洞庭湖四口河系连接了洞庭湖和长江干流，对洞庭湖整体水质有重要影响。开展洞庭湖四口河系地区非点源污染研究有助于四口河系地区水污染防治工作的推进，进而对洞庭湖整体水污染的治理与控制有重要意义。本研究分析了洞庭湖四口河系地区水环境现状，并建立了该地区SWAT模型，为满足SWAT模型在平原河网地区的使用要求，进行了空间离散化过程的改进，并利用“水库”出流的设置模拟了长江干流分流对四口河系地区非点源污染模拟的影响。利用实测日径流资料、氨氮月均负荷资料对模型在洞庭湖四口河系地区的模拟效果进行了率定和验证。分析模型计算结果，四口河系地区氮磷污染负荷排放总量和排放强度较高的子流域均分布在研究区北部和西北部的丘陵地带。洞庭湖四口河系地区氮、磷污染排放强度最高，总量贡献最大的主要是水田、旱地、林地三种土地利用类型。从年内分布规律来看，每年5月-8月入湖氮磷负荷占全年负荷总量的70%左右，而入湖氮磷平均浓度多在春季汛期前达到最高。以总氮作为评价指标，四口河系地区河道多属于III-IV类水，以总磷作为评价指标，河道多为II-III类水。河道中，氮磷浓度非汛期高于汛期。本研究以氮元素的输出为例，分析了林地、水田、旱地中污染负荷的输出特征。以林地为代表的天然流域中，氮输出总量对降雨径流过程的响应并不明显，汛期输出总氮的浓度低于非汛期。在以水田为代表的农业流域中，农业化肥的施加为土壤提供了额外的氮元素，总氮流失浓度在汛期高于非汛期。溶解态无机氮和吸附态有机氮的排放规律方面，在林地区域，地下径流中氮的浓度高于地表径流中氮的浓度，氮的流失形式主要通过吸附的有机氮和地下径流中溶解的无机氮。在农业区域，地表径流中氮的浓度高于地下径流中氮的浓度，溶解态无机氮大量通过地表径流排放进入河道。对于四口河系地区的污染控制，在林地等自然区域因注意避免破坏植被。而在农业区域，应该将防治重点聚焦于控制表层土壤的营养物质流失。

With the practice of comprehensive treatment of water environment, point- source pollution such as industrial wastewater has been gradually controlled. the impact of agricultural non-point source on the water environment have gradually been paid attention to. The four inlets region is the link between Dongting Lake and the Yangtze River, which has an important impact on the water quality of Dongting Lake. Carrying out non-point source pollution research in four inlets region of the Dongting Lake will help promote the prevention of water pollution in the four inlets region, and will be of great significance to the overall water pollution control of the Dongting Lake. In this study, the basic database of the SWAT model in the four inlets region was established. In order to meet the requirements of the SWAT model in the study area, the spatial discretization process was improved, and the setting of the "reservoir" outflow was simulated, to repressent the impact of the divergence of the main stream of the Yangtze River. The measured daily runoff data and the monthly average ammonia load data were used to verify the simulation result of the model.According to the calculation results, the total discharge volume and the discharge intensity of the nitrogen and phosphorus pollution load in the four inlets region are relatively high, and the sub-basins are distributed in the hilly areas in the north and northwest of the study area. The nitrogen and phosphorus pollution emission intensity is the highest, and the land use categories with the largest total contribution are mainly paddy fields, upland field and forest land. Judging from the distribution law during the year, the nitrogen and phosphorus loads entering the lake from May to August each year account for about 70% of the total annual load, and the average concentration of nitrogen and phosphorus entering the lake mostly reaches the highest before the spring flood season. Taking total nitrogen as the evaluation index, most of the rivers in the four inlets region belong to Class III-IV. Taking total phosphorus as the evaluation index, most of the rivers are Class II-III. In the watercourses, the concentration of nitrogen and phosphorus is inversely proportional to the runoff.taking the output of nitrogen as an example, the output characteristics of pollution load in forest land, paddy field and upland field were analyzed. The total nitrogen output in natural watersheds represented by forest does not respond significantly to rainfall-runoff processes, and the concentration of total nitrogen output in flood seasons is lower than in non-flood seasons. In the agricultural watersheds represented by paddy fields, the application of agricultural fertilizers provides additional nitrogen to the soil, and the total nitrogen loss concentration is higher in flood seasons than in non-flood seasons. In terms of the emission of dissolved inorganic nitrogen and adsorbed organic nitrogen, in forest areas, the concentration of nitrogen in underground runoff is higher than that of surface runoff. The form of nitrogen loss is mainly organic nitrogen absorbed in soil colloids and underground runoff. Inorganic nitrogen. In agricultural areas, the concentration of nitrogen in surface runoff is higher than the concentration of nitrogen in underground runoff, and dissolved inorganic nitrogen is largely discharged into the river channel through surface runoff. For pollution control in four inlets region, attention should be paid to avoid damage to vegetation in natural areas such as forest. In agricultural areas, the focus should be on controlling the loss of nutrients from the top soil.