  The objective of this work is to derive the soil water retention from the
soil structure without curve-fitting and only using the physical parameters
found irrespective of an experimental retention curve. Two key points underlie
the work: (i) the soil suction at drying coincides with that of the soil
intra-aggregate matrix and contributive clay; and (ii) both the soil suction
and volume shrinkage at drying depend on the same soil water content. In
addition the two following results are used: (i) the available two-factor
(capillarity and shrinkage) model of clay suction enables one to connect a clay
suction and clay water content using the clay matrix structure; and (ii) the
recent reference shrinkage curve model based on the concepts of intra-aggregate
soil structure permits one to connect the soil water content at shrinkage with
the water content of the contributive clay. With that the available two-factor
model was essentially modified and, in particular, the effect of adsorbed water
film was taken into account. The developed model includes the following input
parameters: the solid density, relative volume of contributive-clay solids,
relative volume of contributive clay in the oven-dried state, soil clay
content, aggregate/intra-aggregate mass ratio, and specific volume of lacunar
pores in the aggregates at maximum swelling. The validation of the model is
based on available data of water retention and the above input parameters for
six soils. A promising agreement between the predicted and observed water
retention curves was found.
