Buckingham (1907) introduced for the first time in the subsurface history, the concept of "capillary potential" (when Tensiometer was not yet invented) and he proposed an equation similar in form to Darcy's law, where hydraulic conductivity was NOT treated as a constant- but rather - K was treated as a function of capillary potential for the studies pertaining to the movement of moisture content in an unsaturated soil.
Why do we hardly find Buckingham's contribution in describing flow through vadose zone - particularly - in the context of Groundwater Engineering?
Of course,
Richards deduced the NON-LINEAR PDE for the first ever time
to address the fluid flow through the vadose zone
by coupling soil moisture retention curves
along with the Buckingham (1907)'s work; and
Richards for the first time introduced the concept of "Moisture Capacity"
(as against hydraulic capacitance).
In fact,
the later suggestion by Childs and Collis-George (1950) to use the concept of "volumetric moisture content" as against the "capillary potential"
as the dependent variable
is ONLY to reduce
the intensity of the NON-LINEARITY of the PDE
deduced by Richards; so that it can be solved easily by numerical means.
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