TPO24阅读(含答案和解析).doc

TPO24阅读(含答案和解析).doc华联教育
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 ram 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 comp