User's guide of Proceessing Modflow 5.0

78 Processing Modflow
and the elastic property of the soil matrix. The specific storage or specific storativity is defined
as the volume of water that a unit column of aquifer releases from storage under a unit decline
-6 -1 -2 -1
in hydraulic head. The specific storage ranges in value from 3.3 × 10 [m ] of rock to 2.0 × 10 [m ]
of plastic clay (Domenico, 1972).
The confined storage coefficient is required by layers of type 0, 2 and 3. PMWIN uses specific
storage and the layer thickness to calculate the confined storage coefficient, if the corresponding
Storage Coefficient flag in the Layer Options dialog is Calculated. By setting the Storage Coefficient
flag to User Specified and choosing Storage Coefficient from the Parameters menu, you can specify
the confined storage coefficient directly.
In a phreatic (an unconfined) layer, the storage term is given by specific yield or drainable porosity.
Specific yield is defined as the volume of water that an unconfined aquifer releases from storage
per unit surface area of aquifer per unit decline in the water table. Specific yield is a function of
porosity (and is not necessarily equal to porosity), becuase a certain amount of water is held in
the soild matrix and cannot be removed by gravity drainage. Specific yield is required for layers
of type 1, 2 and 3.
Refer to Bear (1972 , 1979) or Freeze and Cherry (1979) for more information about the storage
terms and their definitions.
3.5 The Parameters Menu