Simulating Nitrogen Loss In A Paddy Field With HYDRUS-2D Model

In this study,field experiments were carried out on paddy fields with certain surface slope and certain depth of surface water.The surface water depth and the nitrogen concentrations in surface water and soil water at different depths were observed by the points of A1,A2 and A3 on the diagonal of paddy field.The nitrogen loss in surface runoff was fitted by function.And the nitrogen transport in surface water and soil water were analyzed.HYDRUS-2D model was used to predict the surface water depth and nitrogen concentrations in surface water and soil water at different depths.The model was verified by the experimental data,and the sensitivity analysis of the prediction model to the parameters was carried out.The study results showed that the changes of TN and NO₃^--N loss in surface runoff with time in the early runoff period and the whole process were polynomial functions with good fitting effect.The changes of NH₄⁺-N loss in surface runoff with time for the whole runoff process was polynomial function with good fitting effect,which was the logarithmic function fitting in the early runoff stage.According to the analysis of nitrogen concentration changes in paddy field,there were three peak values of TN concentrations in the whole period,corresponding to three fertilization times.The overall variation trend of NH₄⁺-N concentration was basically consistent with that of TN.The NO₃^--N concentration was low,but there were still three concentration peaks.The change of nitrogen concentration in soil water with time at different points of paddy field was also analyzed.It was found that the changes of TN,NH₄⁺-N and NO₃^--N concentrations at different points were different.The concentrations of TN,NH₄⁺-N and NO₃^--N at point A2 were higher than those at points A1 and A3.Through the HYDRUS-2D model simulation analysis,it was found that the model can be used efficiently to simulate the surface water depth with RMSE=0.717cm and NSE=0.805.The simulated NH₄⁺-N concentrations in the surface water at point A1 agreed well with the observed data with RMSE=1.323mg/L and NSE=0.958.At point A2,the simulated values of NH₄⁺-N concentration at 0.1m soil depth were consistent with the measured values.The overall trend of the simulated and measured NO₃^--N concentrations in surface water at point A1 was consistent,but the simulated peak value was larger than the measured value.At points A2 and A3,the simulated NO₃^--N concentrations at different soil depths did not agreed well with the measured data,which became worse with the increase of soil depth.The maximum root accumulation of NH₄⁺-N and NO₃^--N was 68.25mg/L and 2.73mg/L,respectively.The main way of nitrogen loss in the paddy field was the NH₄⁺-N loss through the lateral boundary,with the accumulation amount of 38.94mg/cm,accounting for 67.78%of the sum loss of NH₄⁺-N and NO₃^--N.By the sensitivity analysis of the solute transport reaction parameters to the predicted nitrogen concentrations,it was found that the model output of nitrogen concentration in surface water at the early stage was very sensitive to the the hydrolysis coefficientμ’₁.With the increase of soil depth,the sensitivity of the nitrification coefficientμ’₂ and denitrification coefficientμ₃ to the simulated nitrogen concentrations in soil water became stronger.