Abstract: As a result of the succession of the four seasons in nature, water environmental capacity is obviously characterized by temporal dynamic. However, the use of a simple hydraulic design in calculating water environment capacity is unable to reflect the dynamic variation. Based on characterization of the temporal dynamic of the water environmental capacity, a new method, including a suitable water quality model and a dynamic hydraulic design, was put forth for calculating. Using this method, a numerical water quality model was established with the aid of the hydrodynamic (HD) & advection-dispersion (AD) module of the DHI MIKE11 software for the downstreams of the Ashi River after the Xiquanyan Reservoir in Harbin. Water environmental capacities of its various function zones were worked out using the one-dimensional water environmental capacity model. Results show that the COD and NH₃-H water environmental capacities of various function zones were much lower in the the frozen period (from November to next March) than in the non-frozen period (from April to October). Furthermore, by the comparison between months and between various hydraulic conditions in water environment capacity, both the maximum and minimum water environmental capacity control lines were obtained for every function zone every month, which may serve as reference for total volume control of pollution discharge in the Ashi River.

Key words: water environmental capacity, dynamic, Ashi River, frozen period