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从很长一段时间看,全球水量总值为138598.46亿立方公里,约占地球总质量的万分之四,这是一个不变的定值。但就某一地区、不同时间来看,其储水量则是有变化的,是一个变值。若某一地区初始时储水量为S_0,后来储水量为S_t,那么,储水变化量△S,即S_t—S_0,等于该地区水量收入与支出的差额。
From a long period of time, the total global water volume is 13,858,486 million cubic kilometers, accounting for about four ten thousandths of the total mass of the earth, which is a constant set value. However, in a certain area and at different times, its water storage capacity is a variable and a variable value. If a region initially stores S_0 and then S_t, then the change in water storage △ S, S_t-S_0, is equal to the difference between the income and the expenditure in the region.