Studies On Dynamic Water Environmental Capacity Calculation Of River Based On Distributed Hydrological Model

China is one of the countries of the world with the greatest number water problems.These problems have resulted from poor water endowments and the rapid growth of populations and economies.Water pollution is one of the most serious water issues in China,and is essentially due to a severely overdrawn water environment carrying capacity(WECC).Therefore,it is important to regard the WECC as a rigid constraint for the future sustainable development of China's economy and society.The water environmental capacity(WEC)is an important indicator of environmental management,and is usually calculated in designed hydrological conditions without consideration given to the uncertainty of the hydrology.In this study,the process and mechanism of the water environmental capacity,and the coupling mechanism between it and the hydrological processes,were studied.Then,a dynamic approach based on the hydrologic processes was developed,in which distributed hydrological and water environmental capacity models were coupled in order to solve the problem in a traditonal way.Tieling was taken as the study area for the purpose of demonstrating the proposed method,and the COD(chemical oxygen demand)and NH3-N levels were focuses of the study.The results showed that the dynamic capacities of the COD and NH3-N were 7295t/a to 61717t/a and 356t/a to 3093t/a,respectively.The results also indicated that the water environmental capacity was uneven during different times and spaces.For example,the water environmental capacity during an especially dry year was less than half that of a normal year,or even a quarter of that in an especially wet year.Also,the capacity during flood seasons was found to be nearly 40%,of which August accounted for 200%of the entire year.The capacity of the Liao River made up a significant share(more than half)of the entire region.These results explained that,contrary to the dynamic methods which separately calculated the years with different frequencies,seasons,or months,the traditional method did not take into account the annual and inter-annual variations of the hydrology,and was too conservative to make full use of the water environmental capacity.The WEC thresholds in joint regulating of water quantity and water quality was almost equal to the annual average WEC that the WEC thresholds of the COD and NH3-N were respectively 24531t/a to 25979t/a and 1193t/a to 1386t/a.However,it drastically varied throughout the year,which behaved as more WEC in flood season and less WEC in non-flood season.In the practical management,the runoff adjustment projects and the scheduling operation have a great influence on the hydrological conditions of the river,thus the WEC thresholds in joint regulating of water quantity and water quality considering ecological water demands is more scientific.