单项选择题
Water will evaporate from any wetted surface. A
.significantly large fraction of the rainfall that falls on land is returned to
the atmosphere in this fashion. In addition water is assimilated by root systems
of growing plants and is later transpired from the leaf surfaces by a process
essentially identical to evaporation. The two effects, evaporation and
transpiration, cannot be individually discriminated for their effectiveness in
returning rainfall to the atmosphere, but their sum contribution can be
evaluated and is usually called the evapotranspiration factor. The fraction of
rain falling on the United States that is returned to the atmosphere by
evapotranspiration, for example, is 70 percent; for the world as a whole,
approximately 62 percent. In arid countries such as Australia the fraction is
larger, and in less arid areas such as the United Kingdom it is lower. Water
returned to the atmosphere by evapotranspiration is unavailable to man, except
in, the sense that useful plants may be grown in the place of useless ones. It
cannot be trapped and redistributed for industrial or other purposes. In regions of low rainfall, plant cover will develop to a point where all precipitation is used in evapotranspiration and none remains for stream now. Seasonal rainfalls provide a qualifier for this statement because streams will flow even in the most arid areas during periods of maximum rainfall. In general, if the potential evapotranspiration—that which would result from the maximum plant cover a region could support under ideal circumstances—should exceed the precipitation, overland stream flow ceases. Conversely, if evapotranspiration is less than precipitation, runoff is generated. The amount by which precipitation exceeds evapotranspiration is the perennial yield of stream flow water, and this is the usable fraction of rain and snowfall. Across the entire United States the water yield amounts to 30 percent of the total rainfall, or approximately 1.77×1015 liters per year. But when we consider the distribution of water deficiencies and surpluses we find that essentially the entire eastern half of the United States, together with a small region in the Pacific northwest, enjoys water surplus, while most of the country to west of the Mississippi is water- deficient and arid. |
According to the passage, a greater fraction of rainfall on the globe we live comes from______.