Estimation of residence time of lake water and material circulation on the basis of seasonal variation of river water flowing into the lake

基于入湖河水季节变化估算湖水停留时间和物质循环

• 批准号: 13640436
• 负责人: ENDOH Shuichi
• 金额: $ 1.54万
• 依托单位: Shiga University
• 依托单位国家: 日本
• 项目类别: Grant-in-Aid for Scientific Research (C)
• 财政年份: 2001
• 资助国家: 日本
• 起止时间: 2001 至 2002
• 项目状态: 已结题
• 来源: https://kaken.nii.ac.jp/grant/KAKENHI-PROJECT-13640436/
• 关键词: river water; Lake Biwa; seasonal variation; residence time; material circulation; 湖水の入れ替わり; 水温; 濁度

Seasonal variation of dissolved and suspended material coming into Lake Biwa through main 12 rivers are studied by the successive hydrological and chemical observations. On the basis of these observed data, water quality formation and residence time of lake water are analyzed by the numerical model.River water is characterized by the geological background and land use, and shows a large amplitude of seasonal variation in its water quality. We have an attention on some event such as waste water from paddy field, heavy rainfall, and water level depletion in summer. We also made a successive observation of water quality along the River Yasu, which has a maximum catchment area among river in Lake Biwa basin.After heavy rainfall, electric conductivity is rapidly increased by "washout effect", but is soon decreased by dilution. In this case, river water is flowing into the thermocline depth of the lake because of its lower water temperature than usual.Highly turbid water containing high concentration of phosphorus and nitrogen, which is originated from paddy fields just before rice-planting, flows into the lake. This phenomenon has very bad influence on the lake water quality.Residence time of lake water is estimated by numerical model using chloride concentration as a tracer : As a result, residence time of lake water is calculated little longer than that estimated former because river water flows into the surface layer and goes out fro the lake in spring time. Concentration of chloride also is calculated to be increased about 1 mg/L one year later if the qualities of river water are not improved.

通过连续的水文和化学观测，研究了通过主要 12 条河流进入琵琶湖的溶解和悬浮物质的季节变化。在此基础上，利用数值模型对湖水的水质形成和停留时间进行了分析。河水具有地质背景和土地利用特征，水质呈现出较大的季节变化。我们关注稻田废水、强降雨、夏季水位枯竭等事件。我们还对琵琶湖流域河流中集水面积最大的野须川沿岸的水质进行了连续观测。强降雨后，电导率因“冲刷效应”而迅速增加，但又因稀释作用而很快下降。在这种情况下，由于水温比平时低，河水正流入湖泊的温跃层深处。来自水稻种植前的稻田的含有高浓度磷和氮的高度浑浊的水流入湖泊。湖。这种现象对湖泊水质影响非常恶劣。以氯化物浓度为示踪剂，通过数值模型估算湖水停留时间：结果，由于河水流动，计算出的湖水停留时间比之前估算的稍长一些进入表层并在春季从湖中流出。如果河水水质不改善，一年后氯化物浓度也将增加约1毫克/升。