Water Balance of the Black Sea (2)
  (b) Precipitation
  Precipitation in the Black Sea area is generally caused by cyclonic activity. Cyclones pass mainly from west to east through the southern part of the sea. The eastern and south-eastern regions receive the largest amount of precipitation. The lowest precipitation occurs in the central region, particularly the west. Monthly precipitation over the sea reaches a peak in winter (about 40 percent) and falls to its lowest level in spring and early summer (nearly 15 percent). Along the coast, particularly in the north-west, there is sometimes some summer precipitation caused by thermal convection. In the period under review the average precipitation over the entire surface of the Black Sea was 225 km3 per year (Reshetnikov, 1992).
  The total volume of precipitation is nearly 1.5 times lower than the total volume of river inflow, but its impact on the overall water balance of the sea can vary considerably according to the year and the season. For example, in autumn and winter the volume of precipitation on the surface of the sea exceeds the volume of continental inflow.
  (c) Evaporation
  A general trend can be observed in the seasonal distribution of evaporation over the surface of the Black Sea. The greatest rate of evaporation occurs in the western and eastern zones between latitudes 44 N and 45 N. These regions usually also have the highest wind velocities as a result of cyclonic circulation due to Black Sea morphometry. The depth of the evaporated water layer in the area bordering the coast of the Caucasus is 1,300 mm per year.
  Average evaporation in the Black Sea area during the period under review was 370 km3 per year, ranging from 484 km3 in 1951 to 289 km3 in 1985. The long-term trend of evaporation is negative, particularly over the last 30-40 years, during which time wind velocities have decreased by an average of 0.8-1.3 m/sec per month (Kabatchenko, 1985); and this has been a major factor in the decline in evaporation levels over the surface of the Black Sea. More than 50 percent of annual evaporation occurs during the summer-autumn period. The highest levels of evaporation take place in August, while evaporation are lowest in April. Total evaporation above the surface of the Black Sea in March-June accounts for only 14 percent of the annual total.
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