Abstract: In this study, a combined field experiment and numerical modeling approach is used to examine the water temperature of the lower Lancang influenced by its main tributary the Buyuan River. Water temperature has been observed at three mainstream hydrological stations: Mohan on Buyuan, and Yunjinghong and Guanlei (located downstream to Buyuan confluence) on Lancang. Analysis on these data shows a trend of the water temperatures at Guanlei and Mohan both highly correlated. We make numerical simulations of water temperature at Guanlei for the period of Nov. 2014 to Nov. 2015, and compare the calculations with the corresponding scenarios without Buyuan, revealing the influence of Buyuan’s discharge. Results suggest that during Sept. and Oct. 2014, Guanlei’s water temperature could be effectively lowered by at most 1?C due to Buyuan’s discharge, while in the rest of the study period it was dominated by the temperature of the incoming Lancang mainstream. Actually this was a special two months during which the discharge of Buyuan with temperature lower than Lancang was relatively large, yet still smaller than the incoming Lancang mainstream, and the hydropower plants on the upper Lancang mainstream were operated in impounding mode.

Key words: water temperature, Lancang River, Buyuan River, Delft3D FLOW