Abstract

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The integral water capacity is first introduced as a flexible method to
quantify various soil physical limitations when calculating available water in
non-swelling soils. ‘Weighting’ functions that account for
hydraulic conductivity, aeration, and soil resistance to penetration are
applied to the wet and dry ends of the differential water capacity, and then
integration is performed. The concept is extended to swelling soils by
applying the theory of Groenevelt and Bolt (1972), which enables overburden
pressures to be taken into account. A set of shrinkage lines measured by
Talsma (1977) is analysed using this theory, which enables precise values of
overburden potentials to be calculated as a function of the moisture ratio for
different load pressures. The addition of the overburden potential to the
unloaded matric potential causes minor shifts in the classical limits of
plant-available water (viz. –1/3 bar and
–15 bar). However, when other soil physical restrictions are taken into
account (such as in the concept of the least limiting water range), the
consequence for available water deeper in the root-zone (due to an overburden
pressure) is far more serious. This is primarily because the matric potential
at which aeration begins to be satisfied shifts to a considerably lower value,
making a large quantity of water at the wet end no longer available. Examples
of weighting functions derived from the literature are applied and their
implications for available water in swelling soils are discussed.
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