A transition zone boundary determining method based on MIKE21: A case study of Baiyangdian Lake, North China

River and lake ecosystems converge to form transition zones, which play a key role in the flow of organisms, matter and energy. Although the importance of transition zones has been gradually recognized, there is still a lack of unified understanding on the transition zone boundary division. In this study, we took Baiyangdian Lake, North China as an example, proposed a method for determining the transition zone boundary based on water quality gradient using MIKE21 software, in which the confluence of the river and the lake was taken as the upper boundary, and the lower boundary was determined according to the variation inflection point of the water quality gradient. Total nitrogen (TN) was selected as the critical parameter due to its key role and high percentage among inflow river pollution. The results showed that the transition zone ranges vary with inflow rates and TN concentrations, as well landforms, and the corresponding areas of three transition zones, namely Fu River-Baiyangdian Lake, Xiaoyi River-Baiyangdian Lake and Baigou Canal-Baiyangdian Lake were 37.92, 34.11 and 24.99 km2 under critical flow conditions respectively. And the transition zone boundary was stable when connected flow was higher than critical value. This study explored a theoretical way of determining the boundary of the river–lake transition zones, thus providing a scientific basis for further research on water quality-ecological response and the optimal allocation of water resources in transition zone.