Water Temperature Stratification Research Articles

Quantifying the contribution of external thermal energy and internal hydrodynamic processes to the water temperature of river-valley reservoirs

Hydropower plants utilize hydroelectric energy while also impacting the hydrodynamic and hydrothermal processes within river. To comprehend and address the environmental and ecological impacts of hydropower plants, it is crucial to quantify these processes. The study synthesizes methods for calculating reservoir heat transport and Richardson numbers, presenting a comprehensive framework for understanding the interaction of heat transport and hydrodynamic processes in reservoirs based on EFDC model simulations. It quantifies the external thermal and internal hydrodynamic effects on the water temperature evolution of a deep valley reservoir, using the Jinsha River Xiluodu Reservoir as a case study. Multi-interface heat transport calculations indicate that the Deep River Valley Reservoir is a hydrothermal body dominated by inflow and outflow. During the water temperature stratification period, the reservoir water temperature increases by 0.015 °C/day due to surface heat fluxes, incorporating hydraulic residence time, while the reservoir water temperature increases by 0.06 °C/day due to inflow heat fluxes. Furthermore, this study analyses the interrelationships between reservoir buoyancy and shear stress based on the gradient Richardson number, revealing the interaction between hydrodynamic and water temperature processes. This study not only provides a framework for understanding reservoir thermodynamics but also provides a theoretical basis for effective reservoir management and ecosystem assessment.

Late Cenomanian Plenus event in the Western Interior Seaway

The Late Cenomanian Plenus Cold Event is one of the most enigmatic paleoclimate episodes in Earth history with potential to inform understanding of global climate system variability under greenhouse warming conditions, as well as internal feedback pathways that modulate such variability. Following an interpreted massive addition of volcanic CO2 to the atmosphere and warming that led to a major ocean anoxic event (OAE2), there was a brief interval of cooling recorded in oxygen isotopes and biogeographic data. Here we present evidence that cooling was absent or muted within the Western Interior Seaway (WIS). Clumped isotope data from the basin suggest persistent extreme warmth during the Late Cenomanian, macroinvertebrate fossil assemblages do not record a decrease in temperature, and changes in other paleoceanographic proxies do not correlate temporally with Plenus interval signals from other locales. Using select proxy data to guide construction of GCM model simulations, we explore possible hypotheses to explain these observations. Our results suggest that the paleogeographic configuration of the basin and its gateways to adjoining oceans, which evolved in association with changing pCO2 and sea level, influenced winter sea ice formation at the northern aperture of the seaway, water mass circulation, salinity, temperature, and water column stratification. We propose that northward advection of warm Tethyan water muted expression of Plenus cooling in the seaway. Understanding the unique character of the Western Interior paleoceanographic record provides critical input for the development of robust models of ancient Earth System dynamics and should aid predictions of future climate system dynamics.

Estimating water surface evaporation losses under floating coverage: Modeling and Application

In arid and semiarid regions, floating covers can effectively reduce water surface evaporation to save water. However, the physical process of water surface evaporation greatly changes under floating coverage, and the open water surface evaporation estimation models cannot accurately quantify the water surface evaporation under floating coverage. A novel combination model for estimating water surface evaporation was developed, which can accurately estimate the water surface evaporation under floating coverage. (1) In view of the influence of the size and shape of the pore water formed by floating covers on the wind function in the water surface evaporation process, a wind function with the fetch of the evaporating water surface and wind speed as independent variables was established, and the calculation model of the aerodynamic term was modified. (2) Aiming at the influence of the variations in the energy consumed by water evaporation and water temperature stratification on the estimation of water surface evaporation under floating coverage, the energy balance equation and equilibrium temperature of the water body under floating coverage were established, and the calculation model of the radiation term was modified. Finally, the established evaporation combination model was verified by an experiment on water surface evaporation covered by spheres. The results showed that the estimation model established could accurately quantify the water surface evaporation under floating coverage. For arid and semiarid areas, the new evaporation estimation model can effectively help in decision-making for water surface evaporation prevention and water savings and promote water resource management.

Vertical distribution rules and factors influencing phytoplankton in front of a drinking water reservoir outlet

The phenomenon of algal blooms caused by the excessive proliferation of phytoplankton in drinking water reservoirs is becoming increasingly frequent, seriously endangering water quality, ecosystems, water safety, and people's health. Thus, there is urgent need to conduct research on the distribution rules and factors influencing phytoplankton in drinking water reservoirs. Given that the outflows from reservoirs usually come from the middle and lower layers of the water column and the current studies on phytoplankton in drinking water reservoirs are usually carried out on the surface, an 8-month monitoring of vertical phytoplankton and the corresponding influencing factors in front of the outlet in a drinking water reservoir was conducted. Based on the monitoring results, the distribution rules of phytoplankton and the associated factors were analyzed. The results showed that phytoplankton biomass significantly decreased with increasing water depth, but the biomass near the outlet (40 m depth) still reached the WHO level 2 warning threshold for algal blooms multiple times. During the monitoring period, Cyanophyta, Chlorophyta and Bacillariophyta dominated. The selected multisource environmental factors explained 60.5 % of the spatiotemporal changes in phytoplankton, with thermal intensity (water temperature and thermal stratification intensity) being the driving factor. Meanwhile, excessive TN and TP provided necessary conditions for the growth of phytoplankton. Based on influencing factors, reducing upstream nutrient inflows and thermal stratification intensity are recommended as measures to prevent and control algal blooms. This study provides insights into the vertical distribution rules and factors influencing phytoplankton in a drinking water reservoir, which can provide a reference for the management of drinking water reservoirs and the prevention and control of algal blooms.

The impact of floating photovoltaic power plants on lake water temperature and stratification

Floating photovoltaics (FPV) refers to photovoltaic power plants anchored on water bodies with modules mounted on floats. FPV represents a relatively new technology in Europe and is currently showing a rapid growth in deployment. However, effects on thermal characteristics of lakes are largely unknown, yet these are crucial for licensing and approval of such plants. Here, we quantify FPV impacts on lake water temperature, energy budget and thermal stratification of a lake through measurements of near-surface lateral wind flow, irradiance, air and water temperatures at one of the largest commercial German facilities, situated on a 70 m deep dredging lake in the Upper Rhine Valley, South-West Germany. Underneath the FPV facility, a 73% reduction in irradiance on the lake surface and an average 23% reduction in near-surface wind speed at module height are detected. A three month data set is then used to set up the General Lake Model and simulate scenarios of different FPV occupancies and changing climatic conditions. We observe that a lake coverage with FPV result in a more unstable and shorter thermal stratification during summer, which could mitigate the effects of climate change. The reduction of water temperatures follows a non-linear relationship with increased FPV occupancy. A sensitivity analysis showed that an increased wind reduction by FPV can have a considerable impact on the thermal properties of the lake. However, measurements only suggest small deviations with regard to the thermal properties of the investigated lake. These findings can be used in approval procedures and allow for a more accurate assessment of environmental impacts of future installations.

The influence of geomorphology and environmental conditions on stratification in Intermittently Open/Closed Estuaries

Intermittently Open/Closed Estuaries (IOCE) have entrances that close during periods of low river flow. A major characteristic of IOCE is stratification of salinity, dissolved oxygen and water temperature. After IOCE open (either naturally or artificially), large changes in stratification occur as water drains from the estuary to the ocean. The rapid change in water level and loss of the top oxygenated layer of the water column during drainage often causes fish kills in IOCE globally and is related to stratification. Despite this, there are a lack of studies that statistically analyse the relationships between environmental variables and stratification and that quantify changes to stratification during the draining period across multiple IOCE. To fill these gaps, we (1) analysed the relationships between environmental variables and stratification using distance-based Linear Models (distLM) and distance-based Redundancy Analysis (dbRDA) in five different IOCE across Victoria, Australia, and (2) measured near-continuous physicochemical depth profiles and changes in entrance morphology, fluvial inflows, and estuary water level following four estuary openings.The distLM results revealed that maximum air temperature, mouth state and fluvial inflows showed statistically significant relationships with stratification in the IOCE studied. The dbRDA suggested that high maximum air temperatures were associated with low values of stratification in small IOCE, more commonly during closed periods. High fluvial inflows were associated with low values of stratification in large IOCE during open periods (except at one site, Curdies River). Field observations of changes in stratification during the draining period revealed two distinct responses. First, a high energy opening with discharge at the mouth between 70 and 182 m3s-1 and fluvial inflows of 0.87–1.85 m3s-1, causing the IOCE water column to mix and become uniform. Second, a low energy opening with discharge from the mouth between 6 and 37 m3s-1 and fluvial inflows of 0.02–0.03 m3s-1, causing the IOCE to remain stratified. These findings were summarised into a conceptual model showing the sequence of changes during openings for different types of IOCE. Over longer timescales (days to years), our results suggest that differences between stratification during open and closed periods are reflected at a shorter time-scale during the draining period (hours to days). These differences further reflect differences in geomorphology and hydrology between IOCE. Our findings will be useful for estuary managers to predict how stratification in different types of IOCE will change during artificial openings and provide a proxy for predicting their response to climate change.

Study on the water temperature distribution characteristics of a mixed pumped storage power station reservoir: A case study of Jinshuitan Reservoir

With the development of the social economy and the demand for environmental protection, people's requirements for clean and renewable energy are gradually increasing. Generally, renewable energy, such as wind power and photovoltaic energy, has natural intermittency to varying degrees. Consequently, renewable energy sources, due to their fluctuating nature, cannot provide a continuous supply of power and hence require bulk electricity storage. Additionally, energy storage is important to electrical systems, allowing load levelling, peak shaving, frequency regulation, damping energy oscillations, and improving power quality and reliability. Pumped storage power stations are notable for their ability to efficiently store energy on a large scale. The construction of a reservoir inevitably changes the water temperature situation of the original river channel. The expansion of pumping and storage units on a pre-existing reservoir, namely, a mixed pumped storage power station, is different from a conventional power station in terms of the thermal structure of the reservoir area. This study focuses on the Jinshuitan hydropower station and uses the MIKE3 model to analyse the influence of different outlet elevations and pumping flows on the water temperature structure of the reservoir area. The water pumping had no impact on the surface water body but did have a significant impact on the middle and bottom water bodies. The vertical water temperature stratification intensity weakened under different pumping scenarios, and the temperatures of the middle and bottom water bodies increased. The influence of the outlet elevation on the water temperature structure in the reservoir area was much greater than that of the pumping flow. The lower the elevation of the water outlet was, the greater the impact. For the Jinshuitan hydropower station, when the elevation of the water outlet was 130 m, the water temperature of the bottom column rose significantly, the stratification intensity of the water column weakened significantly, and the reservoir reached a vertically isothermal state in December. In summary, water pumping dramatically changed the original water temperature structure in the reservoir. The final research results can provide an effective reference for follow-up studies on relevant ecological and environmental issues associated with the development of pumped storage power stations and for setting the outlet elevation and pumping flow.

Experimental analysis of wet gas-liquid water mixture within the tank of water in glass evacuated tube solar water heater

ABSTRACT In this investigation, thermal performance characteristics of passive solar water heater with vacuum tube were experimentally studied by using the mixture of wet gas and liquid water. Five different outlet locations of the tank were used to produce five different proportions of wet gas-liquid water mixtures. Experimental results were proposed in the way of water temperature, gas temperature, thermal stratification number, temperature distribution zone and a correlation for thermal efficiency through wet gas proportion. It was concluded that the wet gas-liquid water mixture at a wet gas proportion of 70% ameliorates the water temperature and thermal stratification number by respectively 13.7°C and 10.33 times compared to the mixture at a wet gas proportion of 10%. For different wet gas-liquid water mixtures, temperature distribution zone shows a quadratic parabola. The correlation for thermal efficiency through different proportions of mixtures presents the regularity of linear regression equation. Also, the average R2 and average Root Mean Square Error (RMSE) of regression analysis correlation are calculated as 0.936 and 0.375, respectively. The novelty of this work is evaluating the effect of different proportions of wet gas-liquid water mixture on the thermal performance and thermal stratification of passive vacuum tube solar water heater, and develop a correlation for thermal efficiency through different proportions of mixture.

Intelligent Simulation of Water Temperature Stratification in the Reservoir.

In order to fully make use of limited water resources, humans have built many water conservancy projects. The projects produce many economic benefits, but they also change the natural environment. For example, the phenomenon of water temperature stratification often occurs in deep reservoirs. Thus, effective ways are needed to predict the water temperature stratification in a reservoir to control its discharge water temperature. Empirical formula methods have low computational accuracy if few factors are considered. Mathematical model methods rely on large amounts of accurate hydrological data and cost long calculation times. The purpose of the research was to simulate water temperature stratification in a reservoir by constructing an intelligent simulation model (ISM-RWTS) with five inputs and one output, determined on the basis of artificial neural networks (ANN). A 3D numerical model (3DNM) was also constructed to provide training samples for the ISM-RWTS and be used to test its simulation effect. The ISM-RWTS was applied to the Tankeng Reservoir, located in the Zhejiang province of China, and performed well, with an average error of 0.72 °C. Additionally, the Intelligent Computation Model of Reservoir Water Temperature Stratification (ICM-RWTS) was also discussed in this paper. The results indicated that the intelligent method was a powerful tool to estimate the water temperature stratification in a deep reservoir. Finally, it was concluded that the advantages of the intelligent method lay in its simplicity of use, its lower demand for hydrological data, its well generalized performance, and its flexibility for considering different input and output parameters.

A coupled streamflow and water temperature (VIC-RBM-CE-QUAL-W2) model for the Nechako Reservoir

Study regionNechako Reservoir, British Columbia, Canada. Study focusHydrological regulation affect both hydrological and thermal conditions in the reservoir and downstream reach, subsequently disrupting fish habitats. This paper aims at developing an integrated model simulating physical processes that govern the quantity and quality of inflow, reservoir, and outflow water of the Nechako Reservoir. Such a model would help stakeholders understand the response of in-reservoir water temperature stratification and downstream water temperature to changes in inflow and reservoir operation under future climate change. New hydrological insights for the regionThe model was calibrated against historical reservoir levels and in-reservoir and outlet water temperature field data. The integrated model simulated accurately the wide variation of reservoir levels as well as the in-reservoir water temperature at Kenney Dam and the outlet temperature. Sensitivity analysis shows that reservoir water temperature particularly the epilimnion is sensitive to changes in both meteorological and hydrological forcing. Forcing the model with different outflow scenarios shows the weak sensitivity of temperature of water released to outflow rates. Given epilimnion water releases at the spillway, the Summer Temperature Management Program could be inefficient to provide cool water in the Nechako River during the critical period of salmon migration in a warming climate. However, colder water remains available at depth at Kenney Dam to potentially mitigate and better control downstream water temperature.

Carbon dioxide (CO2) partial pressure and emission from the river-reservoir system in the upper Yellow River, northwest China.

The exchange of carbon dioxide (CO2) flux between rivers and the atmosphere is an important part of the global carbon cycle. Reservoir development and environmental changes gradually transform rivers into river-reservoir systems. However, the current estimates of CO2 exchange flux at the water-air interface in river-reservoir systems, especially in ecologically fragile regions, are still largely uncertain. In this study, the CO2 partial pressure (ρCO2) and exchange flux (FCO2) from river-reservoir systems in the upper reaches of the Yellow River (YeUR) were investigated using the CO2SYS system and a boundary layer approach. The spatiotemporal dynamics and driving factors of the partial pressure of ρCO2 and FCO2 were revealed. Our results demonstrated that, driven by the freeze-thaw cycle of the permafrost active layer and the development of cascade reservoirs, the average ρCO2 in the two water periods was higher in the cascade reservoir section (CR) than in the source region section (SR) and higher in the flood period than in the dry period. Driven by water temperature stratification and light conditions, the ρCO2 of each reservoir in the CR exhibited seasonal variations along with water depth. The environmental factors TN, TP, T, DO, and DOC were the main influencing factors of ρCO2 distribution and could be used as predictors of ρCO2 in the dry period (R2 = 0.40 P < 0.01). In the dry period, the FCO2 in the SR was - 112.91 ± 165.94mmol/(m2·d), which was a sink of CO2, and the FCO2 in the CR was 131.02 ± 156.77mmol/(m2·d), which was a source of CO2. In the flood period, the FCO2 in the SR was 686.54 ± 624.33mmol/(m2·d), and the FCO2 in the CR was 466.10 ± 366.67mmol/(m2·d). Both the SR and the CR were sinks of CO2. Our results contribute to the understanding of CO2 exchange in river-reservoir systems and carbon cycle processes.

Comparative Study on Water Temperature Stratified Flow under Different Vertical Coordinate Systems in Delft3D

Reservoirs often suffer from water blooms, which are likely related to the hydrodynamic and water temperature characteristics of the tributary bays. To obtain the detailed changing process of hydrodynamics and water temperature stratification, it is necessary to choose a suitable vertical coordinate system in order to achieve the required precision. Based on a physical model experiment of cold water flowing into the Generalized Reservoir Hydraulics (GRH) flume, both the σ-coordinate system model and the z-coordinate system model are built for comparison. For the z-coordinate system model, the influences of different grid resolutions and different bottom slopes on the simulation accuracy are also analyzed. The results show that the σ-coordinate system model can simulate cold-water underflow in a reservoir better than the z-coordinate system model, and the numerical errors of the z-coordinate system model can be reduced but not eliminated by increasing the horizontal grid resolution. When the bottom slope of the reservoir is less than 18‰, the z-coordinate system model can also be used to simulate cold-water underflow in a reservoir. The conclusions about vertical coordinate systems can be applied to the development of a three-dimensional hydrodynamic and water temperature model of reservoirs.

Ensemble of models shows coherent response of a reservoir’s stratification and ice cover to climate warming

Water temperature, ice cover, and lake stratification are important physical properties of lakes and reservoirs that control mixing as well as bio-geo-chemical processes and thus influence the water quality. We used an ensemble of vertical one-dimensional hydrodynamic lake models driven with regional climate projections to calculate water temperature, stratification, and ice cover under the A1B emission scenario for the German drinking water reservoir Lichtenberg. We used an analysis of variance method to estimate the contributions of the considered sources of uncertainty on the ensemble output. For all simulated variables, epistemic uncertainty, which is related to the model structure, is the dominant source throughout the simulation period. Nonetheless, the calculated trends are coherent among the five models and in line with historical observations. The ensemble predicts an increase in surface water temperature of 0.34 K per decade, a lengthening of the summer stratification of 3.2 days per decade, as well as decreased probabilities of the occurrence of ice cover and winter inverse stratification by 2100. These expected changes are likely to influence the water quality of the reservoir. Similar trends are to be expected in other reservoirs and lakes in comparable regions.

How has the eutrophication evolved in the tributary bay of the Three Gorges Reservoir area

Abstract The Three Gorges Reservoir on the Yangtze River is the largest water control project in the world. While exerting great benefits (i.e., flood control, hydropower generation, inland river navigation and scenic tourism), the eutrophication of the tributary bay of the Three Gorges Reservoir has become one of the main environmental problems. This paper is to study the causes of water environment evolution in the tributary bay and investigate the driving force of eutrophication succession after the Three Gorges Reservoir enters the regular operation. By considering the Meixi River on the left bank of the mainstream of the Three Gorges Reservoir area (a typical tributary), this paper developed a three-dimensional hydrodynamic, water quality and water eutrophication mathematical model for the Meixi tributary bay, calibrated with measured data about hydrological regime (water level, flow), hydrodynamic factors (velocity) and water quality (water temperature, Chl-a, TP, TN, etc.). The annual variation of Chl-a concentration in the tributary bay was simulated, and the response relationship between the variation of Chl-a concentration and water conditions of the mainstream and tributary bay (e.g., reservoir water level, reservoir bay velocity, exogenous nutrient input, water temperature stratification and other factors) was analyzed. Results show that the water storage operation of the Three Gorges Reservoir contributes to the low flow velocity maintenance (≤0.05 m/s) in the tributary bay, the backward flow of the mainstream and the sufficient nutrients carried by the tributary water; the water temperature stratification is more likely to occur in the slow detention area in the middle-upper part of the bay in spring and summer, which provides a potential driving force for algae blooms. With the continuous decline of pollution load input in the reservoir basin, the algae blooms in tributary bay was the result of the combined action of low water level operation, low flow velocity (≤0.02 m/s), a large number of non-point source loads input with rainfall and runoff (the proportion of wet year is &amp;gt;70%), and obvious water temperature stratification in shallow water area, and the suitable meteorological conditions are the main inducing factors. Therefore, since the ecological regulation cannot be applied to the multitudinous tributary bays, the non-point source pollution control in the tributary bay is the key to controlling factor.

Effects of the Three Gorges Reservoir Operation on Vertical Distribution of Chlorophyll a and Environmental Factors in Tributaries

The hydrodynamics and environmental factors in the Xiangxi River (XXR) and Shennong River (SNR), which are tributaries of the Three Gorges Reservoir (TGR), were monitored from July to August (the low water level period) and in October (the impoundment period) in 2018. The vertical distribution characteristics of chlorophyll a and other indicators of the two tributaries were analyzed during the different operation periods, and the factors that affected the vertical distribution in each period were discussed. The results showed that the vertical distribution of dissolved oxygen, water temperature, pH value, and chlorophyll a of the XXR and SNR during the low water level period was relatively consistent. The indexes 0-10 m (0-5 m for chlorophyll a) from the surface of the XXR and SNR, respectively, showed significant stratification and decreased with increasing water depth; the stability index of thermal stratification (RWCS/H) was 13.71-29.07 m-1, which was stable. After the water depth reached 10 m (5 m for chlorophyll a), the indexes tended to be stable along the water depth. During the impoundment period, there was no obvious stratification for each index; the stability index of thermal stratification was 0-0.5 m-1, the stability of the water body was weak, and the vertical variation of each index was relatively stable. The comprehensive trophic state index (TLI) of the XXR and SNR were 55 and 53 during the low water level period, respectively, indicating that they were in a slightly eutrophic state, and 39 and 46 during the impoundment period, respectively, indicating a mesotrophic state. Linear regression analysis showed that chlorophyll a, dissolved oxygen, water temperature, and pH in the two tributaries were significantly correlated in the vertical direction in the low water level period, indicating that dissolved oxygen, water temperature stratification, and pH were important factors affecting the vertical distribution of chlorophyll a. During the impoundment period, a large amount of backflow from the Yangtze River, a large fluctuation in tributary water level, and the decrease in RWCS/H were the important factors that affected the small vertical change in the water body. The enhancement of vertical mixing and the decrease in Zeu/Zmix were the key factors affecting the nutritional status of the water.

Characteristics of phytoplankton community structure and its response to hydro-meteorology in summer of Qiantang River

河流中浮游植物的动态变化能够较好的指示河流水质状况.为探索水文气象过程对筑坝河流浮游植物群落结构的影响，2020年夏季，以钱塘江干流为例，对包括富春江水库在内的11个河段开展了浮游植物群落结构及相关环境因子的调查分析.结果表明：夏季在钱塘江干流共鉴定出浮游植物6门59属95种，优势属为沟链藻（Aulacoseir）、菱形藻（Nitzschia）、平裂藻（Merismopedia）、长孢藻（Dolichospermum）、假鱼腥藻（Pseudoanabaena）和微囊藻（Microcystis），其中梅雨期优势属主要隶属于硅藻门，梅雨后蓝藻门种属则迅速占优，浮游植物总细胞密度快速增加；整个夏季浮游植物的平均细胞密度为（1.57±2.96）×10⁶ cells/L，其中梅雨期细胞密度为（0.37±0.36）×10⁶ cells/L，显著低于梅雨后的细胞密度（4.06±4.38）×10⁶ cells/L.空间上，大坝拦截形成的河流型水库对浮游植物群落结构和藻密度均产生极显著的影响，浮游植物密度在富春江库区段最高，兰江及以上江段其次，新安江库区段及钱塘江河口较低；梅雨后富春江库区水温分层，显著促进了蓝藻和硅藻的增殖.冗余分析表明，气温和降雨是夏季钱塘江干流浮游植物群落结构变化的主要驱动因子，蓝藻门、绿藻门细胞密度与气温呈显著正相关，与降雨呈显著负相关，而硅藻门细胞密度与之相反；梅雨后气温快速增高显著增加库区段水体的蓝藻门细胞密度，蓝藻水华暴发风险迅速增高，水安全管理中应予以重点防范.;The dynamic changes of phytoplankton in rivers can better indicate the water quality of rivers. In order to explore the effects of hydro-meteorological processes on the phytoplankton community structure of damming rivers, taking the main stream of Qiantang River as an example, the phytoplankton community structure and related environmental factors were investigated and analyzed in 11 river reaches including Fuchunjiang Reservoir in the summer of 2020. The results showed that:A total 95 species belonging to 6 phyla and 59 genera of phytoplankton were identified during the summer in the main stream of Qiantang River. The dominant genera were Aulacoseir, Nitzschia, Merismopedia, Dolichospermum, Pseudoanabaena, and Microcystis. During the Meiyu period, the dominant genera belonged to the diatoms, while after the rain, the cyanobacteria quickly became dominant. The average cell density of phytoplankton in summer was (1.57±2.96)×10⁶ cells/L, and the cell density during Meiyu period was (0.37±0.36)×10⁶ cells/L, which was significantly lower than that in Meiyu period ((4.06±4.38)×10⁶ cells/L). Spatially, the river reservoir formed by dam interception has an extremely significant effect on phytoplankton community structure and algal density. The phytoplankton density is the highest in the Fuchunjiang River reservoir section, followed by the Lanjiang River and its upper reaches, and the Xin'anjiang River reservoir section and the Qiantang River estuary. After the Meiyu period, the water temperature stratification in the Fuchunjiang River reservoir significantly promoted the proliferation of cyanobacteria and diatoms. Redundancy analysis showed that temperature and rainfall were the main driving factors of phytoplankton community structure change in the main stream of Qiantang River in summer. Cyanophyta and Chlorophyta cell densities were positively correlated with temperature and negatively correlated with rainfall, while Diatomophyta cell densities were contrary to Cyanophyta. The rapid increase of temperature after the Meiyu period can rapidly increase the cell densities of cyanobacteria in the reservoir area, and the risk of cyanobacteria blooms increases rapidly. Therefore, the prevention of cyanobacteria blooms should be emphasized in water safety management.

Dissolved oxygen stratification in a small lake depending on water temperature and density and wind impact

The dissolved oxygen (DO) is one of the most important parameters in lakes ecosystem. Variability of DO in lakes is associated with the absorption of oxygen due to the decomposition of organic matter and chemical reactions and the release of oxygen as a result of photosynthesis. The DO concentration also depends on seasonal changes in water temperature and mixing regime. The aim of this work is to assess the influence of seasonal thermal and density stratification on the DO stratification in a small mesotrophic lake and to develop a regression DO model, with water temperature and density and characteristics of wind regime as independent variables. Long-term measurements of water temperature and DO in small Karelian Lake Vendyurskoe in 2007-2020 were used. At the stage of spring-summer heating, three periods are considered when the water column was in the state of homothermy (May 15-June 15), weak stratification (July 15-August 15), and strong stratification (July 15-August 15). The wind load (number of days with wind speed more than 3 m/s for each period) was analysed based on the weather station Petrozavodsk data. As a result of multiple regression analysis, taking into account the wind load, dependences of DO stratification on water temperature stratification (R2 = 0.51) and water density stratification (R2 = 0.61) are found. Obtained regression DO models can be used for solving various environmental tasks.

Performance analysis on combined heat and power of photovoltaic-thermal module integrated with phase change material-water storage

Use of phase change material (PCM) to generate temperature stratification in water storage tank coupled with unglazed photovoltaic-thermal (PVT) module for combined heat and power generation enhancement was investigated by experimental and numerical analyses. In the experiment, RT42 PCM having melting point of 38–43 °C was filled in a packed-bed of 40 mm diameter spherical capsules contained in a 220 L water storage tank connecting with four units of 200 Wp (watt peak) unglazed PVT module in series. The experiments were performed outdoor with the water mass flow rates of 2.4 and 5.8 LPM. One-dimensional finite difference enthalpy method was used to predict the water and PCM ball temperatures in the tank. The PVT thermal performance was evaluated similarly to that of solar thermal collector, and the electrical performance was calculated from nominal operating cell temperature (NOCT) approach. It could be noticed that the simulated results on the water and the PCM ball temperatures including the module temperature and the maximum generated electrical power of PVT modules agreed well with the experimental data. The model was used to predict the electrical and overall (both thermal and electrical) efficiencies of the PVT modules by considering the positions of the PCM packed-bed, the PCM amount, the circulating water mass flow rate, and the PCM type. It could be found that the position of the packed-bed did not give effect on the PVT module performances but increase of the PCM amount generated higher water temperature stratification in the storage tank which resulted in higher PVT module efficiencies especially in the afternoon. The circulating water also did not give strong effect on the total PVT module performances. For PCM with low melting point, the PVT module performances were higher but the water temperature in the storage tank might be too low for hot water utilization.

Three-Dimensional Numerical Modeling of Large-Scale Free Surface Flows with Heat and Contaminant Transfer in Reservoirs

The temperature distribution and pollutant distribution in large reservoirs have always been a hotspot in the field of hydraulics and environmentology, and the three-dimensional numerical modeling that can effectively simulate the interactions between the temperature fields, concentration fields, and flow fields needs to be proposed. The double-diffusive convection lattice Boltzmann method is coupled with a single-phase volume of fluid model for simulating heat and contaminant transfer in large-scale free surface flows. The coupling model is used to simulate the double-diffusive natural convection in a cubic cavity and the temperature distribution of a model reservoir. The mechanism of convection-diffusion, gravity sinking flow, and the complexity of the temperature and the pollutant redistribution process are analyzed. Good agreements between the simulated results and the reference data validate the accuracy and effectiveness of the proposed coupling model in studying free surface flows with heat and contaminant transfer. At last, the temporal and spatial variations of flow state, water temperature stratification, and pollutant transport in the up-reservoir of a pumped-storage power station are simulated and analyzed by the proposed model. The obtained variations of the flow field agree well with the observations in the physical model test and in practical engineering. In addition, the simulated temperature field and concentration field are also consistent with the general rules, which demonstrates the feasibility of the coupling model in simulating temperature and pollutant distribution problems in realistic reservoirs and shows its good prospects in engineering application.

Unravelling climate change impacts from other anthropogenic influences in a subalpine lake: a multi-proxy sediment study from Oberer Soiernsee (Northern Alps, Germany)

Mountain lakes are increasingly impacted by a series of both local and global disturbances. The present study reveals the eutrophication history of a remote subalpine lake (Oberer Soiernsee, Northern Alps, Germany), triggered by deforestation, alpine pasturing, hut construction, tourism and atmospheric deposition, and identifies the intertwined consequences of on-going global warming on the lake’s ecosystem. The primary objective was to disentangle the various direct and indirect impacts of these multiple stressors via down-core analyses. Our multi-proxy approach included subfossil diatom assemblages, carbon and nitrogen stable isotope ratios and subfossil pigments from dated sediments. Shifts within the diatom assemblages were related to variations in trophic state, lake transparency, water temperature and thermal stratification. The organic carbon isotope (δ13Corg) records, the diatom valve density and the pigment concentrations documented the development of primary production and composition. Total nitrogen isotope values (δ15N) are more likely to reflect the history of atmospheric nitrogen pollution than lake-internal processes, also mirrored by the decoupling of δ15N and δ13Corg trends. The composition of sedimentary pigments allowed a differentiation between planktonic and benthic primary production. Concordant trends of all indicators suggested that the lake ecosystem passed a climatic threshold promoted by local and long-distance atmospheric nutrient loadings.