APEX-Paddy model simulation of hydrology, total nitrogen, and rice yield for different agricultural activities in paddy fields

Existing agro-hydrological models to explore the effects of paddy management practices on hydrology and water quality suffer from multiple shortcomings because they were developed using upland characteristics. The Agricultural Policy/Environmental eXtender (APEX)-Paddy model was recently developed to improve the agro-hydrological characteristics of paddy ecosystems. This study explores the hydrology, total nitrogen (T-N) content, and rice yield behaviors of the APEX-Paddy model by simultaneously considering multiple parameters associated with different agricultural activities. Model performances with respect to runoff, T-N, and rice yield were assessed as good level of approved statistical criteria over the calibration and validation periods. Results showed that the APEX-Paddy model can simulate the behavior differences of soil nutrients between different agricultural activities, such as treatment with commercial mineral fertilizer or liquid manure application. Changes in mineral-nitrogen levels in root zones caused by fertilizers can affect T-N load and crop uptake. A high degree of correlation was observed between the amount of nutrients remaining after crop uptake and sediment-transported nitrogen. These findings suggest that the crop-growth parameters of the model can significantly affect biomass, evapotranspiration rate, and T-N load on farmland. The results may help decision makers evaluate water-saving and pollutant-reduction options in paddy fields under future climate scenarios and conservation strategies.