The Water Balance of the Caspian Sea

The Caspian Sea is the world's largest inland lake, a closed reservoir with a surface area of just under 400,000 km2, about equal to the areas of Germany and Austria combined. The sea is fed by several rivers, and about 80% of the runoff is from the Volga River, the largest river in the European part of Russia and in the whole of Europe. Throughout this century the flows of the major rivers have been measured by monitoring stations; the annual mean runoff into the Sea is close t o 300 km3, which corresponds to a depth of about 75 cm at the present surface area. The annual mean precipitation over the surface is about 20 cm. A partial natural regulator of the level of the Sea is the bay of Kara-Bogaz-Go1 (KBG) on its eastern coast. Water flows into the bay via a narrow channel, and is then evaporated intensively in the surrounding desert. The volume of water flow into the KBG depends on the level of the Sea but a t the high levels observed in the first quarter of this century it could consume up to 30 km3 annually, or up to 7.5 cm of the Sea level.

Another debit part of the Sea water balance is evaporation from the sur- face. Using meteorological information from 17 monitoring stations along the shore and on few islands (see Figure I ) , the evaporation from the Sea can be estimated if the sea surface temperature (and, therefore, the absolute humid- ity a t the surface), the humidity a t the level of the measuring instruments, and the wind speed are known. After some extrapolation and interpolation procedures it can be extended over the whole Sea surface (see Golitsyn and Panin, 1989; and Panin et al., 1992) to obtain the annual mean depth of water that is evaporated. Between 1900 and 1975, it was about 90 cm per year. Of course, there are interannual variations in all components of the water balance, leading to fluctuations of the water level.

Knowing the river runoff, the area of the Sea as a function of its level, and with estimates of precipitation (from a limited number of monitoring

Figure 1. Map of the Caspian Sea and the locations of meteorological monitoring stations. T h e numbers in ellipses denote the significant values a t the 95% level of confidence for the linear trend in annual mean wind speeds in m/s/lO years (see text and Figure 4).

stations) and evaporation, one can compare the estimated sea level changes with those actually observed. Figure 2 presents the calculated changes in annual mean sea level (AHest) versus the observed changes (AHobs) for 1930-1986. There is a natural spread of points, but the regression is close t o unity with a correlation coefficient ^T = 0.87. The agreement is better when we calculate the accumulated changes for several consecutive years: for the 1930s the calculated change was 170 cm versus the observed 175 cm, and for

Figure 2. Observed versus calculated values of the changes in the Caspian Sea level, 1930-1986, based on the climatic water balance. The correlation coefficient r = 0.87. Source: Golitsyn and Panin, 1989.

the period 1977-1986 the calculated change was 103 cm versus the observed 117 cm. These calculations support the argument that the changes in sea level have been caused by changes in the climatic water balance, leaving little room for other causes such as geological movements of the sea floor, or the release of water from, or seepage into, the underlying rocks.

T h e instrumental measurements of the Caspian Sea level started in 1837 in Baku and until recently there were eight posts along the shoreline and on some islands in the former Soviet Union. The level also undergoes seasonal fluctuations with a maximum in summer ,and a minimum in winter, with a range of some 40 cm. The record of annual mean values since 1837 is presented in Figure 3; the d a t a from the Baku station are given here because other stations began operations only during this century. During the first 100 years, the level was within -25 and -26 m (minus means that the level is below the global mean sea level, msl), and a dramatic drop occurred in the 1930s. T h e minimum of -29 m was reached in 1977, and since then there has been a steady rise. In 1992 the level was close t o -27 m.

Water is taken from major rivers feeding the sea for irrigation and other purposes; in the 1970s and 1980s estimated abstraction rate was around 40 km3Iyr and less in preceding years. Estimates made by the Institute of Water Problems in Moscow (and supplied t o the author by its Directors, Professor M.G. Khublaryan) indicate that if the water had not been used for

1850 1 900 1950

Figure 3. The annual mean Caspian Sea level for 1837-1992 in Baku (1);

the dashed curve (2) shows the level reconstructed for 1940-1992 for the case if water had not been taken from the rivers feeding the Sea.

these purposes the level of the sea would have started t o rise much earlier, and would be 1.5 m higher than it is now.

Our analysis (Golitsyn and Panin, 1989) has revealed that during peri- ods when fluctuations of the level are greater than the general trend, there is no statistically significant correlation between runoff and estimated evap- oration, while in the period of the most recent changes there has been a strong negative correlation between runoff and evaporation: in the 1930s the reduced runoff was accompanied by increased evaporation and in 1978- 1987 the increased runoff was observed together with reduced evaporation.

About 40% of the rise of 1978-1987 can be explained by increased runoff, 40-45% by increased precipitation (which for the period was 20 mm higher than the average over the preceding 50 years), and 15-20% by the dam that cut off the bay of KBG in 1980.

The construction of the dam closing the channel to the bay is an excellent example of how damaging a large-scale project can be t o the environment without a full understanding of the regional climate and the lack of its fore- cast. In the 1970s and 1980s the Ministry for Water Management of the former Soviet Union always favored grand projects such as big canals, diver- sion of rivers, etc. The fall of the Caspian Sea level in the 1970s (and before) presented a big opportunity to save it from further decreases. As a first step, the dam t o cut off the Kara-Bogaz-Go1 Bay was proposed, although a t the low levels of the late 1970s it was consuming only 1.5-2 cm of the Sea level.

The authorities and scientists of the Republic of Turkmenistan, in whose ter- ritory the KBG Bay lay, were against the dam. They argued that without the flow of water into the bay it would dry up in a few years and thus would kill the nearby chemical industry which used the brine of the evaporating water as a raw material. They also feared that the dam would cause dust and salt storms, raising material from the dried-up bottom and transporting it hundreds of kilometers away. But their arguments were ignored by the central government and the dam was built rather hastily in 1980. (In June 1993, the independent Turkmenistan blew up the dam.) During 1980-1992 the presence of the dam contributed about 40 cm t o the observed rise of the Caspian Sea level by 2 m.

In a n analysis of the causes of the reduced evaporation from the sea Panin et al. (1991) unexpectedly found that the wind speed over most mete- orological stations had decreased since 1960. D a t a from 12 of the analyzed 17 monitoring stations showed statistically significant negative trends since about 1960 (these are shown in the solid ellipses in Figure 1; the two num- bers in broken ellipses on the eastern shore present small negative trends and three numbers in squares on the western shore show small positive trends.

The rate of evaporation is proportional to the differences in humidity just above the surface and at the measurement height (usually a t 2 m), and the wind speed. Neither humidity difference nor the sea surface temperature revealed any significant trend, but the wind speed did. Panin and his col- leagues (Panin, 1992) then analyzed the wind speed d a t a from several dozen meteorological stations in the Volga basin and found that a majority also revealed statistically significant negative trends in wind speed for the last few decades. This finding should be of interest t o energy researchers because the wind speed determines the heat exchange between the atmosphere and buildings, ground surface, etc., and calls for a thorough analysis of the wind field changes. Figure 4 gives an idea of the wind speed changes, based on d a t a from all 17 monitoring stations around the Caspian Sea, averaged over the sea area. Figure 4 clearly shows that the moduli of the wind speeds were positive for the first half of the period and negative for the second half.

Supporting evidence for declining evaporation has been presented by Professor I.A. Shiklomanov, Director of the State Hydrological Institute in St. Petersburg. The Institute has obtained d a t a on direct measurements of evaporation using so-called evaporimeters, dishpans with precise level mea- surements instruments. Figure 5 presents just three cases out of many show- ing statistically significant reductions in evaporation from stations on the upper and middle reaches of the Volga River, and on the nearby Don River, about 200 km west of Volgograd. Both pieces of evidence - the weakening

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Figure 4. Time dependence of the changes in annual mean wind speed (A,) normalized by its variance (o,, recorded a t stations on the Caspian Sea for 1960-1987. (a) Values for all 17 stations; (b) for 12 stations a t northern and southwestern parts of the Sea with considerable negative trends; and (c) for five stations with small negative or positive trends (numbers in broken ellipses and squares, respectively, in Figure 1.)

wind speed and the reduced evaporation - show good correlation, although thorough analyses of cyclone tracks and their intensities, radiation balance, etc., are required before these data can be fully understood.

Paleogeographic reconstructions of the last few thousand years show that the level of the Caspian Sea has fluctuated from -36 to -22 m, although there are large uncertainties associated with both the dating and the amplitude of these changes (Kaplin, 1992). Fluctuations of the level in the present century may not bear any relation t o global warming due t o the increase in greenhouse gas concentrations, although both the fall of the 1930s and the

Figure 5. Evaporation from three reservoirs on the Volga and Don Rivers during the warm season, 1963-1990. (a) Tsymlianskoye reservoir, 48ON; (b) Kuybyshevskoye reservoir, 52ON; and (c) Rybinskoye reservoir, 58ON.

present increase seem to have occurred more rapidly than any reconstructed changes. The situation only stresses the need for a deeper understanding of climate variability on a regional scale. This is precisely the goal of CLIVAR, a new project in the development of the World Climate Research Program.

One projection of the future Caspian Sea level up t o the year 2050 was performed by Budyko et al. (1988). They used a scenario in which an equiva- lent doubling of COz would be reached by greenhouse gases by the year 2030.

The climate behavior was assumed t o be similar t o the paleoanalogues: as an optimum of the holocene in the 1990s, the Eemian in the first quarter of the twenty-first century and the Pliocene in the second quarter. For the Volga basin these periods have been studied in relation t o both temperature and precipitation patterns, providing estimates of the runoff and the water balance of the Caspian Sea. These procedures gave a slight decrease of the level in the 1990s (not yet observed) and an increase of the level up t o about -23 m in the year 2050.