Storage compressibility parameter in unsaturated flow ???

Hello,

I have a question concerning simulations in unsaturated media.

In the parameters it can be seen that in the unsaturated  type of problem it is still possible to enter a storage compressibility value. I am wondering about the effects of this parameter ? as I thought that in unsaturated runs it is only the porosity that should play  this role. So what is the meaning of this storage compressibility parameter in unsaturated models. How does it interact with the porosity ?

Thanks for all

Boris

FEFLOW uses two storage parameters (for saturated flow):

- storage compressibility: This is usually called specific storage and reflects storage by compression of the matrix and water
- storativity: This one is usually called specific yield and is used for the storage in the pore space when the phreatic water level moves.

In unsaturated flow, the storativity is replaced by porosity and the parametric relationship for unsaturated conditions. However, there's still storage by compression. Please see the corresponding equations in the FEFLOW Reference Manual.

Ok,

I thought it would be that. But I am wondering about the strong influence of this storage compressibility parameter on the results. I did parametrical tests and this parameter seems to be very important. Now, how should I manage to calibrate my model ?. Modifiying the storage compressibility results in a change of the global behavior ( more little it is set more nervious is the system)... I can also reproduce it with the porosity. So should I consider this parameter, or simulate this unsaturated system with the default values ?
Now the default values are 10-4, much to small for an unsaturated system !!!

Boris

Usually the compressibility parameter shouldn't have too much influence on the results in an unsaturated system. It mainly affects confined systems where it is the only storage parameter. However, the smaller it is the 'more nervous' the system has to react, as changes in the boundary condtion are less dampened by storage. In an unsaturated system, you'd put the porosity in the unsaturated parameters, and a small compressibility value. The latter depends on the material, it will be much smaller than the default in hardrock systems, but higher in highly compressible material.

In unsaturated flow storage is represented by the water capacity term which is essentially the derivation of the saturation–pressure relationship. A look at this functional relationship reveals, that the water capacity term can decrease strongly at high and at low saturations, depending on the soil hydraulic parameters. However I can not figure out, that the compressibility term should have such a strong influence under unsaturated flow conditions.…