Hello Abdelrazzaq M Al-Tawaha

[(Vi - Vf)/Vi ]/dT should be the simplest equation where, VI and Vf are volumes of water first filled and measured after a given time period, dT being the change in time (preferably in hours); this can be used as % change by simple modification in terms of 100%. VI can be obtained from the dimension of the tank if filled to the brim or the height of the water column needs to be taken into consideration otherwise. Vf is the simply the change in water column height and hence the change in volume of the content of the tank.

Regards,

Dr. Abhishek Mukherjee