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Bruce Campbell

PEST Variables - Drain Conductance

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Hi -- I'm using PEST to estimate MODFLOW Drain Package conductance terms. I have a number of stream gages where I have analysed the data and calculated the groundwater baseflow component and I'm using these data as observations in PEST. In several of the basins, I have 2 (or more) gages and have a question about the PEST variable. In the stream arcs, I'm setting up a negative number in the attribute table for the conductance term (-201, -202, etc). What is the best procedure to use for having the 2 gages in the same basin. What I've done in the past is have 2 separate variables for the 2 gages. However, it seems like that cuts the simulated flow term at the lower gage. Does anyone have a better procedure?

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Hi Bruce, you may try to make one parameter adjustable (parent) and the rest in the basin tied to the parent. Relevant part from PEST manual:

Quote

PEST allows one or more parameters to be tied (i.e. linked) to a “parent” parameter. PEST does not estimate a value for a tied parameter; rather it adjusts the parameter during the inversion process such that it maintains the same ratio with its parent parameter as that provided through the initial estimates of these parameters.

Or, alternatively, you could include the ratio between the pertinent parameters as prior information and adjust its weight accordingly. This way it would still be treated individualy, but the solution would tend to respect the parameter ratios.

It is also also possible, that the contribution to total phi for drain observation group is too low, so it is invisible for the inversion proces. You may try to look at it in the case.rec file. It could be resolved by adjusting the weights for the observation groups. There is a nice utility form the PEST suit, that can help to balance this, its called PWTADJ1.

Edited by Michal

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