Abstract: To solve four key problems existed in the current ecological flow criteria (EFC) spatial transferability, monthly variability, inter-annual variability and scalability, a modified flow-duration curve method is proposed. Based on specific guaranteed rates of average monthly flow series, this approach divides all the years into wet, normal and dry year groups for every month; and then monthly flow-duration curves for every water year group are established based on total-period method and daily flow series, the discharges at the 50-percent-duration and 90-percent-duration points are respectively taken as the optimum and minimum EFC. Finally, multi-level EFC from minimum to optimum are established with progression idea of modified Tennant arithmetic. The new approach is tested in the Hanjiang River and compared with Tennant method, modified Tennant method, 7Q10 method, minimum monthly runoff at 90% guaranteed rate method, minimum monthly runoff for recent ten years method, month-by-month minimum ecological runoff method, minimum monthly average discharge method, Q₉₀ method, modified Q₉₀ method, Q_97,10 method and ecological monitoring data. The results show that the new approach is reasonable and superior to traditional hydrological methods for excellent spatial transferability, temporal variability, gradability of perch condition, meanwhile, by the new approach, we can avoids the influence of extreme flow events and uneven intra-annual distribution.

Key words: ecological flow, flow-duration curve, spatial transferability, temporal variability, gradability, extreme flow events