A two-dimensional model MIKE21 coupled with a modified EcoLab module was applied to model the water quality of surface flow wetlands. In the model, vegetation effects, oxygen production, nutrient consumption by microorganisms and vegetation were set in the solutions of hydrodynamic, chemical, and biological processes. Based on the field investigation and measurements in the Guishui River wetland, the model was established for the downstream reaches of the Guishui River and the Sanli River. The model calculated the hydrodynamics and water quality changes by vegetation type and distribution. The model parameters were calibrated and results were validated using the measurements. The concentrations of ammonia nitrogen, phosphate, and total nitrogen at outflow decreased by 14.29%, 33.33%, and 20.00% in the presence of wetland vegetation compared to no wetland vegetation. During water circulation, the flow rate increased by 0.4 m3 ·s-1 at the inlet of Guishui and Sanli rivers, increasing the water level and velocity in some parts of the rivers. The water areas with vegetation in Sanli and Guishui rivers increased by 144.44% and 13.16%, respectively. The concentrations of ammonia nitrogen, phosphate, and total nitrogen at outflow decreased by 35.71%, 50.00%, and 46.67% compared to no wetlands and no circulation. The circulation strengthened the wetland purification function. The wetland vegetation distribution was organically integrated into the model for water quality calculation, which provides the technical support for the water quality response research under comprehensive measures such as river and lake wetland ecological restoration and water conservancy regulation.
Keywords: Guishui River; MIKE 21-EcoLab; purification function of vegetation; surface wetland; water quality simulation.