I calculated the volume of a reservoir using surface volume option in 3D Analyst but it is so much less than the actual volume and the 2D surface is also much less. Why is it so?
I don't know what the units are. The coordinate system is WGS_1984.
Your volume units are the units of your coordinate system. In your case, WGS 1984, that would be degrees. Cubic degrees are pretty nonsensical, and that's the reason the values are so small.