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We shouldn't have to use 'trade' flows modulated by monthly multipliers to represent seasonally-varying baseflows. Users would benefit from baseflows being modelled in the 'baseflow' field. We should introduce a BIP (Baseflow Infiltration Profile) file that mirrors what the WWG does for foul flows and the TWP does for trade flows. This will improve clarity and also align with the Imax approach taken for treatment works.
There are also good arguments to say that this is a feature that should actively not be implemented.
In reality, ‘seasonally varying’ infiltration is always tied to rainfall, either local or further up the watershed, and due to the wetness of the ground, through downward infiltration, or the water table level. The best-practice way of replicating this in a hydraulic model is by use of the ground infiltration module.
The approach of breaking the connection between infiltration and rainfall, and just tying it to the month, usually leads to unrealistic model predictions. Most years do not perfectly follow the theoretical long-term monthly wetness averages, so using this approach, the model will underpredict on above-average wetness periods, and will overpredict for dryer-than-average periods. Plus the model will show awkward step changes in infiltration at month boundaries.
So in my view, the software should not contain functionality that accommodates, and therefore encourages, this unrealistically oversimplified approach for this modelling challenge. A better solution to this problem should be better guidance for the use of GIM, especially the Ground Store, probably with a set of template profiles included in the guidance.