Modeling On Nitrogen Transport, Transformation And Losses In The Farmland

The impacts of farmland drainage on water environment have received increasingly great attention from both at home and abroad. A number of studies have shown that nitrate-nitrogen (NO3--N) loss through subsurface drainage is a major source of pollution for surface and groundwater bodies and thus threatens the water environment. The complexity of factors and processes evaluating environmental impacts of agricultural activities makes model development an important tool in the agricultural water-fertilizer management and nitrogen pollution controlling.Based on summarization and analysis of available models, this paper developed a mechanism model of nitrogen transport and transformation in the farmland soil that is suitable for organic and inorganic fertilizer application. Coupled with soil water and heat model SWAP or DRAINMOD (DM), an integrated nitrogen-model that simulates vertical transport of water, nitrogen and heat flow and NO3--N loss was developed.By graphical display and statistical comparison of observed and simulated data, three-year field data receiving inorganic fertilizer application, including water table, daily surface runoff and subsurface drainage flow, and NO3--N losses, were used to test the nitrogen-model coupled with DM. In the same way, one-year field data from four test plots receiving liquid hog manure, including daily subsurface drainage flow and NO3--N losses, soil NO3--N content, were used to test the nitrogen-model coupled with SWAP. Results showed that the simulated and observed values were in good agreement. The event rainfall-runoff data from field runoff plots under different land uses were used to test the surface runoff sub-model SCS in the nitrogen-model. The result showed that SCS method could simulate satisfactorily the surface runoff, and the error of SCS application is mainly systemic bias. To reduce the potentially systemic biases, an improved method for runoff process was proposed, namely, a relationship between runoff correction coefficient and rainfall was used to improve the runoff process.The paper also carried out the sensitivity analysis of main parameters influencing nitrogen transport and transformation, discussed the impacts of main parameters variation on nitrogen transport, and provided the judgment for model calibration. Combined with model evaluation, comparison and analysis of experimental data, and previous researches, the impacts of farmland management measures on nitrogen losses were analyzed, including fertilizer and irrigation management, water-table controlling, tillage measures, crop system and drainage reuse, the reasonable farmland management measures were proposed.