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1、 1 / 12Lake Water （TPO24 ）Paragraph 1 Where does the water in a lake come from, and how does water leave it? Water enters a lake from inflowing rivers, from underwater seeps and springs, from overland flow off the surrounding land, and from rain falling directly on the lake surface. Water leaves a l
2、ake via outflowing rivers, by soaking into the bed of the lake, and by evaporation. So much is obvious.Paragraph 2 The questions become more complicated when actual volumes of water are considered: how much water enters and leaves by each route? Discovering the inputs and outputs of rivers is a matt
3、er of measuring the discharges of every inflowing and outflowing stream and river. Then exchanges with the atmosphere are calculated by finding the difference between the gains from rain, as measured (rather roughly) by ram gauges, and the losses by evaporation measured with models that correct for
4、the other sources of water loss. For the majority of lakes certainly those surrounded by forests, input from overland flow is too small to have a noticeable effect. Changes in lake level not explained by river flows plus exchanges with the atmosphere must be due to the net difference between what se
5、eps into the lake from the groundwater and what leaks into the groundwater. Note the word net: measuring the actual amounts of groundwater seepage into the lake and out of the lake is a much more complicated matter than merely inferring their difference.Paragraph 3 Once all this information has been
6、 gathered, it becomes possible to judge whether a lakes flow is mainly due to its surface inputs and outputs or to its underground inputs and outputs. If the former are greater, the lake is a surface-water-dominated lake; if the latter, it is a seepage-dominated lake. Occasionally, common sense tell
7、s you which of these two possibilities applies. For example, a pond in hilly country that maintains a steady water level all through a dry summer in spite of having no streams flowing into it must obviously be seepage dominated. Conversely, a pond with a stream flowing in one end and out the other,
8、which dries up when the stream dries up, is clearly surface water dominated. Paragraph 4 By whatever means, a lake is constantly gaining water and losing water: its water does not just sit there, or, anyway, not for long. This raises the matter of a lakes residence time. The residence time is the av
9、erage length of time that any particular molecule of water remains in the lake, and it is calculated by dividing the volume of water in the lake by the rate at which water leaves the lake. The residence time is an average; the time spent in the lake by a given molecule (if we could follow its fate)
10、would depend on the route it took: it might flow through as part of the fastest, most direct current, or it might circle in a backwater for an indefinitely long timeParagraph 5 Residence times vary enormously. They range from a few days for small lal.es up to several hundred years for large ones: La
11、ke Tahoe, in California, has a residence time of 700 years. The residence times for the Great Lal.es of North America, namely, Lal.es Superior, Michigan. Huron, Erie, and Ontario, are, respectively, 190,100,22,2.5, and 6 years. Lake Eries is the lowest: although its area is larger than Lake Ontarios
12、, its volume is less than one-third as great because it is so shallow-less than 20 meters on average.Paragraph 6 A given lakes residence time is by no means a fixed quantity. It depends on the rate at which water enters the lake, and that depends on the rainfall and the evaporation rate. Climatic ch
13、ange (the result of global warming?) is dramatically affecting the residence times of some lakes in northwestern Ontario. Canada. In the period 1970 to 1986, rainfall in the area decreased from 1,000 millimeters to 650 millimeters per annum, while above-average temperatures speeded up the evapotrans
14、piration rate (the rate at which water is lost to the atmosphere through evaporation and the processes of plant life). The result has been that the residence time of one of the la4.es increased from 5 to 18 years during the study period. The slowing down of water renewal leads to a chain of further
15、consequences: it causes dissolved chemicals to become increasingly concentrated, and this, in turn, has a marled effect on all living things in the lake. Questions Paragraph 1 Where does the water in a lake come from, and how does water leave it? Water enters a lake from inflowing rivers, from under
16、water seeps and springs, from overland flow off the surrounding land, and from rain falling directly on the lake surface. Water leaves a lake via outflowing rivers, by soaking into the bed of the lake, and by evaporation. So much is obvious.1. The phrase “So much” in the passage refers to the negative effects of overland flow, ram, and evaporation on river water levels water that a lake loses to outflowing rivers.to the lake bed, and to evaporation the importance of rivers t
