Abstract:

This paper combined the SWAT model of advantage on pollution load and MIKE21 model of advantage on water quality simulation to build coupled models. The coupled models overcome the shortcoming of inaccurate simulation of SWAT in river and inadequate hydrologic computing of MIKE21 in watershed. This study established the connection of SWAT HRUs and MIKE21 boundary in space, and the pollutant load distribution relationship between SWAT model and MIKE21 model. The models were applied to study water pollution in the Pengxi watershed, which consist of Donghe, Taoxi, Nanhe, Puli and mainstream subwatersheds. Based on the data from digital elevation, soil types, vegetation types, meteorology, hydrology, water quality, the Pengxi watershed was divided into twenty five computing subwatersheds and 225 hydrological response units (HRUs). The SWAT HRUs of five computing subwatersheds of Pengxi watershed were taken as backwater area, the SWAT HRUs of twenty computing subwatersheds were considered as the boundary conditions of river terrain grid from MIKE21 model, the SWAT HRUs of two computing subwatersheds were considered as the point sources of Pengxi river terrain grid from MIKE21 model, and SWAT HRUs of three computing subwatersheds were considered as the linear sources of Pengxi river terrain grid from MIKE21 model. The simulated results of monthly sediment, phosphorus and nitrogen discharge in 1994-2010 at the Pengxi watershed outlet showed that average annual sediment discharge was 456 million tons; The maximum and minimum annual nitrogen loads were 11 37558 tons and 3 11420 tons, respectively. The maximum and minimum annual phosphorus loads were 75837 tons and 20761 tons respectively. Nitrogen and phosphorus loads were high from July to September, the mean monthly loads of nitrogen and phosphorus in the flood period were 92867 tons and 17347 tons, respectively. The nitrogen and phosphorus loads from December to February is low, the mean monthly loads of nitrogen and phosphorus in the dry period were 6191 tons and 1156 tons respectively. The simulated results showed that the average concentrations of TP in the sections of Quma, Gaoyang, Huangshi and Shuangjiang bridge were 0012-0191 mg/L, the average concentrations of TN of four sections were 064-324 mg/L， the average contents of Chla of four sections were 019-6108 μg/L. The Pengxi backwater was mesotrophic or mesoeutrophic water type. High concentrations of the TN and the TP were easy to be developed in high water season. The Chla whose concentration was higher than 30 ug/L was easy to be developed from March to May. Soil erosion and nonpoint pollution were important sources of Pengxi backwater eutrophication. The study results showed that the coupled models had high accuracy and reliability, the SWAT and MIKE21 coupled models were suitable for study on watershed pollution