I realized that FePEST doesn't support time-varying material properties. I was hoping that using multipliers instead of directly adjusting the parameters themselves will solve the problem. FePEST, however, doesn't even let me to get to the point of assigning multipliers. I wonder if there is someone who could give me clues as of how to overcome this problem. I have to confess that I'm quite new to working with FEFLOW, but I'm well experienced with finite difference models and using PEST outside of any GUI. Nevertheless, FePEST "enforces" me to stay inside the GUI for many reasons, Thanks.