How could falling water level mark the passing of time?

A water clock can mark time by letting water drain from a vessel. As water leaves through a small opening, the surface level falls. Marks inside the vessel connect selected levels with elapsed time.

That connection needs care because drainage does not automatically keep a constant rate. A deeper column of water creates greater pressure at the outlet. As the level falls, the changing pressure can change the flow.

The shape of the vessel and spacing of marks can account for this behavior. Comparing the clock with a known time interval allows calibration. A carefully shaped bowl can make the falling level more useful than an arbitrary container would.

Some ancient clocks also used different scales for the changing length of nighttime hours across seasons. The device therefore joined a physical process with a human definition of time. Flow created the changing level; observation and chosen markings gave that level a timekeeping meaning.