Re: Definition of Water Holding Capacity

[Martin]

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Hello Rick

I'm not sure if this is a helpful contribution. In my way of thinking, the
most important cause of lack of clarity surrounds our continued use of "field
capacity". Sorry I currently don't have text books to hand.

Meanings which I think are ascribed to "field capacity" include water retained
in the soil matrix after internal drainage for 24-48 hours or water retained
when internal drainage drops below a measurable or "significant" flux viz 0.1
mm/day, at a specific depth. For some soils, these are definitely not the same
thing.

Might terminology be agreed which identifies a proportion of the soil pore
space which contains water which might be lost either by internal drainage or
evaporation/ root water extraction and a proportion of the soil pore space
containing water which is only removed by evaporation/root water extraction.
Such a distinction might help with validation of crop water uptake models.

Martin


> >From: owner-sowacs@aqua.ccwr.ac.za
> >Date: Mon, 21 Feb 2000 18:22:15 GMT
> >X-Authentication-Warning: aqua.ccwr.ac.za: majordom set sender to
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> >Subject: Definition of Water Holding Capacity
> >Sender: owner-sowacs@aqua.ccwr.ac.za
> >To: <martinparkes@gn.apc.org> ....... moved!!
> >
> >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perhaps outdated term, as well as are field capacity and wilting point.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
>