find the coordinates of a point in which the magnitude of derived variable at a specific time is minimum?

Hello,

I am working on a 2D transient problem. I have the results and want to post processing them.
I want to plot my results like this:

Detect the minimum of a derived variable all over the domain at a specific time.
then, again detect that variable in another time (and repeat it for my time period)

I could do that. Now I wanna to have the coordinates of that point in which the derived variable is minimum at that specific time.
Any idea?

Thanks
Hooman