[Seasonal Changes of the Pathways of Nitrogen Export from an Agricultural Watershed in China]

Nonpoint source pollution has become a major factor influencing the water quality. Identifying the pathway of nitrogen (N) transport from the source to the watershed mouth is a critical step in taking measures to control this pollution. However, it is difficult to identify the pathway of N transport because the transport pathway varies among different watersheds depending on the difference in the terrain, hydrology, and land cover etc and changes over time. Additionally, there is little knowledge about the major pathway of N transport through agricultural watersheds in the Yunnan Plateau lake area. The pathways of N export and their temporal variations over time were investigated in this study based on a typical agriculture-dominated watershed in a plateau lake area, Yunnan Province, and two-year monitoring data (June 2011-May 2013) in combination with a base flow separation program. The results show that the base flow accounts for most of the streamflow discharge (80.0%) and N export (71.1%). The proportion of the stream flow discharge via storm flow increases significantly with increasing rainfall. Therefore, the percentage of total N (TN) export via storm flow increases with increasing storm flow, which is closely related to rainfall. The major pathway of N export shifts toward storm flow when the storm flow proportion of the stream flow discharge increases up to 40%. During the monitoring period, the proportion of the TN export via storm flow increases up to 65.6% in the rainy season. This study provides important information for the improvement of the management of nonpoint source pollution at the watershed scale.