A New Modified Tennant Method with Spatial-Temporal Variability

Abstract

Tennant method gets more widely applied worldwide via diverse modified forms, yet the spatial-temporal variability of ecological flow (EF) criteria has frequently been questioned, because the EF criteria are based on constant percentages of average periodic flow. To solve this problem, a modified Tennant method with EF criteria changing with flow regime is proposed in this research. The improved approach consists of four steps: (1) monthly and year-to-year flow variability considered, natural daily flow series are grouped into 12 monthly categories, and every category is subdivided into wet, normal and dry year groups; (2) taken the influence of extreme inter-annual flow events and uneven intra-annual distribution on average periodic flow into account, the median daily flow for every month and water year is taken as the ceiling of optimum EF; (3) spatial flow variability considered, the daily average streamflow with 90 % exceeding probability in monthly flow duration curves is taken as the lower limit of minimum EF; (4) the EF criteria from minimum to optimum are designed by using the idea of arithmetic progression from Tennant method, and the classification number is appropriately readjusted. Finally, to verify the rationality of the improved method, the EFs of a large river (Hanjiang River), a middle river (Meijiang River) and a small river (Panlonghe River) with different flow regimes are assessed and compared with common hydrological methods. The results show that the proposed approach is rational and has excellent monthly and inter-annual rich-poor variability and transferability in different riverine ecosystems.