Effects of Extreme Rainfall on the Export of Nutrients from Agricultural Land

In this study, plot and watershed scale experiments were conducted to study the effect of some extraordinary rainfalls on the export of nutrients from agricultural land. The results of three-year plot experiments show that more than 50% of annual nitrogen and phosphorus losses via surface runoff happened between June and August, when the monsoon and typhoon rains are heavy and frequent. During the three largest storms, 35.7%-52.4% of the annual N and 46.8%-66.4% of the annual P were exported by surface runoff from different land use types. Nutrient concentrations increased with hydrological process during storm flow, decreased with the prolongation of storms yet remained at higher concentrations compared with base flow time. About 30% of N and more than 40% of the annual P loads were exported from the watershed by the four largest storms. Reducing agricultural practices such as tillage and fertilization during frequent heavy rainfall period are effective ways for mitigating agricultural nonpoint source pollution.