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cansudemir

MODFLOW-USG PEST calibration for 1-layer : Residual is high (around 7000)

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Hello,

For a very long time, I have been trying to create a numerical model of an aquifer with complex hydrogeology (by using borehole-to-Ugrid option). I had several layers of 4 different materials. But all my efforts were trash. Because, I have never obtain a successful Modflow-usg run. They never converged. Although wetting option was on, the aquifer had so many dry cells. And these dry cells were always at same locations (not near/at the location of wells). It was meaningless. I couldn't run the PEST. So I decided to make my system more basic and created a one-layer model with HK zones. It worked well without the wetting. I have applied pilot points approach for the calibration. And pest is working because the forward run has a successful result. 

However, this time the calculated residual is too high. I have done many things to decrease the error. But the best result until now is about 7000 (Sum of Squared Weighted Residual Head+Flow) and for Root mean squared residual it is 16. These values are so big. Interestingly, the computed vs. observed head plot (attached) looks not so bad that there should be a way to fit them to the line. I only have head observations in my hand and I don't have any flow observations data. May this be the problem? Or what may be the problem? Could you please help me on this? I would be so happy to hear your recommendations to lower my calibration error to an acceptable value.

Note: The computed heads are higher than the measured heads. The boundary conditions are ; specified head along the coast and GHB condition was assigned to some part of the boundary. The model is steady-state. 

Thank you!

Best,

Cansu

Edited by cansudemir

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Just for kicks, you might try lowering your specified head to see if you can achieve a reasonable calibration.  Have you accounted for evapotranspiration?  Could your recharge be too high?  Are your calibration measurements being influenced by any tidal effects (you said "coast," but are running steady state, which is an average condition....perhaps your measurements were taken under a lower tidal influence)?  Just some things to consider.

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It seems your model overestimates at all obs. points. You may think about the inflow and outflow of your system and add more complexity to your model, such as surface recharge/discharge. The transient state might also be an issue, as Sean mentioned. Finally, John Doherty suggests to have more pilot points close to the BCs than the middle of the system. It might control the total inflow/outflow.

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4 hours ago, Farhad said:

It seems your model overestimates at all obs. points. You may think about the inflow and outflow of your system and add more complexity to your model, such as surface recharge/discharge. The transient state might also be an issue, as Sean mentioned. Finally, John Doherty suggests to have more pilot points close to the BCs than the middle of the system. It might control the total inflow/outflow.

Yes, clearly all your modelled heads are all overestimated.  Obvious quick fixes to try would include increasing K and/or lowering recharge.  Try playing with PEST ranges.

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