104) 日記 「琵琶湖の水位 Water Level of Lake Biwa」

<Japanese Version> 琵琶湖には水が275億トンあると言い、平均して毎秒150万トン前後の水量が瀬田川洗い堰から排出され、下流域の1,500万人の人を潤している。しかし、渇水期には排出量は減量され、増水期には増量される。琵琶湖に流れ込む川は多数あるが、琵琶湖から流れ出る川は南部の瀬田川(後に宇治川・淀川と名前を変える)1本だけで、その水流出量などは国が滋賀県大津市の南郷洗い堰を守る国土省の近畿地方整備局によって調整管理されている。そしてその管理政策の基本は流水を利用する滋賀県と下流の京都府・大阪府・奈良県・兵庫県が流出量の増減・汚染問題などを協議して決めているそうだ。昨年秋からの琵琶湖周辺の降雨量が少ないため今の時期の琵琶湖数位は59cmと若干例年より低目らしいが、湖北地方に雪の備蓄もあるし、全体的にそれ程心配した量でもなさそうだ。下流での取水制限をすると京都・大阪では世間が騒がしくなるからタイミングは難しいが、これから春季雨季シーズンに入る筈だから何とか持ち返すだろう。今琵琶湖の水が多いと4~5月の大雨シーズンに下流域で洪水が起こって琵琶湖の放水が出来難くなるから今の時期琵琶湖水位がー60cmは丁度手頃な量であろう。


<English Version>   Lake Biwa is said to hold 27.5 billion tons of water, and on average, about 1.5 million tons of water per second is discharged from the Setagawa washing weir, which supplies 15 million people in the downstream area. However, the discharge is reduced during drought periods and increased during periods of high water.    Although there are many rivers flowing into Lake Biwa, only one river, the Seta River (later renamed the Uji River and the Yodo River) in the southern part of the lake, has a flow out of Lake Biwa, and its water runoff is regulated and managed by the Kinki Regional Development Bureau of the Ministry of Land, Infrastructure and Transport, which protects the Nango-Washigam in Otsu City, Shiga Prefecture.    The basic management policy is decided by Shiga Prefecture, which uses the water, and the downstream prefectures of Kyoto, Osaka, Nara, and Hyogo, through discussions on the increase or decrease of the outflow volume and pollution problems.    Due to the lack of rainfall around Lake Biwa since last autumn, the level of the lake at this time of year is said to be slightly lower than usual at 59 cm, but there is a stockpile of snow in the northern part of the lake, and overall, the amount does not seem to be that worrisome.      The timing is difficult to determine, since restricting water intake downstream would cause a public uproar in Kyoto and Osaka, but since the spring rainy season is expected to begin from now on, the situation should somehow improve.    If the water level in Lake Biwa is too high now, flooding will occur in the downstream areas during the heavy rain season in April and May, making it difficult to discharge water into Lake Biwa.    When I moved to Shiga Prefecture in the late 1965's, the Japanese economy was advancing and people were abandoning garbage and detergents, and Lake Biwa was very dirty. However, new problems have emerged, such as red tides and the overgrowth of non-native weeds due to increased plankton and phosphorus elements. Lake Biwa is said to be deep in the northern part of the lake and shallow in the southern part.    It is said that a large amount of water flowing in from the northern part of the lake enters the deepest part of the lake in the north, and if the water is not replaced, the water quality will deteriorate and the ecosystem will change or deteriorate, making it a difficult problem to solve. It is not just a simple matter of water coming in and going out.


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