The bulk of the water is contained in the oceans, which contain about 30000 times more water than atmosphere and continents combined, cover approximately 70% of the Earth's surface and are on average 3800 m deep. The remainder of the water is found in ice caps & glaciers (3%), groundwater (1%), and rivers and lakes (0.01%). The latter two reservoirs constitute the terrestrial fresh water supply. Thus, only a very small fraction of the overall water supply is suitable and available for human use. The water transfer between these reservoirs is accomplished by the processes of evaporation, transpiration, precipitation, and flow of water (following gravity).
Two minor reservoirs by size, the atmosphere (0.001%) and the biosphere (the totality of living tissue; 0.0001%) play a role that is not reflected by their size. As we shall see below, cycling of water through the atmosphere is an important factor for energy transfer in the atmosphere, and in Chapters 7 through 11 we will look at the important role that organisms play in driving the evolution of the climate and atmosphere evolution through the production and consumption of the greenhouse gases carbon dioxide and methane.
Every year about 30000 to 40000 cubic kilometers (a cube 30-35 km in size) of water move across the surface of the continents to the oceans, profoundly shaping the surface of the continents. Evaporation by the sun effects lifting of water into the atmosphere, and the counterforce to this process is gravity that forces rain to fall back on the earth and causes water move back to the oceans in streams (river systems), on the way eroding soils, cutting canyons, and transporting solids (silt, sand, clay) and dissolved salts to the oceans. The transfer of water vapor from the oceans to the atmosphere goes hand in hand with the transfer of tremendous amounts of thermal energy to the atmosphere and is very important for atmospheric circulation (see below). For this reason atmospheric circulation and winds can be considered part of the hydrologic cycle.
In cold climates water in the form of glaciers (glacial systems) moves downslope due to gravity, erodes bedrock by abrasive action, and transports sediment. Glaciers may either enter the sea directly, or melt at the end and add their water to the continental surface runoff.
Water may also seep into the porous portions of the earth's surface and is then called groundwater (groundwater systems). It may dissolve rocks in the subsurface and create karst systems (caves in limestone) and sinkholes.
Along the shores of oceans (shoreline systems) water that is agitated by waves (wind, atmospheric circulation) erodes and moves sediment and forms beaches, sandbars, lagoons, and related coastal features.
Winds (eolian system) can carry considerable amounts of sand and dust and can profoundly shape landscapes (sand seas of deserts) as well as human destiny (the highly fertile loess plains of southern China, without them it would be much more difficult to feed a population of 1 billion people).
Thus, the hydrologic cycle affects a broad range of landscape-shaping dynamic systems. We therefore consider river systems, glacial systems, groundwater systems, shoreline systems, and eolian systems as subsystems of the hydrologic system. In terms of overall importance for the shaping of the Earth surface, river systems are the primary force. Glaciers, although very powerful in modifying the land surface, were only abundant for brief intervals in Earth history.
Rain on the Cumberlands
Through the stricken air
Through the buttonwood balls suspended on twig strings
The rain-fog circles and swallows
Climbs the shallow plates of bark, the grooved trunks
And wind pellets go hurrying though the leaves
Down, down the rain
Down in plunging streaks of watered gray.
Rain in the beechwood trees
Rain upon the wanderer whose breath lies cold upon the mountainside
Caught up with broken horns within the nettled grass
With hooves relinquished on the breathing stones eatened with rain strokes
Rain has buried her seed and her dead
Through the buttonwood balls suspended on twig strings
The rain-fog circles and swallows
Climbs the shallow plates of bark, the grooved trunks
And wind pellets go hurrying though the leaves
Down, down the rain
Down in plunging streaks of watered gray.
Rain in the beechwood trees
Rain upon the wanderer whose breath lies cold upon the mountainside
Caught up with broken horns within the nettled grass
With hooves relinquished on the breathing stones eatened with rain strokes
Rain has buried her seed and her dead
