Hydrologic cycle and the climate system

In ancient Greek, dtp-tg (atmis water vapors) denotes the transfer of water (by evaporation) from the telluric form (hydrosphere) into drip (aer), the water vapor of the atmosphere and its return as precipitation to the earth, (with water) one of the two lower elements. We know from Herodotus that in the fifth century B.C. this theory was known and accepted and described by Hippocrates. 

The water or hydrological cycle is the continuous circulation of water throughout the earth and between its systems. At various stages, water moves through the atmosphere, the biosphere, and the geosphere, in each case performing functions 

The water cycle is driven by processes that force the movement of water from one reservoir to another. Evaporation from the oceans and land is the primary source of atmospheric water vapor (Fig. 2.36). Water vapor is transported, often over long distances (which characterize the type of air masses), and eventually condenses into cloud droplets, which in turn develop into precipitation. Globally, there is as much water precipitated as is evaporated, but over land precipitation exceeds evaporation and over oceans evaporation exceeds precipitation (Fig. 2.35). The excess precipitation over land equals the flow of surface and groundwater from continents to the oceans. Flowing water also erodes, transports and deposits sediments in rivers, lakes and oceans, affecting the quality of water. 

This natural cycling of water is now perturbed by human activities. Together with changing vegetation patterns due to land-use management (see Chapter 2.6.5.3), these factors complicate the prediction of the consequences of climate change on the global water cycle. 

The already mentioned feedback between the water and carbon cycle (see also Fig. 2.31) is not only given by the essential role of water in sustaining all forms of life. Observational evidence indicates that transpiration rates of plants are high at the same time as CO2 fixing by the plants, and hence CO2 flux from the atmosphere to the plant canopy is large. When an environment is humid, plants grow more rapidly, draw more CO2 from the atmosphere, and release more water to the atmosphere. 

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