水循环是多环节的自然过程，全球性的水循环涉及蒸发、大气水分输送、地表水和地下水循环以及多种形式的水量贮蓄降水、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。蒸发是水循环中最重要的环节之一。由蒸发产生的水汽进入大气并随大气活动而运动。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。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。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。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。全球的大气水分交换的周期为10天。在水循环中水汽输送是最活跃的环节之一。径流是一个地区（流域）的降水量与蒸发量的差值。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。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。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。它们是中国东南、华南、华南、东北及西北内陆的水汽来源。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。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。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。流域径流是陆地水循环中最重要的现象之一。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。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。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。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。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。在这些循环水中，约有4%成为陆地降水，这其中的约三分之二又从陆地蒸发掉了。但总算蒸发量小于降水量，这才形成了地面径流。The water cycle is the process of natural process， the water cycleinvolves evaporation， atmospheric moisture transmission， surface water and groundwater circulation and water storage in various forms of precipitation， evaporation and runoff is the three most important aspects of the global water cycle， water cycle path consisting of these three sizedetermines the global water balance， also determines the total amount of water resources in a Evaporation is one of the most important link in the water Produced by evaporation of water into the atmosphere and with the atmospheric activity and Water vapor in the atmosphere mainly come from the sea， but also from the mainland part of the surface Water vapor in the atmosphere of the cycle is the evaporation condensation precipitation and evaporation process go round and begin The ocean water can be transported to the land above the condensation precipitation， precipitation， known as external water vapor; direct condensation of water vapor over the precipitation，water vapor within said The ratio of total amount ofprecipitation and precipitation of the external water vapor to the water cycle called the Global atmospheric water exchange cycle for10 Water vapor transport in the water cycle is one of the most active Runoff is a regional (basin) difference between precipitation The average ocean water balance equation: evaporation = precipitation - runoff; the average land water balance equation is:precipitation = runoff + However， both ocean and land， the geographical distribution of precipitation and evaporation is uneven， this difference is most obvious differences between different Atmospheric water circulation path Chinese of Pacific Ocean， India， the South China Sea， Okhotsk Sea and inland water circulation system， They are Southern China， Southern China， China southeast， northeast and northwest inland water vapor And the prevailing westerly winds and cyclones eastward to the northwest of the Atlantic a small amount of water The land (or a watershed) occurred in the water cycle is a complex process of precipitation and evaporation of the surface and Exchange between the terrestrial atmospheric precipitation，surface runoff and underground runoff and water The runoffis one of the most important phenomena in the water The movement of groundwater is mainly related with the molecular force，gravity and thermal properties， the gap， the movement is Water upward into the atmosphere through the vegetation and soil evaporation， transpiration; infiltration through thedownward movement can recharge groundwater; horizontal direction through the river water can also become a part of Groundwater reserves are high， but after Months and years pass or even thousands of years into storage， a long period of water exchange，circulation is extremely Conversion between surface water and groundwater is one of the main contents of the research on waterrelations， it is also an important issue of modern water According to estimates， the total global water cycle is about 496 '1012 cubic meters / year， less than 4/10000 of the total storage capacity of the In the circulating water， about 4% to become land precipitation，of which about 2/3 of the evaporated from the But at least less than the precipitation evaporation， which formed the surface
