Design
Dew ponds occur mainly on porous soils where the rainfall is about 1m (40”) per year, and the annual evaporation from a body of open water is in the range of 450mm (18”). The traditional dew pond design ensures an adequate water supply from rainwater alone, by maximising the pond’s catchment area in relation to its evaporation area. The average dew pond is about 18m (20 yards) across its water surface with a further collection margin of at least 3.6m (4 yards).
A typical dew pond contains over 273,000 litres (60,000 gallons) of water, and even under conditions of drought, should last three months before going dry. However, if stock are watered at the pond, the rate of water loss will greatly increase.
The dew pond has a built up rim, which acts as a water catchment area. The cross section shows how the evaporation area shrinks as the water level drops. The vertical scale is exaggerated in the diagram. The maximum depth can vary from 1.2-2.4m (4-8’).
The various layers indicated are those of the ‘average’ dew pond, although Pugsley (1939), found that there were many variations. Straw is sometimes claimed to be a ‘non- conducting’ material critical to the pond’s performance, but there is no consistency in its use and many successful ponds have no straw. It is likely, however, that a thick layer of damp straw spread over the puddled clay would prevent its drying and cracking before the pond filled. It might also act as binding. The straw would rot eventually, but once the pond filled this would not matter. Some ponds have a bottom layer of chalk or lime, about 50mm (2”) thick, which may serve to keep earth-worms from penetrating the puddled clay. Some have a thick top layer of rubble, broken chalk or stone which helps protect the impervious layer from animals’ hooves.
Some ponds are made of concrete, and one has a layer of gas tar, showing the influence of more modern technology on an ancient craft.
Siting
In times past, the siting as well as construction of dew ponds was something of a mystery, a closely kept secret among professional pond-makers, who combined water- dousing with pond-making.
However, most so-called dew ponds are in fact aided by surface drainage, and locating the pond in a shallow depression will increase the catchment. Too much runoff is not desirable, as the pond will rapidly silt up. Ponds receiving runoff from cultivated land will silt up much more quickly than ponds in permanent pasture.
It may be possible to take advantage of runoff from minor roads and tracks, although this may also cause silting as well as problems from pollutants such as oil residues.
Site dew ponds well away from trees, as their roots may damage the pond lining.
