Rivers are nourished by direct precipitation, which usually contributes extremely small to

the water surface.

A．Y

B．N

C．NG

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更多“Rivers are nourished by direct…”相关的问题

第1题

RiversBy original usage, a river is flowing water in a channel with defined banks. Modern

Rivers

By original usage, a river is flowing water in a channel with defined banks. Modern usage includes rivers that are multichanneled, intermittent, or ephemeral in flow and channels that are practically bankless. The concept of channeled surface flow, however, remains central to the definition.

Rivers are nourished by precipitation, by direct overland runoff, through springs and seepages, or from meltwater at the edges of snowfields and glaciers. The contribution of direct precipitation on the water surface is usually minute, except where much of a catchment area is occupied by lakes. River water losses result from seepage into shallow or deep aquifers (沙石含水层) and particularly from evaporation. The difference between the water input and loss sustains surface discharge or streamflow. The amount of water in river system at any time is but a tiny fraction of the Earth's total water; 97 percent of all water is contained in the oceans and about three-quarters of fresh water is stored as land ice; nearly all the remainder occurs as groundwater. Lakes hold less than 0.5 percent of all fresh water, soil moisture accounts for about 0.05 percent, and water in river channels for roughly half as much, 0.025 percent, which represents only about one four-thousandth of the Earth's total fresh water.

Water is constantly cycled through the systems of land ice, soil, lakes, groundwater (in part), and river channels, however. The discharge of rivers to the oceans delivers to these systems the equivalent of the water vapour that is blown overland and then consequently precipitated as rain or snow, e., some 7 percent of mean annual precipitation on the globe and 30 percent of precipitation on land areas.

The historical record includes marked shifts in the appreciation of rivers, numerous conflicts in use demand, and an intensification of use that has rapidly accelerated during the 20th century. External freight trade became concentrated in estuarine ports (河口港) rather than in inland ports when ocean-going vessels increased in size.

Demand on open-channel water increases as population and per capita water use increase and as underground water supplies fall short. Irrigation use constitutes a comparatively large percentage of the total supply.

Present-day demands on rivers as power sources range from the floating of timber, through the use of water for cooling, to hydroelectric generation. Logging in forests relies primarily on flotation during the season of melt-water high flow. Large power plants and other industrial facilities are often located along rivers, which supply the enormous quantities of water needed for cooling purposes. Manufacturers of petrochemicals, steel, and woolen cloth also make large demands. Hydroelectric power generation was introduced more than 100 years ago, but the majority of the existing installations have been built since 1950.

The ever-increasing exploitation of rivers has given rise to a variety of problems. Extensive commercial navigation of rivers has resulted in much artificial improvement of natural channels, including increasing the depth of the channels to permit passage of larger vessels. In some cases, this lowering of the river bottom has caused the water table of the surrounding area to drop, which has adversely affected agriculture. Also, canalization, with its extensive system of locks and navigation dams, often seriously disrupts riverine ecosystems.

An even more far-reaching problem is that of water pollution. Pesticides and herbicides are now employed in large quantities throughout much of the world. The widespread use of such biocides and the universal nature of water makes it inevitable that the toxic chemicals would appear as stream pollutants. Biocides can contaminate water, especially of slow flowing rivers, and are responsible for a number of fish kills each year.

In ag

A．Y

B．N

C．NG

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第2题

Your failure in the business is ___________ unless you change your way of management a

A.nourished

B.average

C.permissive

D.inescapable

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