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World Water Balunce
World water balance (General report)
Professor M. I. Lvovitch, Institute of Geography of the AS of the USSR, Moscow
RESUME : Cette communicatjon expose la notion de bilan hydrique mondial en gnral, ainsi que l'importance qu'elle revt notamment pour accrotre notre connaissance des processus hydrolo- giques et pour valuer les ressources en eau du globe terrestre. Elle contient un historique des diffrentes valuations faites par plusieurs chercheurs pour diverses parties du cycle hydrologique, ainsi que des tables correspondantes. L'auteur a procd i une tude critique dcs diverses mthodes utilises pour arriver ces valuations, dgageant ainsi les lments pour lesquels il y a lieu de poursuivre les recherches.
BILAN HYDRIQUE MONDIAL (RAPPORT GENERAL) SUMMARY: The paper presents the concept of world water balance in general and its importance, in particular for the advancement of knowledge of hydrological processes occurring on the earth and for estimating world water resources. A historical review of various estimates made by different scientists for difcrent parts of the hydrological cycle is given with relevant tables. The author has made a critical study of various methods used for such estimates thus separating those elements which need further research.
BALANCE HIDROL~CICO MUNDIAL (tlljurtne generai), RESUMEN: En este documento se presenta el concepto del balance hidrolgico mundial en generai y, en especial, su importancia para el progreso del conocimiento de los procesos hidrolgicos que se producen en la tierra, as como para calcular los recursos hidrulicos mundialcs. En las tablas correspondientes se hace un examen retrospectivo de las diversas estimaciones efectuadas por diversos cientficos para diferentes partes del ciclo hidrolgico. EI autor ha realizado un estudio crtico de los diversos mtodos utilizados para estos cilculos, separando, por lo tanto, aquellos elementos quc necesitan de ulterior investigacin.
M. I. Li~ooitcli
World luafel- balance should be considered as a quantitative expression of a great process called the iouler cycle which takes place on the earth. The two ternis characterize different sides of the same process. That is why the differentiation in the two concepts which some investigators resort to cannot be considered well grounded. In the past, the hydrological science was limited by qualitative notions of water cycle but when, due to the progress in water resources use, the hydrometric and atmospheric precipitation data became availa- ble, it became possible to comprehend this process qualitatively. Water cycle, as it is known, connects together all parts of the hydrosphere: ocean and
water on land, i.e. surface, soil and ground waters; as well as water in other components of nature, e.g. climate, soils, geological structure and biosphere. In the process of water cycle erosion occurs; relief is formed; dissolved matters change place; large quantities of heat are transferred and a most important biological process-transpiration-occurs. One of the links in the water cycle is the important economic link wherein water is
used to meet mans needs. The term ioater balance cannot be considered literally as balancing, i.e. equality
between income and output elements of water balance. Jt is obvious that in the studying of water balance, it is indispensable to observe the law of matter conservation. The water cycle for the whole world is not watertight and, therefore, a perfect balance is not possible, either on a global basis or on a regional basis. The study of world water balance, as a whole as well as separate continents and oceans,
depends on the state of knowledge of two major water balance elements-precipitation and river runoff. It cannot be said that present knowledge of these elements is quite perfect, Nevertheless, available data, even if insufficient, together with scientific theory make it possible to solve a whole set of water balance problems as well as to estimate world water balance, provided suitable methods for investigating are applied. The importance of studying world water balance lies in the fact that the development of
knowledge of any processes occurring on the earth, including the water cycle, is a contri- bution to general scientific and ecmcmic progress. It plays an important part in estimating the world water resources, which are continuously being used within the process of water cycle. The level of our knowledge on the water cycle and world water balance also indicated the level of our understanding of world water resources and, in this way, partly suggests the right ways of their use.
DEVELOPMENT OF THE COMPREHENSION OF WORLD WATER B AL AN CE^
Already in accient times, a certain ncticn of water cycle existed in a very crude form which was far frcm reality. A better qualitative picture of water cycle appeared during the Renaissance (Leonardo da Vinci) period. Significant contributions to promote such knowledgp, was made in the XVII-XVIIIth centuries by persons like P. Perrault, M.V. Lomonosov, Lh. Buffon, P.I. Ritchkov, J.Dalton and others. But a lack of information n tfe fcrm c discerned data made their contributions less effective. The present estimations of world water balance were initiated by E.Ya. Brikner of
USSR (1905). The method of estimating world water balance suggested by him has
I. The Rusiaii terni umler circiilrrtio/z ~orrcsponds to the Anglo-American term miter cycle
2. The section is prcpared in collaboration with Prof. A A . Sokolov, Statc Hydrological Institutc. or *h.vdrnlu,yicul c.vc.Ie.
World Wciter Boltitice
probably a greater importance than Lhe actual estimate of balance. The essence of the nxthod lies in the following equations:
- For the peripheral part of the land feeding the Ocean with stream water: E,, = P,, - R,; - for closed areas without runoff to the Ocean: Eo = P,; - for the Ocean: E,,, = P,,, + R,,; - for the Earth: E = E, +E,,, ; where: E,, evaporation frem the peripheral part of the land; P, precipitation on the peripheral part of the land: R, river runoff to the Ocean; E, and P, evaporation aiid precipitation of closed areas and E,,, and P,,, E E,
evaporation and precipitation of the Ocean; evaporation from the surface of the land; evaporation from the whole of the land.
The next step was made by R. Fritzsche (1906), who was the first to obtain values of river runoff close to the present ones. G. Wst (1920), improved upon it by adding runoff from Polar glaciers to river runoff. Both results of G. Wusts estimation (1920 and 1936) of the world water balance are characterized by low values of precipitation and evapora- tion of the Ocean. Attention was drawn to this by W. Meinardus (1934) who probably prepared the first reliable map of atmospheric precipitation for the Ocean.
TABLE 1. World Water Balance from the data of different authors (cm)
Author, year Precipitation Evaporation Total river - runoff from
Land Ocean Total Land Ocean Total land
E. Brikner (1005) R. Frizchc 11906) G. Wst (I 920) W. Meinardus ( 1934) G. Wust (1936) M.I. Lvovitch (1945) M.I. Lvovitch (1964) M.I. Budiko (1956) F. Albrecht (1961) R. Nace (1968) M. 1. Lvovitch (1969)
85 15 75.2 67 66.4 12 13 10
98 94 61.3 98 91 54.5 74 74.3 50.7 114 100.2 42 82.3 11.1 41.6 114 101.5 41.6 114 107 48.7 1C2.4 92.8 44.6 104.6 88.5 82.5 41.0 113.1 1C2.0 48.5
I05 1 o5 84.3 124.2 92.5 I24 I24 112.7 114.4 91 124.0
94 91 74.3 100.2 17.7 101.5 107.0 92.8
17 20.5 24.9 25 24.8 24.4 24.9 25.4 22.4 28.5 25.0
A principal distinction of our world water balance investigations lies in the fact that they are based on the first map of world river runoff (Lvovitch, 1954) and on the fairly detailed map of precipitation on the land, prepared by O.A. Drosdov in 1939. M y subsequent world water balance estimations are based on making river runoff maps (fig. i) and data for the runoff of Polar glaciers more accurate. M. I. Budiko (1965) and later F. Albrecht (1961) in their research estimated evaporation
from heat balance, and river runoff fro in the difference between precipitation and evaporation. Attcntion is drawn to the fact that world water balance elements have not substantially
M. I. Luouitch
changed throughout thc history of research in this field. This shows a great insight of our predecessors in solving the problem on the basis of very poor information. The results of the various estimations of separate water balance elements are basically
dependent on the accuracy of estimation for precipitation, especially in the Ocean. Thus, beginning from W . Meinardus (1934) precipitation on the Ocean area, (excluding G. Wst's estimation) is assumed to be more than 100 cm. Precipitation on the surface of the Ocean should be assumed not less than I14 c m provided corrections for gage records are applied. The methods of correction have been developed by the Iiydrometeorological Service of USSR (M. J. Budiko's paper for the present Symposium). River-runoff value, as indicated above, has remained almost unchanged for the past
fifty years. This does not imply the lack of progress in the study of this world water balance element. For instance, runoff data obtained from the first map of world river-runoff (1945) differ from the results of Fritzsche Wst's calculations (1906, 1920) for selected 10"- latitude zones by 90% to 116%. For the world, as a whole, the results of both calculations have agreed. Here the law of great numbers reveals itself, i.e. compensation for errors for large areas. However, the understanding of river-runoff distribution over the area has substantially improve