Water is essentially required for different life forms such as plants animals. Birds etc. For cell building and other purposes. The main source for the water is ocean. The water from the oceans is evaporated, clouds are formed and carried away by wind and they precipitate. The water received from precipitation is lost to the ocean back by different processes such as run- off evaporation from soil, lakes and ponds, streams, etc evapotranspiration from plants the water which is absorbed in the ground is also lost by direct or indirect way to the ocean, for example, some water which is absorbed in ground is utilized by plants and then evaporated, the ground waer which is absorbed in ground is utilized by plants and then evaporated. The ground water flows to the streams and the stretch finally lost in the oceans etc. Thus, we find that there is a constant circulation of water from oceans to the air and back again to the oceans. This process has not end beginning and therefore it is termed as hydrological cycle or water cycle. The hydrological cycle can be briefed by the following equation.
P = ET + DST + S
The total amount of water present on the earth surface remains constant but undergoes continuous transformation from water vapour to liquid. This equation is also called as water balance equation. Where P is the water received by precipitation, ET is loss by evapotranspiration, dst is the gain on loss by storage in the soil and S is the surplus run-off of water, from this mathematical relation, we can find out the value of other elements.