Modflow Sf1, sf2, ss and porosity

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My notes: Modflow Sf1, sf2, ss and porosity

Sf1: the primary storage coefficient, the storability. Read only for a transient
simulation (steady-state flag, ISS, is 0). SF1 would normally be confined storage coefficient.
The primary storage coefficient divided by the block thickness is equal to the specific storage Ss.
Usually in the range of 1e-3 to 1e-6. Represent the spongy characteristics of a confined aquifer.

Sf2: secondary storage coefficient, the specific yield. SF2 is read only if
the simulation is transient (steady-state flag, ISS, is 0). SF2 is approx. to porosity for a unconfined layer, but is smaller than porosity for a confined aquifer or an aquifer under pressure. sf2 is used to simulate water table in an aquifer.
sf2 is usually in the range of 0.1~0.4.

Ss: Specific storage
Sf1: storativity = Ss*b (depth of a confined layer)
Sf2: Specific yield
Wikepedia
Specific storage and specific yield
Main article: Specific storage
Specific storage (Ss) and its depth-integrated equivalent, storativity (S=Ss*b), are indirect aquifer properties (they cannot be measured directly); they indicate the amount of groundwater released from storage due to a unit depressurization of a confined aquifer. They are fractions between 0 and 1.
Specific yield (Sy) is also a ratio between 0 and 1 (Sy ≤ porosity) which indicates the amount of water released due to drainage, from lowering the water table in an unconfined aquifer. Typically Sy is orders of magnitude larger than Ss. Often the porosity or effective porosity is used as an upper bound to the specific yield.

Specific storage is typically very small, generally 0.0001 1/ft or less
Storativity is a dimensionless quantity, and ranges between 0 and the effective porosity of the aquifer; although for confined aquifers, this number is usually much less than 0.01
Specific yield, also known as the drainable porosity, is a ratio, less than or equal to the effective porosity, indicating the volumetric fraction of the bulk aquifer volume that a given aquifer will yield when all the water is allowed to drain out of it under the forces of gravity

Typical Specific yield values: (low mid high)
Unconsolidated deposits
Clay: 0 2 5
Sandy clay (mud): 3 7 12
Silt: 3 18 19
Fine sand: 10 21 28
Medium sand: 15 26 32
Coarse sand: 20 27 35
Gravelly sand: 20 25 35
Fine gravel: 21 25 35
Medium gravel: 13 23 26
Coarse gravel: 12 22 26

Consolidated deposits
Fine-grained sandstone: 21
Medium-grained sandstone: 27
Limestone: 14
Schist: 26
Siltstone: 12
Tuff: 21
Other deposits: Dune sand 38
Loess : 18
Peat: 44
Till, predominantly silt: 6
Till, predominantly sand: 16
Till, predominantly gravel: 16