Monthly Ecological Water Level Schemes for Tangxun LakeBased on Improved Intra-annual Distribution Method

[Objective] Tangxun Lake is an important lake ecosystem in Wuhan City, Hubei Province. The rational determination of its ecological water level is crucial for maintaining regional ecosystem balance, landscape functions, and flood control security. [Methods] The daily water level data from Tangxun Lake during 2016-2024 were utilized. Five methods—annual assurance rate method, low water level method, natural water level data method, 7q10 method, and minimum ecological space requirement method—were employed to calculate the allocation coefficient required for the intra-annual distribution method. This coefficient was then used to calculate the monthly ecological water levels. Using the range of variability approach, water level values corresponding to the 25% and 75% frequencies were selected as thresholds. The variation patterns of water levels in Tangxun Lake were analyzed, and the high- and low-water-level thresholds, along with their durations, were identified. [Results] The allocation coefficient derived from the minimum ecological space requirement method was identified as the most suitable for determining the ecological water level of Tangxun Lake using the intra-annual distribution method. The calculated average annual ecological water level of outer Tangxun Lake ranged from 17.77 to 17.87 m, while that of inner Tangxun Lake ranged from 17.58 to 18.02 m. The duration of high- and low-water-level periods exhibited significant inter-annual variation in both lakes. In outer Tangxun Lake, the high-water-level period lasted only 16 days in 2022 but extended to 121 days in 2019, with durations in other years ranging between 36 and 119 days. The low-water-level period lasted 196 days in 2021 (the maximum duration recorded), while in other years it ranged between 0 and 136 days. In inner Tangxun Lake, the high-water-level period lasted 141 days in 2018, with durations in other years ranging between 0 and 124 days. The low-water-level period lasted only 2 days in 2021 but reached 197 days in 2017, with durations in other years ranging from 0 to 104 days. The differences in high- and low-water-level durations between the inner and outer lakes were attributed to the greater hydrological connectivity of the outer lake, whereas the inner lake was highly enclosed, relied more heavily on artificial regulation, and exhibited lower drainage efficiency. The inter-annual fluctuations in high and low water levels were primarily caused by extreme meteorological conditions and human regulatory interventions. [Conclusions] This study provides a scientific basis for the ecological restoration and landscape planning of Tangxun Lake. Based on the results, recommendations for water level regulation and management are proposed. The inner lake has high enclosure and weak hydrological connectivity, leading to limited drainage capacity during flood seasons, as evidenced by high-water-level durations ranging from 20 to 141 days over the study period. Therefore, measures to enhance the hydrological connectivity of inner Tangxun Lake are necessary, such as implementing connectivity projects to enhance water exchange with the outer lake and mitigate drainage challenges during flood seasons. In outer Tangxun Lake, the low-water-level duration exceeded 100 days in 2017, 2020, and 2021, posing risks to ecological flows. It is recommended to develop more scientifically grounded water level regulation schemes to shorten the duration of low-water-level periods in both lakes and effectively protect the ecological health of the Tangxun Lake area. As a multi-functional lake system, Tangxun Lake faces conflicts between flood control objectives and ecological/landscape objectives. Therefore, there is an urgent need to formulate water level regulation schemes that integrate both flood control objectives and ecological/landscape objectives.