Study on the water and nitrogen balance in paddy fields with irrigation and drainage dual-purpose channel mode using the tank model

In South China's paddy irrigation area, irrigation and drainage dual-purpose channel mode (IDDCM) is applied to conserve farmland resources in rice cultivation areas with relatively flat topography. However, this mode makes it difficult to quantify and analyze the water and nutrient balance in rice fields. To solve this problem, a tank model considering both on-farm water management under IDDCM and nitrogen (N) transformation processes was reconstructed and applied in Xiaochang County, Hubei Province, South China in 2015 and 2016. The model performed well in 2015; however, in 2016, the model's N load prediction performance was poor due to the cumulative effect of parameter errors and the uncertainty of parameter values. The simulation results in 2015 show a considerable quantity of outflow from upstream paddy fields in the dual-purpose channel. The water could not be preserved and reused by paddy fields in the experimental zone. When there was no rain or irrigation, paddy fields were a source of water release due to the continuous water loss by lateral seepage, but the fields could mitigate flooding during rainfall. The main pathway for N loss in the fields was ammonia volatilization and lateral seepage, which usually occur within 1 week of fertilization. The water and N balance were successfully simulated by the reconstructed tank model. This provided guidelines for rational field management in paddy fields with IDDCM, such as reducing the outflow from upstream paddy to reenter downstream paddy, applying slow-release N fertilizers, and strengthening the linings of ditches.