| The study of ecological water demand in estuary is not only the major project and developing trend of water resource of estuary, but also the active demand of ecological and environmental health. The paper systemically studies ecological water demand in macro-tidal estuary according to ecosystem's service function.Ecological water demand in estuary is the water which is used to maintain the basic structure and function and satisfy the ecosystem's service function of estuary. It contains two parts: one part is the basic ecological water demand which is used to maintain the basic structure and function of estuary; the other part is the water demand which is used to satisfy the ecosystem's service function such as produce, purify the environment, accommodate hydrological circle, eliminate wave and decrease disaster, accommodate atmosphere, maintain biological diversity, epeirogenesis and society and culture et al. The runoff's annual and inter-annual changement, the characteristic of the water quality, the characteristic of the sediment transport and salinity mixture et al bring ecological water demand of estuary has the characteristics of temporal, special, threshold, coherence of water quality and quantity and variational.Based on the ecosystem's service function of estuary, ecological water demand of estuary can be divided into several parts: water demand for balance of sediment transport, water demand for balance of salinity, water demand for dilution of pollutant loads to satisfy the water quality requirement, water demand for hydrological cycles consume and other water demand. According to the characteristics of various ecological water demands, the calculation methods and models and its merits and drawbacks and applicability are studied, moreover, Oujiang Estuary is regarded as an example to study the ecological water demand. The results indicate that the range of the basic ecological water demand is 5.02×109～1.17×1010 m3/a; water demand for balance of sediment transport is about 2.74×109～9.34×109m3/a; water demand for balance of salinity needs to ensure mean monthly discharge greater than 26.8m3/s considering the mixed character of salt and fresh water; water demand for dilution of pollutant loads to satisfy the water quality requirement needs to ensure mean monthly discharge greater than 20.7 m3 /s considering the character of pollutant dispersion and the division of water function zone; water demand for hydrological cycles consume is 4.11×109m3/a according to the character of the evapotranspiration of the drainage basin. From what mentioned above, it is concluded that the ecological water demand in Oujiang Estuary is about 4.11×109～9.34×109m3 every year, the water requirement is 29%～66% of the natural runoff, in which the water demand for balance of sediment transport and the water demand for hydrological cycles consume play important roles.
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