In the literature there are some notions
that have never been defined precisely.
One example is the so called

field capacity.

It is absolutely unclear which quantity
is supposed to be described.

Ulrich Hornung, 1986






Field capacity is usually defined
as that value of water content
remaining in a unit volume of soil
after downward gravity drainage has ceased...

A difficulty inherent in this definition is
that no quantitative specification
of that is meant by materially ceased is given.

Jacob Bear, 1977



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