| {{B}}Water{{/B}} The second most important constituent(构成成份)of the biosphere(生物圈)is liquid water. This can only exist in a very narrow range of temperatures, since water freezes at 0℃ and boils at 100℃. Life as we know it would only be possible on the surface of a planet which had temperatures somewhere within this narrow range. The earth's supply of water probably remains fairly constant in quantity. The total quantity of water is not known very accurately,but it is about enough to cover the surface of the globe to a depth of about two and three-quarter kilometers. Most of it is in the form of the salt water of the oceans -- about 97 percent. The rest is fresh,but three-quarters of this is in the form of ice at the Poles and on mountains, and cannot be used by living systems until melted. Of the remaining fraction, which is somewhat less than one percent of the whole,there is 10～20 times as much stored underground water as there is actually on the surface. There is also a tiny, but extremely important fraction of the water supply which is present as waer vapour in the atmosphere. Water vapour in the atmosphere is the channel through which the whole water circulation(循环)of the biosphere has to pass. Water evaporated(蒸发)from the surface of the oceans,from lakes and rivers and from moist(潮湿的)earth is added to it. From it the water comes out again as rain or snow,falling on either the sea or the land. There is,as might be expected,a more intensive evaporation per unit area over the sea and oceans than over the land, but there is more rainfall over the land than over the oceans and the balance is restored by the runoff from the land in the form of rivers. |