Hydrological Cycle

Water continuously between the atmosphere and the earth’s surface (lithosphere) and known as the hydrological cycle. The energy for energy for driving the  cycle and thus ensuring a constant supply of fresh-water on land comes from sun. Solar heat evaporates water from the ocean which is the great reservoir of water. A lesser amount of water is also evaporated from the surface of the land and from plants, a process known as evapotranspiration.

All these vaporized water forms clouds which moved by winds, may pass over land where they are called enough to precipitated water soaks into grounds, some runs off the surface into streams and goes directly back to the sea. The ground water is returned  to the surface by springs, by pumps from roots to leaves. Water inevitably ends up back in the sea, but it may become incorporated into the bodies of several different organisms one after another.

Water of oceans, atmosphere and lands moves in a great series of continuous interchanges of both geographic position and physical state. A particular molecule of water might, if we could trace it continuously, travel through any one of a number of possible circuits involving alternatively the water vapor state and the liquid or solid state.

Global water in Storage (Hydrological Cycle)

The total global water resource is stored in the gaseous, liquid and solid state. Water of the oceans constitutes over 97 percent of the total, as we would expect. Next comes water in storage in lakes and streams is a very small quantity indeed. But it is surface water together with the very small quantity of soil water, that sustains life of the lands.

Some environmentalist consider that fresh surface water will prove to be the limiting factor in the capacity of our planet to support the rapidly expanding human population. The quantity of water vapor held in the atmosphere is also very small- only about 10 times greater than that held in streams: but this atmospheric moisture is the source of all fresh water of the lands.

Global water balance and Climate Change

Long term changes in water- balance quantities are associated with atmospheric environmental changes. For example, atmospheric cooling on global scale would bring a reduction of atmospheric water-vapor storage. This change would in form reduce precipitation and runoff generally.

But at the  same time, a greater proportion of that precipitation would be in the form of snow, so the storage in ice accumulations would rise and the storage in ocean waters would fail. These changes describe the changing water balance associated with onset of an ice age, or glaciation-a major environmental change already experiences by our planet at least many times in the past two million years.

Infiltration and Runoff (Hydrological Cycle)

Most soil surfaces in their natural states are capable of absorbing the water from light or moderate rains. This absorption process is known as infiltration. Most soils have natural passageways between poorly fitting soil particles, as well as larger openings, such as earth cracks resulting from soil drying, brings of worms and animals, cavities left from decay of plants roots, or openings made by growth and melting of frost crystals.

Evaporation and Transpiration

Between the periods of rain, water held the soil is gradually given up by a twofold drying process. First, direct evaporation into the open air occurs at the soil surface and progresses downward. Air also enters the soil freely and may actually be forced alternatively in and out of the soil by atmospheric pressure changes. Even if the soil did not breathe in this way, there would be a slow diffusion of water vapor surface ward through the open soil pores.

Second, plants draw the soil water into their systems through vast networks of tiny rootlets. This water after being carried upward through the trunk and branches into the leaves, is discharges through leaf pores into the atmosphere in the form of water vapor. The process is called transpiration.

Soil Moisture and Ground Water

The role of soil moisture in the water budget is highly variable; it also depends on the capacity of a soil to hold water grained by infiltration. The amount of water a saturated soil can retain against the pull of gravity is its field capacity. It varies mainly with soil texture but also with structure, organic matter content, and the depth of the soil. Fine clays have high field capacities, whereas sandy soil hold little moisture. For agricultural purposes, it is useful to consider the minimum amount of soil water that is necessary in the root one to allow extraction by plants.

When soil zone reaches field capacity, excess soil water percolates downward under the force of gravity through a subsurface zone of aeration to a zone of saturation known as ground water. Depths to the upper surface of ground water, that is the water table, can be measured in wells and are used to estimate ground water supplies. Ground water may also be treated as part of the surplus in the water budget, and when other values in the budget are known as the amount transported to ground water can be calculated.