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Impact of intra-annual runoff uniformity and global warming on the thermal regime of a large reservoir
... Many researchers have conducted numerous studies on the effects of climate change on the thermal stratification of lakes and reservoirs by combining historical data and mathematical models, showing that climate warming will extend the stratification period and increase thermal stability (He et al., 2019a;Sahoo et al., 2016;Yang et al., 2021b). The strengthening of thermal stratification in lakes and reservoirs leads to intensified oxygen stratification, increasing the number of harmful algae, promoting eutrophication, and affecting the structure and function of lake and reservoir habitats (Labaj et al., 2018;Piccolroaz et al., 2018;Rühland et al., 2015). ...
... The buoyancy frequency significantly increases, and the vertical stratification strength of the entire water column increases. This also meant that although the mixing state of the reservoir area has changed and the mixing area in the reservoir has become larger under the RCP 8.5 scenario, the increase in the buoyancy effect makes it more difficult for the substances and energy entering the reservoir with the incoming water to be exchanged and transferred throughout the entire water column, increasing the ecological risk in the reservoir area (He et al., 2019a). ...
... The basic equations of this model were shown in Eq. (A.1-A.11) [21]. It is suitable for hydrodynamic and water temperature simulation of the XJB reservoir [18]. ...
... Thirdly, the FPV system has the possibility to mitigate climate change impacts on reservoir temperature and stratification. Studies have shown that climate change will increase the temperature and thermal stability of lakes and reservoirs, prolong the duration of water temperature stratification, and then change the biochemical process and aquatic ecology of lakes and reservoirs [21,26]. The increase of reservoir water temperature will inevitably lead to the increase of discharge water temperature, which will affect the downstream river ecology and environment [36], and induce carbon dioxide emissions [37]. ...
As green energy, the floating photovoltaic (FPV) system has many advantages. Taking Xiangjiaba reservoir as an example, a Ce-qual-W2 model was established to evaluate the potential impact of the FPV system on water temperature and its potential benefits. The result showed that: (1) FPV system could reduce the water temperature, water age, and relative water column stability of the reservoir; (2) the influence range of FPV on water temperature will not exceed 20% of reservoir length outside the coverage area in the flow direction. (3) FPV system would delay the reaching time of critical temperature threshold and accumulated temperature threshold for Coreius heterodon spawning, shorten the spawning period. The delay time of AT and CT under 100% coverage are 20.3 and 7 days respectively compared with no FPV condition; (4) FPV system can play a great role in power generation and emission reduction. Under 100% coverage, the power generation can reach 17.2 billion kW·h, reduce 14.4 million tons of carbon emissions and save 35.12 million m³ of water. This study can provide a reference for the impact assessment of the FPV project on the water environment and the formulation of the FPV coverage scheme.
... The heat absorption of the water surface has an important influence on the heating process of the thermocline during the heating period. In the cooling period, the heat loss of the surface water drives vertical mixing and the inflow of low-temperature water forms an intrusion flow, which jointly controls the cooling process of the thermocline [20,21]. Temperature, radiation and local winds are the most important factors in the thermal processes in lakes or reservoirs [22][23][24]. ...
... A stratified thermal structure is a basic physical feature of deep lake reservoirs that affects the material exchange inside the water body, such as that of nutrients and dissolved oxygen [19,22,63]. Climate warming would change the thermal regime and mixing of lakes and reservoirs, strengthen thermal stratification during the summer, increase thermal stability, and lengthen the stratification period [20,64]. The strengthened thermal stratification in summer would cause a series of environmental impacts, especially those involving dissolved oxygen [65]. ...
The Qinghai-Tibetan Plateau region has unique meteorological characteristics, with low air temperature, low air pressure, low humidity, little precipitation, and strong diurnal variation. A two-dimensional hydrodynamic CE-QUAL-W2 model was configured for the Pangduo Reservoir to better understand the thermal structure and diurnal variation inside the reservoir under the local climate and hydrological conditions on the Qinghai-Tibetan Plateau. Observation data were used to verify the model, and the results showed that the average error of the 6 profile measured monthly from August to December 2016 was 0.1°C, and the root-mean-square error (RMSE) was 0.173°C. The water temperature from August 2016 to September 2017 was simulated by inputting measured data as model inputs. The results revealed that the reservoir of the Qinghai-Tibetan Plateau was a typical dimictic reservoir and the water mixed vertically at the end of March and the end of October. During the heating period, thermal stratification occurred, with strong diurnal variation in the epilimnion. The mean variance of the diurnal water temperature was 0.10 within a 5 m water depth but 0.04 in the whole water column. The mixing mode of inflow changed from undercurrent, horizontal-invaded flow and surface layer flow in one day. In winter, the diurnal variation was weak due to the thermal protection of the ice cover, while the mean variance of diurnal water temperature was 0.00 within both 5 m and the whole water column. Compared to reservoirs in areas with low altitude but the same latitude, significant differences occurred between the temperature structure of the low-altitude reservoir and the Pangduo Reservoir (P
... The intra-annual flow distribution varies constantly from year to year due to differences in the values of water discharge during the same phases of the water regime (peaks of high water, floods, low water level) and due to shifts in the time of occurrence of single-valued phases of the regime in different years (Christodoulou et al., 2020). Data on the intra-annual flow distribution is used in the development of flood control measures, in irrigation, in the development of industrial and economic water supply projects (He et al., 2019;Komilova et al., 2021). ...
Climate change and intensive economic activity in the river basins lead to a restructuring of the water regime. Therefore, this study aims to study the current intra-annual flow distribution in the plain rivers of the Yesil river basin, and to determine the estimated intra-annual distribution for different time periods. The results showed that the decrease in spring runoff is most noticeable in the upper stream of the Yesil river, in the lower stream of the river the spring runoff for the period with the disturbed regime is much higher than the spring runoff for the natural period, there is an increase in seasonal runoff. These findings indicate that as a result of an increase in the coefficient of natural flow regulation, the winter runoff of the Yesil river in the middle stream has increased by two or three times. KEYWORDS climate change, intra-annual flow distribution, coefficient of natural flow regulation, water regime, flow hydrograph
... The wind-induced flow is common in the shallow lakes. Wind induces reverse/compensation flow and consistent flow field, determining the transportation of pollutant and nutrient, and there exists strong correlation between the wind field and flow velocities He et al., 2019;Wei et al., 2022). The wind-induced flow is mainly impacted by the wind field, lake bathymetry, and boundary input and output runoff discharge (Soulignac et al., 2021;Zhao et al., 2021). ...
... The most suitable ecological flow supports the spawning, survival, and reproduction of indicative species, thereby ensuring the stability and integrity of the river ecosystem. When the flow is significantly lower than the most suitable ecological level, the river water quality may deteriorate, and the river may dry up or even disappear [34,35]. Conversely, if the flow substantially exceeds the suitable ecological level, flooding, soil submersion, and swamping can occur [36]. ...
Due to shortage of water resources and increasing water demands, the joint operation of multireservoir systems for balancing power generation, ecological protection, and the residential water supply has become a critical issue in hydropower management. However, the numerous constraints and nonlinearity of multiple reservoirs make solving this problem time-consuming. To address this challenge, a deep reinforcement learning approach that incorporates a transformer framework is proposed. The multihead attention mechanism of the encoder effectively extracts information from reservoirs and residential areas, and the multireservoir attention network of the decoder generates suitable operational decisions. The proposed method is applied to Lake Mead and Lake Powell in the Colorado River Basin. The experimental results demonstrate that the transformer-based deep reinforcement learning approach can produce appropriate operational outcomes. Compared to a state-of-the-art method, the operation strategies produced by the proposed approach generate 10.11% more electricity, reduce the amended annual proportional flow deviation by 39.69%, and increase water supply revenue by 4.10%. Consequently, the proposed approach offers an effective method for the multiobjective operation of multihydropower reservoir systems.
... Microsoft Excel version 2016 was used for statistical analyses, including the calculation of mean and linear regression analysis. The mean absolute error (MAE) and the root mean square error (RMSE) were used to evaluate the goodness of the fitted temperature profile and the difference between the measured and simulated temperatures (He et al., 2019a;Lian et al., 2015). Correlation analysis was performed using Statistical Program for Social Sciences (SPSS) 26.0. ...
Selective withdrawal is an effective strategy to alleviate the adverse impact of reservoir construction on the downstream thermal regime. However, most studies mainly focus on the water temperature improvement effect of a single reservoir, while few studies focus on the impact of selective withdrawal of upstream cascade reservoirs on downstream reservoirs. This study combined measured data and a two-dimensional hydrodynamic model (CE-QUAL-W2) to analyze the response of the downstream reservoir to selective withdrawal of the upstream reservoir in the operation mode of cascade reservoirs. The purpose of this study was to optimize reservoir operation to minimize the impact of cold water and satisfy the water temperature requirements of downstream fish spawning and breeding. The results indicated that the proposed index of the response to selective withdrawal (IRSW) could reflect the response of the downstream reservoir to selective withdrawal of the upstream reservoir. There existed a statistically significant positive correlation between the improvement in the withdrawal water temperature (WWT) in the Xiangjiaba Reservoir and the IRSW value (R = 0.9257, P < 0.05). The impact of the Xiangjiaba Reservoir on the incoming water temperature during the selective withdrawal period of the upstream reservoir imposed a negative superposition effect, and the selective withdrawal effect of the upstream reservoir could be maintained at 87.0%. The negative superimposition effect could be alleviated via independent operation of the upper water intake and maintenance of a low water level, and the IRSW could be increased by 6.6% and 10.4%, respectively, compared to that under the designed scenario, which could even approach the equilibrium effect (100%) and could be helpful to improve the WWT. This study provided a helpful reference for the evaluation of selective withdrawal and reservoir management in similar cascade reservoirs.
... The reservoir was formed as a result of the construction of the Sanbanxi power station, which is the second large hydropower station on the mainstem of the Yuanjiang River. The reservoir has a total capacity of approximately 4.09 billion m 3 [32]. It commenced impounding water in January 2006, and the water level rose to 475 m. ...
The construction of a reservoir dam alters the environment within its basin, including
composition of the fish community, fish biodiversity, and the river ecosystem itself. This study was conducted in the Sanbanxi Reservoir and used eDNA metabarcoding technology comprising eDNA capture and extraction, PCR amplification, sequencing and database comparison analysis, and other environmental DNA metabarcoding standardized analysis processes to characterize the composition and diversity of fish communities and assess their current status. A total of 48 species of fish were detected. Previously, 68 species of fish were screened and identified in this reservoir based on the reports of Dai and Gu. The results for fish community composition showed that species of the order Cypriniformes are still the most dominant in the Reservoir with 38 species
of cyprinids, accounting for 90.81% of all OTUs. Carp were no longer the dominant species, and Spinibarbus denticulatus, Homalopteridae, Cobitidae, and Sisoridae were not detected, with the exception of Misgurnus anguillicaudatus (Cobitidae). These families have the common characteristic of being adapted to survive in fast-water, sandstone substrate habitats. The results also show that two of the sampling sites, sbx03 and sbx10, significantly differed from other sampling sites due to their geographical environment. The impact of the construction of reservoirs on freshwater fish communities is extreme, since the transformation from a lotic to a lentic habitat contributes to habitat destruction and constrains fish in movement. The change in the aquatic environment before and after the storage of water in the Sanbanxi Reservoir has reduced the number of fish species found in the reservoir, and species characteristically found in fast moving, rapids habitats are virtually absent. The profound change in the aquatic environment from that of a lotic to a lentic habitat leads to changes in the composition of fish populations in the reservoir and to a certain extent a reduction
in the ecological stability and species diversity within the reservoir. Therefore, the protection of fish diversity in the reservoir is of great significance to the stability of the ecosystem.
... Global warming increases the reservoir temperature, released water temperature, and thermal stability, and its influence is larger than that of intra-annual runoff uniformity in Sanbanxi Reservoir. The increased water temperature will likely promote algae blooms, and the colder outflow may delay fish spawning in spring and summer in the 2090s (He et al. 2019). ...
The thermal regimes of rivers play an important role in the overall health of aquatic ecosystems. Modifications to water temperature regimes resulting from dams and reservoirs have important consequences for river ecosystems. This study investigates the impacts of the impoundment of the Three Gorges Reservoir (TGR) on the water temperature regime of fish spawning habitats in the middle reach of the Yangtze River, China. Mike 11 model is used to analyze the temporal and spatial variation of water temperatures of the expanse of 400 km along the river, from Yichang to Chenglingji. The water temperature alterations caused by the operation of the TGR are assessed with river temperature metrics. The impact on spawning habitats due to water temperature variation was also discussed in different impoundments of the TGR. The results show that the TGR has significantly altered the downstream water temperature regime, affecting the baseline deviation and phase shift of the water temperature. Such impacts on the thermal regime of the river varied with the impoundment level. The effects of the TGR on the water temperature regime decreased as the distance from the structure to the sample site increased. The water temperature regime alterations have led to the delay of the spawning times of the four famous major carp (FFMC) species. The results could be used to identify the magnitudes of water temperature alterations induced by reservoirs in the Yangtze River and provide useful information to design ecological operations for the protection of river ecosystem integrity in regulated rivers.
... Carbon emission from aquatic ecosystems plays an important role in the global carbon cycle (Yang and Flower, 2012b;Yang et al., 2020c). However, there are still quite few modelling studies on CO 2 in reservoirs (He et al., 2019a). To the best of our knowledge, this study made the first attempt to apply CE-QUAL-W2 model in the Wujiangdu Reservoir, Southwest China. ...
Carbon emissions from aquatic ecosystems, including reservoirs, play an important role in the global carbon budget, while the carbon cycle in reservoirs is still far from clearly understood. In this study, a vertical two-dimensional quality-hydrodynamic model (CE-QUAL-W2) was established to simulate the fluxes of CO2 and the transport of carbon in the Wujiangdu Reservoir in Southwest China, verified by field measurement in 2017. Thanks to overall good accuracy of multi-indicator simulation of the model (mean RRE=23.4%, mean NES=0.67), CE-QUAL-W2 is considered as a valid module to simulate carbon flux in the reservoir. Based on the model, the average CO2 flux across the air-water interface of the reservoir was 4.37 mmol m⁻² d⁻¹. Temporally, CO2 fluxes in the cold season were higher than those in the warm season, indicating the important effect of seasonal temperature change on CO2 flux, as well as the effect of other parameters, such as DO, Chla, TN, TP and pH. Spatially, CO2 emission fluxes were lower in the water inlet area of the reservoir (2.74 mmol m⁻² d⁻¹) and in front of the dam (3.40 mmol m⁻² d⁻¹), but higher in the middle part of the reservoir (highest 5.42 mmol m⁻² d⁻¹ at Segment 54). Different discharge elevation scenarios have been set up to model the influences of hydrodynamics on CO2 emission fluxes. When the water discharge elevation increased by 30m, CO2 emission increased by 39.31% and discharged downstream carbon decreased by 1.9% compared with the actual working condition. Finally, the total mass of carbon released by the reservoir (the combination of carbon diffused into the atmosphere and carbon in released water) decreased by 1.85% compared with the actual working condition. When the water discharge elevation decreased by 30m, no obvious change of CO2 emission happened, but the total mass of carbon released by the reservoir increased by 1.44%. This research confirmed the validity of CE-QUAL-W2 in simulating carbon flux in reservoirs. Our modelling results shed light on the influence of hydrodynamics on carbon emission in reservoirs, and they can serve to mitigate carbon emission from reservoirs by optimizing water discharge elevation.
... The comparisons of the observed and modeled temperature distributions are shown in Fig. 5, in which the MAE and RMSE values in the 9 validation scenarios are all smaller than 1.0 ℃. This indicates that the model-simulated vertical temperature distributions were in good accordance with the observed data (He et al., 2017;He et al., 2019;Wang et al., 2020) and that the model can accurately simulate the reservoir water temperatures associated with cascade P. Zhang et al. hydropower. The final calibrated parameters for the reservoir water temperature model are listed in Table A1. ...
The use of environmental flow (e-flow) regimes has been widely implemented to improve fish habitat quality in river restoration efforts. However, e-flow designs focusing only on one key life stage (e.g., spawning) without considering potential bottlenecks in other stages (e.g., hatching) can result in little to no improvement, especially when targeting the restoration of fish with drifting eggs. Few e-flow assessments are available that closely link the spatial-temporal dependence of the hydrodynamic effect of drifting eggs on hatching habitats. Moreover, an understanding of how to allocate e-flows to achieve the best possible outcomes for biological diversity conservation is still lacking. In this study, a new framework was developed to assess e-flows, aiming to satisfy the requirements of multiple fish species with different spawning patterns concerning streamflow requirements during spawning and hatching periods. In this framework, the final weighted usable area (FWUA) was proposed by linking the spawning demand to the hydrodynamic effect of drifting eggs to assess habitat quality for fish that produce drifting eggs, and water temperature was used to guide when and how to shift from fixed to moving protection targets in allocating e-flows. Here, we used the Xiangjiaba Reservoir, located in the lower Jinsha River, as a case study to design e-flows for the conservation of multiple fish species with different spawning patterns, which was beneficial for increasing the probability of restoration success. By testing scenarios with an absence of drifting passage, the ecological base flow considering only spawning habitat appears to be lower than that considering both quality of spawning habitat and hatching passage. The ecological benefits (FWUA) generated from the ecological base flow identified by traditional models represent only 64.91% of our framework and are thus anticipated to have cascading deviant effects on ecological patterns and processes in riverine ecosystems. This underlying difference in FWUA generated due to different ecological base flows determined from traditional models and our framework, however, has been overshadowed in previous research. We highlight that the highest fish population density recovery potential will be reached at only certain ratios for both sets of habitat benefits. This work provides a tool that can help managers evaluate e-flows and compare different river restoration scenarios to protect degraded rivers or develop strategies to build resilience to climate change.
... At the same time, many large-deep reservoirs have been built around the world for power generation and water supply in the past 20 years, (Deng, 2016;He et al., 2018;Long et al., 2018;Zhang et al., 2019). These large-deep reservoirs usually have higher SWT than the small reservoirs and natural rivers in similar climatic conditions due to their lentic environment and stable thermal stratification (He et al., 2019;Wang et al., 2018;Xie et al., 2017). Previous studies have shown that rising SWT can cause some environmental problems such as harmful algae blooms (Huisman et al., 2018;Kosten et al., 2012;Paerl and Paul, 2012;Wang et al., 2018). ...
Surface water temperature (SWT) is a key indicator to characterize the ecological health of a reservoir. Many newly built large-deep reservoirs, however, lack enough SWT observation data and high-efficient SWT predicting methods for water ecosystem management. This paper proposed a Long Short-Term Memory (LSTM) based SWT predicting method by surrogating a Delft3D hydrodynamic model. The Delft3D model calibrated by a handful of measured data was used to generate 30-year daily SWT data for training the LSTM model. The LSTM model that uses the air temperature, relative humidity, radiation, and water level data as input variables, can significantly improve the efficiency of SWT prediction. The SWT predicting method was implemented in the Nuozhadu reservoir, a large deep reservoir located in southwest China. The results showed that the LSTM model could predict the SWT generated by Delft3D accurately with an R² value of 0.99, and had a dramatic reduction in computational burden. Meanwhile, the R² between the LSTM model results and measured data was also over 0.93. Based on the SWT predicting method, we analyzed the sensitivity of SWT to the air temperature and water level to reveal the impacts of climate change and reservoir operation policies on the SWT. The major contribution of this study is that we greatly improve the computational efficiency of the SWT predicting method so that it can easily be coupled with reservoir operation optimization models, thereby enabling reservoir managers to identify optimal operation rules simultaneously considering the water temperature targets and other targets such as hydropower generation and water supply, and to eventually support reservoir management strategies that aim to reduce the potential aquatic ecosystems risk from abnormal water temperature.
... Crucially, climatic warming will affect the thermal and mixing regimes of lakes and reservoirs. It is generally believed, for example, that warming will lengthen the stratification period and make stratification more stable (He et al., 2019;Sahoo et al., 2016). The enhancement of thermal stratification will have a negative impact on aquatic organisms, thereby affecting the uses and health of affected waterbodies Weber et al., 2017). ...
Climate change is one of the most serious threats to aquatic ecosystems in lakes and reservoirs. Aquatic organisms on the Qinghai-Tibetan Plateau in China are particularly susceptible to changes in water temperature and hydraulic status. To better characterize the evolution of the thermal and mixing regimes of deep reservoirs under climate change on the Qinghai-Tibetan Plateau, a two-dimensional hydrodynamic model (CE-QUAL-W2) of Pangduo Reservoir was established, calibrated, and verified using observational data. We used representative concentration pathways (RCP) 2.6, 4.5, 6.0, and 8.5 based on the GFDL-ESM2M output data after bias correction using the ISIMIP2b dataset to simulate the water temperature distribution from 2021 to 2099. Our results show that Pangduo Reservoir’s annual mean surface water temperature has small differences under RCP 2.6 and RCP 4.5, but there are significantly increases of 0.12 °C and 0.22 °C decade⁻¹ under RCP 6.0 and RCP 8.5, respectively. The water age in the hypolimnion also increases by 0.05, 0.92, 1.73, and 3.2 d decade⁻¹ under RCP 2.6, RCP 4.5, RCP 6.0, and RCP 8.5, respectively. The delay in offset of summer stratification became a key feature in response to climate warming (the longest delay change was 3.21 d decade⁻¹ under RCP 8.5). Furthermore, simulated climate warming caused the compression of the spring convective periods and the strengthening of stratification in summer, which commonly weakened or even eliminated the replenishment of dissolved oxygen (DO) to the hypolimnion via vertical mixing. The depth of inflow intrusion also increased significantly (0.71 m decade⁻¹ under RCP 8.5), enhancing summer thermal stratification. The increase in the depth of inflow intrusion and the extension of the age of hypolimnetic water pose potential threats to water quality and ecological health in the reservoir. The concentration of DO in the hypolimnion also significantly decreased under RCP 4.5, RCP 6.0, and RCP 8.5. These issues may also exist in other dimictic reservoirs and lakes on the Qinghai-Tibet Plateau.
... However, the design of LNWLs requires longterm water level series, which usually show characteristics of nonstationarity under changing environments (Milly et al. 2008). These nonstationary characteristics appear not only in water levels at different timescales (e.g., annual, seasonal, dry or wet period, month, and day) but also in intra-annual distributions of water levels (Shiau and Wu, 2007;He et al. 2019;Yao et al. 2020). Under this circumstance, the LNWLs designed in the past or based on stationary assumptions may be unreasonable. ...
The Lowest navigable water level (LNWL) is an important indicator used for navigation design to balance the relationship between navigation safety and economic benefits of a waterway. However, it is a challenge of accurately estimating LNWLs due to the nonstationary characteristics of observed water level data series. In this study, a comprehensive framework was developed for handling this issue. In this framework, inter-annual variabilities in both the mean and variance of water level series were described by decomposing original series and were eliminated by composing new series. Intra-annual variability was determined by detecting indicators describing intra-annual water level distributions. Considerations of inter- and intra-annual variabilities were combined by designing annual water level processes for the past and current environments. Shipping risks during both annual and multi-annual periods were considered in the framework as well. The framework was demonstrated in estimating LNWLs at the Gaodao and Shijiao stations in the North River basin, southern China. The recommended LNWLs at the Gaodao station were 22.32 m for 95% guaranteed rate and 21.84 m for 98% guaranteed rate; LNWLs at the Shijiao station were 0.27 m for 95% guaranteed rate and 0.15 m for 98% guaranteed rate. The impact of variance variability on estimations of LNWLs was also evaluated. Results indicated that the recommended LNWLs would have errors of 0.11 ~ 0.48 m at the Gaodao station and 0.03 ~ 0.04 m at the Shijiao station if the variance variability was not considered. The proposed framework was then compared with Nonstationary synthetic duration curve (NSDC) method, and results illustrated that the duration curves plotted by NSDC method were unreasonable, leading to inaccurate design values. Overall, the developed framework is more reasonable and suitable for designing LNWLs of waterways where the variabilities of the water levels at different time scales are different or where the historical water level data contain various variations .
... Reservoirs are influenced by upstream inflows and downstream dam intakes, and the water exchange rate and water age change greatly over different horizontal regions (Wu et al., 2016). Moreover, for some large and deep reservoirs, the vertical thermal stratification is significant, and the water exchange differs obviously between the epilimnion and hypolimnion (He et al., 2017(He et al., , 2019. There exist some prospects of this study. ...
The hydrodynamic conditions of estuary channels are crucial to water exchanges, saltwater intrusions, and sediment movements. A comprehensive hydrodynamic study considering above multi-environmental impacts is imperative for the evaluation of local hydrodynamic strengths and the effects of water diversion projects. Aiming at this, this paper has proposed a comprehensive hydrodynamic fitness (CHF) framework, including the setup of numerical model, the calculation of the satisfactory percentage of a single index (SPSI), and the determination of the CHF. A hydrodynamic-dye-salinity-sediment model based on the finite-volume coastal ocean model (FVCOM) is built to elaborate the CHF using a case study of the Rongjiang River Estuary (RRE), and the effects of a freshwater inflow of 20 m3/s from a water diversion project on the CHF are revealed. The results show that the average turnover time, water salinity, and sediment deposition are 7.10 days, 9.39‰, and − 0.006 m, respectively, for the whole estuary channel in a normal year, and their SPSIs are 34.9%, 57.5%, and 69.4%, respectively. The CHFs are 55.8%, 73.9%, and 35.2% for normal, wet, and dry years, respectively, and these values can increase to 64.0%, 81.3%, and 43.5% with water diversion inflow. As sea level rises by 0.175 m, the CHF decreases from 55.8 to 52.9%, and can increase to 60.4% with water diversion inflow. The effects of the water diversion inflow on the CHF should not be linearly judged by the increase in freshwater runoff. The CHF framework can be extended to lagoons, bays, and reservoirs to support water resource management.
... The highest linear correlation was found between inflow temperature and mean air temperature (r = 0.78, p value <0.001; Fig. S1). Hence, bimonthly inflow temperature was converted into the daily frequency, following the same procedure found in previous studies (Hornung 2002;Rolighed et al. 2016;He et al. 2019;Silva et al. 2019). Considering that direct tributaries contribute to a small fraction of the total annual inflow, variations in their water temperature were assumed to have a negligible impact on the Barra Bonita Reservoir's thermal dynamics. ...
Lake surface warming and thermal responses to climate change have been widely reported, especially in temperate regions. Evidence of reservoir response in low latitudes is still limited. In this study, the vertical profile of water temperature in the Barra Bonita Reservoir (Brazil) is simulated using the one-dimensional General Lake Model (GLM), calibrated, and validated using in-situ data. Water temperature and reservoir hydrodynamics are simulated over 26 years (1993–2018) to investigate warming trends, seasonal patterns, Schmidt stability, and the number of stratified days per year. Results indicate that the reservoir has experienced significant warming since 1993 related to increasing air temperature and decreasing wind speed. Water temperature increases (p value <0.001) from the surface (+ 1.02 °C per decade) to the bottom (0.33 °C per decade). Higher warming rates are detected during the dry and cold season. Significant increasing trends are found for Schmidt stability and in the number of stratified days per year. Deepwater warming is directly related to increasing air temperature and frequent mixing episodes which transfer heat from surface to bottom waters. A deep outlet structure and an artificially controlled water level may enhance deepwater warming during the dry season. Our findings contribute to the understanding of subtropical reservoirs’ response to climate change and help to guide planning strategies for ensuring the security of water storage and ecosystem services they provide.
... In fact, seasonal storm runoffs can temporarily cause destratification (Huang et al., 2014) while increased inflows associated with heavy rains can totally destroy the thermocline in the riverine zone (Liu et al., 2020). Another numerical simulation study showed that extreme flood events can significantly enhance vertical mixing in the reservoir, resulting to lower thermal stability (He et al., 2019). In cases of normal inflows, it is also interesting to analyze by numerical simulation the effect of stratification to interflow travel time and the transport of sediments and nutrients as affected by the facilities. ...
This study investigates the thermal stratification responses of a monomictic reservoir operated under different facilities. The analysis of 60-year long data showed that the reservoir’s thermal regime varies with season and withdrawal scheme and is affected by upstream reach control through the vertical curtain. Isothermal conditions exist during winter (December-March) while stratification onsets in spring (starting April), intensifies in summer (August) and weakens during fall (October-November). Considering summer stratification, deep hypolimnetic withdrawals through the penstock intake promoted thicker epilimnion, with low values of thermal stability (Schmidt Stability Index, SSI) and thermocline strength index (TSI). Meanwhile, shallow withdrawals using selective outflow system resulted in narrower epilimnion, with larger TSI for no curtain scenario and larger SSI for with curtain scenario. Strongest thermoclines do not necessarily translate to largest magnitudes of thermal stability. Longer duration of stratification is associated with shallow withdrawals. Depending on the outflow depth and the occurrence of prolonged hot or cold atmospheric conditions, the onset of stratification could be likely shifted early or late. The 3D numerical simulation determined the individual effects of each operation, which strongly supported the results of the long term analysis. Since thermal stratification directly influences the reservoir’s water quality regime, this study can be a helpful reference in optimizing the water quality management of the reservoir.
... In the past decade, a model called CE-QUAL-W2 has been extensively used in research and applied projects, which demonstrates its high acceptability among experts of the field. For example, this model has been used in research on the algae of Xiangxi Bay (Chuo et al. 2019), impact of intraannual runoff uniformity and global warming on the thermal regime of Sanbanxi Reservoir (He et al. 2019), a seasonal ice cover lake for drinking water in a part of Canadian shallow prairie (Terry et al. 2018), simulation of algae condition of a reservoir based on variable chlorophyll α/algal biomass ratios , the effect of dam structure on the ice cover of Fengman Reservoir (Tuo et al. 2018), temporal and depth variations of water quality due to thermocline of Karkheh Dam Reservoir (Noori et al. 2018a, b), development of a new method for optimizing the water quality monitoring network of lakes and reservoirs (Maymandi et al. 2018), modeling of Powell Lake (Williams 2007), study of a monomictic reservoir (Chung and Oh 2006), and simulation of Shihmen Dam Reservoir (Wu et al. 2004) and Henry Hagg Lake (Sullivan and Rounds 2004). ...
Continuous monitoring of water quality in dam reservoirs is a typically difficult and costly operation. In this study, the results of computer modeling with the CE-QUAL-W2 model were combined with data mining techniques to develop a new method called “delta-normal stress” for identifying the critical temporal and spatial monitoring ranges. For this purpose, long-term variations of three quality parameters including nitrite-nitrate level, dissolved oxygen (DO) level, and water temperature near the outlet of the dam, which is the point of interest for reservoir exploitation, were analyzed. Based on this analysis, the time intervals and depth ranges with the highest frequency of significant variations in terms of each parameter were identified. The results showed that given the difference between the delta-normal stress trend of temperature and that of other parameters in Karkheh Dam Reservoir, temperature can be monitored at much lower sampling resolutions and using cheaper methods and equipment without sacrificing accuracy. Based on the frequency of occurrence of delta-normal stress of more than 20% above the total average, the key sampling times and locations for nitrite-nitrate and DO levels were determined to be the periods of January–February, February–March, and March–April, and depths of 60, 55, 50, and 5 m, respectively.
... Water temperature plays a vital role in the overall health of aquatic ecosystems. An inappropriate temperature threatens fish spawning and growth and influences the irrigation and water quality of a basin (Caissie, 2006;He et al., 2019). Thermal stratification widely exists in reservoirs and causes the outflow temperature to differ from the inflow temperature, which imposes a negative effect on the downstream environment and ecology (Gao et al., 2014;Rheinheimer et al., 2015;He et al., 2018). ...
... Fish habitats are mainly affected by the hydrological conditions of the river, where flow and water temperature are one of the two main important factors affecting fish spawning [13]. Thermal stratification is common in narrow channel reservoirs and greatly influences the aquatic animal habitat environment [14]. It causes a significant difference between the reservoir outflow temperature and the inflow temperature [15]. ...
The study of fish habitats is important for us to better understand the impact of reservoir construction on river ecosystems. Many habitat models have been developed in the past few decades. In this study, a fuzzy logic-based habitat model, which couples fuzzy inference system, two-dimensional laterally averaged hydrodynamic model, and two-dimensional shallow water hydrodynamic model, is proposed to identify the baseline condition of suitable habitat for fish spawning activities. The proposed model considers the reservoir and the downstream river channel, and explores the comprehensive effects of water temperature, velocity, and water depth on habitat suitability. A real-world case that considers the Ctenopharyngodon idella in the Xuanwei Reservoir of Qingshui River is studied to investigate the effect of in- and outflow of reservoir on fish habitat and the best integrative management measure of the model. There were 64 simulations with different reservoir in- and outflows employed to calculate the weighted usable area and hydraulic habitat suitability. The experimental results show that the ecological flow for Ctenopharyngodon idella spawning can satisfy the basic demand when the reservoir inflow is greater than 60 m3/s and the reservoir outflow is greater than 100 m3/s. The habitat ecological suitability is the best when the reservoir outflow is 120 m3/s. A more reasonable and reliable ecological flow range can be obtained based on the habitat model in this paper, which provides the best scenario for water resources planning and management in the Qingshui River Basin.
Utilizing the thermal stratification of reservoirs to obtain cold water for cooling green data centers (GDCs) is a new mode of energy conservation and emission reduction. However, global warming is expected to alter this phenomenon in deep reservoirs and thus may affect the digitalization process. While the frequency and intensity of extreme climate (EC) events are increasing under a changing climate, the impacts of these highly destructive events on thermal stratification and the related cascading effects on GDC cold-water supplies are still unclear. A two-lateral-dimensional thermohydrodynamic model was established to determine the characteristics of reservoir thermal stratification changes and its potential water environment impacts based on the heat extreme experienced by the Jinshuitan Reservoir, a large, pumped storage power station (PSPS) with a GDC in southeastern China. Fourteen different PSPS inlet/outlet schemes under 2 climate scenarios were designed, and two new indexes, namely, the risk index of cold-water shortage and the index of thermal stratification, were proposed to explore the impact of extreme climates on GDC water extraction. The results showed that compared to the present climate conditions, the reservoir experienced stronger thermal stratification with a 20.6 % average increase in thermal stability, a longer thermal stratification duration with a transitional trend from seasonal to permanent stratification, and a thinner cold-water layer with a 15-m average decrease. Changes in the thermal stratification of reservoirs under EC conditions will benefit some species, such as diatoms (except for early July and the period from mid-August to early October), green algae and cyanobacteria and the migration of warm-water fish, but adversely impact dissolved oxygen, diatoms and cold-water fish. Furthermore, the risk of cold-water shortages under EC conditions was consistently greater than that under average climate (AC) conditions (traditional assessment) when the elevation of pumped storage intake/outlet was less than 145 m. According to the traditional assessment method, the intake/outlet elevation of pumped storage reservoirs resulted in a 60.27 % increase in the risk of cold-water shortages during climate extremes. Furthermore, the demand for green cold-water withdrawal and optimal pumped storage power generation benefits were met when the PSPS intake/outlet elevation was 145 m. Beyond solving this specific case, this study elucidates the importance of linking the thermal stratification and data center of cold-water withdrawal to ECs. These findings provide new insights for increasing climate change resilience.
Following the Three Gorges Reservoir (TGR) impoundment, many tributaries were turned into bays; hydrodynamic conditions of TGR profoundly changed the residence time, temperature, and nutrient distributions of bays, and nutrient enrichment occurred in these bays. However, little research has been done on the effects of water level fluctuations (WLFs) of TGR on the bay. In this study, Xiangxi Bay (XXB), one of the tributaries of TGR, was selected as the delegate to construct and calibrate a two-dimensional hydrodynamic–temperature–tracer–water quality model based on the CE-QUAL-W2. The results were the following: 1) In spring, as total nitrogen (TN) in the TGR tended to be higher than that in the XXB, the downward WLF increased water exchange, TGR-XXB nutrient flux and TN in the epilimnion of the XXB, and decreased the water exchange and TN in the hypolimnion of the XXB. The upward WLF did the opposite. The situation would be reversed in autumn. 2) Under a larger magnitude or a shorter period of WLF, its corresponding effects on the water exchange and TN increased. 2) Both the downward and upward modes of WLF helped to decrease the thermal stratification of XXB. 4) The upward/downward WLF could be used to decrease the epilimnetic TN of XXB in spring/autumn, and was suggested to reduce the local algal bloom. The WLFs by the TGR regulation could profoundly change the water exchange and nutrient distribution in the bay, which helped to control nutrient concentrations and prevent algal blooms.
Large reservoirs and dams can fundamentally alter downstream flow and thermal regimes by resetting the flow and water temperature release boundary conditions, resulting in significant impacts on fish habitat suitability. Therefore, it is critically important to understand how the volume and temperature of dam releases to influence downstream hydraulic and temperature dynamics and predict the ecological responses of fish habitat suitability to these changes. In this study, a physically‐based coupled model framework was developed to identify the temporal and spatial variations of hydraulic and water temperature regimes and the spawning habitat suitability of four major Chinese carps (FMCC) in the middle reach of the Yangtze River below Three Gorges Reservoir (TGR) responded to different upstream boundary conditions established by dam operation. The results showed that the dam discharge volume from TGR mainly affected the downstream hydraulic regime, while dam discharge temperature would have most impact on downstream water temperature. The spawning habitat suitability of FMCC obviously decreased due to TGR operation, and the declined water temperature suitability and the delayed and narrowed window of suitable water temperature were the main influencing factors. For the purpose of maximizing the spawning habitat suitability of FMCC in the downstream reaches below TGR, dam discharge volume ranged from 10000 m3/s to 22500 m3/s and dam discharge temperature varied from 22 °C to 23.5 °C would be recommended as the outflow conditions in the ecological operation of TGR.
Study region
Dongqing Reservoir located in Guizhou, China.
Study focus
Zoning the RWTF (reservoir water temperature field) is of great significance and is an effective way to analyze RWTF’s nature. In practice, the conditions of RWTF fluctuate greatly as time goes on, which leads to the existing RWTF zoning methods can’t give a steady zoning result. In consequence, this paper creates a kind of zoning method to study the properties of RWTF in Dongqing Reservoir, which has two main steps: firstly, numerical simulation is used to obtain the whole data of RWTF, and then the K-means clustering algorithm is executed based on the numerical simulation results.
New hydrological insights for the region
This paper proved that the zoning method developed in this paper, which combined numerical simulation and unsupervised machine learning, can be effectively applied and divided the RWTF into four zones without using experimental parameters in Dongqing Reservoir. Moreover, on the base of the four zones’ spatial borders, the influencing factors of water temperature in each zone of Dongqing Reservoir were able to be found, which would be of value in further research of RWTF.
Temperature stratification increases the thermal stability (TS) of the reservoir area, decreasing the vertical water exchange and going against the water environment improvement. It is affected by the reservoir operation scheme and should be considered with multiple objectives. Hence, considering the early flood season of Xiluodu Reservoir as a case, this study built a multiobjective optimization model of temperature stratification by coupling the hydro-temperature model CE-QUAL-W2. The non-dominated sorting genetic algorithm II and multiobjective particle swarm algorithm were used to solve the multiobjective optimization model, and the Pareto frontiers were evolved by reducing the peak rate of outflow (PRO), increasing total hydropower generation (THG), and decreasing TS. This paper shows that (1) the maximum-THG individual tends to release water late to increase the water level and hydraulic head, which is opposite for the minimum-TS individual. A temporally uniform outflow favors a decrease in PRO. (2) In the Pareto frontiers, THG can be increased from 47.42×10⁸ to 52.15×10⁸ kw·h (+9.97%), and TS can be reduced from 15448.27 to 14627.01 J/m² (-5.32%). (3) When the inflow rate increases, the temperature stratification is further weakened, and the minimum TS is reduced from 14627.01 to 13233.54 J/m². For large cascade reservoirs with low flood risk in the early or late flood season, their THG and thermal regime can be improved under a moderate outflow and should be considered for the administrations.
Hydropower plays an extremely important role in the energy market with the increasing energy demand. The intra-annual runoff distribution (IARD) in the hydrological year has an important impact on the energy generation of cascade hydropower plants (CHP). This study evaluated the non-uniformity of IARD of the Datong River basin (DRB) in the hydrological year. The optimal operation model of CHP was constructed based on the genetic algorithm to investigate the impact of the non-uniformity of IARD on the energy generation of CHP. This study shows the following: 1) The observed runoff series has a decreasing trend, the non-uniformity of IARD has an increasing trend overall. 2) The energy generation of CHP shows a weak decreasing trend affected by the decrease of runoff and the increase of non-uniformity. 3) With or without water diversion, the CHP energy generation decreases with the increase of non-uniformity of IARD. 4) The difference of energy generation of CHP between consideration or not the water diversion is about 400 GWh when the inflow frequency is less than 50%. 5) The impact of non-uniformity of IARD on energy generation will be greater with the inflow increasing. This study can provide theoretical support for the water resources management and operation of CHP.
Saltwater intrusions occur widely in tidal river channels, significantly changing the vertical distribution of mainstream velocity and density (salinity). They are vital to the hydrodynamic conditions of lateral withdrawal. In this study, a 3-D numerical model was constructed and validated through experimental tests, and the hydrodynamic characteristics of lateral withdrawal under saltwater intrusion were investigated. Additionally, the vertical distribution effects of mainstream velocity, withdrawal velocity, and density stratification were analyzed. Under non-uniform mainstream velocity conditions, with larger surface velocity than bottom velocity, the division width of the upper layer is smaller, while the local secondary circulation is stronger than that of the lower layer. Secondary circulation is top-shaped and weakens as the withdrawal velocity increases. When salinity stratification exists, the division width of the lower layer decreases, and the secondary circulation is stronger than that of the upper layer; meanwhile, the vertical water exchange increases. The influence of mainstream velocity is greater than that of salinity stratification. Under these two factors, the sediment entering the intake increases, and submerged vane, sill, and weir measures should be adopted. This research represents an advance in lateral withdrawal hydrodynamic research and provides support for engineering design in estuarine areas.
During the early flood season of the Yangtze River, which encompasses the spawning season of four major Chinese carps, reservoirs tend to remain at a low limiting water level in case of large floods. However, a low limiting water level, which is the initial water level of the spawning season, decreases the reservoir volume, causing insufficient outflow for downstream fish spawning. Therefore, the Limiting Water Level of Early Flood Season (LWLEFS) must be optimized to achieve a comprehensive benefit considering fish spawning, apart from hydropower generation and flood control. Using the Three Gorges Reservoir (TGR) as a case study, this study aims to combine multiobjective optimization scheduling using the non-dominated sorting genetic algorithm and the copula joint distribution function, and propose an optimizing framework to determine the LWLEFS. The optimizing framework is as follows: first, multiple inflow time series during the spawning season are randomly simulated based on historical hydrological wavelet analysis, to support optimization scheduling. Next, considering three objectives, i.e., hydropower generation, flood control, and fish spawning, multiobjective optimization scheduling is conducted, and the optimal individual for each inflow time series is obtained at different LWLEFSs. Subsequently, the copula joint distribution function of the three objectives is established, and the Probabilities Achieving the Established Objective Values (PAEOVs) are calculated. Additionally, the relationships between different LWLEFSs and PAEOVs are obtained, and the LWLEFS that leads to the maximum PAEOV is identified. For the TGR, the optimal LWLEFSs are 154, 149, 148, 147, and 145 m for extremely dry, generally dry, normal, generally wet, and extremely wet hydrological conditions, respectively. The optimizing framework can be extended to other reservoirs with local targets, and it offers technical support for ecological and environmental management.
Plain language summary
The fish in Yangtze River, China often spawns during June 15th to July 20th, which is located in the early flood season. In these days, the basin reservoir such as Three Gorges Reservoir always keeps a low water level for flood control. However, under the low operation water level, the reservoir volume is small and cannot release adequate water for the downstream fish spawning. The above water level needs be changed considering downstream fish spawning. Therefore, we have analyzed the multi objectives of reservoir operation, including hydropower generation, flood control and fish spawning. And we propose an optimizing framework to determine the operation water level based on their trade-off relationship. The optimizing framework is practical for ecological and environmental management, and can be applied for other reservoirs with different operation targets.
Flood risk increases with climate change and rapid urbanization, which urgently needs to improve the capacity of urban drainage systems. In the study, a staged optimization model considering climate change and hydrological model uncertainty (SOCU) was proposed for urban drainage system design. The SOCU model refers to the construction of drainage system by stages instead of traditional implement-once plans, which was established based on an integration of staged optimization policy, urban hydrological model, generalized likelihood uncertainty estimation (GLUE) method and chance-constrained programming (CCP) model. The staged optimization policy was employed to deal with the uncertainty of climate change. The GLUE method was adopted to analyze the uncertainty of urban hydrological model established by PCSWMM. Considering the uncertainty from urban hydrological simulation, the CCP model (one of the main methods of stochastic mathematical programming) was used to deal with the uncertainty of the optimization model. Subsequently, a case study of the Haidian island of Hainan Province in China was used to demonstrate the proposed model. The result shows that the optimal investment is 183 million Yuan with a pumping capacity of 28.3 m³/s for the first stage and 38.4 m³/s for the next stage. The SOCU model is not only more flexible to adapt to climate change, but also is economically efficient (10% lower than implement-once plans). The urban drainage system design obtained from the SOCU model is more reliable and robust than traditional implement-once plans since the climate change and hydrological model uncertainty are simultaneously taken into account. Furthermore, the investment increases from 166 million Yuan to 195 million Yuan when the flood constraint satisfaction probability increases from 0.75 to 0.95. Therefore, the model could provide richer decision-making information than traditional implement-once plans and help decision makers seek a trade-off between system investment and acceptable flood damage. The study outcomes provide a reliable optimization model for urban drainage design and may have profound implications and contributions for urban flood management.
Accurate vertical diffusivity estimates at different stratification conditions are essential to correctly model vertical mixing of discharges into lakes. This study presents calculated variations in vertical mixing at different depths in Boulder Basin, Lake Mead, a deep reservoir over a four-year period using hourly weather data and 6-hourly measured temperature, conductivity, and DO profiles. Turbulent Kinetic Energy (TKE) and mixing intensities within Boulder Basin, calculated based on surface heat flux and wind speed were compared to water column stability and diffusivity over the study period. Analysis of surface heat fluxes showed that evaporation and longwave radiation were the main heat loss mechanisms in summer and winter, respectively. The lake showed strong summer stratification with stability numbers N 2 > 10 − 4 s − 2 , followed by increased water column instability during fall and eventually winter over- turn, resulting in gradient Richardson numbers < 0.25 in the water column’s top 50 m. The average calculated Wedderburn number was 45 during summer stratification, indicating that local winds were not sufficiently strong to generate upwelling. Burger numbers ( S < 1) show that the Coriolis force significantly affects vertical mixing in Boulder Basin over the entire annual cycle. Diffusivities seasonally varied by 1 to 1.5 orders of magnitude (typically 5 ×10 − 5 to 10 − 3 m 2 s − 1 ) in the upper water column, and typically varied by about 1.5 orders of magni- tude (typically 3 ×10 − 6 to 10 − 4 m 2 s − 1 ) in the deeper layers. Increases in winter diffusivities caused deep water dissolved oxygen (DO) concentrations to increase from 6.0 to 8.5 mg L − 1 . Analysis of DO profiles and chloride and sulfate concentrations in the epilimnion and deep hypolimnion showed marked differences between epilimnetic and hypolimnetic concentrations during stratification. Similar epilimnetic and hypolimnetic concentrations dur- ing January and February confirm increased vertical mixing during these months. Use of hourly-based computed TKEs, and water column vertical diffusivity estimates in stratified and unstratified conditions over the entire annual cycle can help modelers to more accurately predict vertical mixing in large lakes.
One of the most important current issues in the management of lakes and reservoirs is the prediction of global climate change effects to determine appropriate mitigation and adaptation actions. In this paper we analyse whether management actions can limit the effects of climate change on water temperatures in a reservoir. For this, we used the model EOLE to simulate the hydrodynamic and thermal behaviour of the reservoir of Bimont (Provence region, France) in the medium term (2036-2065) and in the long term (2066-2095) using regionalised projections by the model CNRM-CERFACS-CNRM-CM5 under the emission scenarios RCP 4.5 and RCP 8.5. Water temperature projections were compared to simulations for the reference period 1993-2013, the longest period for which we had year-long data for both hydrology and meteorology. We calibrated the model using profile measurements for the period 2010-2011 and we carried an extensive validation and assessment of model performance. In fact, we validated the model using profil
Environmental flow is fundamental to ecological health and integrity of a riverine environment. River delta systems have become more and more complicated due to climate change and human activities and these have made a significant impact on significant changes in hydrological processes and the ecological environment. Highly intense human activities and most economically developed regions in the Pearl River Delta (PRD), China, was selected as case study. Based on observed daily flow data with a length of 50 years from seven control stations, inter-annual and intra-annual streamflow alterations in this region were analyzed by using the indicators of hydrologic alteration (IHA) method, the range of variability approach (RVA), and the histogram matching approach (HMA), and quantitative impact of main factors on inter- and intra-annual streamflow alterations were derived. Results showed the following: (1) Combination of RVA and HMA can better reveal changes of IHAs, so as to more comprehensively evaluate environmental flow alteration of river systems. (2) Discharge diversion due to changes in river channel geometry is the main factor causing inter-annual streamflow alteration in the Northwest River of PRD, whose contributions were 122.35% and 90.08% at Makou and Sanshui stations, respectively. (3) Change in upstream flow is the main factor causing intra-annual streamflow alteration in the Northwest River of PRD, while reservoir operation is the main factor causing intra-annual streamflow alteration in the East River of PRD. (4) Climate change and reservoir operation can make intra-annual distribution of monthly discharge more concentrate and even, respectively. This study contributes to an improved understanding of environmental flow alteration and associated underlying causes of flow regime variations in the river delta region.
The Poyang Lake Basin has been suffering from severe water problems such as floods and droughts. This has led to great adverse impacts on local ecosystems and water resource utilization. It is therefore important to understand stream flow changes and their driving factors. In this paper, the dynamics of stream flow and precipitation in the Poyang Lake Basin between 1961 and 2012 were evaluated with the Mann-Kendall test, Theil-Sen approaches, Pettitt test, and Pearson's correlation. Stream flow was measured at the outlets of five major tributaries of Poyang Lake, while precipitation was recorded by fourteen meteorological stations located within the Poyang Lake Basin. Results showed that annual stream flow of all tributaries and the precipitation over the study area had insignificant (P > 0.1) temporal trends and change points, while significant trends and shifts were found in monthly scale. Stream flow concentration indices (SCI) at Waizhou, Meigang, and Wanjiabu stations showed significant (P < 0.05) decreasing trends with change points emerging in 1984 at Waizhou and 1978 at Wanjiabu, while there was no significant temporal trend and change point detected for the precipitation concentration indices (PCI). Correlation analysis indicated that area-average stream flow was closely related to area-average precipitation, but area-average SCI was insignificantly correlated with area-average PCI after change point (1984). El Niño/Southern Oscillation (ENSO) had greater impacts on stream flow than other climate indices, and La Niña events played a more important role in stream flow changes than EI Niño. Human activities, particularly in terms of reservoir constructions, largely altered the intra-annual distribution of stream flow but its effects on the amount of stream flow were relatively low. Results of this study provided a useful reference to regional water resource management and the prevention of flood and drought disasters.
This work presents a methodology for extracting optimal operational rules for selective reservoir water withdrawal by considering fixed levels of reservoir water outlets for thermal control. The outlet water temperature of the Karkheh reservoir, Iran, is simulated with the CE-QUAL-W2 model. A data-mining model (the LIBSVM model) is applied as a surrogate model of the CE-QUAL-W2 model and coupled with a genetic algorithm (GA), resulting in the LIBSVM-GA algorithm. The selective withdrawal approach considered four fixed reservoir outlets, located at 120, 140, 163, and 181 m above sea level, to account for reservoir thermal stratification. This paper's methods are evaluated with nonselective and selective withdrawal operations through different scenarios in which single outlet, fixed withdrawal proportions, fixed monthly variable proportions, continually variable (10-day) proportions using total monthly LIBSVM input data, and continually variable (10-day) proportions using separated monthly LIBSVM input data are considered. The highest outlet (at 181 m) was found to be the best level for the nonselective withdrawal scenario. The best selective withdrawal operations scenario was the continually variable (10-day) proportions using separated monthly LIBSVM input data, which minimize the root-mean-square deviation (RMSD) between upstream and downstream temperatures during the operating period.
The microbial communities within reservoir ecosystems are shaped by water quality and hydrological characteristics. However, there are few studies focused on the effects of thermal stratification on the bacterial community diversity in drinking water reservoirs. In this study, we collected water samples from the Jinpen Reservoir around the re-stratification period. To explore the functional diversity and bacterial community composition, we used the Biolog method and 16S rRNA-based 454 pyrosequencing combined with flow cytometry. The results indicated that stratification of the reservoir had great effects on temperature and oxygen profiles, and both the functional diversity and the composition of the bacterial community strongly reflected the significant vertical stratification in the reservoir. The results of the Biolog method showed a significantly higher utilization of carbon sources in the hypolimnion than in the epilimnion. The result of pyrosequencing also showed a significantly higher species diversity and richness in the hypolimnion than in the epilimnion with different dominant phylum. Redundancy analysis also indicated that the majority of environmental variables, especially pH and dissolved oxygen, played key roles in shaping bacterial community composition. Our study provides a better understanding of the functional diversity of bacterial communities, and the response of microorganisms to seasonal thermal stratification.
Since the first impoundment of Three Gorges Dam in 2003, algal blooms occurfrequently in the near-dam tributaries. It is widely recognized that the impoundment-inducedchange in hydrodynamic condition with the lower current velocity will make theeutrophication problem even more severe when an excessive amount of nutrients is alreadyloaded into a reservoir and/or its tributaries. Operation tests carried out by Three GorgesCorporation in 2010 point to some feasible reservoir operation schemes that may havepositive impacts on reducing the algal bloom level. In our study, an attempt is made toobtain, through a numerical hydrodynamic and water quality modeling and analysis, thereservoir operation rules that would reduce the level of algal blooms in the Xiangxi River(XXR), a near-dam tributary. Water movements and algal blooms in XXR are simulatedand analyzed under different scenarios of one-day water discharge fluctuation or two-weekwater level variation. The model results demonstrate that the reservoir operations canfurther increase the water exchange between the mainstream of the Three Gorges Reservoir(TGR) and the XXR tributary and thus move a larger amount of algae into the deep waterwhere it will die. Analysis of the model results indicate that the water discharge fluctuationconstituted of a lower valley-load flow and a larger flow difference for the short-termoperation (within a day), the rise in water level for the medium-term operation (e.g., over weeks), and the combination of the above two for the long-term operation (e.g., overmonths) can be the feasible reservoir operation rules in the non-flood season for TGR.
Individual impacts of climate change, land use/coverage change, the regulation of water reservoir and water use to intra-annual distribution of streamflow of the Dongjiang River basin, South China, are quantitatively separated. Data on precipitation, streamflow, regulation of water reservoirs, and water use are used in the analysis. Changes in the concentration degree and non-uniformity coefficient of intra-annual distribution of streamflow over time scales from daily to monthly are evaluated. Trend and change point analyses are applied to determine characteristics of intra-annual distribution of streamflow. By comparison with the difference among region precipitation, naturalized streamflow and observed streamflow, the contribution of main impact factors to the intra-annual distribution of streamflow are quantified. Results demonstrate the non-uniformity coefficient decreases with a negative logarithmic linear function of the time scale. Change points of the concentration degree and non-uniformity coefficient occurred in 1973 exhibited a significant downward trend. In the period posterior to the change point the former and the latter decrease approximately by 15–34 % and by 29–40 %, respectively. The decrease in concentration degree and non-uniformity coefficient is due to the regulation of water reservoirs and land use/cover change; while the increase is the result of water use and climate change. The individual contributions to the impact on the intra-annual distribution of streamflow from regulation of water reservoirs, land use/cover change, water use, and climate change are approximately −33.5, −9.0, 4.5 and 1.0 %. It is observed that the impacts of the regulations of the Xinfengjiang, Fengshuba, and Baipenzhu reservoirs account for −21, −10, and −2 %, respectively. There is an increasing tendency in the impacts of land use/cover change and water use in the past 30 years.
This study uses a multidisciplinary approach to simulate the spatial and temporal patterns of hydrodynamics and water quality in a thermally stratified reservoir in the southern side of the Mediterranean Sea in response to water withdrawal elevation using the 2D water quality and laterally averaged hydrodynamic model CE-QUAL-W2. The withdrawal elevation controls largely the transfer of heat and constituents in the dam in particular during thermal stratification. Fifteen scenarios of withdrawal elevation are possible. To identify the most effective scenarios, a hierarchical clustering technique was performed and only four scenarios were clustered. Deep withdrawals deepen the hypoxia, increase the thickness of the metalimnion, and weaken the stratification stability, which facilitate the vertical transfer of heat and dissolved oxygen mainly. Surface withdrawals, however, shrink the metalimnion and tend to strengthen the stratification, resulting in less transfer of matter from the epilimnion to the hypolimnion. Most of the bottom sediment is overlaid by the hypolimnion. The oxygen depletes significantly and waters become anoxic at a few meters depth. For all scenarios, the reservoir experiences a summer hypolimnetic anoxia, which lasts from 42 to 80 days and seems to decrease as withdrawal elevation increases. At the end of stratification, waters below the withdrawal elevation showed a noticeable release of iron, nutrients, and suspended sediments that increases with depth and near-bottom turbulence. Attention should be drawn to shallower withdrawals because they accumulate nutrients and silts continuously in the reservoir, which may deteriorate water quality. Based on these results, a withdrawal elevation rule is presented. This rule may be adjusted to optimize water withdrawal elevation for dams in the region with similar geometry.
From January 2010 to March 2014, detailed depth profiles of water temperature, dissolved oxygen (DO), and chromophoric dissolved organic matter (CDOM) were collected at three sites in Lake Qiandaohu, a large, deep subtropical reservoir in China. Additionally, we assessed the changes in DO stratification over the past 61 years (1953-2013) based on our empirical models and long-term air temperature and transparency data. The DO concentration never fell below 2 mg/L, the critical value for anoxia, and the DO depth profiles were closely linked to the water temperature depth profiles. In the stable stratification period in summer and autumn, the significant increase in CDOM in the metalimnion explained the decrease in DO due to the oxygen consumed by CDOM. Well-developed oxygen stratification was detected at the three sites in spring, summer and autumn and was associated with thermal stratification. Oxycline depth was significantly negatively correlated with daily air temperature and thermocline thickness but significantly positively correlated with thermocline depth during the stratification weakness period (July-February). However, there were no significant correlations among these parameters during the stratification formation period (March-June). The increase of 1.67 °C in yearly average daily air temperature between 1980 and 2013 and the decrease of 0.78 m in Secchi disk depth caused a decrease of 1.65 m and 2.78 m in oxycline depth, respectively, facilitating oxygen stratification and decreasing water quality. Therefore, climate warming has had a substantial effect on water quality through changing the DO regime in Lake Qiandaohu.
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Warming-induced changes in lake thermal and mixing regimes present risks to water quality and ecosystem services provided by U.S. lakes and reservoirs. Modulation of responses by different physical and hydroclimatic settings are not well understood. We explore the potential effects of climate change on 27 lake “archetypes” representative of a range of lakes and reservoirs occurring throughout the U.S. Archetypes are based on different combinations of depth, surface area, and water clarity. LISSS, a one-dimensional dynamic thermal simulation model, is applied to assess lake response to multiple mid-21st century change scenarios applied to nine baseline climate series from different hydroclimatic regions of the U.S. Results show surface water temperature increases of about 77 % of increase in average air temperature change. Bottom temperature changes are less (around 30 %) for deep lakes and in regions that maintain mid-winter air temperatures below freezing. Significant decreases in length of ice cover are projected, and the extent and strength of stratification will increase throughout the U.S., with systematic differences associated with depth, surface area, and clarity. These projected responses suggest a range of future challenges that lake managers are likely to face. Changes in thermal and mixing dynamics suggest increased risk of summer hypoxic conditions and cyanobacterial blooms. Increased water temperatures above the summer thermocline could be a problem for cold water fisheries management in many lakes. Climate-induced changes in water balance and mass inputs of nutrients may further exacerbate the vulnerability of lakes to climate change.
An important factor in aquatic environment is water temperature, which influence many important processes in an aquatic ecosystem. In this paper, a 3-D model of thermal-hydrodynamic was developed to simulate the thermal structure in stratified reservoir, taking YE reservoir, a typical canyon reservoir in south-west China as an example. The simulated results showed that YE reservoir is a stratified reservoir, and water column in reservoir have two overturning periods per year separated by periods of stratification. In this case, 7-days flood process (from Jul.17th to Jul.23th) during the normal flow year was taken as a inflow charge, the results showed if the temperature of flood is the same as that of the outflow from the reservoir, the thermal structure will maintain the former status, and if the temperature of flood is lower than that of the outflow, the flood will cause thermocline erosion to some extent. And in this case, the effect time can last two months. By changing the outlet location, simulations demonstrate that the outlet elevation is the main factor controlling the thermal structure in YE reservoir.
Thermal stratification is strongly associated with hydrodynamics, and plays an important role in the dynamics of water quality and the ecosystem of stratified water bodies. Changes in the climate or hydrological conditions can alter thermal regimes. This study aims to investigate the effects of climate change on the thermal structure of a reservoir. To quantify these effects, a hydrodynamic and water quality model was applied to the Latian dam reservoir in Iran. The outputs of the Canadian global climate model version 3 for B1, A1B and A2scenarios were used to obtain future (year 2100) air temperature trends. Results from these scenarios were compared with those of base conditions to investigate the effects of climate change. The maximum surface water temperature increase was about 2°C for the S2 scenario, which was the scenario with the highest air temperature increase (5.7°C). The increase in water temperature indicated some important effects on thermal stratification; for example, thermal gradient across the metalimnion was increased. It was also inferred that the Latian dam was severely sensitive to inflow variations as well as temperature changes.
This paper studied the spatial and temporal variability of the statistical structures of precipitation across Xinjiang, China, by analysing the time series of daily precipitation from 50 weather stations during the period from 1961 to 2008. Three indices precipitation concentration index (CI), precipitation concentration degree (PCD) and precipitation concentration period (PCP) were used to detect precipitation concentrations and the associated spatial patterns. The results show that higher precipitation CI values were mainly observed in Southern Xinjiang, whereas lower precipitation CI values were mostly detected in Northern Xinjiang. The precipitation CI values are noticeably larger in places where both annual total precipitation and number of rainy days are lower. The Mann–Kendall trend test demonstrates that the most parts of Xinjiang are characterized by no significant trends of precipitation CI at the 0.05 significance level. The periodicity characteristic of precipitation CI time series in Xinjiang could be detected by wavelet power spectrum analysis, and significant periods of that in most of Xinjiang were concentrated on 2–5 years band. The results of PCP reveal that rainfall in Xinjiang mostly occurs in summer, and the rainy season arrives earlier in Eastern Xinjiang than Western Xinjiang, whereas the results of PCD indicate that the rainfall in Northern Xinjiang was more dispersed within a year than that in Southern Xinjiang. Copyright © 2010 Royal Meteorological Society
The properties of water withdrawn from a stratified reservoir are investigated in a field study conducted in Lake Burragorang, Australia. It is shown that temperature and turbidity fluctuations of the extracted water are directly correlated to the vertical displace-ment of the thermal structure of the reservoir immediately in front of the offtake and the thickness of the selective withdrawal layer. Scaling of the unsteady withdrawal revealed that the time scale associated with the formation of selective withdrawal is an order of magnitude smaller than the typical period of the internal wave. This means the withdrawal layer is acting as a filter, extracting water of a particular quality as it is swept past the outlet by the internal seiches; the steady-state theory of the selective withdrawal can be used to predict outflow temperature fluctuations in reservoirs where long internal waves are present. To correctly interpret other outflow water parameters, such as turbidity or dissolved oxygen, it is important not only to know the stratification conditions in front of the offtake, but also to understand the local flow dynamics in the lower reaches of the reservoir.
In this study, the effect of thermal stratification on water quality in a reservoir has been investigated by field observations
and statistical analysis. During the summer period, when stratification is evident, field observations indicate that the observed
dissolved oxygen concentrations drop well below the standard limit of 5mgl−1 at the thermocline, leading to the development of anoxia. The reasons for variations in the dissolved oxygen concentrations
were investigated. Variations of air temperature and other meteorological factors and lateral flows from side arms of the
lake were found to be responsible for the increase of dissolved oxygen concentrations. It was also observed that turbidity
peaked mostly in the thermocline region, closely related to the location of the maximum density gradient and thus low turbulence
stabilizing the sediments in the vertical water column. Relatively cold sediment-laden water flowing into the lake after rain
events also resulted in increased turbidity at the bottom of the lake. Nondimensional analysis widely used in the literature
was used to identify the strength of the stratification, but this analysis alone was found insufficient to describe the evolution
of dissolved oxygen and turbidity in the water column. Thus correlation of these parameters was investigated by multivariate
analysis. Fall (partial mixing), summer (no mixing), and winter (well mixed) models describe the correlation structures between
the independent variables (meteorological parameters) and the dependent variables (water-quality parameters). Statistical
analysis results indicate that air temperature, one day lagged wind speed, and low humidity affected variation of water-quality
parameters.
Based on the time series data from the Aral hydrological station for the period of 1958–2005, the paper reveals the long-term trend and fractal of the annual runoff process in the mainstream of the Tarim River by using the wavelet analysis method and the fractal theory. The main conclusions are as follows: 1) From a large time scale point of view, i.e. the time scale of 16 (24) years, the annual runoff basically shows a slightly decreasing trend as a whole from 1958 to 2005. If the time scale is reduced to 8 (23) or 4 (22) years, the annual runoff still displays the basic trend as the large time scale, but it has fluctuated more obviously during
the period. 2) The correlation dimension for the annual runoff process is 3.4307, non-integral, which indicates that the process has both fractal and chaotic characteristics. The correlation dimension is above 3, which means that at least four independent variables are needed to describe the dynamics of the annual runoff process. 3) The Hurst exponent for the first period (1958–1973) is 0.5036, which equals 0.5 approximately and indicates that the annual runoff process is in chaos. The Hurst exponents for the second (1974–1989) and third (1990–2005) periods are both greater than 0.50, which indicate that the annual runoff process
showed a long-enduring characteristic in the two periods. The Hurst exponent for the period from 1990 to 2005 indicates that the annual runoff will show a slightly increasing trend in the 16 years after 2005.
Streamflow observations from near-natural catchments are of paramount importance for detection and attribution studies, evaluation of large-scale model simulations, and assessment of water management, adaptation and policy options. This study investigates streamflow trends in a newly-assembled, consolidated dataset of near-natural streamflow records from 441 small catchments in 15 countries across Europe. The period 1962-2004 provided the best spatial coverage, but analyses were also carried out for longer time periods (with fewer stations), starting in 1932, 1942 and 1952. Trends were calculated by the slopes of the Kendall-Theil robust line for standardized annual and monthly streamflow, as well as for summer low flow magnitude and timing. A regionally coherent picture of annual streamflow trends emerged, with negative trends in southern and eastern regions, and generally positive trends elsewhere. Trends in monthly streamflow for 1962-2004 elucidated potential causes for these changes, as well as for changes in hydrological regimes across Europe. Positive trends were found in the winter months in most catchments. A marked shift towards negative trends was observed in April, gradually spreading across Europe to reach a maximum extent in August. Low flows have decreased in most regions where the lowest mean monthly flow occurs in summer, but vary for catchments which have flow minima in winter and secondary low flows in summer. The study largely confirms findings from national and regional scale trend analyses, but clearly adds to these by confirming that these tendencies are part of coherent patterns of change, which cover a much larger region. The broad, continental-scale patterns of change are mostly congruent with the hydrological responses expected from future climatic changes, as projected by climate models. The patterns observed could hence provide a valuable benchmark for a number of different studies and model simulations.
Watershed models are powerful tools for simulating the effect of watershed processes and management on soil and water resources. However, no comprehensive guidance is available to facilitate model evaluation in terms of the accuracy of simulated data compared to measured flow and constituent values. Thus, the objectives of this research were to: (1) determine recommended model evaluation techniques (statistical and graphical), (2) review reported ranges of values and corresponding performance ratings for the recommended statistics, and (3) establish guidelines for model evaluation based on the review results and project-specific considerations; all of these objectives focus on simulation of streamflow and transport of sediment and nutrients. These objectives were achieved with a thorough review of relevant literature on model application and recommended model evaluation methods. Based on this analysis, we recommend that three quantitative statistics, Nash-Sutcliffe efficiency (NSE), percent bias (PBIAS), and ratio of the root mean square error to the standard deviation of measured data (RSR), in addition to the graphical techniques, be used in model evaluation. The following model evaluation performance ratings were established for each recommended statistic. In general, model simulation can be judged as satisfactory if NSE > 0.50 and RSR < 0.70, and if PBIAS + 25% for streamflow, PBIAS + 55% for sediment, and PBIAS + 70% for N and P. For PBIAS, constituent-specific performance ratings were determined based on uncertainty of measured data. Additional considerations related to model evaluation guidelines are also discussed. These considerations include: single-event simulation, quality and quantity of measured data, model calibration procedure, evaluation time step, and project scope and magnitude. A case study illustrating the application of the model evaluation guidelines is also provided.
Thermal stratification frequently occurs in deep reservoirs, and discharged water temperature (DWT) is detrimental to the downstream regions. Source tracking of discharged water reveals the flow regularity of stratified reservoir, and provides scientific basis for the prediction of DWT. Taking Sanbanxi Reservoir as a case, a 3-D hydro-thermal-tracer model based on Flow-3D is built, validated and used to investigate the source of discharged water, and a rapid quantitative method of DWT is further proposed. The result shows that: 1) DWT is closely related to the vertical temperature distribution, negatively related to the water level, and positively related to the flow rate. 2) Withdrawal zone is located within 30 m near the intake, which contributes approximately 82% to the discharged water. Among all water layers, the most effective layer which contributes the most to the discharged water is located 7.5 m above the top of intake. Contribution of the surface and hypolimnetic layers to the discharged water is small. 3) Based on source tracking, a forecasting formula of DWT with five parameters is proposed and verified, including elevation of most effective layer hmain=h422.5, upper characteristic elevation hup=h430, lower characteristic elevation hdown=h410, maximum Qfull=870m³/s and actual (q) flow rate of intake. This formula systematically considers the withdrawal zone, most effective layer and discharged tracer proportion, provides a rapid and accurate method predicting DWT, and can be a reference for other deep reservoirs. © 2018 International Association for Hydro-environment Engineering and Research, Asia Pacific Division
Thermal stability (Schmidt stability) and water age, which are significantly related to water quality and algae bloom in deep reservoirs, are two crucial indicators of stratification strength and pollutant transport time, respectively. Here, the original Schmidt stability, which was derived from a one-dimensional assumption, was theoretically extended to a three-dimensional water body. In addition, a three-dimensional model was verified for the case study of Hongfeng Reservoir in China based on data from 2009 and 2010. Although the revised stability was similar to the original stability of Hongfeng Reservoir, which occurred at a relatively low level, the greater stratification in other deep water bodies would enhance their difference. Air temperature and water depth were the most important factors of the temporal variation in stability and the spatial variation in stability, respectively. The pollutant transport processes in the Hongfeng Reservoir was very complex with alternate appearances of overflow, interflow and underflow, depending on the season. The spatial water age was primarily determined by the morphometry and the inflow/outflow (with the highest water age in North Lake), whereas the vertical difference in the water age among the layers was primarily controlled by thermal stratification. Negative linear relationships between the average stability and the water ages of the bottom layers in three representative sites during summer were observed. Positive linear relationships between the average stability and the water ages of the surface layers were also observed. These findings enable a better understanding of the hydrodynamic and pollutant transport processes in a deep reservoir.
The hierarchy and definition of the precipitation-concentration degree and precipitation-concentration period of annual precipitation have been proposed by using the so-called vector method of annual distribution of precipitation, so that the two relevant parameters can represent the annual distribution of total precipitation correctly and indeed accurately. The relationship between the spatial and temporal distribution patterns and variations of the two parameters and the annual precipitation amount in China has been further investigated. Results demonstrate that the precipitation-concentration degree and the precipitation-concentration period increase from southeast to northwest gradually. Moreover there obviously exists a belt pattern: The largest variability of the precipitation-concentration degree and the precipitation-concentration period occurs in the Yellow River Valley and the middle and lower reaches of the Yangtze River, corresponding to the significant zones in which flood and drought take place frequently. It is found that there exist high correlations between the precipitation-concentration degree and precipitation-concentration period and the annual precipitation amount in Northeast China, North China, the middle and lower reaches of the Yangtze River. Furthermore, 8-year and 22-year periodic oscillations in the precipitation-concentration degree and 6-year and 12-year cycles in the precipitation-concentration period are identified by use of their Morlet wavelet analysis.
In order to investigate the relationship between phytoplankton community structure and environmental factors in Sanbanxi Reservoir, a nitrogen-limited deep reservoir of Guizhou Plateau, phytoplankton and water were sampled in December (wet season) 2012, and in April (normal season) and August (dry season) 2013. There existed 87 genera and 6 phyla phytoplankton in the samples, which were mainly composed of the Chlorophyta, Bacillariophyta and Cyanobacteria. In wet, normal and dry seasons, the phytoplankton abundance ranged from 0. 064×104 to 1. 17× 104 cells • L-1, from 8. 21×104 to 422. 47×104 cells • L-1, and from 9. 08×104 to 2903. 33×104 cells • L-1 respectively. The phytoplankton abundance of dry and wet seasons peaked in Jiachi and was minimized in Nanjia, and that of normal season peaked in Daba and was minimized in Nanjia. The dominant species were Melosira granulata, Fragilaria capucina and Microcystis marginata in dry, normal and wet seasons, respectively. The phytoplankton mainly distributed at the surface of 0 to 10 m, and its abundance decreased with water depth. The concentrations of total phosphorus averaged at 0. 403 mg • L-1 in Sanbanxi Reservoir, ranging from 0. 281 to 1. 139 mg •L-1 in dry season, from 0. 394 to 0. 639 mg • L-1 in normal season, and from 0. 054 to 736 mg • L-1 in wet season. The mean concentration of total nitrogen was 1. 38 mg • L-1. The N/P ratio (3.7:1) was lower than the most appropriate ratio 7: 1 of phytoplankton, indicating that nitrogen is the limiting factor in Sanbanxi Reservoir. RDA analysis and Pearson analysis showed that water temperature was the key environmental factor, and N/P ratio was the important factor, regulating the phytoplankton community composition in the Sanbanxi Reservoir. Nutritive salt can affect phytoplankton community composition via improving the growth of Bacillariophyta and limiting the growth of Cyanobacteria. © 2014, editorial Board of Chinese Journal of Ecology. All rights reserved.
Saltwater intrusion is detrimental to water utilization. It is of vital significance to study the joint impact of runoff and tide on salinity and the risk of saltwater intrusion. To analyze the risk of saltwater intrusion, this paper proposes two concepts: critical runoff-tide level line and guarantee rate of freshwater. Taking Nandu River Estuary in China as a study case, a three-dimensional (3-D) hydrodynamic and salinity numerical model is built. Critical runoff-tide level lines are obtained to determine the occurrence of saltwater intrusion. To quantify the guarantee rates of freshwater, copula joint distribution is utilized, which connects the numerical model and daily hydrological characteristics. Guarantee rates of freshwater are obtained under different amounts of water intake (0,10 m3/s, 20 m3/s, 30 m3/s). In addition, critical locations of water intake that satisfy different guarantee rates (80%, 85%, 90%, 95%, 99%) are identified. All the results will provide technical support for risk evaluation of saltwater intrusion and decisions on water intake location.
The assessment of urban stormwater quantity and quality is important for evaluating and controlling the impact of the stormwater to natural water and environment. This study mainly addresses long-term evolution of stormwater quantity and quality in a French urban catchment using continuous measured data from 2004 to 2011. Storm event-based data series are obtained (716 rainfall events and 521 runoff events are available) from measured continuous time series. The Mann-Kendall test is applied to these event-based data series for trend detection. A lack of trend is found in rainfall and an increasing trend in runoff is detected. As a result, an increasing trend is present in the runoff coefficient, likely due to growing imperviousness of the catchment caused by urbanization. The event mean concentration of the total suspended solid (TSS) in stormwater does not present a trend, whereas the event load of TSS has an increasing tendency, which is attributed to the increasing event runoff volume. Uncertainty analysis suggests that the major uncertainty in trend detection results lies in uncertainty due to available data. A lack of events due to missing data leads to dramatically increased uncertainty in trend detection results. In contrast, measurement uncertainty in time series data plays a trivial role. The intra-event distribution of TSS is studied based on both M(V) curves and pollutant concentrations of absolute runoff volumes. The trend detection test reveals no significant change in intra-event distributions of TSS in the studied catchment.
Selective withdrawal systems can take advantage of thermal stratification in reservoirs to manage downstream temperatures. Selective withdrawal might also help adapt operations to environmental changes, such as increased stream temperatures expected with climate change. This exploratory study develops a linear programming model to release water from different thermal pools in reservoirs to minimize deviations from target downstream temperatures. The model is applied with representative thermal dynamics to Lake Spaulding, a multipurpose reservoir on the South Fork Yuba River in California with climate warming represented by uniform increases in air temperature. Optimization results for thermal pool management with selective withdrawal are compared to a single, low-level outlet release model. Optimal selective withdrawal hedges the winter release of cold water to decrease summer stream temperatures. With climate warming, selective withdrawal can help lessen stream warming in the summer but at a cost of warmer stream temperatures in winter. As numerous assumptions are made, particularly regarding representation of thermodynamics, modeling improvements are needed to further develop selective withdrawal optimization models; several improvements are discussed. (C) 2014 American Society of Civil Engineers.
Hydrological changes of the Ili River were analyzed concerning the changes of annual runoff and its distribution features within a year measured by coefficient of variation and concentration degree. Abrupt changes were detected by the heuristic segmentation method. Possible causes of the hydrological changes were investigated considering climate changes and human activities (especially the reservoir operation and irrigation extension). The Mann-Kendall method was applied to estimate whether the temperature and precipitation were changed. The increased temperature and the decreased precipitation in the flood season may decrease the runoff. At the middle reaches, the impact of the reservoir is significant and may be the main factor leading to the abrupt decreases in annual runoff and its intra-annual variability and concentration. The increased water surface area of the reservoir aggravates the evaporation and leads to annual runoff reduction. The reservoir regulates runoff by storing water in the flood season and releasing water in the dry season.
Long term (1921–2011) yearly and seasonal hydrological regime of 23 Alpine rivers in Northern Italy (ca. 10²–10⁴ km²) was investigated here. First, for regulated catchment, the potential effect of flow storage was investigated using an index of potential flow regulation, and pre and post reservoirs’ installation flow analysis. For catchments displaying little regulation effect, non stationarity was studied using linear regression, including variable (segmented) slope analysis, and Mann Kendall test, traditional and progressive. The link of the observed trends against descriptive physiographic variables was then investigated, to highlight geographic and topographic patterns of changes of the hydrological cycle. Dependence upon global thermal and North Atlantic Oscillation NAO anomalies were analysed to highlight potential impact of large scale climate drivers against regional hydrological regimes. Also, the correlation between stream flows and climatic drivers of precipitation and temperatures in nearby stations was investigated, to highlight climate trends potentially driving hydrological changes, and potential changes in the nexus between climate and hydrology given by reservoirs’ operation.
Large dams often alter flow and thermal regimes downstream, resulting in fish spawning delays and larval abundance declines. Accurate prediction of the delayed spawning period under changed thermal regime is critical for selecting a correct timing to modify dam operation and to release the environmental flow needed to enhance fish spawning. We used correlation analysis to investigate the relationship between thermal regime alteration and spawning delay of the four major Chinese carps (FMCC) below the Three Gorges Dam in the Yangtze River, China, using a 13-year ecological data series (1997–2009). Eighteen variables were defined to quantify thermal regime and related to two variables representing spawning timing. Our results demonstrate that the start of FMCC spawning has been delayed from early May to middle June since the Three Gorges Dam initiated operation in 2003. Water temperature declines of 2 °C–4 °C in March, April and May (a critical period for gonad development) were the principal reason for the observed spawning delay. The variable most associated with spawning timing was the arrival date of the cumulative temperature needed for gonad development from stages IV to V (DDegDayIV–V), which describes the long-term impacts of the timing, magnitude and duration of thermal regime upon gonad development. Only the cumulative temperature for gonad development and the minimum temperature for FMCC spawning (18 °C) are both satisfied, the occurrence of suitable flow conditions, that is, flow increase or flash flood, would produce a successful spawning event. Consequently, we suggest that the experimental flow increase process of the Three Gorges Reservoir aimed at enhancing FMCC spawning should initiate after 15 June when the requisite thermal regime can be met. Copyright © 2013 John Wiley & Sons, Ltd.
Highlights
► Streamflow seasonality differs substantially from precipitation seasonality over US. ► Aridity and within-year moisture and energy imbalance control streamflow seasonality. ► Regional differences in streamflow seasonality is also explained. ► Eigenvectors from principal component analysis is used to identify dominant processes.
This research investigates the effect of climate change on the thermal structure of lakes in response to watershed hydrology. We applied a hydrodynamic water quality model coupled to a hydrological model with a future climate scenario projected by a GCM A2 emission scenario to the Yongdam Reservoir, South Korea. In the climate change scenario, the temperature will increase by 2.1°C and 4.2°C and the precipitation will increase by 178.4 mm and 464.4 mm by the 2050 and 2090, respectively, based on 2010. The pattern changes of precipitation and temperature increase due to climate change modify the hydrology of the watershed. The hydrological model results indicate that they increase both surface runoff itself and temperature. The reservoir model simulation with the hydrological model results showed that increasing air temperature is related to higher surface water temperature. Surface water temperature is expected to increase by about 1.2°C and 2.2°C from the 2050 and 2090, respectively, based on the 2010 results. The simulation results of the effects of climate warming on the thermal structure of the Asian Monsoon Area Lake showed consistent results with those of previous studies in terms of greater temperature increases in the epilimnion than in the hypolimnion, increased thermal stratification, and decreasing thermocline depths during the summer and fall. From this study, it was concluded that the hydrodynamic water quality model coupled to the hydrological model could successfully simulate the variability of the epilimnetic temperature, changed depth and magnitude of the thermocline and the changed duration of summer stratification.
Much of the discussion on climate change and water in the western United States centers on decreased snowpack and earlier spring runoff. Although increasing variability in annual flows has been noted, the nature of those changes is largely unexplored. We tested for trends in the distribution of annual runoff using quantile regression at 43 gages in the Pacific Northwest. Seventy-two percent of the stations showed significant (alpha = 0.10) declines in the 25th percentile annual flow, with half of the stations exceeding a 29% decline and a maximum decline of 47% between 1948 and 2006. Fewer stations showed statistically significant declines in either median or mean annual flow, and only five had a significant change in the 75th percentile, demonstrating that increases in variance result primarily from a trend of increasing dryness in dry years. The asymmetric trends in streamflow distributions have implications for water management and ecology well beyond those of shifted timing alone, affect both rain and snow-dominated watersheds, and contribute to earlier timing trends in high-elevation watersheds.
mainly focused on the analysis of the inhomogeneity, with the disadvantage of unintuition and the poor comparability of the indicators from the view of methods that used nowadays, few of the studies considering the alteration of the inhomogeneity happened or not under the changing environment. Therefore, the Gini Coefficient with the advantage of well description of inhomogeneity is used to set up the analysis method of hydrological alteration of the inner-annual distribution. In this method, the series of homogeneity of inner-annual hydrological elements distribution will be calculated first of all, then the Hydrological Alteration Diagnosis System is used to analyze whether the alteration is happened in the series or not, finally, the mean difference method will be employed to analyze the attributions if the alteration is existing. At the end of this study, with the monthly runoff data from the year of 1954 to 2005 of Longchuan Station at Dongjiang River, the alterational characteristics of inner-annual runoff distribution is calculated and the attribution analysis is conducted in succession. The results show that: at the year of 1973, the increasing jump alteration happened in the series of inner-annual runoff distribution, which means the tendency of inner-annual runoff distribution becomes equalization. About 60% of the alteration is induced by the human activities, especially the construction and operation of Fengshuba Reservoir, and the factor of climate change only accounts for about 40%.
To better understand the factors influencing the thermal structure of tributaries in the Three Gorges Reservoir (TGR), a well validated three-dimensional hydrodynamic and water temperature model was proposed to simulate the water temperature distribution in the Xiangxi Bay, a representative tributary of TGR. The numerical results show that water temperature stratification seasonally occurred in the Xiangxi Bay, with stable vertical temperature profiles. It is found from the numerical experiments that three key factors are responsible for the formation of water temperature structure: (1) very often, the locations of thermocline are mainly determined by wind speeds, and the higher the wind speed is, the deeper the thermocline is located beneath the water surface, which could be expressed by a fitted exponential function, (2) the thermal structure is affected by static stability of water column, and the thermocline becomes closer to the water surface and its thickness increases with the increase of temperature, (3) due to the effect of the thermal density inflow, the water temperature of the hypolimnion tends to be uniform, however, even under the condition of larger inflow discharge, the influence of the inflow on the epilimnion and the thermocline is not significant.
Multipurpose reservoirs can be used to improve water utilization where this resource is limited. Many reservoirs in Israel were and are being built to simultaneously store water for field crop irrigation and for fish culture. One of the main problems of reservoirs as fish culture units is thermal stratification, which may leave a large fraction of the water volume unavailable and even dangerous for the fish. In previous works (e.g.: Milstein et al., 1992; Krambeck et al., 1994; Zoran et al., 1994) it was found that in Israel stratification is a daily process in fish ponds of up to 3–4 m depth, while in deeper reservoirs seasonal stratification develops in addition to the daily one. Seasonal stratification leads to the development of a warm oxygenated epilimnion appropriate for fish life, and a cool anoxic hypolimnion where toxic metabolites accumulate. In water bodies deep enough a metalimnion with intermediate temperature may be formed between those two masses. While in natural warm monomictic lakes seasonal stratification terminates when physical stability decreases due to autumn cooling, in fish culture and irrigation reservoirs this occurs due to water level decrease resulting from water withdrawal for irrigation. With destratification the oxygen of the epilimnion is consumed by the whole water column. If the total oxygen demand of the whole water column is higher than the total available dissolved oxygen of the epilimnion, heavy fish mortality might occur (Milstein et al., 1995). Since the irrigation period is in summer, destratification in dual purpose reservoirs is expected when fish biomass is large and fish are almost ready for marketing.
According to the concept of “natural flow regime,” introduced and developed in the 1990's in aquatic ecology, streamflow can be described using five basic characteristics: magnitude, frequency, duration, period of occurrence and variability. A sixth could be added to these, namely, distribution curve. Our study, which focused exclusively on the temporal variability of these six characteristics, had these objectives: (1) to compare their stationarity, (2) to determine the links among these characteristics and (3) to analyze, for the first time, their relationship to six climatic indices (Atlantic Multidecadal Oscillation, Artic Oscillation, North Atlantic Oscillation, Niño3.4, Pacific decadal Oscillation and Southern Oscillation Index). To do this, we used the Vermillon River (2670 km²) as an example, analyzing the streamflow of heavy spring floods (equal to or greater than the annual flood streamflow), as measured between 1934 and 2000.
Hydrological processes in river systems have been changing under the impacts of both climate variability and human activities. The non-parametric Mann-Kendall statistic was used to identify change trends and points in the annual streamflow in the Hun-Tai River basin in northeast China. The identifications were based on streamflow records from six hydrological stations during 1961-2006, and the purpose was to analyze the change characteristics of the hydrological processes. The results indicated that all hydrological stations presented downward trends in annual streamflows. Abrupt changes in the annual streamflow occurred around 1978 in the Hun River basin, and around 1998 in the Taizi River basin. The impact of climate variability on the mean annual streamflow was also analyzed based on the relationships among streamflow, precipitation, and potential evapotranspiration. Precipitation and meteorological data from 22 rainfall stations and 10 weather stations within the basin were employed in the analysis. Daily potential evapotranspiration was calculated using the Penman-Monteith equation. Climate variability was estimated to account for 43% of the reduction in the annual streamflow, and human activities accounted for about 57%.
In this study, a 1D model of reservoir hydrodynamics DYRESM has been applied to Sau Reservoir, a river valley reservoir in the North-Eastern Spain. Simulation is undertaken for 3 years (1995–1997). Meteorological input data measured at the dam are only available from May of 1997. In this case the simulation results fit measured temperatures very well. In the remaining periods, some meteorological data (radiation, wind and rainfall) were obtained from two nearby stations. Simulated temperature distribution in 1996 is close to the observed one. In 1995, however, the simulated result is far from the observed data. Inflows , outflow and local meteorological events such as storms and gusts of wind seem to be responsible for the differences. By changing some parameters, the effects of flow, light extinction coefficient and outlet elevation on thermal stratification are investigated. Simulations demonstrate that the inflow with high temperature is the main factor controlling the thermal structure in Sau Reservoir and demonstrate that the effect of residence time on thermal stratification is manifested mainly by the changes in the depth of thermocline.
Thermal regime is strongly associated with hydrodynamics in water, and it plays an important role in the dynamics of water quality and ecosystem succession of stratified reservoirs. Changes in both climate and hydrological conditions can modify thermal regimes. Liuxihe Reservoir (23°45'50″N; 113°46'52″E) is a large, stratified and deep reservoir in Guangdong Province, located at the Tropic of Cancer of southern China. The reservoir is a warm monomictic water body with a long period of summer stratification and a short period of mixing in winter. The vertical distribution of suspended particulate material and nutrients are influenced strongly by the thermal structure and the associated flow fields. The hypolimnion becomes anoxic in the stratified period, increasing the release of nutrients from the bottom sediments. Fifty-one years of climate and reservoir operational observations are used here to show the marked changes in local climate and reservoir operational schemes. The data show increasing air temperature and more violent oscillations in inflow volumes in the last decade, while the inter-annual water level fluctuations tend to be more moderate. To quantify the effects of changes in climate and hydrological conditions on thermal structure, we used a numerical simulation model to create scenarios incorporating different air temperatures, inflow volumes, and water levels. The simulations indicate that water column stability, the duration of the mixing period, and surface and outflow temperatures are influenced by both natural factors and by anthropogenic factors such as climate change and reservoir operation schemes. Under continuous warming and more stable storage in recent years, the simulations indicate greater water column stability and increased duration of stratification, while irregular large discharge events may reduce stability and lead to early mixing in autumn. Our results strongly suggest that more attention should be focused on water quality in years of extreme climate variation and hydrological conditions, and selective withdrawal of deep water may provide an efficient means to reduce internal loading in warm years.
CE-QUAL-W2: A Two-Dimensional, Laterally Averaged, Hydrodynamic and Water Quality Model, Version 3.72 User Manual
- T M Cole
- S A Wells
Cole, T.M., Wells, S.A., 2015. CE-QUAL-W2: A Two-Dimensional, Laterally Averaged, Hydrodynamic and Water Quality Model, Version 3.72 User Manual. Department of
Civil and Environmental Engineering, Portland State University, Portland, OR.
Climate change 2013: The Physical Science Basis (10 pp)
IPCC, 2013. Climate change 2013: The Physical Science Basis (10 pp).
Research on the Runoff's distribution characteristics of the Year in the Yalong River
- B Wang
- L Wang
- C Li
- Y Ma
- F Chen
Wang, B., Wang, L., Li, C., Ma, Y., Chen, F., 2015. Research on the Runoff's distribution characteristics of the Year in the Yalong River. China Rural Water Hydr. 11, 81-84 (in
Chinese).
Analysis of rainfall trend and extreme events in Guizhou
- Z Zhang
- X Chen
- W Wang
- P Shi
Zhang, Z., Chen, X., Wang, W., Shi, P., 2007. Analysis of rainfall trend and extreme events
in Guizhou. Earth Environ. 35 (4), 351-356 (in Chinese).
Analysis the runoff variation of Yangtze River in Yichang
- J Zhao
- J Li
- Z Dai
- Y Wang
- A Zhang
Zhao, J., Li, J., Dai, Z., Wang, Y., Zhang, A., 2012. Analysis the runoff variation of Yangtze
River in Yichang. Resources Science 12, 2306-2315 (in Chinese).