Simulating Water And Nitrogen Loss In A Paddy Field With Hydrus-1D Model

Agricultural non-point source pollution（ANPSP）is a main factor in surface water and groundwater pollution,especially for nitrogen（N）pollution.In order to reduce ANPSP and improve the efficiency of N fertilizer utilization,field experiments were conducted in a paddy field with ponded water and plow pan under traditional continuously flooded irrigation（CFI）to study N loss by runoff and leaching from soil profile.The loss process of N in surface runoff was studied and fitted with a function formula.At the same time,the distribution of N in soil profile was studied.On this basis,using Hydrus-1D model to simulate the water movement in rice field and the loss of N through surface runoff and subsurface drainage,it was possible to predict N loss in paddy fields.Under the condition of CFI,field experiments of natural rainfall runoff were conducted on the fertilized paddy fields to study the loss of N through surface runoff.The numerical simulation of N loss in runoff was carried out.The total N（TN）,ammonium（NH₄⁺-N）and nitrate（NO₃^--N）concentrations in runoff during the decline stage on the day of fertilization and the NO₃^--N concentration in runoff on the 18th day after fertilization were fitted well by power function.While the TN and NH₄⁺-N concentrations in runoff on the 18th day after fertilization were fitted well by exponential function.During the early half runoff processes,the relationship between cumulative surface runoff and accumulation of N loss in runoff was fitted well by parabolic function.While in the late half process,the parabolic function was slightly better than the logarithmic function.The Hydrus-1D model was used to simulate the water movement and N transport and transformation processes in paddy fields.The field test data in 2016 were used for calibration and verification,and the data of 2017 trials were applied and predicted.The model based on 2016 data had a total water balance error of 0.236 cm and a relative error（error/input total water）of 0.23%.For the solute transport model,the N balance error and relative error（error/input TN）were 0.36 kg ha^-1 and 0.40%,respectively.The results show that the plow pan plays an important role in the vertical infiltration of paddy water.Surface runoff and subsurface drainage were the major water loss pathways,accounting for 32.33%and 42.58%of the total input water,respectively.In 2016,the water productivity of rice fields was 0.36 kg m^-3,and that of 2017 was 0.64 kg m^-3.The results of the simulation based on 2016 test data showed that the NH₄⁺-N concentration was the main form of N absorption in rice（95%of total N uptake）,and its concentration was much greater than NO₃^--N.Denitrification and volatilization were the main ways of N consumption,accounting for 23.18%and 14.49%of the total consumption,respectively.N leaching（10.28%）and surface runoff loss（2.05%）were the main ways for N to enter the water environment with water flow.Studies have shown that Hydrus-1D simulation is an effective method to predict water flow and N transformation process.The results could provide a theoretical basis for further research on water migration and water management,nitrogen transport and nitrogen loss in paddy field,which could also reduce ANPSP and improve nitrogen use efficiency.