Question

though caspian sea is enclosed ,its salinity is just 14 to 17 parts per thousand.why it is so

Answer 1

Caspian sea which is actually considered as a lake is the biggest in the world is surrounded on the south by Persia.

Seas are usually determined by association to the ocean or different sea through salt water, which the Caspian Sea is not. Seas are normally or partly enclosed by land, according to the National Oceanic and Atmospheric Administration, but the Caspian Sea is completely enclosed by land. Seas commonly consist of salty water. While the Caspian Sea is not fresh water, its salty water is diluted by the inflow of fresh water, especially in the north which reduces its salinity.

As Caspian sea,  is part of an internal drainage system due to this the Caspian lake is slightly saline although it is enclosed. Ordinary salinity ranges between 12.80 and 12.85 percent but its sharply drops to less than 1 % across the delta of the Volga river. However, it extends usually from north to south and from west to east. Hence average salinity is thus only 9.28 percent in the northern basin, 13 percent in the central basin, and 13.1 percent in the southern basin.

Answer 2

The water of seas are are usually salty, but the water of Caspian Sea is not fresh water, but its salty water is diluted by the inflow of fresh water, especially in the north side. 

Secondly, seas are partly or more enclosed by land but Caspian Sea is entirely closed by land.