Nationwide estimates of nitrogen and phosphorus losses via runoff from rice paddies using data-constrained model simulations

Abstract Quantifying the surface runoff losses of nitrogen (N) and phosphorus (P) at the regional scale is still challenging, mainly due to the model deficiencies and the lack of high-resolution data of agricultural management practices. Here, we re-evaluate N and P runoff losses from single, early and late rice systems in China, using a newly-developed model that was constrained by 127 rainfall-runoff events in 11 sites and driven by the fertilization and irrigation schemes from national surveys covering most of rice-producing provinces. Results show that national N and P runoff losses from rice paddies were 272.6 ± 101.2 Gg N yr−1 and 17.0 ± 6.4 Gg P yr−1, which were 38% and 12% less than previous model estimates, respectively, but 43% more than those of the first National Pollution Census Program (NPCP). The hotspots of N and P runoff losses were identified, most of which occurred in south China (60% of the total), generated during non-fertilization period (>65%), and took place in single rice system (∼50%). These losses might be mitigated by ∼90% through the increase in weir outlet height and the application of shallow-wet irrigation, but remain relatively stable even if optimizing fertilization scheme. The detailed assessment of mitigation potentials highlights an urgency in environmental policy regulation as well as farmers’ accessibility to advanced irrigation technologies, which is expected to inform promotion towards more effective non-point source pollution controls in China and other rice-producing countries.