Flux value at the transfer boundary didn't change

Hi! Now I'm modeling a big spring in a karst area. I used transfer boundary to represent the spring. The spring's outflow elevation was used to be reference hydraulic head. The parameter of transfer rate was calibrated accoring to the observed spring outflow.

But when I checked the outflow of spring through Budget analyzer to check the flux at the transfer boundary, the flux value wasn't changed too much even if I changed several orders of magnitude of transfer rate.

So my question is why the flux value at my transfer boundary wasn't changed when I changed the transfer rate severely?

Looking forward to your reply! Thanks!

two possibilities I can think of: 1) your model is poorly converged and so error controls the result, or 2) setting the transfer rate larger than the K of the surrounding elements will result in similar flow rates because the surrounding K will control the flow.  In the second case, if you decrease the transfer rate to a value that is based on a K that is 0.1 or smaller than the surrounding K you should see a decrease in the flow.

Thanks for your reply, Pete!

According to your annalysis, I think the reason is in the second case. Because the big spring's flux (160,000 m3/d) is so large that I have to use a large value of transfer rate.

Now I'm thinking of using fixed drawdown pumping well to represent the big spring. But I don't know how to set fixed drawdown pumping well in FEFLOW.

So could you tell me how to set fixed drawdown pumping well in FEFLOW?

Looking forward to your reply!

If you have:
1. tried a transfer boundary with a very large conductance;
2. induced drawdown to the desired level; and
3. not acheived the desired rate of spring discharge.

Then changing the boundary condition type will not solve your problem.

The issue then lies with the flow balance of your model.  Check the following:
1. Does your conceptual model adequately describe the mechanisms involved in generating your spring discharge, and does your numerical model adequately represent this conceptual understanding?
2. Is the conductivity of your model too low?
3. Have you introduced sufficient recharge into the system?
4. Is the geometry of the system adequately represented?
5. Have you checked the flow data from the spring and confirmed its representativeness of average discharge from the aquifer system?

"Fixed drawdown" wells in feflow (a constrained "Well BC") are essentially the same as fixed head ("Hydraulic-head BC").  You set them up by adding a minimum (and/or maximum) constraint to a Well BC. When the head at the well node exceeds the constraint you set, the constraint, which is a fixed head equal to the value of the constraint, is put in effect.  If you want to fix the drawdown at the spring, set a hydraulic head BC.  If this does not help then the problem lies with convergence error, and so Blair's recommendations will likely help solve the problem.

Thanks! I have got it :)