Simulation of the water and nitrogen dynamics for paddy field-ditch pond system and model construction of irrigation and drainage regulation

Abstract: The runoff nitrogen (N) loss of paddy fields is one of the major sources for agricultural non-point pollution (AGNPS) in the rice areas of southern China. In an effective way to reduce the AGNPS risk, a ditch-pond system can be used to intercept farmland drainage in recent years. It is necessary to quantify the water and N processes of the system for appropriate management practices. In this study, a new regulation model of water and nitrogen was developed for the paddy field-ditch pond system. The water and nitrogen processes of the ditch system incorporated the regulation of irrigation and drainage into the soil-rice system model (soil water heat carbon-nitrogen simulator for rice, WHCNS_Rice). A global Morris sensitivity analysis was adopted to evaluate the output responses of the model to different input parameters. The model was verified to utilize the dataset from a two-years (2009-2010) field experiment with the combination of two irrigation regimes (FI, traditional flooding irrigation; CI, controlled irrigation) and two N management (FP, farmer's N practice; SP, site-specific N practice) in the Taihu Lake Basin. The specific parameters included the ponding water depth, soil water content, runoff, N runoff loss, ammonia volatilization, crop N uptake, and crop yield. The results showed that the Relative Root Mean Square Error (RRMSE), Index of Agreement (IA), and Nash-Sutcliffe Efficiency (NSE) ranged from 4.6% to 29.7%, 0.758 to 0.996, and 0.073 to 0.983, respectively. The model performed well to simulate the water and N balances, as well as the rice growth for paddy field-ditch pond system under different irrigation regimes and N management practices. Morris sensitivity analysis showed that the soil hydraulic parameters of the paddy field and the leakage rate of the ditch (kr) presented the greatest influence on the simulation of water depth in the ditch, while the sensitivity of crop parameters was relatively low. The nitrate concentration in the ditch was also more sensitive to the hydraulic parameters of the paddy field, the coefficient of N reduction (RD) in ditches, and the first-order kinetic coefficient of ammonia volatilization (Kv). At the same time, the improved model was utilized to clarify the effects of ditch pond/paddy field area (β), irrigation regimes, and N management practices on water consumption, N fate, and crop growth in the paddy field-ditch pond system. Furthermore, the calibrated and validated model was selected to evaluate the effects of different water and N management on water and N balances of the paddy field-ditch pond system. It was found that the combination of controlled irrigation and site-specific N management significantly reduced irrigation water use and N runoff loss by 32.1%-36.2% and 36.7%-67.3%, respectively. Meanwhile, the nitrate concentration in the ditch pond was reduced by 55.1%, leading to a significant decrease in the N loss risk of the paddy field-ditch pond system. Consequently, the incorporated model can provide a powerful tool to regulate irrigation and drainage of paddy field-ditch pond system, and thereby to control agricultural non-point source pollution.