Water Balance


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Water balance

An accounting of the inflow to, outflow from, and storage in a hydrologic unit.

Water Balance

 

the quantitative characterization of all forms of income and expenditure of water in the atmosphere and on the earth’s surface and in individual areas. It is the quantitative expression of the evaporation-condensation cycle on earth. The calculation of the components of the water balance is widely used in hydrology and in meteorology for the study of the hydrological regime.

The water balance of landmasses is characterized by the following basic relationship: the quantity of atmospheric precipitation falling on a given territory is equal to the sum of the evaporation, drainage, and accumulation (or loss) of water in the upper layers of the lithosphere. For the entire surface of the earth over the period of a year and for a long-term average for its individual territories, the last term of the water balance is equal to zero.

In calculating the water balance over a specific portion of the earth’s surface, the water loss from precipitation is equal to the sum of the evaporation from the earth’s surface and the gain or loss of water vapor as a result of its horizontal trans-port by air currents and as a result of changes in the quantity of water in the atmosphere. (The last term is usually small in comparison to the other terms of the water balance.) The water balance of the atmosphere essentially depends on the conditions of atmospheric circulation of moisture, in the course of which water vapor is carried from one region to another. Although evaporation from the land surface ac-counts for approximately two-thirds of the precipitation on the continents, most of the precipitation falling on land is actually formed by water vapor carried by air currents from the ocean. This relationship is explained by the fact that atmospheric circulation carries off a considerable portion of the water vapor formed by local evaporation from the continents to the oceans. The difference between evaporation and precipitation over the continents is equal to the difference between the gain and loss of water vapor in the atmosphere

Table 1. Water balance off the earth
 Volume
(km2)
Layer
(mm)
Landmass with drainage into the ocean
precipitation...............
102,000870
river discharge...............38150320
evaporation...............65000550
Landmass without drainage into the ocean
precipitation...............
7,400230
evaporation...............7,400230
The world’s oceans
precipitation...............
411,0001140
influx of river water...............38,150100
evaporation...............448,0001,240
The earth as a whole
precipitation...............
520,0001020
evaporation...............520,0001020

over the continents and, at the same time, is equal to the magnitude of river discharge from the continents into the oceans.

If the water equilibrium is examined for the earth’s surface as a whole, as well as for the entire atmosphere, the total precipitation for a year is equal to the amount of the evap-oration, which according to current data is equal to 100 cm per year (see Table 1).

The components of the water balance are precipitation, evaporation, and drainage; they are measured at meteorological and hydrological stations. Methods of estimation are widely used in determining evaporation, drainage, and other terms of the water balance.

REFERENCES

Velikanov, M. A. Gidrologiia sushi, 5th ed. Leningrad, 1964.
Drozdov, O. A., and A. S. Grigor’eva. Vlagooborot v atmosfere. Leningrad, 1963.

M. I. BUDYKO

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