Abstract: The ecological water demand in the plenty polders of the Dongting Lake are difficult to calculate due to the lack of observation data and inadequate water circulation caused by severe eutrophication. To achieve the goal of reaching grade III water quality in 25 polders in the northern part of Dongting Lake in three years, we arranged 88 surface water sampling points and monitored the concentration of ammonia nitrogen pollutants based on field survey and investigation. By using the environmental dilution method, we established a calculation model for the ecological water demand of polders and obtained the ecological water demand of polders in north Dongting Lake in the presence of water supplement as 3.216 billion m³ in comparison with the result of water exchange cycle method. The research finding is of reference value for improving the ecological environment in polders of the Dongting Lake area.

Key words: ecological water demand, water quality target, environmental dilution method, polder, Dongting Lake