Official24 托福阅读Passage1 Lake Water文本+题目原文+答案解析【雷哥托福】2019-02-26 09:45:32 发布 来源：雷哥托福
TPO24 托福阅读Passage1 Lake Water文本+题目原文+答案解析
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 lake via outflowing rivers, by soaking into the bed of the lake, and by evaporation. So much is obvious.
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 matter 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 rain gauges, and the losses by evaporation, measured with models that correct for 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 seeps 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.
Once all this information has been gathered, it becomes possible to judge whether a lake’s 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 tells 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, which dries up when the stream dries up, is clearly surface water dominated. [■]
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 lake’s residence time. The residence time is the average 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) 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 time.
Residence times vary enormously. They range from a few days for small lakes up to several hundred years for large ones; Lake Tahoe, in California, has a residence time of 700 years. The residence times for the Great Lakes of North America, namely, Lakes Superior, Michigan, Huron, Erie, and Ontario, are, respectively, 190,100,22,2.5, and 6 years. Lake Erie’s is the lowest: although its area is larger than Lake Ontario’ s, its volume is less than one-third as great because it is so shallow-less than 20 meters on average.
A given lake’s 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 change (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 evapotranspiration 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 lakes increased from 5 to 18 years during the study period. The slowing down of water renewal leads to a chain of further consequences; it causes dissolved chemicals to become increasingly concentrated, and this, in turn, has a marked effect on all living things in the lake.
TPO24托福阅读 Passage1 Lake Water 题目
Question 1 of 14: The phrase “So much” in the passage refers to
A. the negative effects of overland flow, rain, and evaporation on river water levels
B. water that a lake loses to outflowing rivers, to the lake bed, and to evaporation
C. the importance of rivers to the maintenance of lake water levels
D. the information given about ways that water can enter or exit a lake
Question 2 of 14: The word “gains” in the passage is closest in meaning to
Question 3 of 14: Which of the following can be inferred from paragraph 2 about the movement of water into a lake?
A. Heavy rain accounts for most of the water that enters into lakes.
B. Rainfall replaces approximately the amount of water lost through evaporation.
C. Overland flow into lakes is reduced by the presence of forests.
D. Seepage has a smaller effect on water level than any other input.
Question 4 of 14: Why does the author use the phrase Note the word "net" in the passage?
A. To emphasize the impact of seepage on water levels
B. To point out that seepage is calculated differently from river flows and atmospheric exchanges
C. To compare the different methods of calculating seepage
D. To emphasize the difficulty of obtaining specific values for seepage inputs and outputs
Question 5 of 14: The word “Conversely” meaning to
A. on the other hand
B. in the same way
C. in other words
D. on average
Question 6 of 14: According to paragraph 3, which of the following best describes a seepage-dominated lake?
A. A lake that is fed by streams but still has fluctuating water levels
B. A lake with a constant water level that has no streams or rivers as inputs
C. A lake with a stream flowing into it and a stream flowing out of it
D. A lake that has surface and underground inputs but loses water during dry seasons
Question 7 of 14: It can be inferred from paragraph 4 that the length of time a given molecule of water remains in a lake
A. depends entirely upon the average speed of a lake' s currents
B. can be measured by the volume of the lake alone
C. can be greater or lesser than the residence time
D. is similar to the length of time all other molecules remain in that lake
Question 8 of 14: According to paragraph 5, Lake Erie's residence time is lower than Lake Ontario's for which of the following reasons?
A. Lake Erie has a larger area than Lake Ontario.
B. Lake Ontario is shallower than Lake Erie.
C. Lake Ontario has a greater volume than Lake Erie.
D. Lake Erie receives less rainfall than Lake Ontario.
Question9 of 14 ：Why does the author discuss the Great Lakes in paragraph 5?
A. To demonstrate the extent to which residence times vary from lake to lake
B. To illustrate how residence times are calculated for specific lakes
C. To argue that the residence time of a lake increases with area
D. To emphasize that Lake Tahoe' s residence time is unusually long
Question10 of 14: The word “further” in the passage is closest in meaning to
Question11 of 14: According to paragraph 6, which of the following explains the increase in residence northwestern Ontario?
A. The amount of water flowing into the lakes has increased.
B. The rate of evaporation has decreased more sharply than the amount of rainfall.
C. The renewal of the lakes' water has slowed due to changes in climate.
D. Plants have required less water from the lakes.
Question 12 of 14: According to paragraph 6, residence time is affected by all of the following EXCEPT
A. amount of rainfall
B. rate of evaporation
C. temperature of surrounding air
D. concentration of chemicals in lake water
Question13 of 14: Look at the four squares III that indicate where the following sentence could be added to the passage.
Of course, a lake may be neither surface-water-nor seepage-dominated if, for example, its inputs are predominantly surface and its outputs are predominantly seepage.
Where would the sentence best fit? Click on a square to add the sentence to the passage.
A. If the former are greater, the lake is a surface-water-dominated lake; if the latter, it is a seepage-dominated lake.
B. Occasionally, common sense tells you which of these two possibilities applies.
C. 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, which dries up when the stream dries up, is clearly surface water dominated.
Question14 of 14: Water enters, remains, and eventually leaves a lake in a variety of ways.
A. By measuring the water quantities at each of a lake's inputs and outputs, it can be determined whether water enters the lake mainly from surface or groundwater sources.
B. Changes in lake level and volume are caused principally by the amount of evaporation of water into the atmosphere.
C. It is sometimes possible to decide whether a lake is surface water dominated or seepage dominated by simple observation at different seasons.
D. The average period of time that molecules of water spend in a lake—the residence time—varies from lake to lake and overtime within a particular lake.
E. The residence times of surface-water-dominated lakes are usually longer than those of seepage-dominated lakes.
F The residence time of a lake frequently depends on the kinds of organisms to be found in the lake.
Question 1 of 14
解析：So much指代前文，提到water是怎么进入怎么离开的，所以正确答案是D。注意B代入之后有问题，因为water不是obvious的，“明显的”是信息，因此so much不仅仅指代它的前句，还指代了再往前的一句，正确答案是D。
Question 2 of 14
Question 3 of 14
解析：此题可以采用排除法。A的heavy rain和most of the water原文完全没有相关信息，错误；B的evaporation和rainfall做关键词定位至第三句，但文章明确提出可以通过蒸发跟雨水获得的差值计算湖水与大气的交换，也就是说二者是有差异的，B与原文相反；C的forests做关键词定位至第四句，提到周围有森林的湖，陆上来水很难有明显影响，推出C说水量减少是正确选项，因果关系；D的seepage做关键词定位至最后，但原文没有任何比较的相关信息，错误。
Question 4 of 14
Question 5 of 14
解析：conversely“相反地”，所以正确答案是on the other hand。原句提到有河水流入流出的是surface water dominated，前一句提到一个没有地表水流入的pond是seepage dominated，那么后文当然与之相对，所以正确答案是A，in other words表解释，不正确；其他的不合文意。
Question 6 of 14
Question 7 of 14
解析：以a given molecule of water做关键词定位至最后一句，提到residence time是平均值，一个分子在lake里的时间取决于它的route，有可能很快也有可能很慢，所以正确答案是C，或者多或者少。A和B原文未提及；D与原文相反，原文都说了一个具体的分子和residence time不一样，错误。
Question 8 of 14
解析：以两个专有名词做关键词定位至最后一句，说Erie的residence time是最小的，尽管Erie面积比Ontario大，但volume小，所以正确答案是C，注意不要深挖一层选B，因为shallow是volume少的原因，但它不能直接导致residence time变小。A明显不是作者强调的点，D未提及。
Question 9 of 14
解析：修辞目的题，先看例子所在句，发现整句话都是例子，所以往前看，前一句也是例子，看开头句，开头句说residence time变化很大，也就是说，后面举了这些湖都是为了说明它们的residence time不同，从而证明变化很大，所以正确答案是A，注意提到五大湖不是为了跟Lake Tahoe对比，因为Lake Tahoe本身也是个例子，所以D错误，其他的不合文意。
Question 10 of 14
Question 11 of 14
解析：以northwestern Ontario和increase双关键词定位至倒数第二句，但这句话只是提到增加，没给出具体的解释，往下看。下一句说slowing down of water renewal会带来一系列后果，也就是前文说的降水减少也好蒸发量增加也罢都属于renewal减慢，所以正确答案是C，其他答案都未提及。
Question 12 of 14
Question 13 of 14
解析：如果只是以surface water dominated和seepage-dominated做过渡点会发现所有答案都可以，但由于待插入句中有neither……nor，因此必须在前文把这些概念都说完之后才可以说“既不是……也不是……”，所以正确答案是D。
Question 14 of 14