Hi all,

I wonder if anyone could clarify what exactly happens in FEFLOW when a multilayer well goes partially “dry”.

To better explain what I mean, suppose I inserted a vertical multilayer well in a given position of a model with its interval from layer 5 to layer 10, with a constant pumping rate of 1000 m3/day. Let’s suppose that during a transient simulation, the groundwater level (line of zero pressure) undergoes drawdown, setting, after a given period, at layer 7. In this case, part of the multilayer well interval will be above the groundwater level, and part below.

My question is, what happens with the actual pumping rate that the model simulates? Does it remain at 1000 m3/day, or does reduce proportional to the interval of the multilayer well that is still below groundwater level, as it does in MODFLOW (say, in the case I gave, the pumping rate would go down to something around 400 to 600 m3/day)? Regarding this question, would it make any difference if the model is run in confined/free/phreatic/Richard’s equation mode?

Looking at the FEFLOW manual:
“While using physical properties of the well for input and data storage (e.g., screen top and bottom, well radius), internally during the simulation such Multilayered wells are represented combining an extraction or injection of water on the lowermost node (like a well boundary condition) and a linear discrete feature element along the screen interval with properties according to the well geometry.”

If I understood correctly, provided that the lowermost node of the multilayer well is still below groundwater level, the pumping rate should not change regardless of what happens in the layers above. Is this correct?

Thank you all very much in advance,

Rafael
My experience has been that it's best to represent wells like this as seepage face BCs because the multilayer wells tended to have numerical issues (i.e. go dry, stop pumping, and in the following timestep have water, start pumping, and repeat) due to the drawdown.

In this case, I imagine it would help to represent your problem as fully saturated so it continues pumping, or represent your well differently

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