In the original proposal rain was the sole source of water.
I have a memory of England as a rainy place, an idea that seems ill founded now. In chalk lands, where there are no springs or streams, water is collected in dew ponds, depressions dug in the ground and lined with clay.
Gradually they fill up from what rain there is and from dew. Situating a pond under leafy trees both shades them from the sun, slowing evaporation, and increases the local precipitation, gathering moisture.
The pond acts as a reservoir for the hole. A pipe runs between the two, downhill, under the ground, from the bottom of the pond to the top of the hole, where eventually it will be buried among loose flints, and provide a steady drip, falling into the hole, striking water and percussion.