant in the extreme dry season of 2016. The seasonal regulation of river discharge by the reservoirs in the Lancang River makes the seasonal distribution of downstream river discharge more uniform, favoring downstream freshwater utilization and alleviating flood disasters and saltwater intrusion in the MRD.

Experimental study of dam-break flow in cascade reservoirs with steep bottom slope

Dam construction alters flow regimes and can change the composition of aquatic communities. Using data from three Brazilian hydrographic basins, we tested the hypothesis that reservoir cascades act as environmental filters for fish traits. This dataset included information on different environmental variables and fish traits (diet, migration, fecundation, parental care, position in the water column, and body size), and we used multivariate analysis (partial RLQ) to quantify the relationships between environmental variables, species abundance and traits. We found that the abundance of migratory species declined towards downstream reservoirs, which tend to be smaller and less turbid with a shorter water residence time than upstream reservoirs. We also found evidence of an association between reservoir age and the domination of fish communities by small-sized species with parental care, external fecundation, and benthic habits. Our findings suggest that particular fish traits are selected for across reservoir cascades.

Activity and structure of methanogenic microbial communities in sediments of cascade hydropower reservoirs, Southwest China

Despite the fact that cascade reservoirs are built in a large number of river basins nowadays, there is still an absence of studies on sequential embankment dam-break in cascade reservoirs. Therefore, numerical simulations and risk analyses of cascade reservoir dam-break are of practical engineering significance. In this study, by means of contacting the hydraulic features of upstream and downstream reservoirs with flood routing simulation (FRS) and flood-regulating calculation (FRC), a numerical model for the whole process of cascade reservoir breaching simulation (CRBS) is established based on a single-embankment dam-break model (Dam Breach Analysis—China Institute of Water Resources and Hydropower Research (DB-IWHR)). In a case study of a fundamental cascade reservoir system, in the upstream Tangjiashan barrier lake and the downstream reservoir II, the whole process of cascade reservoir dam-break is simulated and predicted under working schemes of different discharge capacities, and the risk of cascading breaching was also evaluated through CRBS. The results show that, in the dam-break of Tangjiashan barrier lake, the calculated values of the peak outflow rate are about 10% more than the recorded data, which are in an acceptable range. In the simulation of flood routing, the dam-break flood arrived at the downstream reservoir after 3 h. According to the predicted results of flood-regulating calculations and the dam-break simulation in the downstream reservoir, the risk of sequential dam-break can be effectively reduced by setting early warnings to decrease reservoir storage in advance and adding a second discharge tunnel to increase the discharge capacity. Alongside the simulation of flood routing and flood regulation, the whole process of cascade dam-break was completely simulated and the results of CRBS tend to be more reasonable; CRBS shows the great value of engineering application in the risk assessment and flood control of cascade reservoirs as an universal numerical prediction model.