The x component of the velocity at the centre of the face ABCD above is

The x component of the velocity at the centre of the face EFGH above is

The volume of fluid crossing ABCD per unit time is

The volume of fluid crossing EFGH per unit time is

The rate at which water is lost is the difference of these two and equals

Similarly the net loss of fluid in the y and z directions areandrestively.

Adding these gives the rate of loss of fluid overall as