Research on the Ecological Flow and Water Replenishment Thresholds for Diversion Rivers Based on the MC-LOR Algorithm

Diversion rivers are important water and biological connection channels that provide valuable ecological services. Quantifying and guaranteeing the ecological flow of diversion rivers is necessary for sustaining riverine ecosystems. However, there is a lack of methods available to verify the reliability of the existing hydrological series length when calculating ecological flow using a hydrological approach. This study proposes a Monte Carlo-length of record (MC-LOR) algorithm to analyze the accuracy of the existing hydrological series. Considering the Dongjing River (DJR) in the lower Han River as an example, the result shows that the 15-year hydrological series can achieve an accuracy level of 10/90 and 15/99. Changes in flows in the Qianjiang hydrological station (QJS) and the Tianguan River (TGR) were analyzed. The flow variation in the QJS is large and shows a downward trend, while that in the TGR displays a significant upward trend. This paper establishes an ecological flow threshold calculation method based on kernel density curves and quantiles. Compared with six traditional hydrological methods, the method proposed here can reduce the influence of extreme flow events and is suitable for the diversion river. Finally, the rates of ecological water replenishment needed to meet the ecological flow target of DJR under different flow conditions in the Han River were analyzed. The results show when the flow rates of Han River are