Simulation Study on Water Quality of Paddy Field Ditches Considering the Effects of Rainfall and Sediment Release

Since the early 1980s, the water of the Taihu Lake Basin has been seriously polluted, and the phenomenon of eutrophication has been frequent [1]; this is mainly caused by serious agricultural non-point source pollution in the Taihu Lake region from farmland drainage. Currently, there is abundant research being conducted on farmland drainage. Hatcho et al. [2] found that it is possible to reduce the nitrogen load in paddy field drainage by means of circular irrigation. Tootoonchi et al. [3] monitored the water quality of paddy field drainage under four different flooding treatments and found that all flooding treatments reduced the phosphorus concentration in paddy field drainage. Varol et al. [4] found that paddy field drainage is the main source of nutrients in the downstream water body, and suggested that the use of fertilizer be controlled to reduce the nutrient load in paddy field drainage. Ghane et al. [5] found that the amount of nitrate migration in the drainage of unfertilized farmland was significantly lower than that of fertilized farmland, and the nitrate and total nitrogen load in the drainage of farmland increased correspondingly with an increase in flow depth. Badrzadeh et al. [6] found that fertilizer is the main factor affecting the pollution of farmland drainage, and the content of nitrate and phosphate in farmland drainage is also reduced by reducing the consumption of urea and phosphate fertilizer. Liu L. et al. [7] simulated the drainage of paddy fields and found that increasing the maximum standing water level could reduce phosphorus loss. By simulating the situation of nitrogen interception in farmland ditches with or without vegetation, Li X. et al. [8] found that the nitrogen interception rate in vegetated ditches was higher than that in non-vegetated ditches, and the amount of nitrogen interception was affected by the species and growth stage of plants in the ditches. Wang X. et al. [9] conducted a comprehensive evaluation of the drainage of farmland ditches, and found that the nitrogen index had a great impact on the water quality of ditches. Li S. et al. [10] analyzed the drainage of ecological ditches in paddy fields in the Taihu Basin, and found that plants in ditches and permeable dams played an important role in reducing nitrogen pollutant concentrations. Currently, the research on farmland drainage focuses on the analysis, control, and treatment of pollutants, and there is more research on field monitoring and less research on the simulation of drainage in paddy fields. However, the real-time monitoring of farmland drainage usually requires significant manpower and material resources, the monitoring may also face various problems, and it is difficult to study different drainage conditions. The use of mathematical model tools can easily solve many problems, and can contribute to the safe and sustainable use of farmland drainage.

The drainage of paddy fields ditches is mainly affected by rainfall. He S. et al. [11] found that the total nitrogen (TN) concentration in runoff induced by rainfall (the annual rainfall is 1468 mm) from paddy ditches in the Yangtze River Delta region of China ranged from 0.31 to 7.89 mg/L, and the total phosphorus (TP) concentration from 0.01 to 1.18 mg/L. Li X. et al. [12] found, by monitoring natural rainfall for two consecutive years (the annual rainfall is 544.4 mm and 433.5 mm), that the average concentration ranges of TN and TP in the drainage of a paddy field ditch in Dezhou City were 2.63–18.84 mg/L and 0.81–13.14 mg/L, respectively, and that the dominant form of nitrogen in the runoff was NO₃⁻-N, and the dominant form of phosphorus was particulate phosphorus. Wang L. et al. [13] conducted 17 rainfall (ranging from 7–101.7 mm) monitoring experiments on the ditches of the Baisha paddy field in Fuzhou, Fujian Province, China, and found that the runoff flux of natural rainfall was closely related to NO₃⁻-N, TP, and other losses, and the loss of nitrogen was greater than that of phosphorus. Yu Y. et al. [14] observed two typical natural rainfall events (with rainfall of 19.8 mm and 20.1 mm) in paddy field ditches in Jiangsu Province, and found that the water quality of ditches was seriously polluted during rainfall; the runoff carried a large amount of nitrogen at the early stage of rainfall, increasing its concentration until it reached its peak. Currently, simulating different rainfall conditions is one of the hot spots in the study of non-point source pollution, but there are few studies on the simulation of water quality in paddy field ditches. The drainage of paddy fields is the erosion of paddy fields by rainwater. The discharged water can enter rivers, lakes, and other water bodies, which has a great impact on downstream water bodies. Therefore, it is of great significance to study the water quality of paddy fields under different rainfall conditions.

Sediment release is also an important factor affecting water quality. Zhang Y. et al. [15] studied the release rules of nitrogen and phosphorus in sediments in shallow waters of the Yellow River Delta under different hydraulic actions, and found that sediment release increases the concentration of pollutants, and has a great impact on water quality. Komissarov et al. [16] found that the lake bottom sediments of small lakes in the Miyagi Prefecture contained about 3–4 times the Cs content of the surrounding soils, giving the overlying waters of the catchment area a rather high concentration of Cs. Xiang W. et al. [17] studied nitrogen and phosphorus pollution in the surface sediments of Baitan Lake, and found that the sediment release had a significant impact on the increase in nitrogen and phosphorus concentrations. Zhu Y. et al. [18] analyzed the contents of total nitrogen and ammonia nitrogen in the surface sediments of a reservoir in East China, and found that the sediment release was closely related to the increase in nitrogen concentration in overlying water. The drainage ditch of a paddy field is the only way for the drainage of the paddy field to enter the surrounding water. Long-term accumulation leads to large amounts of nitrogen and phosphorus in the sediment of the ditch. Therefore, the release of nitrogen and phosphorus in the sediment is bound to have a certain impact on the water quality of the ditch. Currently, there are relatively few studies using mathematical models to simulate sediment release in paddy field ditches, most of which focus on lakes, rivers, reservoirs, and other fields. As the connection channel between farmland water and downstream water, pollutants in paddy field ditches accumulate very easily in sediment, and it is easy to cause secondary pollution. Therefore, conducting simulations to study the effect of sediment release on water quality in the ditches of paddy fields can be helpful to understand the drainage pollution situation of paddy fields more accurately, and to put forward reasonable treatment measures.