Definition of Water Holding Capacity

[Rick Allen]

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I know that this is "old ground", but I need help in clarifying 
definitions.  The Irrigation Association is currently refining a glossary 
of irrigation terminology.  The glossary includes soil water terms.

My question concerns the best definition to use with the term "water 
holding capacity."  In my mind, water holding capacity is defined as 
the amount of water that the soil can "hold" above oven-dry.  It is 
expressed in terms of mm/m (in the US as in/ft).

WHC contrasts with the term "available soil water" in that available 
soil water is defined as the difference between field capacity and 
wilting point.

I realize and appreciate that water holding capacity is an old and 
perhaps outdated term, as well as are field capacity and wilting point. 
 These latter two have been redefined as dynamic rather than static 
values and have sometimes been given the names drained upper limit 
and crop extractable limit, etc.  

My main question is whether the above definition for WHC is valid 
and widely held, or if it should be modified.

WHC is contrasted with AM as indicated by their calculation:

In the classical approach:
AM = FC - WP
WHC = FC

I believe that it is useful to have a term as WHC that describes the 
total amount of measurable water that we can expect to see retained 
in a soil profile.  This is useful for comparing against methods that 
measure total water such as neutron meter, TDR, etc.  In addition, 
many old soil surveys contain soil water holding information that is 
often equivalent to the above definition of WHC.  Therefore, it is useful 
to keep a term around that is compatible with the old data.

Thanks.

Rick Allen
Univ. Idaho