Abstract
Reservoirs change downstream thermal regimes by releasing water of different temperatures to that under natural conditions, which may then alter downstream biodiversity and ecological processes. The hydropower exploitation in the mainstream Lancang-Mekong River has triggered concern for its potential effects on downstream countries, especially the impact of the released cold water on local fishery production. However, it was observed recently that the annual water temperature downstream of the Jinghong Reservoir (near the Chinese border) has increased by 3.0 °C compared to its historical average (1997–2004). In this study, a three-dimensional (3D) model of the Jinghong Reservoir was established to simulate its hydro- and thermodynamics. Results show that: (1) the impoundment of the Jinghong Reservoir contributed about 1.3 °C to the increment of the water temperature; (2) the solar radiation played a much more important role in comparison with atmosphere-water heat exchange in changing water temperatures; and (3) the outflow rate also imposed a significant influence on the water temperature by regulating the residence time. After impoundment, the residence time increased from 3 days to 11 days, which means that the duration that the water body can absorb solar radiation has been prolonged. The results explain the heating mechanism of the Jinghong Reservoir brought to downstream water temperatures.
Highlights
The riverine environment can be influenced by reservoirs and dams, as well as their operations in many forms, including the changing of riverine thermal regimes and downstream water temperatures [1,2,3]
Covering a distance of 105 km, the Jinghong Reservoir has a water depth roughly increasing along the direction of the river flow
The thermal structure along the reservoir from 10 km down the Nuozhadu Dam to the Jinghong Dam is shown in Figure 9 (3 days corresponding to different months were selected for demonstration)
Summary
The riverine environment can be influenced by reservoirs and dams, as well as their operations in many forms, including the changing of riverine thermal regimes and downstream water temperatures [1,2,3]. The cold water is released to the downstream thermal regimes [2,10,11,12]. This case was rarely reported, but some small dams release water from above the thermocline, namely the epilimnetic layer, so temperatures of the downstream water increase [7]. The simulated results were in good agreement with measurements, with the absolute error limited to 1.5 ◦ C
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