The dew pond is another mystery commonly encountered while out walking hilly areas.The photo shows an old dew pond on land above Monyash in Derbyshire. I guess it is about 1100 feet above sea level.
How dew ponds work still seems uncertain even though they appear to be a very ancient way of watering livestock in hilly areas. I suspect rain is the main water source rather than dew. Not for any scientific reason but somehow it isn't easy to see dew creating enough water. So it's rain for me. Not only normal rain but that misty rain often encountered in hills. More info here.