Study On Dynamic Water Environmental Capacity Of Qingyi River Basin(Xuchang)

With the rapid development of economy, the amount of pollutants discharged into river is continuously increasing, which goes gradually beyondthe capacityofself-clearance of water and results in water deterioration. Carrying out the research of water environmental capacity is becoming an essential initiative for water environment management. Water environmental capacity reflects the water tolerance to pollutants in the particular function and is the fundamental basis for water-quality target management, and it is great significant to improve the water quality of the environment and promote economic, socialand environmental development.Based on the basic theory and research status of water environmental capacity, this paper divides control units in Qingyi River(Xuchang), analyses pollutants emissions, establishes the relationship between pollutants discharge and water quality response, and calculates water environmental capacity for each control unit, which provides the scientific basis for the comprehensive treatment of Qingyi River basin an water-quality target management. The main results are as following:(1) Qingyi River was divided into control units. First, the indicators of control units division were built and relevant basic data was collected, and when building the indicators the factors of physical geography, social economy and water environment in Qingyi River were considered. Second, combined with the distribution of control sections and the water quality objectives of water environmental function zoning, the waters part of control units was determined in accordance with clean boundary principle and section principle. Third, on the basis of the waters part division and taking DEM as the basic data, the catchment area of Qingyi River was divided through using the SWAT model based on GIS. Last, based on the layer overlay analysis of GIS, Qingyi River basin was divided into 11 control units in accordance with the principle of control units, and there were 4 control units in the trunk stream of Qingyi River, 2 control units in Shiliang River, 3 control units in Xiaoni River and 2 control units in Xiaohong River.(2) The water quality of Qingyi River’s main stream was evaluatedand the pollution source in Qingyi River basin was analyzed. Through the water quality assessment, COD and ammonia nitrogen as water environmental capacity calculation factor was calculated. Through the analysis of pollution source, the results are as: The biggest COD pollution areais Qingyi River-3(Hongteng bridge-Xinxing bridge). The biggest ammonia nitrogen pollution area is Qingyi River-1(source- Nanwaihuan bridge). Thesetwo control units wereas priority unit to control.(3) Water environment capacity of Qingyi Riverwas calculated.The Delft 3d software was used to meshing. Based on EFDC, one-dimensional water quality model of Qingyi Riverwas established. Through the multiple model calibration and verification, the watershed model parameters was determined: roughness of Qingyi River is 0.03;The degradation coefficient of COD is 0.15 mg/d;The degradation coefficient of ammonia nitrogen is 0.18 mg/L.Thus the response relationship of the Qingyi River basin water quality and water pollutant discharge was established.Hydrologic analogy method and the runoff coefficient method is used to estimate the flow of Qingyi River. The flow that hydrological assurance was 90% was as the design flow.Through the years distribution, we got 12 months of design flow rate.Water function regionalization and “much starker choices-and graver consequences-in planning objectives” was designed for water quality goals, and water environmental capacity was calculated. The largest water environmental capacity is in August. The minimum water environmental capacity is in January. Based on water function zoning target, the value of minimum capacity of COD is 291.88 t;The value of minimum capacity of ammonia nitrogen is 14.36 t. Based on “much starker choices-and graver consequences-in planning water quality target”, the value of minimum capacity of COD is 392.17 t;The value of minimum capacity of ammonia nitrogen is 19.61 t.