Abstract: Water diversion projects are currently an important means to improve lake water quality. However, the water diversion route, incoming water volume, and incoming water quality are all crucial factors to achieve the improvement. This study develops a two-dimensional hydrodynamic water quality model to simulate the hydrodynamic conditions of a lake area, and applies it in a case study of the Yangtze-to-Chaohu Water Transfer Project. Based on the shallow lake characteristics, we consider that water flow diverted into Chaohu Lake not only improves directly the lake water quantity, but may cause changes in the hydrodynamic conditions in the lake area and endogenous releases. Thus, this model simulates the impacts on water quality improvement caused by different water diversion routes and different water volumes that change the lake’s hydrodynamic conditions, by different incoming water quality, and by strong release of nitrogen and phosphorus from endogenous sources. The results show that water diversion imposes weak disturbances to the lake flow in wet years and strong ones in dry years. A larger diversion flow rate affects more the lake water quality. The incoming water quality is the key to determining whether or not the lake water quality can be improved. Overall, the water transfer affects less the water quality in the western lake area. In contrast, water quality in the eastern lake area is an indicator of the changes in water quality across the entire lake. A water diversion plan should be formulated based on the lake's real water volume and water quality status, and take into account the flow rate and water quality of the water diversion project.

Key words: Yangtze River Water Transfer Project, Chaohu Lake, MIKE21, water diversion routes, water environment management