地球上的水,是在不停地運(yùn)動(dòng)著的.它無處不在,通過蒸發(fā)、冷凝、降水等連續(xù)不斷地循環(huán).水的循環(huán)過程具體可以分為以下三個(gè)步驟：
第一步是蒸發(fā)和蒸騰的水分子進(jìn)入大氣.
吸收太陽輻射熱后,水分子從海洋、河流、湖泊、潮濕土壤和其他潮濕表面蒸發(fā)到大氣中去；生長在地表的植物,通過莖葉的蒸發(fā)將水?dāng)U散到大氣中,植物的這種蒸發(fā)作用通常又稱為蒸騰.據(jù)估計(jì),在一個(gè)生長季中0.4公頃的谷物幾乎就可以蒸騰200萬升的水,等于同等面積內(nèi)43厘米深的水層.通過蒸發(fā)和蒸騰的水,水質(zhì)都得到了純化,是清潔水.
第二步是以降水形式返回大地.
水分子進(jìn)入大氣后,變?yōu)樗S氣流運(yùn)動(dòng),在適當(dāng)條件下,遇冷凝結(jié)形成降水,以雨或雪的形式降落到地面.降水不但給地球帶來淡水,養(yǎng)育了千千萬萬的生命,同時(shí),還能凈化空氣,把一些天然的和人為的污物從大氣中洗去.
降水是陸地水資源的根本來源.我國多年來平均年降水量為632毫米,而全球陸地平均年降水量是834毫米.
第三步是重新返回蒸發(fā)點(diǎn).
當(dāng)降水到達(dá)地面時(shí),一部分滲入地下,補(bǔ)給地下水；一部分從地表流掉,補(bǔ)給河流.地表的流水,即徑流可以帶走泥粒,導(dǎo)致侵蝕；也可以帶走細(xì)菌、灰塵和化肥、農(nóng)藥等,因而徑流常常是被污染的.最后千流歸大海,水又回到海洋以及河流、湖泊等蒸發(fā)點(diǎn).這就是地球上的水循環(huán).
推動(dòng)這種循環(huán)的永恒動(dòng)力是太陽輻射.進(jìn)入到地球上的太陽能約有23％消耗于海洋表面和陸地表面的蒸發(fā)上,當(dāng)水汽凝結(jié)時(shí),這些能量又重新釋放出來.就全年平均情況來看,大約從北緯40度到南緯30度是一個(gè)廣大的輻射過剩區(qū)域,而極地周圍的高緯度地區(qū)是輻射虧損區(qū).海陸之間,在不同的季節(jié)有著不同的虧損和盈余.只有把熱量從盈余的地區(qū)向虧損的地區(qū)輸送,才能達(dá)到全球的能量平衡.而水分循環(huán)是這種能量輸送的主要途徑之一.水在海洋中能夠形成洋流,水又能夠以氣液相變的形式來大量地儲存和輸送能量.這種能量輸送保持了全球的能量平衡,使得輻射的虧損區(qū)不至于太冷,輻射過剩區(qū)不至于太熱,為生物提供了一種適宜的生活環(huán)境.
有時(shí)水循環(huán)會(huì)出現(xiàn)一些較特殊的情況.在高緯度和高海拔區(qū),自大氣層降下的不是水而是雪.落在極地區(qū)或山地的雪積久可成冰,水因此得到保存,算是退出循環(huán),退出時(shí)間一般為幾十年、幾百年或幾千年.因此,冰雪的固結(jié)與消融,影響著參與水循環(huán)的水的總量,進(jìn)而影響全球海面變化.
南極冰蓋和格陵蘭冰蓋是世界上最大的冰庫.如果全部融化,海洋的水位就會(huì)上升大約60米,這意味著各大洲的沿海地區(qū)、包括許多世界級大城市都將被淹沒,海平面將達(dá)到紐約曼哈頓摩天大樓的20層樓那么高.
水分循環(huán)的過程是非常復(fù)雜的.除了這種海陸之間的水分循環(huán)外,海洋有自己的洋流等水圈內(nèi)部的水循環(huán)；大氣圈里有隨著大氣環(huán)流進(jìn)行的大氣內(nèi)部水循環(huán)；大氣圈與陸地之間,大氣圈與洋面之間,有著水汽形成降水,降落的水分又被蒸發(fā)的直接循環(huán)；巖石圈上存在著地表水與地下水之間的轉(zhuǎn)換與循環(huán)；生物體內(nèi)也有著生物水的循環(huán)等.
水分循環(huán)把地球上所有的水,無論是大氣、海洋、地表還是生物圈中的水,都納入了一個(gè)綜合的自然系統(tǒng)中,水圈內(nèi)所有的水都參與水的循環(huán).像人體中,從飲水到水排出體外只要幾個(gè)小時(shí)；大氣中的水,從蒸發(fā)進(jìn)入大氣,到形成降水離開大氣,平均來說,完成一次循環(huán)要8～10天；世界大洋中的水,如果都要蒸發(fā)進(jìn)入大氣,完成一次水分循環(huán)的過程,需要3000～4000年.
水分循環(huán)系統(tǒng)是一個(gè)水的自然凈化系統(tǒng).水不斷地從潮濕的表面蒸發(fā),或者從植物表面蒸騰,當(dāng)水蒸氣進(jìn)入大氣時(shí),大部分雜質(zhì)留下來.雨水到了地面經(jīng)過沙石的過濾和沉淀,成為潔凈的水.在這個(gè)凈化過程中,海洋起著巨大的作用.太陽、海洋和大氣像一個(gè)巨大的蒸餾裝置,時(shí)刻不停地運(yùn)轉(zhuǎn)著.
由于水分循環(huán)的存在,使得水成為地球上最活躍的物質(zhì),使全球的水量和熱量得到均衡調(diào)節(jié).正是由于這種年復(fù)一年、日復(fù)一日永不停息的水分循環(huán),才使得大氣圈氣象萬千,使得地球表面千姿百態(tài),生機(jī)盎然.假如水分循環(huán)停止,將再也看不到電閃雷鳴、雨雪霜雹；再也沒有晴、雨、陰、云的天氣變化；再也看不到江、河、湖、沼；當(dāng)然更不會(huì)有森林、草原；動(dòng)物與人類也將不存在.
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On Earth's water, is in does not stop the movement. Its there is noplace in, through does not evaporate, the condensation, theprecipitation and so on continuously unceasingly circulates. The watercyclic process specifically may divide into following three steps:
First step is evaporates with the transpiration hydrone enters theatmosphere.
After absorbs the solar radiating heat, hydrone from sea, rivers,lake, moist soil and other moist surface evaporation to atmosphere in;The growth in the surface plant, proliferates through stem leaf'sevaporating the water to the atmosphere in, plant's this kind ofevaporation usually is called the transpiration. It is estimated, in agrowth season 0.4 hectares grain nearly may the transpiration 200myrialiters water, be equal to in the same level area 43 centimeterdeep water level. Through evaporated with the transpiration water, thewater quality all obtained the purification, was the clean water.
Second step is returns to the earth by the precipitation form.
After the hydrone enters the atmosphere, becomes the water vapor alongwith the air current movement, under the suitable condition, meets thecondensation knot to form the precipitation, descends the ground bythe rain or the snow form. The precipitation not only brings the freshwater to the Earth, has raised the great amount life, simultaneously,but also can purify the air, washes off some natural and theartificial contamination from the atmosphere.
The precipitation is the land water resources radical origin. Ourcountry for many years the average year precipitation has been 632millimeters, but the global land average year precipitation is 834millimeters.
Third step is rereturns 蒸發(fā)點(diǎn).
When the precipitation arrives the ground, part of permeatingunderground, military supplies ground water; A part falls from theplace sheet current, military supplies rivers. The surface runningwater, namely the runoff may carry off the putty grain, causes thecorrosion; Also may carry off the bacterium, the dust and the chemicalfertilizer, the agricultural chemicals and so on, thus the runofffrequently is polluted. Finally thousand class turn over to the sea,the water to return to the sea as well as the rivers, the lake and soon 蒸發(fā)點(diǎn). This is on Earth's water cycle.
Impels this kind of circulation the eternal power is the solarradiation. Enters the solar energy approximately has 23% consumptionto the Earth on to the sea surface and on land surface evaporating,when the water vapor congeals, these energies rerelease. Looked on thewhole year average situation that, probably from north latitude 40 tosouth latitudes 30 is general radiations too much regions, but aroundthe polar high latitude area radiates the loss area. Between the sealand, has the different loss and the earnings in the different season.Only has the quantity of heat from the earnings area to the loss localtransportation, can achieve global the energy balance. But thehydrologic cycle is one of this kind of energy transport main ways.The water can form the ocean current in the sea, the water can be madthe liquid phase changes the form comes massively the storage and thetransportation energy. This kind of energy transport maintained theglobal energy balance, causes the radiation the loss area not as fortoo coldly, the radiation area as for too is not too much hot, hasprovided the living conditions for the biology which one kind issuitable.
Sometimes the water cycle can have some more special situation. In thehigh latitude and the high elevation area, the arrogant aerospherelowers is not Shui Er is a snow. Falls accumulates in extremely localor the mountainous region snow may become the ice, the water thereforeobtains the preservation, is withdraws from the circulation, thewithdrawal time generally is several dozens year, several hundred yearor several millenniums. Therefore, snow and ice solidifying withmelts, is affecting the participation water cycle water totalquantity, then affects the global sea level change.
The south pole ice cap and the Greenland ice cap are in the world thebiggest ice bunker. If completely melts, the sea water level can risethe about 60 meters, this meant each big continent the coastal area,is all submerged including many world-classs big city, the sea levelachieved the New York Manhattan skyscraper 20 buildings are that high.
The hydrologic cycle process is extremely complex. Besides this kindof sea land between hydrologic cycle, the sea has ocean currenthydrosphere interior the and so on own water cycle; In the aerospherehas the atmospheric internal water circulation which carries on alongwith the atmospheric circulation; Between the aerosphere and the land,the aerosphere with 洋面 between, has the water vapor to form theprecipitation, descends the moisture content the direct circulationwhich evaporates; On the lithosphere has between the surface water andthe ground water transformation and the circulation; Biological invivo also has the biological water the circulation and so on.
Hydrologic cycle Earth on all water, regardless of is the atmosphere,the sea, the surface or in the biosphere water, all has bought intoline with in a comprehensive natural system, in the hydrosphere allwater all participate in the water the circulation. In the picturehuman body, discharges in vitro from the potable water to the water solong as several hours; In the atmospheric water, from evaporatesenters the atmosphere, to forms the precipitation to leave theatmosphere, on average, completes a time of circulation to want 8 ~ 10days; In world ocean water, if all must evaporate enters theatmosphere, completes a time of hydrologic cycle the process, needs3000 ~ in 4000.
The hydrologic cycle system is a water nature purification system.Water unceasingly from moist surface evaporation, or from plantsurface transpiration, when the steam enters the atmosphere, themajority of impurities stay behind. The rain water arrived the groundafter grit filtering with the precipitation, becomes the pure water.In this purification process, the sea is playing the huge role. Thesun, the sea and the atmosphere like a huge distilling apparatus, thetime does not stop is revolving.
As a result of the hydrologic cycle existence, causes Shuicheng forthe Earth on the most active matter, causes global the water volumeand the quantity of heat is under the proportional-plus-floatingcontrol. Is precisely because this kind never ceases year after year,day after day the hydrologic cycle, only then causes the aerosphere tobe spectacular, causes the earth's surface differ in thousands ofways, vitality abundant. If the hydrologic cycle stops, again also theblind lightning flash will be thunderous, the sleet frost hail; Againalso does not have clearly, the rain, to be cloudy, the cloud weatherchange; Again also the blind river, the river, the lake, moor;Certainly cannot have the forest, the prairie; The animal and thehumanity also will not exist.
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其實(shí)也就是 蒸發(fā)>降雨>地表徑流>流入大海>蒸發(fā)
Evaporates > Rains > Surface runoff > Inflow sea >Evaporates