Heating Impact of a Tropical Reservoir on Downstream Water Temperature: A Case Study of the Jinghong Dam on the Lancang River

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.