Contrary to what you might think, the science of hydrology has nothing to do with fork-lift trucks or fluid-operated braking systems. That is the province of the hydraulic engineer, the science of hydraulics.
Hydrology, on the other hand, is the science of water, and in particular its movement in relation to land, where it comes from, where it goes to, and how it gets there. Rain and snow are the raw materials of the hydrologist; he or she follows the water through rivers and lakes, and through the processes of evaporation and condensation, until it returns to the sea again whence it originally came. This is the hydrological cycle.
There are about 1,500 million cubic kilometres of water on this planet, enough to fill a giant cubic tank with sides 700 miles long. About 94 per cent of this water is contained in the world's oceans; the remaining 6 per cent comprises ice sheets and glaciers, rivers and lakes, and water on or underneath the surface of the land. A very small proportion of it - about 14,000 cubic kilometres - is present in the atmosphere in the form of clouds or invisible water vapour.
The driving force behind the hydrological cycle is the radiant energy of the sun. Heating of the sea causes evaporation, by means of which water is changed from the liquid into the gaseous state, and absorbed as part of our atmosphere. In due course, this water vapour is converted back into water again through the process of condensation, and falls back to earth as precipitation, mainly in the form of rain or snow.
Some of the snow accumulates in the polar regions or on high mountains. Here it is often consolidated into ice, and may remain captive in this state for a very long time. In the more temperate regions, rainfall may be intercepted by vegetation, from which it evaporates again, or it may reach the ground, over which it may flow along the surface into streams and rivers.
The rest of the water falling on a land surface infiltrates the soil, percolating downwards until it reaches the water table, the level where the soil becomes saturated. Water collected like this, deep down below the surface of the Earth, is called groundwater; it often flows underground, reappearing here and there in the form of springs, and joins the surface run-off flowing into rivers and streams.
The by now much-travelled liquid may be held up in lakes for a time before completing its journey, but it finally flows back into the ocean, ready for the whole process to begin again.