新托福阅读TPO24解析.doc

Tpo24 Lake Water Where does the water ina e 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. 河水的来源和去向 The questions e plicated when actual volumes of water are considered: how much water enters and leaves by each route? Discovering the inputs and outputs of rivers isa 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 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 plicated matter than merely inferring their difference. 如何定义流入和流出河流的水量 Once all this information has been gathered, it es 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 isa surface-water-dominated lake; if the latter, it isa 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 str