Stephen Buck

Hello,

As a follow-up to questions around errors in the overall model water balance I have the following observation:

If the options setting under the menu Options->Specific Option Settings->(E) Free surface(s) if "TOUCHING THE TOP SURFACE" is set to "Constrained" then if the "In(+)/out(-) flow on top" value is such that it causes the head to be constrained to the model top the budget imbalance reports the full value of recharge to the model (even though it is not being applied due to the constraint).  This is clearly incorrect since the constrained option limits the quantity of recharge to prevent groundwater mounding above the top model layer.

This can be easily demonstrated by revising the option from "Constrained" to "Unconstrained" and observing that the imbalance is now close to zero for a steady-state model, as it should be.

I would consider this a bug in the software requiring a fix.

Regards,

Stephen Buck

Hi Peter,

Thank-you for clarifying this.  I did a quick review of the forum posts and I do not see any other reference to first type of boundary condition constraints other than the one you mention.

Regards,

Stephen

I want to clarify a point regarding boundary conditions versus constraints.  In the FEFLOW user manual (p. 57) it is stated that "Outfluxes are defined as positive for boundary conditions.  However, for constraint conditions, the definition is opposite, here influxes are positive".  Therefore, if you are using first type of boundary conditions to represent wells  (i.e. constant head is set to the well screen bottom elevation) then the constraint to limit flow out of the model to the well rate should be Qmin = well rate NOT Qmax = well rate.  Similarly, to prevent discharge into the model when the aquifer head drops below the boundary condition elevation Qmax should be set to zero.  I have tried the WASY recommended method of setting constraints of Qmax = well rate, Qmin = 0 and monitored flows using observation point groups.  This clearly indicates that no flow occurs out of the model at the drain locations under this scenario.

Can you confirm I am correct on this point since there are several posts by WASY where this incorrect advice is given?