As one of the largest hydropower projects in the world, the Three Gorges Reservoir (TGR) is located at the upper and middle catchment of the Yangtse River. It is about 660 km long and 58,000 km2
in the watershed area [1
]. The water level of TGR was impounded to the elevations of 135 m, 156 m, and 175 m in 2003, 2006 and 2010. Then, the TGR was fully operational with water levels fluctuating between 145 m (from April to September, wet season) and 175 m (from October and mid-April, dry season) annually [2
]. It is key to engineering the exploitation and harnessing of water resources of the Yangtse River, which brings many obvious benefits, such as electrical energy, flood control, drought relief and economic development. It also has many negative effects on the environment and ecology [2
After the first impoundment of the TGR in 2003, the water flow velocity decreased and the water retention time increased. It has changed the spatiotemporal distributions of water resources and nutrient compositions in the TGR mainstream and its tributaries, even resulting in the occurrence of algal blooms in some tributaries of the TGR [4
]. Water circulation is a complex process based on precipitation, runoff and evaporation. However, due to its operation modes, the hydrological conditions of TGR are different from that of ordinary rivers [6
]. For example, in the wet season, the rainfall is abundant while the water level of the TGR is lower; in the dry season, the rainfall is insufficient while the water level is higher. Thus, it might weaken the effect of precipitation while strengthening the characterization of runoff on water cycle of the TGR, which would lead to some specific hydrological conditions in the reservoir [7
]. Consequently, it is of great importance to explore the processes of water cycles in the TGR to understand the runoff characteristics and the hydrodynamic conditions between the mainstream and its tributaries.
Stable hydrogen and oxygen (H-O) isotopes (denoted as δD and δ18
O) can serve as conservative natural tracers in hydrological environments and have been widely used in many aspects of tracing the water cycle [8
]. For example, intensive evaporation enriches the isotopic composition of surface water and this signal has been identified in the downstream rivers [11
] and applied to estimate evaporation over large lake systems [12
]. Xiao et al. [13
] reported that the stable isotopic mass balance model was of high precision and very practical in the case study of Taihu Lake. In addition, it is useful to identify water sources and mixing process by tracers of stable H-O isotopes, since different water sources often contain different isotopic compositions. Mortathi et al. [14
] detected that the average surface runoff and baseflow contributed to about 30.3% and 69.7% of the Amazon River, respectively.
Since the 1980s, the isotopic tracing technology was used to study the hydrological processes in the catchment of Yangtse River and previous studies have concluded that precipitation, evaporation and inflows of tributaries were the main causes for the spatial variations of H-O isotopes in the Yangtse River [15
]. In recent years, emerging isotopic studies have focused on the TGR. Deng et al. [17
] reported the phenomenon of small fluctuations of deuterium excess (d-excess) downstream the Three Gorges Dam (TGD) after 2008 and inferred that the retention effect of reservoir greatly altered the water cycle. Zhou et al. [11
] found that the TGR operation had a certain effect on the isotopic composition of the river water, especially in the dry season. The inflow of tributaries water also significantly influenced the variation of the H-O isotopic values of the TGR water. For instance, the river water in Yichang region was mainly influenced by the tributaries in the north bank, which was originally supplied by the meltwater with highly-depleted H-O isotopes at a high altitude of Shenlong Mountain [11
]. In addition, the H-O isotopic technology has been used to analyze the mixing processes and nutrient distributions between the mainstream and its tributaries, like Daning River [5
] and Xiangxi Bay [18
]. They found that there were significantly layered and bidirectional density currents in the confluence zone and concluded that nitrogen and phosphorus nutrients contributed from the mainstream accounted for more than 80% and 60%, which were regarded as one of the most likely causes of eutrophication in tributaries [5
]. Nevertheless, few studies have investigated the H-O isotopic variations and runoff characteristics in the TGR mainstream. As such, these studies may not provide full recognition on the TGR.
To complete the knowledge system, this study presented H-O isotopic data of the TGR mainstream and a typical tributary (Zhuyi Bay) in August and November of 2015, aiming to (1) Characterize the water isotope composition of TGR; (2) Clarify the spatial and temporal variations of water isotope composition in TGR mainstream; (3) Investigate the interaction process between the mainstream and tributaries under different climate conditions. We expected that the results would help develop water resource and exert better control over the eutrophication in tributaries.
The study presented water H-O isotopic compositions in the mainstream and a tributary (Zhuyi Bay) of the TGR in 2015 and showed that the H-O isotopes and d-excess can be used to indicate the water cycle of the reservoir and reveal the stratification phenomenon that occurred in the confluence zone between the mainstream and tributary. Isotopic analysis showed that linear regressions of the TGR were δD = 7.19δ18O + 0.42 and δD = 7.67δ18O + 0.12 in the wet and dry seasons, respectively and suggested that precipitation was the major source of water for the TGR ultimately.
The fluctuations of δD and δ18O values along the runoff direction of TGR mainstream displayed a spatial difference, showing that the water from the downstream was generally enriched with heavier isotopes, which primarily resulted from the continuous evaporation effect along the runoff direction and different water supplies from nearest tributaries. Meanwhile, the spatial distributions of d-excess decreased from upstream to downstream, which indicated that kinetic fractionation effect was an important process in the TGR.
The observation of water H-O isotopes in ZYB showed that the water mass from the TGR mainstream flowed backward to the confluence zone via the upper-middle layers in the wet season. However, in the dry season, the water reversed through the middle and bottom layers with smaller intrusion ranges. These findings matched well with the results of flow velocities. This stratification phenomenon might hamper the exchanges of water bodies in the tributaries and help to accumulate algae to form algal blooms within the surface water, which requires further research.