Development Of Nitrogen Migration And Transformation Distributed Model (SWAN-N) In Water And Soil Environment

All over the world,nitrogen pollution is becoming more and more serious.With the research on the source,route and mechanism of nitrogen pollution in different environments and media,the model development and application of nitrogen migration and transformation process have made some progress.At present,most nitrogen migration and transformation models were established for the vadose zone,groundwater and surface water respectively,while relatively few models were established for the integration of soil and water environment.It is extremely important to understand and evaluate the whole process of nitrogen entering the soil layer and underground aquifer from the atmosphere and surface environment.The whole process of nitrogen entry into the soil layer and underground aquifer from the atmosphere and surface environment is very important for the quantitative assessment of agricultural non-point source pollution.Therefore,it is necessary to develop a regional-scale quantitative model of the migration and transformation process of nitrogen in soilsurface water-groundwater.Based on the comprehensive comparison of several(semi)distributed hydrological models widely used in the world,a distributed watershed hydrological model(The Soil and Land-use based rainfall-runoff and recharge Model,SLIM)was selected to simulate the hydrological process;Based on the migration and transformation of nitrogen in water and soil environment,a distributed model in water and soil environment was developed by embedding the nitrogen migration and transformation process in the SLIM hydrological process;A systematic technical scheme for the parameters optimization and calibration of the distributed hydrological model was proposed;Taking the Eden catchment of England as the study area,the rationality and reliability of the model were tested.The main contents of this paper are presented as follows:(1)Conceptual model construction of nitrogen migration and transformation in water and soil environmentFrom the perspectives of research progress on nitrogen migration and transformation theory in water and soil environment,conceptual model of nitrogen migration and transformation,development and application status of nitrogen simulation model,systematically summarized the mechanism of nitrogen migration and transformation in water and soil environment,and constructed the concept of nitrogen migration and transformation.The model provides theoretical support for the numerical simulation of the migration and transformation of "three nitrogen" in the water and soil environment.(2)SLIM model structure and distributed model parameter calibration scheme designThe principle,structure and characteristics of SLIM distributed hydrological model were described.Through a series of technical methods,such as GLUE analysis model simulation ability,sensitivity analysis identifies key parameters,self-organizing mapping network determine parameter value space,genetic algorithm optimize parameters,to complete model parameter optimization calibration,and propose to solve distributed model parameter optimization calibration systematic technical solutions.(3)The coupling mechanism of nitrogen migration and transformation process and hydrological processWith water as the carrier and hydrological cycle as the migration path,based on the principle of water balance,the main reaction process of nitrogen in soil,groundwater and river channels was embedded in the hydrological process,which was the physical description of the distributed model of nitrogen migration and transformation(SWAN-N)process.(4)Program development of distributed model of nitrogen migration and transformation(SWAN-N)On the basis of the source code of the SLIM hydrological model program,the input format of some data and the flow generation process program were improved.Based on the discussion of the nitrogen migration and transformation mechanism in the water and soil environment,the nitrogen migration and transformation module were independently developed.The model consists of a main program and many independent subprograms.The model can be divided into grid cell sizes according to needs,input and output files set default items and optional options,the codes were written in C#language,and it was completed on the Visual Studio 2015 platform.(5)Application and test of SWAN-N modelThe model was applied to simulate nitrogen migration and transformation in Eden catchment,England,values of the four gauging stations(Great Musgrave Bridge,Temple Sowerby,Great Corby and Sheepmount)from upstream to downstream in the Eden catchment were all greater than 0.7.In addition,the average error values of the nitrate nitrogen and ammonia nitrogen concentrations at the four gauging stations were all small,which can be within the acceptable range.The rationality and reliability of the model was demonstrated,and a model that meets the needs of the BGS and the DEFRA.The fully distributed SWAN-N can directly use the GIS data layer,which greatly simplifies the modeling process and improves modeling efficiency.SWAN-N was suitable for application exploration in China,the United Kingdom or other countries.At the same time,the SWAN-N code also provides a basis for flexibly solving the complex situations of various research areas,further improving models,and coupling with other models.