Development of a Hydrological Boundary Method for the River-Lake Transition Zone Based on Flow Velocity Gradients, and Case Study of Baiyangdian Lake Transition Zones, China

The river-lake transition zone is affected by many environmental factors, leading to significant dynamics and complexity. This makes the boundary unclear, and not enough attention has been paid to this problem by scholars, even though it has great significance for research on water quantity, water quality, and the aquatic environment. In this paper, we define this transition zone, define its upper and lower boundaries, and develop the method for defining the hydrologic boundary. It includes a method for defining the upper boundary, based on the flow velocity mutation point, and a method for defining the lower boundary, based on the velocity gradient field. We then used this approach to examine the transition zones between the Fu River, Baigou Canal, and Baiyangdian Lake in China as a case study. We found that the upper boundary of the Fu River-Baiyangdian Lake transition zone was 2.35 km upstream of the lake's inlet; the lower boundary was farthest from the lake's inlet in July, and the maximum area of the transition zone was 2.603 km(2). The lower boundary was closest to the lake's inlet in March, when the minimum area was 1.598 km(2). The upper boundary of the Baigou Canal-Baiyangdian Lake transition zone was 2.18 km upstream from the lake inlet, and the lower boundary was farthest from the lake's inlet in August, when the maximum area was 2.762 km(2). The lower boundary was closest to the lake's inlet in April, when the minimum area was 0.901 km(2).