Water balance

WATER BALANCE (EN: balance of water; DE: Wasserbilanz; FR: bilan d'eau; ES: balance de agua; RU: водный баланс) is the quantitative expression of the water cycle within the atmosphere, hydrosphere, on the Earth as a whole, or within its individual regions. The water balance characterizes all the types of the income and expenditure of the water in the liquid, vaporous, and solid (the water ice) forms.

The annual water balance of the Earth (according to M. I. Lvovich)
The elements of the balance The volume, thousand cubic kilometres The layer, millimetres
The peripheral part of the dry land (million square kilometres)
The atmospheric precipitations 106 910
The riverine runoff 41 350
The underground runoff into the ocean (bypassing the rivers) 2.5 22
The evaporation 62.5 538
The enclosed part of the dry land (million square kilometres)
The atmospheric precipitations 7.5* 238
The evaporation 7.5 238
The world ocean (million square kilometres)
The atmospheric precipitations 411.6 1140
The inflow of the riverine waters 41 114
The inflow of the underground waters into the ocean (bypassing the rivers) 2.5 6
The evaporation 455.1 1260
The Earth's globe (million square kilometres)
The atmospheric precipitations 525.1 1030
The evaporation 525.1 1030
*Including 0.83 thousand cubic kilometres, or 26 millimetres, of the riverine runoff

For the dry land of the Earth's globe (which has the runoff into the ocean), the evaporation is equal to the quantity of the falling precipitations minus the riverine runoff and underground runoff into the ocean (which is bypassing the riverine network), and for the world ocean, the evaporation is equal to the atmospheric precipitations, riverine runoff, and inflow of the underground waters from the mainlands, while within the enclosed ("without the runoff") regions of the dry land, and on the entire Earth as a whole, the evaporation corresponds to the precipitations (see the Table). For the balance evaluation of the water resources on the large territories, which are renewable during the process of the water cycle, and for the forecast of the changes of the water balance, there are used the equations

R = U + S; W = P - S = E + U; P = U + S + E, where

U is the underground runoff from the zone of the active water exchange of the Earth's crust into the rivers;

S is the superficial (of the freshet type) runoff;

W is the gross moisturing of the territory (in general, the resources of the soil moisture);

R is the riverine runoff;

P are the atmospheric precipitations;

E is the evaporation.

The water balance of the lakes is determined according to the inflow of superficial waters, underground waters, and atmospheric precipitations, which are incoming into the water area, and also of the expenditure portions, namely, of the riverine runoff from the lakes, and of the evaporation from the surface of these lakes. In this case, there is taken into account the change of the water level of the lakes during the time interval, which is explored. The analogous scheme is used for the calculation of the water balance of the water reservoirs.

The income portion of the balance of the underground waters is determined according to the infiltration of atmospheric precipitations, the inflow from other aquifers, the filtering of the waters from the rivers, lakes, canals, and water reservoirs; the expenditure portion is determined according to the underground runoff from the specified aquifer. During the calculation of the water balance of the limited place of the layer of the underground waters, there are also taken into account the inflow from the part of this layer, which is situated above the specified place, and the expenditure of the water into the part of this layer, which is situated below the specified place. For the ground waters, which are situated near to the surface, the condensation of the moisture is taken into account as the income, and the evaporation from the aquifer is taken into account as the expenditure. The water balance is subject to the natural perennial and seasonal fluctuations, and also to the changes, which emerge as the result of the activity of the human being.

The usage of the riverine, lacustrine, and underground waters for the irrigation, and water supply of the population and industry, changes the ratio of the elements of the water balance. The agrotechnical and agroforestry activities increase the infiltration capability of the soil, which fact decreases the superficial runoff from the fields, increases the resources of the soil moisture, its evaporation and transpiration, and also somewhat enforces the feeding of the ground waters. The development of the cities, the construction of the industrial enterprises and roads, decrease the infiltration properties of the soil, increase the superficial runoff, and weaken the feeding of the underground waters. The level of the ground waters changes substantially under the influence of the amelioration activities, of the pumping of the underground waters from the underground mines and open-pit mines.

The calculation of the water balance is conducted for the forecast of the water inflows into the underground mines and open-pit mines, for the design and development of the activities for the control of the water regime at the mining-extracting enterprises. The elements of the water balance are measured at the hydro-meteorological and hydro-geological stations.