Hello,
I am using WEAP with the Soil Moisture Method - GW Wedge and the built-in PEST calibration tool. I would like to improve the calibration of low flows. At present, the calibration on raw daily streamflow appears to be strongly influenced by peak flows, while the model still underestimates streamflow during recession periods.
I would like to know whether it is possible to calibrate PEST using transformed streamflow values, for example a Box-Cox or log transformation.

Can the standard WEAP-PEST wizard calibrate against a transformed streamflow variable? If yes, how should both observed and simulated flows be transformed consistently? If not, is it possible to modify the generated PEST files or use an external post-processor? What alternatives would you recommand?

Thank you for your help,
Guillaume

Good question. I'd also be interested to know if anyone has tried using log-transformed flows with WEAP-PEST and how well it worked for improving low-flow calibration.

I have not done this myself. However, to do this, you would have to edit the .pst file that is created before running PEST. Make sure the uncheck the last option in the PEST window to indicate that you do NOT want to run PEST after building PEST input files. The .pst file contains the observed values, which you would need to manually transform. These are at the end of the file after the line “* observation data”. You would also have to identify the parameter in lines 3-9 that would also transform the WEAP result. Consult the PEST manual for this. The PEST manual is included with the WEAP installation and can be found in the same folder where you’ve installed the WEAP executable. The PEST executable (pest.exe) is also found there.

Also, it is worth noting that there are other algorithms available with PEST. These are described in the PEST Addendum document that you will also find in the WEAP program installation folder. WEAP defaults to the standard solution method. However, you can download other utilities and then edit the RunPEST.bat file to call this other executable (rather than pest.exe). One utility of note is the Complex Evolution Solution (SCE-UA) global optimization algorithm that is used for solving complex, non-linear, and non-convex calibration problems.