Research papersQuantifying the impacts of the Three Gorges Reservoir on water temperature in the middle reach of the Yangtze River
Introduction
Water temperature is a crucial physical property of rivers, having a direct impact on almost all ecological and biogeochemical processes, including chemical reaction rate, oxygen solubility, primary production and fish habitat (Caissie, 2006, Webb et al., 2008). The complexity of river thermal response is strongly related to large-scale climate changes (e.g., air temperature, precipitation and solar radiation) (Caissie, 2006, Rice and Jastram, 2015, Chen et al., 2016) and human activities (e.g., agricultural irrigation systems, power generation, dam construction etc.) (van Vliet et al., 2011, Ding et al., 2015).
Large dams can influence temperature regimes of rivers by impounding water for prolonged periods. Reservoirs in temperate regions often stratify in response to changing atmospheric conditions through heat transfer at the surface and internal thermodynamics (Elçi, 2008). Stratification results in a warm surface (epilimnetic) layer and a cool bottom (hypolimnetic) layer. In view of the development of epilimnetic and hypolimnetic layers in reservoirs, which may have different thermal characteristics than the flowing river upstream of a reservoir, the impact of reservoirs on downstream water temperature can be important if the release of water from the reservoir substantially modifies natural thermal conditions suitable for native aquatic biota (Kedra and Wiejaczka, 2018). Previous work has mainly focused on qualitative evaluation of the role of reservoirs in affecting downstream thermal regimes in river systems (e.g., Erickson and Stefan, 2000, Steel and Lange, 2007, Olden and Naiman, 2010, Casado et al., 2013). These studies have revealed that changes occur to all aspects of water temperature, including reduction in thermal variability, changed frequency and duration of temperature extremes, and weakening of air–water temperature interaction. Quantifying the impact of reservoirs on water temperature is vitally important for developing adequate strategies to minimize adverse effects of thermal changes on aquatic habitat.
Temperature in a stream is the product of heat energy exchange between the stream and its environment, including the atmosphere and the riverbed (Risley et al., 2010). Air temperature is commonly used as a predictor variable for water temperature because it can be viewed as a surrogate for net changes in heat flux that affect the water surface, and also because it approximates the equilibrium temperature of a water course (Webb et al., 2003, 2008). Alternative modeling approaches used to investigate air–water temperature relationships include linear and nonlinear regression models, wavelet models, a time-varying coefficient, and time series models (Mohseni et al., 1998, Cho and Lee, 2012, Li et al., 2014, Gu et al., 2015, Rice and Jastram, 2015, Jackson et al., 2018). Linear regression models linking water temperature and air temperature have been developed successfully at multiple time scales (Erickson and Stefan, 2000, Webb et al., 2003, Jackson et al., 2018). For example, Jackson et al. (2018) formulated a large-scale spatio-temporal model in which a linear function is used to relate maximum daily water temperature to air temperature, showing that the relationship between these two variables is linear. Besides air temperature, changes in runoff volumes are also known to affect water temperature (Langan et al., 2001, Webb et al., 2003, van Vliet et al., 2012). However, the relationship between discharge and water temperature remains poorly understood. A critical need exists to determine how air temperature and discharge influence the water temperature of rivers.
Three Gorges Reservoir (TGR), the largest water control project in the world, provides numerous benefits that contribute to economic prosperity and social well-being. On the other hand, the TGR has substantially altered the hydrological and thermal regimes of the Yangtze River by changing the amount and timing of flow and by producing thermal stratification within the reservoir that results in the release of hypolimnetic water (Wang et al., 2012, Chen et al., 2016, Long et al., 2016, Cai et al., 2018). It also markedly affects the behavior and distributions of aquatic species (Long et al., 2016, Wang et al., 2017). For these reasons, water temperature variations downstream associated with the TGR have been of considerable interest. However, previous analysis attributed differences of water temperature downstream and upstream of the reservoir to operation of the TGR, without considering effects of other factors unrelated to the TGR (e.g., changing climate condition) on changes in downstream water temperature (Zou et al., 2011, Long et al., 2016). To fully assess the effects of TGR on water temperature, reconstruction of water temperatures from meteorological and hydrological data in the absence of the reservoir is necessary. As the basis for reconstruction, natural river flow without the reservoir needs to be simulated using a general regression neural network (GRNN) which outperforms other neural network methods (Kim et al., 2013, Tayfur et al., 2014).
The main objective of this study is to determine the extent to which construction of the TGR has changed water temperatures within the Yangtze River downstream of the reservoir. To achieve this objective, the study: (1) develops a predictive regression model of water temperature in the river based on air temperature and discharge; (2) uses the model to reconstruct water temperatures for the post-impoundment period in the absence of impoundment; and (3) assesses the influence of the TGR on water temperatures by comparing observed and predicted values of water temperature in the post-impoundment period. Quantitative assessment of the impact of the reservoir on river water temperature informs future science-based management of the reservoir aimed at minimizing adverse ecological effects.
Section snippets
Study area and data
The Yangtze River is the longest river in Asia and the third longest river in the world. It passes through its source in Qinghai Province eastward to the East China Sea at Shanghai. The river is about 6300 km long and its catchment covers 1,800,000 km2 (Xu et al., 2006). The basin includes zones of subtropical and temperate climate (Li et al., 2011). The TGR is located along the main stream of Yangtze River between Chongqing and Yichang (Fig. 1). The TGR started to impound water in 2003 and
Research design and methods
The research design to address the main objective of the study involves several sequential components (Fig. 2). First, observed daily data on water temperature (WT), air temperature (Ta), and discharge (Q) were used to develop a set of multivariate regression models predicting water temperature on the basis of air temperature and discharge. Although past work has indicated that multivariate relations between water temperature and independent variables can be nonlinear (Mohseni et al., 1998),
Inter-annual variation
Since 1956 mean annual air temperature at Yichang has varied between about 16 °C and 18 °C. A slight downward trend is evident between 1956 and 1984, whereas an upward trend occurred between 1985 and 2006 (Fig. 3). Yichang experienced an extreme hot spell and drought during the summer of 2006 (Wu et al., 2012), followed by a slight downward trend in annual air temperature between 2007 and 2015.
The annual water temperature variation at Yichang generally exhibits a similar pattern to air
Discussion
The framework developed and implemented in this study provides an improved method for determining the effects of natural factors versus reservoir impoundment on water temperature. Specifically, it isolates the separate contributions of natural factors versus impoundment using a water temperature regression model that captures the changes induced by varying external conditions. This research has refined the analysis of the effects of dams on water temperature variations by accounting for factors
Conclusions
This study has reconstructed water temperature in the absence of the Three Gorges Reservoir to examine the separate impacts of natural change in air temperature and discharge versus dam construction on the water temperature of the middle reach of the Yangtze River following impoundment. The results reveal that the TGR has had a greater impact on water temperature than natural changes in air temperature and discharge. However, factors not explicitly accounted for by the model (β0) have produced
CRediT authorship contribution statement
Yuwei Tao: Methodology, Formal analysis, Writing - original draft, Writing - review & editing. Yuankun Wang: Conceptualization, Writing - review & editing, Supervision. Bruce Rhoads: Validation, Formal analysis, Writing - review & editing. Dong Wang: Writing - review & editing, Project administration. Lingling Ni: Data curation, Investigation. Jichun Wu: Project administration.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This study was supported by the National Key Research and Development Program of China (2017YFC1502704, 2016YFC0401501), and the National Natural Science Fund of China (51679118, 41571017, and 91647203), and Jiangsu Province“333 Project” (BRA2018060).
References (47)
- et al.
Changes in monthly flows in the Yangtze River, China-With special reference to the Three Gorges Dam
J. Hydrol.
(2016) - et al.
A spatio-temporal statistical model of maximum daily river temperatures to inform the management of Scotland’s Atlantic salmon rivers under climate change
Sci. Total Environ.
(2018) - et al.
Climatic and dam-induced impacts on river water temperature: Assessment and management implications
Sci. Total Environ.
(2018) - et al.
Variation in river water temperatures in an upland stream over a 30-year period
Sci. Total Environ.
(2001) - et al.
Impacts of the Gezhouba and Three Gorges reservoirs on the sediment regime in the Yangtze River, China
J. Hydrol.
(2011) - et al.
Coupling soil moisture and precipitation observations for predicting hourly runoff at small catchment scale
J. Hydrol.
(2014) - et al.
Climate effects of the Three Gorges Reservoir as simulated by a high resolution double nested regional climate model
Quat. Int.
(2012) Reflections on the Three Gorges Project since its operation
Eng.
(2016)Predicting temperatures of small streams
Water Resour. Res.
(1969)- et al.
Quantifying the impact of the Three Gorges Dam on the thermal dynamics of the Yangtze River
Environ. Res. Lett.
(2018)
The thermal regime of rivers: a review
Freshw. Biol.
Impact of variable reservoir releases on management of downstream water temperatures
Water Resour. Res.
Influence of dam-induced hydrological regulation on summer water temperature: sauce grande river, argentina
Ecohydrology
Development of an air–water temperature relationship model to predict climate-induced future water temperature in estuaries
J. Environ. Eng.
Impacts of land use on surface water quality in a subtropical river basin: a case study of the Dongjiang River Basin
Southeastern China. Water.
Effects of thermal stratification and mixing on reservoir water quality
Limnology
Linear air/water temperature correlations for streams during open water periods
J. Hydrol. Eng.
Air-stream temperature correlation in forested and urban headwater streams in the southern appalachians
Hydrol. Processes.
Development of weather dependent flow requirements for river temperature control
Environ. Manag.
Yangtze dams increasingly threaten the survival of the Chinese Sturgeon
Curr. Biol.
Disturbance of water-air temperature synchronisation by dam reservoirs
Water Environ. J.
Estimating daily pan evaporation using different data-driven methods and lag-time patterns
Water Resour. Manage
Effects of elevated water temperature on fish and macroinvertebrate communities below small dams
River Res. Appl.
Cited by (59)
Anthropogenic intensification of the eco-hydrothermal regime transition in regulated rivers: The cumulative effect of cascade reservoirs
2024, Journal of Environmental ManagementFuture projections of thermal regimes and mixing characteristics in a monomictic reservoir under climate change
2024, Science of the Total EnvironmentComprehensive evaluation of the hydrological health evolution and its driving forces in the river-lake system
2023, Ecological InformaticsRegional thermal index model for river temperature frequency analysis in ungauged basins
2023, Environmental Modelling and Software