Simulation study on performance of nitrogen loss of an improved subsurface drainage system for one-time drainage using HYDRUS-2D

Improved subsurface drainage has been verified as a more efficient drainage technique for solving problems of farmland shortage and frequent floods in South China. However, the environmental effects of improved subsurface drainage with different parameters have rarely been studied. In this study, nitrogen loss in outflow and soil nitrogen content under improved subsurface drainage for one-time drainage is evaluated based on a calibrated HYDRUS model, considering the precipitation process, initial groundwater table depth, initial soil nitrogen content, submerged pipe height, drain spacing, drain depth, filter width, and filter height during a 3-day individual drainage. The results indicate that HYDRUS performed well for the simulation. Under conditions of homogeneous soil and nitrogen content distribution, when rainfall occurred on the first, second, and third days, the total ammonia nitrogen (NH4-N) and nitrate nitrogen (NO3-N) losses under a rainfall of 141 mm are approximately 1.26, 1.31, 1.39 and 1.2, 1.25, 1.36 times that under a rainfall of 44 mm, respectively. The NH4-N and NO3-N losses in the outflow of the improved subsurface drainage increased significantly 0.5 days after the rain began. Compared with the 10 cm initial groundwater table depth case, the nitrogen loss under a 30 cm initial groundwater table depth reduced by approximately 23%. The NH4-N losses of the improved subsurface drainage with 20 and 40 cm submerged heights of the pipe reduced by 23% and 48%, respectively, whereas the NO3-N losses reduced by 24% and 51%, respectively, compared with those under a free outflow. The initial soil nitrogen content contributed primarily to the nitrogen loss of the outflow. The relationship between nitrogen loss per pipe and drain spacing/drain depth can be described as second-degree parabolic/linear. The effects of filter width and height are similar to that of drain spacing. In the improved subsurface drainage, the maximum decrease percentage of soil nitrogen mass occurred in the filter profile at all times, whereas the NH4-N mass increased at the 0-20 cm soil layer, and the soil NO3-N mass increased slightly at the 60-80 cm soil layer with increasing drainage time. The results of this study provide a scientific basis for nitrogen management.