• Re: Time-step lengths of simulations with multi-layer wells in conf. vs. unconf. aq.

    Thanks, Peter.

    I tested the Unconfined-aq case with different solution convergences. With large convergence criteria, the solutions of the Unconfined aq. case were not correct. Considering the analytical solution and the Confined-aq case (see below), the modeled pumping rate might not be too big.

    This modeling is for a simple pumping-well simulation:
    - A single well in the homogeneous isotropic medium using a constant pumping rate.
    - A steady-state analytical solution is available (Thiem-Dupuit formula, see (1)).

    For your review, I have attached three files:
    (1) Model setting and results - comparison between the analytical solution and the results of Confined-aquifer case. (pdf file)
    (2) Model file:  Confined-aquifer case
    (3) Model file:  Unconfined-aquifer case  --> the dt plot is shown in (1)

    Could you review the above three files and help me understand the following?
    (a) Are there some missing items/settings in the model file (2)?
    (b) How to solve the convergence issue in (2)?

  • Time-step lengths of simulations with multi-layer wells in conf. vs. unconf. aq.


    I am conducting simple simulations using multi-layer pumping wells. For your review, the model domain and other settings are shown in the attached.
    Two cases are considered:
    - Case 1. The model is solved by assuming the aquifer is confined.
    - Case 2. The model is solved as an unconfined aquifer system: Slice #1 is set as "Phreatic" and Sy=0.1
    Cases 1 and 2 use the same settings except the aquifer type.

    [Case 1]
    The simulation of Case 1 run well (e.g., dt increased from 1.0e-5 (a few steps at the beginning) up to 31day), and the modeled hydraulic heads look ok (see the attached).

    [Case 2]
    For Case 2, the model has difficulties in solving the unconfined well problem. The length of modeled time-steps stays at 1.0e-5 day, which is too small to run for the whole modeling period. My understanding of key difference between Cases 1 and 2 is the magnitude of relative conductivity (k): k= 1 constant for Case 1 while k= variable for Case 2, depending on the elevation of water table in model elements (saturation degree). In Case 2, the residual water depth is set as 0.25 (about 10% of the layer thickness).

    My question is what causes the convergence difficulty in Case 2?
    Could you share any ideas, suggestions, or experience to solve this issue?