RE: Definition of Water Holding Capacity

[Tim]

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Rick,

my feeling is that Water Holding Capacity is not only outdated, but also an
ambiguous terms and has been used to mean different things by different
people. Perhaps the glossary should recommend avoiding the use of this term?

Tim

> -----Original Message-----
> From: owner-sowacs@aqua.ccwr.ac.za 
> [mailto:owner-sowacs@aqua.ccwr.ac.za]
> Sent: 21 February 2000 18:22
> Subject: Definition of Water Holding Capacity
> 
> 
> NOTE:  To get off this list, send email to majordomo@aqua.ccwr.ac.za
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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 perhaps outdated term, as well as are field capacity and 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
> 
> 
>