Abstract:Nitrogen runoff loss from paddy fields is one of the main sources of agricultural non-point source pollution. In order to reduce the risk of nitrogen runoff loss from single-crop paddy fields in the climatic zone of the middle and lower reaches of the Yangtze River, a suitable fertilization and water level control model for this region was sought and the model of the SMNRL was built. This article used the historical meteorological data from 1957 to 2019 in the Chaohu Lake area by setting the rice transplanting interval (June 6 to 25) and the water level (3, 10, 20 cm) to simulate nitrogen loss in paddy fields. The result showed that: (1) After fertilization, the nitrogen concentration in the paddy field surface decreased exponentially. Nitrogen decay time of field water was 9 days at base fertilizer stage, and 7 days at tillering and earing fertilizer stage. (2) According to the order of nitrogen runoff loss that accounts for the whole growth in each fertilization period, in the combination of LW and HW, it was the basal fertilizer stage, the tiller fertilizer stage, and the panicle fertilizer stage. The nitrogen loss of basal fertilizer, tiller fertilizer, and panicle fertilizer was respectively 72.4% to 98.4%, 1.9% to 27.6%, and 0 to 8.3% in the combination of LW, and the basal fertilizer period was the period of high nitrogen runoff loss. (3) Controlling water level was more effective than reducing planting time to reduce nitrogen runoff loss. Under the same water level, the total nitrogen runoff loss during the transplanting period were reduced by 0.4 to 4.5 kg/hm2 and 32.8% to 80.3% during the whole growth period. Under the same transplanting time, the nitrogen runoff loss of the LW and MW combinations were reduced by 8.8 to 13.1 kg/hm2 and 92.1% to 98.8% compared with the HW combination. (4) In the three combinations of LW, MW, and HW, June 6 to 25 was divided into the early and late stages of rice transplanting, with June 19, June 11, and June 17 as the boundaries respectively. The average nitrogen runoff loss in the early stage could be reduced by 37.0%, 25.0%, and 21.7% compared with the later stage. (5) The effective measures to reduce nitrogen runoff loss in rice fields in Chaohu Lake region were to control the water level during the fertilization period to 3 cm and choose a rice transplanting time between June 6th and June 19th.