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Thenaturalwatercycleisamulti-partprocess,theglobalwatercycleinvolvesevaporation,atmosphericmoisturetransport,surfacewaterandgroundwatercirculationandstorageofwaterstorageinvariousforms.降水、蒸发和径流是水循环过程的三个最主要环节，这三者构成的水循环途径决定着全球的水量平衡，也决定着一个地区的水资源总量。Precipitation,evaporationandrunoffarethethreemostimportantaspectsofthewatercycle,watercycleconsistingofthesethreewaystodeterminetheglobalwaterbalance,butalsodeterminesthetotalwaterresourcesofanarea.蒸发是水循环中最重要的环节之一。Evaporationisthewatercycleinoneofthemostimportantaspects.由蒸发产生的水汽进入大气并随大气活动而运动。Generatedbytheevaporationofwatervaporintotheatmosphereandwiththeatmosphericactivityandexercise.大气中的水汽主要来自海洋，一部分还来自大陆表面的蒸散发。Watervaporintheatmospheremainlyfromtheoceans,butalsofromthemainlandpartofthesurfaceevapotranspiration.大气层中水汽的循环是蒸发－凝结-降水-蒸发的周而复始的过程。Thecycleofwatervaporintheatmosphereisevaporation-condensation-precipitation-evaporationcycleprocess.海洋上空的水汽可被输送到陆地上空凝结降水，称为外来水汽降水；大陆上空的水汽直接凝结降水，称内部水汽降水。Watervaporovertheoceanscanbetransportedtothecondensationofprecipitationoverland,knownasexternalwatervaporprecipitation;directcondensationofwatervaporoverthecontinentofprecipitation,watervaporwithinsaidprecipitation.一地总降水量与外来水汽降水量的比值称该地的水分循环系数。Anexternalwatervaportothetotalprecipitationandtheratioofprecipitationtothewatercyclecalledthecoefficient.全球的大气水分交换的周期为10天。Globalatmosphericwaterexchangecycleis10days.在水循环中水汽输送是最活跃的环节之一。Watervaportransportinthewatercycleisoneofthemostactivepart.径流是一个地区（流域）的降水量与蒸发量的差值。Runoffisaregional(basin)andthedifferencebetweenprecipitationandevaporation.多年平均的大洋水量平衡方程为：蒸发量=降水量+径流量；多年平均的陆地水量平衡方程是：降水量=径流量+蒸发量。Theaverageyearsofoceanwaterbalanceequation:evaporation=precipitation+runoff;annualaveragewaterbalanceequationistheland:precipitation=runoff+evaporation.但是，无论是海洋还是陆地，降水量和蒸发量的地理分布都是不均匀的，这种差异最明显的就是不同纬度的差异。However,bothseaandland,precipitationandevaporationaretheunevengeographicaldistribution,thisdifferenceismostobviousdifferencesbetweendifferentlatitudes.中国的大气水分循环路径有太平洋、印度洋、南海、鄂霍茨克海及内陆等5个水分循环系统。China'satmosphericwatercyclepathtothePacific,IndianOcean,SouthChinaSea,OkhotskSeaandinlandwatercirculationsystem,etc.5.它们是中国东南、误南、华南、东北及西北内陆的水汽来源。TheyareChina'ssoutheast,misuseSouth,South,NortheastandNorthwestinlandwatervaporsource.西北内陆地区还有盛行西风和气旋东移而来的少量大西洋水汽。NorthwestprevailingwesterlywindsandinlandareasaswellasmallnumberofAtlanticcycloneseastwardfromwatervapor.陆地上（或一个流域内）发生的水循环是降水－地表和地下径流－蒸发的复杂过程。Land(orawatershed)occurredinthewatercycleisprecipitation-runoffofsurfaceandunderground-thecomplexprocessofevaporation.陆地上的大气降水、地表径流及地下径流之间的交换又称三水转化。Land-basedatmosphericprecipitation,surfacerunoffandundergroundrunoff,alsoknownastheexchangebetweentheSanshuitransformation.流域径流是陆地水循环中最重要的现象之一。Runoffisthewatercycleisoneofthemostimportantphenomena.地下水的运动主要与分子力、热力、重力及空隙性质有关，其运动是多维的。Themovementofgroundwatermainlymolecularforce,heat,gravityandnatureofthegap,themovementismultidimensional.通过土壤和植被的蒸发、蒸腾向上运动成为大气水分；通过入渗向下运动可补给地下水；通过水平方向运动又可成为河湖水的一部分。Soilandvegetationbyevaporationandtranspirationofwaterupwardintotheatmosphere;canrechargebydownwardmovementofgroundwaterinfiltration;horizontaldirectionthroughtheriverwatercanalsobecomeapartof.地下水储量虽然很大，但却是经过长年累月甚至上千年蓄集而成的，水量交换周期很长，循环极其缓慢。Groundwaterreserves,whilesignificant,butitistheresultoflongyearsandeventhousandsbuildandsettogetherforalongperiodofwaterexchange,circulationisextremelyslow.地下水和地表水的相互转换是研究水量关系的主要内容之一，也是现代水资源计算的重要问题。Betweengroundwaterandsurfacewaterconversionisthemaincontentoftherelationshipbetweenwater,isoneoftheimportantissuesofmoderncomputingresources.据估计，全球总的循环水量约为496′1012立方米/年，不到全球总储水量的万分之四。Itisestimatedthatthetotalglobalwatercycleisabout496'1012cubicmeters/year,lessthanthetotalstoragecapacityoftheworld'sfourtenthousandths.在这些循环水中，约有22.4%成为陆地降水，这其中的约三分之二又从陆地蒸发掉了。Inthesecyclesthewater,about22.4%tobecomelandprecipitation,ofwhichabouttwothirdsoftheevaporatedfromtheland.但总算蒸发量小于降水量，这才形成了地面径流。Butatleastlessthantheprecipitationevaporation,thisformedthesurfacerunoff.