A friend of mine who farms in England recently took this photo of an ancient dewpond on his land and commented that,
"It's not so very long ago that this pond would have been a vital water supply for animals wintered in an outlying building at the top of the hill. These days this is about as much water as we ever see in it."
Being an interfering busybody, I immediately suggested that he figure out why it was drying up and attempt to restore it, and posted a couple of links that might or might not help.
"In geography at school I was taught that local lore suggested that the dewpond name was misleading and they were carefully sited on the top of the water table just back from the edge of escarpments. This would stack up quite well with the siting of this example. Due to this I'd always assumed that drier winters and water abstraction had lowered the water table which along with silting had stopped this one filling. Your link along with some that I found last night suggests that it may be more complicated. I think that some careful spadework may be in order"
Then true to his word, he promptly set out with a spade and dug a hole, reporting back that,
"I dug a 3 foot deep hole without finding anything. 24 hours later this is what it looked like. I suspect that the "magic" is down to the water table in this particular case."
This is a contour map of the area for anyone who might be interested.
"Most of the contour to the north and west is manmade. I think that the natural contour line would sit between the pond and the hedge to it's south-east side.
We're unlikely to recreate an open pond here as any poaching around it could encourage liver fluke in the livestock. However if I could drop in a cistern and pipe the water it should raise the level at which our free farm water supply can operate by just over 25 metres. That's a pretty enticing prospect!
At present I'm considering digging deeper and putting in a vertical 6 inch pipe to monitor water levels throughout the year. The major obstacle could be the fact that I'm fairly certain that the water table has risen over the last decade and it could easily fall again over the coming years."
Does anyone have any experience or relevant information that might be of use in the restoration project? Is the water table likely to be the only factor involved in this dew pond or does it rely on a combination of factors? Is digging out the silt all that is going to be necessary to restore it or will it need lining with layers of clay and straw as suggested by some of the links?
A true dew pond is a waterproof bowl whose surface is insulated from subsoil heat so that on clear nights it can cool off to temps below dew point and condense moisture on it. This moisture accumulates at the bottom of the bowl. The water level in a dewpond is self regulating in that as the water level rises, less of the insulated surface is exposed to the night sky so less dew is generated. As the water level falls, more insulated surface is exposed and more dew is generated. Dewponds in foggy locations are often positioned where fog drip from surrounding trees can drip onto it, increasing its water collecting ability.
The first thing that needs to be done with that pond is remove those trees growing in it whose roots have likely grown through and perforated its waterproof bottom. After removing the roots, reseal the bottom. Also if that pond is a true dewpond rather than one normally filled by rainwater runoff, you will also need to dig down and identify the waterproof layer (traditionally puddled clay) and the insulating layer (traditionally straw or peat) and remove any material that has accumulated on top of the original puddled clay. This additional material adds thermal mass to the insulated layer, reducing the rate at which it cools off at night. If the original insulating and waterproof layers are past repair, you could rebuild it using modern materials (styrofoam or other fairly non-compressable foam for the insulator and rubber pond liner for the waterproof layer).
I recently read about a similar situation that faced Vicktor Schauberger when he was in a forest at a spring. The problem was the lack of flora shading the spring.
As mentioned above, in order for this to accumulate dew it needs moisture, and by the look of the photos, the plants are removed sans a few trees. I know this sounds like a simple explanation, but I am following my intuition and coincidences in this case. Thank you for the links.
With a dew pond there is no spring involved. It is a perched (non-water table) pond that derives its water mostly from condensate. To get condensate, you need an open sky to which heat can be radiated at night so the temperature of the insulated (from geothermal heat) pond bottom can cool to temperatures below the dew point and start condensing moisture. A tree canopy over the pond would hold in the heat and prevent dew from accumulating (except for any dew forming and dripping off the leaves, a less efficient process of filling the pond since the tree adsorbs a lot of that moisture). Having transpiring vegetation nearby to raise the local relative humidity is good, but it can't over shadow the dewpond for the reason mentioned above.
The whole reason for making a dew pond is so you can have a non-water table water source that isn't dependent on a watershed to feed rain water to it. It can generate water even in the absence of precipitation as long as the relative humidity is high enough that an insulated surface exposed to a clear sky can drop to temperatures below the dew point. You can place a dew pond up on the top of a ridge and have a self-refilling, self-regulating water source without the need for pumps or other fancy hardware.
The main maintenance required by a dew pond is to maintain the waterproof layer (keep tree roots and heavy livestock from puncturing it) and the insulating layer underneath it (in some climates, rot and termites can degrade the hay often used to create the insulating layer and you might need to use some non-decaying alternative). The sky above the dew pond needs to remain open of trees so it radiate heat efficiently at night. The only time you would want to maintain some trees overhanging the dew pond was if the pond was located up on a ridge that was often enveloped in low clouds where the fog drip generated by the trees more than made up for the loss of open sky condensed dew. Especially in locations that don't get very many clear nights. Also if too much muck accumulates on the insulated pond floor over time, this added material increases the thermal mass of the insulated layer, slowing down its rate of temperature drop and the amount of time at night that its temperature is below dew point. So, depending on the biological activity of organisms living in the dew pond, its bottom might need to cleared of muck every few decades to main its efficiency. Ideally the insulated surface of a dew pond should be black so it radiates heat better at night.
Don't suppose anyone would be interested in fixing one or knows someone who's an expert in fixing them? We have a lovely dew pond which I think must have been punctured by my horses and by plants that had grown up through it. It used to be a great place for tadpoles which we now sorely miss!