V. D. Komarov and E. G. Popov,
Hydrometeorological Research Centre of the USSR, Moscow, U S S R
SUMMARY: The influence of snow cover on river water regime and hydrological cycle of the Earth is considered in the paper. Much attention is given to the methods and results of snow cover study on the territory of the USSR.
LA COUVERTURE D E NEIGE C O M M E
ELEMENT
D U BILAN D ’ E A U SUR LE TERRITOIRE D E L’U.R.S.S.RESUME : Le rapport décrit l’influence de la couche de neige sur le régime hydraulique des rivières et sur le cycle hydrologique dans le monde entier. O n accorde une attention particulière aux méthodes et aux résultats de l’étude de la couchc de neige sur le territoire de l’URSS.
LA CAPA D E NfEVE EN EL TERRITORIO D E LA URSS C O M O ELEMENTO DEL BALANCE H I D R O L ~ G I C O
RESUMEN: En este documento se estudia la influencia de la capa de nieve en el régimen de aguas de los ríos y en el ciclo hidrológico de la tierra. Se presta gran atención a los métodos y a los resultados de los estudios relativos a la capa de nieve en el territorio de la URSS.
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In many regions of the Earth a considerable part of precipitation occurs as snowfall and during the winter season snow cover exists. The area annually covered with snow in Asia, North America and Europe amounts approximately to 60-70 per cent, Besides, snow covers Greenland, the Antarctics, many islands in the Arctic zone and higher mountains in the southern hemisphere. From this alone one can see the significance of the snow cover as an element of the World Water Balance.
S n o w cover plays an important role in the river flow regime. Besides, it has an essential influence on heat and moisture exchange between the continents and atmosphere, soil water regime, ground water and on a number of other natural processes and phenomena, as well as on the national economy.
It follows Froin the above that the study of snow cover is of great scientific and practical significance. Further development of this problem will undoubtedly call for the expansion of international co-operation, and first of all, within the framework of the International Hydrological Decade.
T h e climatic conditions within the territory of the USSR vary considerably; while some areas of the country (not mentioning the higher mountains terrains) are covered with snow for inore than half a year, in other regions snow cover exists only for less than a month.
But over a inajor part of the lowland territory of the USSR snow cover accumulates and remains for a period of 2 to 7 months [15].
For this period of time the average precipitation in the northern parts of the country amounts to 40-50 per cent of the annual volume, in central parts the corresponding figure is about 30 per cent and in the south, about 15 per cent. In plain regions snowmelting coinpletes in spring and in mountains, during spring and summer seasons with the excep- tion of higher mountains where the annual storage of snow exceeds the amount of snow- melting and evaporation which results in the formation of glaciers. In the USSR, glaciers are mainly located in the mountainous regions of Central Asia and the Caucasus.
In Russia, snow cover observations have been conducted since 1892. However at the beginning only snow depth was measured at meteorological stations, with the use of 2 or 3 permanent snow-stakes. The measurements of snow density were commenced in 1903.
Since 1936 snow surveys have been made at snow courses of 1-2 kin long, with measuring snow depth and density [lo]. Early development of snow measurements in Russia were stjmulated by the investigations made by A.I. Voeikov, the famous Russian geographer and climatologist w h o showed the great scientific and practical significance of snow cover studies [2, 31.
S n o w surveys in mountains are being made a1 lhe courses passing through river valleys, chiefly, LIP to the elevation of 3 500-3 O00 m. In the inountains or Soviet Central Asia snow surveys have been conducted from 1926 and in the Caucasus from 1931.
Airsurvey and satellite surveillance are also used n o w for mountain snow studies. There are all reasons to believe that these techniques will play an ever inore significant role in the studies of snow cover in mountains.
At present, snow surveys are being inade systematically at 3 400 observation stations over the European part of lhe USSR and at 3 900 stations over the Asiatic part.
The inethod or estimating the snowpack water equivalent in plains by means of air- survey of natural soil gainina-radiation was developed in thc Soviet Union in 1963. The nicasurements were mndc at thc same specified course bcfore snow cover was formed and
Stio~ couer o n tite territory of tlie USSR LIS u water balance elemrni
at a fixed moment in winter-tilne after snow cover was formed. The relation between these two values of gamma-radiation being known, it is possible to estimate for a given moment of time the mean value of the snow cover water equivalent within the limits of a stretch 100-300 in wide, with the altitude of a flight equal to 50-100 m and its length equal to that of a course. The minimum length of the course is equal to the distance which a plane covers for 30 sec. The measurerneiits of natural soil gamma-radiation make it possible to estimate the water equivalent of snow cover up to 300 mm [6].
The new method of snow survey is of a rather high accuracy and takes but little time to measure snow cover over large territories. The standard error of the measured snowpack water equivalent at a JO kin course ranges from 5 to 7 rnm or less than 10 per cent.
During the last few years the new method of snow cover survey has been systematically used in the basins of a number OF rivers (the Volga, the D o n , the Dnieper and others) on the area of more than I inln kin'.
For the purposes of water balance, the annual maximum water equivalent of snowpack is of the greatest interest. In the European part of the USSR, long-time mean values of the annual maximum snowpack water equivalent ranges from 140-160 mm in the north to 30-40 m m in the south.
But for certain years, the annual inaxiinum water equivalent of snow cover amounts here correspondingly to 260-280 mm and 80-100 m m and in the Urals to 500 mm [S, 10, 12, 151. The distribution of the mean value of the maximum snowpack water equivalent over the Asiatic territory of the USSR is rather complicated in connexion with climatic conditions and topography variations. In the West-Siberian lowland, this value amounts to 200-240 m m in the north and 50-70 mm in the south; in East Siberia it ranges from 60 to 120 inni. In snowy winters the annual maximum value of snowpack water equivalent reaches here 200-300 m m and higher.
In mountainous terrains the distribution of seasonal snowpack water equivalents is more uneven but, naturally, their value almost always increases with elevation. In some basins of Central Asia at the elevations of 3 000-3 500 m they range from 1 O00 to I 500 mm, in the Caucasus, from 400 to 500 mm and in Siberia they amount to 500-700 mm.
It is believed that data obtained through snow survey in mountains somewhat under- estiniate the real seasonal snowpack water equivalents.
As it has already been stated, snow cover spreads over a major part of the territory of the USSR and remains there for a long period of time. Snow causes changes in the underlying surface properties and, which has already been mentioned, in heat and moisture exchange between land and atmosphere. Tn particularly, evaporation changes considerably. It can be seen from observations and computations that the value of eva- poration from snow cover in plains over the USSR for winter season ranges from IO to 25 min [7, 91. It is very little, taking into consideration that snow cover exists for a long time and annual evaporation from land is above 300 nim
[?l.
The amount of evaporation from snow cover in mountains is larger than that in plains. In the mountains of Central Asia at' the elevation of 2 000-3 O00 m it amounts to 30-50 mm for a winter season (according to data collected by single measurements) [5].It is evident from the above that since snow lies for a long time and covers a major part o€ the land, it reduces considerably the annual evaporation from continents.
it should be noted that the study of evaporation froni snow cover is complicated by the fact that up to now no techniques have been available to achieve sufficient accuracy of measurements of snow cover surface temperature and roughiiess. It is probable that in the near future noncontact methods of determination of snow cover surface teinperature will be widely practised, based on the measurements of its infrared radiation.
The duration of snowmelt period is rather brief almost everywhere, with the cxception of higher mountains, and this causes intensive inflow of snowmelt to rivers and further on to seas and oceans. In plain areas of the USSR and in the mountains at elevations u p to 800-1 O00 inthe snowmelt period lasts from 5 to 70 days in open country and from 20 to
Y, D. Komarov und E. G. M o p o u
30 in forested areas. A n increase in snowmelt rtinofï is caused by soil freezing in winter.
in the west and south of the East-European plain, soil freezing penetrates to the depth of 30-40 c m and 60-100 c m on the remaining territory of the USSR. Full thawing of soil occurs after snowmelting. In Siberia, permafrost is spread over a considerable area and thawing occurs only in the upper layers of soil.
The amount of evaporation from snow cover in plain regions during snowmelting is not large; it averages about 1 0 m m . Thus, in plains almost all amount of snowmelt is divided between runoff and infiltration. But the relation of these water balance compo- nents varies considerably in different areas, influenced by different factors. The main ones are the following: type of soil, soil moisture deficit in root layers, depth of freezing before start of snowmelting, topography, percentage of swampy and forested areas. In northern and central regions 70-80 percent of snowmelt and rainfall inflow for the period of snowmelting is spent on runoff, and in southern regions the corresponding percentage is 20 to 30. In mountains the runoff coefficient in the period of snowmelting varies from 0.65 to 0.75.
Jn plain regions the snowmelt period is brief, but snowmelting occurs simultaneously over a very large area. For example, in the East-European plain snowmelt occurs simul- taneously on the area of more than 1 million square kilometres [7]. This also proves the effect of snow cover on heat and moisture exchange between land and atmosphere.
Since a major portion of snowmelt is not absorbed by soil, snowmelting causes high spring snowmelt floods (“ polovodje” is the corresponding Russian term). The duration of spring snowmelt floods depends on the areal extent of the basin, a number of hydro- meteorological factors and morphometric properties of the river system. O n lowland rivers the basin areas of which are several thousands square kilometres, the spring snowmelt floods period usually Jasts from 20 to 40 days. The runoff of lowland rivers in the U S S R for this period, accounts for 50 to 80 per cent of the total annual runoff. The annual runoff of many mountain rivers contains not a lesser portion of snowmelt. The highest values of water level on the majority of rivers are registered in the period of spring snowmelt floods.
Thus, the accumulation of snowpack and its rather quick melting have a great influence on the regime and annual distribution of river runoff almost all over the territory of the USSR and cause a little amount of runoff in winter and a very large one in spring, and in mountains, in spring and summer.
Snowmelt is the main, or rather substantial, component of river runoff not only over a large part of Asia and Europe, but of North America too [il].
The estimated total annual inflow of snowmelt to seas and oceans in the northern hemisphere amounts approximately to 6 O00 km3.
In case the antecedent hydrometeorological conditions cause moisture deficit in a root layer, snowmelting provides for substantial soil moistening and replenishment of ground water resources. The latter is confirmed by the annual variation of the ground water level in many geographical zones of the USSR. Soil moistening and replenishment of ground water resources through snowmelting are caused by the uneveness of soil freezing and melting and by slight freezing of soil in forested areas. The above runoff coefficients give an idea of the amount of snowmelt spent on soil moistening and replenishment of ground water sources for the period of snowmelting.
All the above serves to prove the conclusion that snow cover is not only an important element of annual and seasonal water balances, but also a significant factor of heat and water exchange between land and atmosphere and of the whole hydrological cycle of the Earth. It follows from this that the studys of snow cover on a global scale is very urgent.
N o w , we will dwell on the usage of snow cover current data for the purposes of hydro- logical forecasts and snow-retention.
The relation between river regime and snowmelting offers considerable possibilities for long-term forecasting of snowmelt flow which forms more than half thc total annual runoff on the territory of the USSR. For plain rivers, these forecasting methods are based
Snow cover on the ferritory of the USSR u s (I rouler holnnce element
on water balance relationships which express the dependency of the runoff depth, for the period of spring snowmelt floods in a concrete basin, on the maximum water equi- valent of snowpack, the acmount of precipitation for the period of snowmelting and absorbability characteristics of the basin. The latter is mainly determined by the antece- dent moistening of the basin and the soil freezing depth. These relationships can be deter- mined for each basin on the basis of data collected at the hydrometeorological stations for a time series. As regards the mountain rivers, the main factors of snowmelt flow here are the water storage accumulated in snowpack for snow-fall and the amount of precipi- tation for the period of snowmelting.
Long-term Forecasts of runoff for the period of spring snowmelt floods are of great importance for water resources management; they allow a more efficient use of the water resources. These forecasts have been prepared for many years. To improve the existing methods of forecasting, it is necessary not only to develop the appropriate techniques for snow cover measurements, particularly in mountainous terrains, but also to carry investigations of the absorbability of river basins.
Considering the possibilities offered by up-to-date techniques, action could be taken on a wide scale for the regulation of areal-snow distribution with the purpose of increasing the moistening of agricultural areas and feeding the ground water through snowmelting.
There are also some other aspects of snow retention, and much attention is given to their development in arid zones of the USSR.
Thus, the study of snow cover opens the way for improving the use of fresh water resources in many countries.
The primary problems in further studies of snow cover for the purposes of the water balance of land, in our opinion, should be the following :
(a) The development of more accurate and usable methods of measurements of the snowpack water equivalent over large plain territories, including forested areas, and at all elevations in mountains; and among them, methods of the determination of the altitude of snow line and snow-areal extent.
(b) Development of more accurate methods of the calculation of snowmelt intensity and evaporation from snow cover.
(e) Study of snow cover as a factor of heat and moisture exchange between land and atmosphere.
The study of the replenishment of soil and ground water resources in the period of snowmelting is also a very important problem. The potential improvement of accuracy of long-range snowmelt runoff forecasts depends on the solution of this problem.
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1 1 . L’VOVICH, M.I. (1945) : Elementy fodiiogo rczhima rek zemnogo shara. (Wdter regime elements of rivers on the Earth), Moscow.
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