Yes, they are same.

Dearest J.D.,

                            they are not synonymous. Available water is the difference between field capacity which is the maximum amount of water the soil can hold and wilting point where the plant can no longer extract water from the soil.

Water holding capacity is the total amount of water a soil can hold at field capacity.

The field capacity is the amount of water remaining in the soil a few days after having been wetted and after free drainage has ceased.

and

The soil water holding capacity is the portion of water that can be absorbed by plant roots. By definition it is the amount of water available, stored, or released between field capacity and the permanent wilting point water contents.

Kwamina is right, there are a number of different concepts tied up together here, and specific terminology is important.  There is a simple description of some of these concepts at the link below which may assist in identifying the correct terminology to use.

http://bettersoils.soilwater.com.au/module2/2_1.htm

Thank you all for your help. Now I differentiate these confusing concepts. Biologists are not sufficiently trained in soil concepts. Thanks!