specific storage in unconfined aquifers can be much greater than typically observed values in confined aquifers – possibly due to entrapped air bubbles or poorly consolidated shallow sediments. It is clear the elastic properties of unconfined aquifers are too important to be disregarded.

Delayed Water Table Unconfined Response
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Boulton’s ([-@boulton1954b; -@boulton1963]) models encountered conceptual difficulty explaining the physical mechanism of water release from storage in unconfined aquifers. [@neuman1972] presented a physically based mathematical model that treated the unconfined aquifer as compressible (like @boulton1954b [@boulton1963] and @streltsova1972a [@streltsova1972b]) and the water table as a moving material boundary (like @boulton1954a and @dagan1967). In Neuman’s approach aquifer delayed response was caused by physical water table movement, he therefore proposed to replace the phrase “delayed yield” by “delayed water table response”.

[@neuman1972] replaced the Laplace equation of [@boulton1954a] and [@dagan1967] by the diffusion equation; in dimensionless form it is $$\label{neuman_diffusion}
  \frac{\partial ^2 s_D}{\partial r_D^2}+
  \frac{1}{r_D}\frac{\partial s_D}{\partial r_D}+
  K_D\frac{\partial ^2s_D}{\partial z_D^2} =
  \frac{\partial s_D}{\partial t_D}.$$ Like @boulton1954a and @dagan1967, Neuman treated the water table as a moving material boundary, linearized it (using ), and treated the anisotropic aquifer as three-dimensional axis-symmetric. Like @dagan1967, @neuman1974 accounted for partial penetration. By including confined storage in the 