Phil L wrote:
Yes, providing the head of water doesn't drop too low it should be
fine...bad things might happen if it drys up completely though, so
you may want to top it up from time to time

Forgot to add here

it doesn't require a fantastic amount of pressure, the machine simply opens
a valve for a set length of time, it can't 'know' how much water has passed
through in that time, unless it uses some sort of float valve, in which case
it doesn't matter how high or low the pressure is.