Water Balance of the Black Sea (1)
	The high degree of isolation from the world ocean, the extensive drainage basin and the large number of incoming rivers all contribute to the unique water balance of the Black Sea. The water balance is an important factor in the Black Sea marine ecosystem. Changes in the water regime have a significant impact on salt and water balances, which are the most vulnerable to any anthropogenic changes, particularly in the shallow, biologically highly productive north-western region. Fresh water input and exchange of Mediterranean water via the Bosphorus are critical elements in the hydrography and ecosystem of the Black Sea. The water balance of the sea has been studied for many years, and there is still considerable controversy over the magnitude of its terms (Table 1). Its major components can be summarised as follows: river discharge; precipitation; evaporation; exchange via the Bosphorus ; inflow from the Azov Sea.
	(a) River discharge (Black Sea Drainage Basin)
	The largest volume of river flow entering the Black Sea comes from the north-western part of its basin and from the Caucasus, Turkey and the Bulgarian and Romanian coasts. The total catchment area of all the rivers discharging into the sea is 1,874,904 km2, including 215,625 km2 of small river flow. The average total discharge for the period from 1921 to 1988 was 353 km3 per year (Reshetnikov, 1992).
	Drainage from the north-western coast. The catchment area of the rivers on the north-western coast is about 1,500,000 km2, including 20,000 km2 of small river flow. Most of the inflow comes from four principal rivers; namely the Danube, the Dniester, the Dnieper and the Southern Bug (Table 2). The average total annual discharge into the north-west of the sea for the period 1921 -1988 was 261 km3 per year, rising from an average of 153 km3 in 1921 to 389 km3 in 1970.
	Drainage from the Crimea. The rivers of the Crimea do not play an important role in the water balance of the Black Sea. The largest rivers are the Chernaya, the Belbec, the Al'ma and the Kacha. The total catchment area of rivers running from the Crimean mountains into the Black Sea is only 2,729 km2. Their average annual flow over the period 1921-1988 was 0.32 km3, ranging from a minimum of 0.07 km3 in 1930 to a maximum of 0.67 km3 in 1968
	Drainage from the Caucasus. The total catchment area of the rivers in the Caucasus is about 75,000 km2. The major rivers are the Rioni, the Chorokh, the Inguri, the Kodori, the Bzyb, the Supsa and the Mzymta (Simonov, and Altman, 1991). The average annual flow during the period under review was 43 km3, ranging from 31 km3 in 1969 to 57 km3 in 1922 (Reshetnikov, 1992).
	Drainage from Turkey. Many rivers debouch into the Black Sea along the Turkish coast. Their total catchment area is 259,550 km2 (Reshetnikov, 1984). The major rivers include the Yesilrmak, the Kizilrmak and the Sakarya. Average annual flow during the period 1930-1986 was 36 km3, ranging from a minimum of 25 km3 in 1949 to a maximum of 51 km3 in 1940.
	Drainage from the Bulgarian and Romanian coasts. The major rivers in Bulgaria are the Duda, Kamchia, Provodiyska, Rezovska, Veleka, Ropotamo and Fakiyska. The total catchment area is 6,292.10 km2. The total discharge into the sea averages 1.83 km3/year. The total catchment area of all the rivers debouching along the Romanian coast (excluding the Danube) is 4,589 km2. Their total annual discharge is 0.12 km3 (Simonov, and Altman, 1991).
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