Research on Ecological Flow of Chishui River Based on Probabilistic DFM Method

River ecological flow is a key hydrological element for maintaining the structural stability of river ecosystems and ensuring the functionality of aquatic organisms' habitats, and it holds great significance for achieving the coordinated development of basin water resources management and ecological protection. Integrating the laws of annual and monthly runoff wet-dry encounter and the characteristics of flow probability density distribution, this study proposes a probabilistic distributed flow method (DFM) for calculating riverine ecological flow based on the traditional DFM. This method takes the conditional probability of annual and monthly runoff wet-dry encounter as the inflow weight and obtains the monthly ecological flow through weighted integration. Taking the Chishui River Basin as the study area, an application comparison was conducted between this proposed method, the original DFM, the Tennant method, and the monthly frequency curve method. Results showed that: the monthly frequency curve method and the original DFM exhibited similar monthly variation trends in their results, with a smaller fluctuation range than the Tennant method; the original DFM covers more comprehensive hydrological information as it considers both intra-monthly flow variations and inter-annual trends. The ecological flow calculated by the probabilistic DFM showed more significant fluctuations compared to the original DFM, which is attributed to the incorporation of factors such as the inflow encounter probability and the minimum monthly average flow. According to the evaluation criteria of the Tennant method, the ecological flow grades of the proposed method at various stations have been elevated to ranges such as "moderate to excellent" and "good to excellent", improving by 1 to 2 grades compared to the original DFM. This method better matches the adaptive needs of river ecosystems for hydrological regimes and can provide effective support for the healthy development of river ecosystems.