流域氮素主要输出途径及变化特征

流域农业面源输出氮素是湖泊流域水体中氮的主要来源之一,对水环境安全造成了极大威胁.不同流域间地形、水文、植被覆盖等因子的差异及气象条件的变化,造成氮的输出途径具有流域异质性及时间变化特征.本研究以高原湖泊典型流域——凤羽河流域为例,基于2011-06~2013-05期间径流水量、水质高频监测数据,应用基流分割的方法,通过分析流域产流与氮素输出途径的季节性变化,探讨了流域氮素输出的主要途径及变化特征.结果表明,基流是高原湖泊流域水量输出的主要形式和总氮输出的主要途径;基流输出水量及总氮负荷分别占流域总输出水量的80. 0%和流域总氮输负荷的71. 1%;降雨显著增加了流域水量和总氮负荷的输出量,同时改变了总氮的输出途径;降雨导致地表径流量增加,进而使流域输出水量和总氮输出负荷中地表径流途径的比重升高;当地表径流占流域输出水量的比重约达40%以上时,流域总氮输出的主要途径由基流转变为地表径流;监测周期内,地表径流输出的总氮负荷占流域总输出负荷的比重最高达65. 6%.

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.