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.
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I am currently reading Gilbert White's "Natural History of Selborne", and here, for what it is worth, is what he thinks about them. Whether the science stacks up I'll leave to you to decide. There are, of course, modern ones formed by a sort of shallow concrete dish; but lots of them now have plastic piping supplying them. The ones that go dry always seem to have lost part of their liner - clay in the case of the originals, and presumably bitumen etc. in the concrete ones. Anyway, here's Gilbert:
To a thinking mind few phenomena are more strange than the state of little ponds on the summits of chalk-hills, many of which are never dry in the most trying droughts of summer. On chalk-hills I say, because in many rocky and gravelly soils springs usually break out pretty high on the sides of elevated grounds and moun|tains; but no person acquainted with chalky districts will allow that they ever saw springs in such a soil but in vallies and bottoms, since the waters of so pervious a stratum as chalk all lie on one dead level, as well-diggers have assured me again and again.
Now we have many such little round ponds in this district; and one in particular on our sheep-down, three hundred feet above my house; which, though never above three feet deep in the middle, and not more than thirty feet in diameter, and containing perhaps not more than two or three hundred hogsheads of water, yet never is known to fail, though it affords drink for three hundred or four hundred sheep, and for at least twenty head of large cattle beside. This pond, it is true, is over-hung with two moderate beeches, that, doubtless, at times afford it much supply: but then we have others as small, that, without the aid of trees, and in spite of evaporation from sun and wind, and perpetual consumption by cattle, yet constantly maintain a moderate share of water, without overflowing in the wettest seasons, as they would do if supplied by springs. By my journal of May, 1775, it appears that
"the small and even considerable ponds in the vales are now dried up, while the small ponds on the very tops of hills are but little affected."
Can this difference be accounted for from evaporation alone, which certainly is more prevalent in bottoms? or rather have not those
elevated pools some unnoticed recruits, which in the night time counterbalance the waste of the day; without which the cattle alone must soon exhaust them? And here it will be necessary to enter more minutely into the cause. Dr. Hales, in his Vegetable Statics, advances, from experiment, that
"the moister the earth is the more dew falls on it in a night: and more than a double quantity of dew falls on a surface of water than there does on an equal surface of moist earth."
Hence we see that water, by it's coolness, is enabled to assimilate to itself a large quantity of moisture nightly by condensation; and that the air, when loaded with fogs and vapours, and even with copious dews, can alone advance a considerable and never-failing resource. Persons that are much abroad, and travel early and late; such as shepherds, fishermen, &c. can tell what prodigious fogs prevail in the night on elevated downs, even in the hottest parts of summer; and how much the surfaces of things are drenched by those swimming vapours, though, to the senses, all the while, little moisture seems to fall.
Sam - thanks for that, I'm sure Gilbert White is at least partly right when he writes of
"shepherds, fishermen, &c. can tell what prodigious fogs prevail in the night on elevated downs"
During the day we have experienced this as a very fine rain if we are walking through it and it can generate so much moisture that full waterproofs are needed. Another aspect is grass. If the grass is lush then animals may not drink much from the dew ponds because they don't need to.
Thank you to Sam Vega. The lady had ancestors in Selborne in that period. We have paid our respects at White's grave. His idea's seem as good as any so far.
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