Water is indispensable to any kind of terrestrial life. It spreads a 3/4 of our planet's surface. The water is present in the atmosphere as well as in the terrestrial magma.
The three existing forms of the water on our planet are : solid ( ice, snow, hail), liquid (seas, oceans, lakes, rivers, clouds) or gas ( water vapour).
As a main component of our environment, the water is constantly moving in different succession of transformation:
The different status of water in volume (km ) are distributed as follows:
The amount of precipitation on the ocean is estimated to an average year of 870 mm whereas the evaporation is 970 mm and over the continents the average year precipitation is 670 mm for 420 mm of evaporation and 250 mm of runoff.
The water transfer in the form of liquid, solid and gas - hydrological cycle - comprises different processus:
The evapotranspiration is depending on a big number of parameters as the temperature, wind, humidity, radiation, etc.
The amount of water which not returns to the atmosphere moves on different forms:
The water cycle pursuits : the water
bodies and lands are humidifying the mass of the air transported by wind
through the evaporation and evapotranspiration. The condensation forms
the clouds and these pass again to precipitations on the continents, islands
and, of course, on the oceans.
Water balance in a drainage basin
To establish the water balance for
an area and a given time period means to suppose that we are able to asses
both the input and the output water volumes in the different drainage basins
composing the concerned area .
The drainage basin ( catchment area, watershed ) of a river is the surface area where the precipitation is drained by the river . The outlet of a drainage basin is the opening through which the water arising from him flows out .
The water balance on a drainage basin might be expressed by the equation:
dR = P - E - Q - I - U
where dR, P, E, Q, I and U are the corresponding water volumes :
dR: the variation of the surface and underground water resources
P : precipitation diminished by interception
E : effective evapotranspiration
Q : river runoff
I : water loss by deep infiltration
U : used water
The terms of the water balance equation are affected by different climatic and geografical parameters related to the concerned area of study . Thus, the temperature is one of the main factors of the evaporative capacity of the atmosphere, the relief is conditionning the cloud capacity to precipitate and the vegetation cover has influence in the interception and transpiration process;
The stay of the water in different forms of the hydrological cycle is very inconstant. The mean water stay in the atmosphere is of about one week , of several days in the streams or rivers, of about thirty centuries in the oceans and from centuries to the millenaries in the bigs aquifers .