Abstract: The construction and operation of reservoirs, such as the Xiangjiaba dam, provide invaluable water resources and clean energy for human development, but alter the thermal energy balance of the natural river channels. This study uses the water balance equation and energy balance equation to develop a box model, supplemented by a one-dimensional hydrodynamic model for the reservoir area, to reduce the errors in calculating the water profiles of the reservoir channels under dam-free conditions. Scenario calculations reveal the proportion and difference in each component of the heat budget with and without the presence of the dam. The findings indicate that compared with the natural river case, reservoir operation has enhanced the temporal stability of water thermal storage process; the total thermal energy of the water body is 7 to 35 times that of the natural state. The operation imparts a certain "thermal inertia" effect to the water body in the area, with the longest time lag reaching 42 days. Solar radiation received by the water body presents a maximum increase of 388% and a minimum increase of 283% compared to the natural state; for heat loss in evaporative energy, its maximum increase reaches 391% and minimum increase 284%. Of both thermal energy increments, the peak and trough occur in February and August, respectively. As a result, the overall effect has been an increase in net heat income. This study lays an important basis for thermal budget estimation during planning, construction, and regulation of cascade reservoirs, and helps protect the water ecosystems of large reservoirs.

Key words: Yangtze River, Xiangjiaba, heat budget, quantitative solution, box model