I am trying to calculate surplus water level in tank, which would be equal to previous month's total data added up with the current month's data.

e.g. Tank level (month) = surplus water (month) + Tank level (month-1))
i.e. Tank level (Feb) = surplus water (Feb) + Tank level (Jan)


how can I do that in WEAP?
Thanks

To reference a previous month's data-

You can do this using Expression Builder when creating an expression. In Expression Builder, go to Functions, and you can use the function for PrevTSValue. Then, go to Branches within the Expression Builder and you can reference the model object with you want to pull the PrevTSValue for from.