In this article we will discuss about the hydrologic cycle with the help of a suitable diagram.
Most of the earth’s water sources, such as rivers, lakes, oceans and underground sources, etc. get their supplies from rains, while the rain water itself is the evaporation from these sources. Water is lost to the atmosphere as vapour from earth, which is then precipitated back in the form of rain, snow, hail, dew, sleet or frost, etc.
This evaporation and precipitation continues for ever, and thereby, a balance is maintained between the two. This process is known as Hydrologic Cycle. It can be represented graphically as shown in Fig. 15.13.
Hydrologic equation is expressed as follows:
P = R + E …(15.4)
P = Precipitation,
R = Run-off, and
E = Evaporation.
It includes all the water that falls from atmosphere to earth surface.
Precipitation is of two types:
(i) Liquid precipitation (rain fall).
(ii) Solid precipitation (snow, hail).
Run-Off and Surface Run-Off:
Run-off and surface run-off are two different terms and should not be confused. Run-off includes all the water flowing in the stream channel at any given section. While the surface run-off includes only the water that reaches the stream channel without first percolating down to the water table.
Run-off can, therefore, also be named as Discharge or Stream flow. Rainfall duration, its intensity and a real distribution influence the rate and volume of run-off.
Transfer of water from liquid to vapour state is called evaporation.
The process by which water is released to the atmosphere by the plants is called transpiration.