AK - I am guessing here since I don't know who installed this one, but usually these are closed system water features. Underground is either a plastic basin or a piece of pond liner. This creates a hidden reservoir for the water. The submersible pump is underneath the rocks. If the basin is sized appropriately to be larger than the splash zone, the only water loss will be due to evaporation. The water features can run for weeks or even months without needing to add water to the system.
This is a water feature we installed on a recent project, it has a catch basin under the pavers and is a closed system we do install auto fill valves on all water features.
Thanks @ Southern Trillium, I took this photo at a garden in Nashville and I'm sure you are correct. I like the simplicity and it doesn't use much water.
Hope this helps.