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阅读真题及答案
Lake Wat
er
。我们建议考友们，先做真题，后看答案进行验证自己的正确率。之后，环球托
福还会进一步推出托福
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阅读真题
Lake Water
Lake Water
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
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.
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阅读真题
Lake
Water
题目
1.
The phrase So much in the passage
refers to
negative effects of overland
flow, rain, and evaporation on river water levels
that a lake loses to outflowing
rivers, to the lake bed, and to evaporation
importance of rivers to the
maintenance of lake water levels
information given about ways that water can enter
or exit a lake
2.
The word
gains in the passage is closest in meaning to
s
ses
ces
s
3.
Which of the following can
be inferred from paragraph 2 about the movement of
water into a
lake?
rain
accounts for most of the water that enters into
lakes.
ll replaces approximately the
amount of water lost through evaporation.
nd flow into lakes is reduced by the
presence of forests.
e has a smaller
effect on water level than any other input.
4.
Why does the author use the phrase Note
the word
emphasize the impact of
seepage on water levels
point out that
seepage is calculated differently from river flows
and atmospheric exchanges
compare the
different methods of calculating seepage
emphasize the difficulty of obtaining
specific values for seepage inputs and outputs
5.
The word Conversely meaning to
the other hand
the same
way
other words
average