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The current difficulties in determining the ecological flow of rivers in northern China are the poor pertinence of methods and insufficient consideration of ecological factors. On the basis of systematic analysis of river hydrological situations and ecological protection objectives, the existing water ecological problems in river reaches were identified, a nested calculation framework for ecological flow of rivers was given based on Nature-based Solutions (NbS), considering the ecological integrity, biological diversity, morphological diversity and ecological service functions. With help of physical habitat simulation and the Penman-Monteith formula, indicator species were selected to research the natural process of restoring river function and realizing river runoff. To meet the water requirements of riparian vegetation during the growth period of the indicated species, the water demand law of typical vegetation changing with the growth time was studied, the upper and lower water levels were set, and the ecological water requirements of vegetation at each growth stage were determined. The results showed that the ecological basic flows required for the spawning period (April-June), the juvenile period (July-November) and the growth period (December-March) of Pseudorasbora parva in the Long River (typical section) were 3.14 m3/s, 4.32 m3/s and 2.16 m3/s, respectively. At the end of the spawning period, a 7-day (90% guarantee rate) pulse flow with a peak flow of 11.31 m3/s, a small flood process with a peak flow of 24.95 m3/s and a small flood process in the flood season lasting for 3 days provided effective external stimulation for the spawning of Pseudorasbora parva. The minimum annual ecological water requirement of reed was 15,100 m3. The current ecological flow of the Long River can meet the water requirements of most aquatic organisms and plants in the river ecosystem. The ecological flow calculation method based on NbS is widely applicable, and the results are representative. The method can be used to calculate the ecological flow of small and medium-sized northern rivers with a small flow and low flow rate.
