Water in various components of our planet is in a state of continuous circulation from one compartment to another. The cycle is nearly in a state of dynamic equilibrium with the inputs in each compartment balancing the output. Major pathways of the cycle involve an exchange between earth’s surface (both land and oceans) and the atmosphere through evaporation and precipitation.

Energy needed for the operation of the cycle is derived from sun, which vaporises water from oceans and the land surface. Oceans contribute 4.49 x I017 kg of water per year to the atmosphere through evaporation. However, they receive only 4.12 x 1017 kg per year as precipitation. From land surface only 0.71 x io17 kg of water evaporates annually but it receives 1.05xl017 kg per year.

Of this water about 0.37x 1017 kg goes back to oceans as surface run-offs through streams, rivers and there . ‘Butteries to equalize the deficit. The rest of the water is retained on the land surface. It goes to the biosphere, replenishes ground water stock or may get locked up in ice deposits for considerable period of time.

As a matter of fact, the cycle is not so simple as described above. Some atmospheric vapours go to the snow-clad mountains and glaciers being directly deposited as ice. Ice-flakes disappear even at freezing temperatures due to sublimation. A considerable amount of water is used by living organisms in terrestrial systems for protoplast synthesis while a lot of water is contributed by plants through transpiration. The exact quantitative significance of these processes in the hydrological cycle is not well understood.