由较缓慢的非地表径流汇流过程引起的非点源氮污染水文滞后效应是不少流域经过多年的氮污染控制努力后河流水质未见成效的主因之一。然而，已有研究大多忽略了水文滞后效应的影响，氮源解析结果可能“张冠李戴”了，阻碍了氮污染控制进程。本项目将通过对农业流域水文过程和氮输移过程的协同监测分析，联合应用流域水文模型、水文图解、数字滤波、端元模型、基于氢氧同位素的正弦波分析、氚和氯氟烃分析等，揭示流域地表径流、侧向流、地下径流汇流规律，动态分割不同类型径流对河流流量的贡献，解析不同类型径流和河水的年龄构成；以此为基础，进一步联合应用常规统计方法、贝叶斯稳定同位素混合模型、氮氧同位素分析、N2:Ar法等，阐明不同类型径流和河流氮输移动态特征，揭示反硝化速率动态规律及其对氮输移负荷的影响，解析不同类型径流和河流氮的来源，从而定量阐释河流氮源构成、主要输移途径及其水文滞后时长，为突破氮污染控制困境提供关键科学依据。

Due to long residence times (ranging from months to decades) of watershed lateral flow and groundwater runoff, nitrogen loading delivered by them may be mainly derived from historical nitrogen inputs and would not be significantly decreased in response to decline of current anthropogenic nitrogen inputs to terrestrial surface. Such hydrologic lag effect has been one of major causes of failing to yield the expected improvements in river water quality after implementing nitrogen nonpoint source (NPS) pollution control measures for decade or several decades in many watersheds. However, little quantitative knowledge is available concerning watershed hydrologic lag effect of nitrogen NPS pollution, resulting in considerable uncertainties in riverine N sources apportioned results in previous studies. To address the hydrologic lag effect, monitoring works for hydrologic and nitrogen delivery processes will be conducted synergistically for a typical agricultural drainage watershed in eastern China. By adopting and integrating various methods and technologies (e.g., the watershed hydrologic model, hydrologic diagram approach, digital filtering method, end–number model, water stable isotopes based sine wave method, and 3H and chlorofluorocarbon analyses), the research will provide new and reliable insights on the confluence principles of watershed surface runoff, lateral flow and groundwater runoff, their individual contributions to river water discharge, and the age constructions of each type of runoff and river water. Combining the conventional statistical methods, Bayesian stable isotope mixture model, 15N and 18O dynamics analyses, and the N2:Ar ratio method with membrane inlet mass spectrometry, the research will further explore the nitrogen dynamics of different types of runoff and river, quantify denitrification rates for different hydrologic pathways, and identify nitrogen sources in each type of runoff and river. Coupled with these two aspects of achievements, riverine nitrogen sources, their main hydrologic delivery pathways, and their associated hydrologic lag times will finally quantified and determined, guiding the development of efficient watershed nitrogen NPS pollution control strategies.