Adew pond is an artificialpond usually sited on the top of a hill, intended for watering livestock. Dew ponds are used in areas where a natural supply of surface water may not be readily available. The name dew pond (sometimescloud pond ormist pond) is first found in theJournal of theRoyal Agricultural Society in 1865.[1] Despite the name, their primary source of water is believed to be rainfall rather thandew ormist.[2][3]
They are usually shallow, saucer-shaped and lined withpuddledclay,chalk ormarl on an insulating straw layer over a bottom layer of chalk or lime.[5] To deter earthworms from their natural tendency of burrowing upwards, which in a short while would make the clay lining porous, a layer of soot would be incorporated[6] or lime mixed with the clay.[7] The clay is usually covered with straw to prevent cracking by the sun[7] and a final layer of chalk rubble or broken stone to protect the lining from the hoofs of sheep or cattle. To retain more of the rainfall, the clay layer could be extended across the catchment area of the pond.[3] If the pond's temperature is kept low, evaporation (a major water loss) may be significantly reduced, thus maintaining the collected rainwater. According to researcher Edward Martin, this may be attained by building the pond in a hollow, where cool air is likely to gather, or by keeping the surrounding grass long to enhance heat radiation.[8] As the water level in the basin falls, a well of cool, moist air tends to form over the surface, restricting evaporation.[9]
A method of constructing the base layer using chalk puddle was described inThe Field 14 December 1907. A Sussex farmer born in 1850 tells how he and his forefathers made dew ponds:
The requisite hole having been excavated, the chalk was laid down layer by layer, while a team of oxen harnessed to a heavy broad-wheeled cart was drawn round and round the cup shaped hole to grind the chalk to powder. Water was then thrown over the latter as work progressed, and after nearly a day of this process, the resultant mass of puddled chalk, which had been reduced to the consistency of thick cream, was smoothed out with the back of a shovel from the centre, the surface being left at last as smooth and even as a sheet of glass. A few days later, in the absence of frost or heavy rain, the chalk had become as hard as cement, and would stand for years without letting water through. This old method of making dew ponds seems to have died out when the oxen disappeared from the Sussex hills, but it is evident that the older ponds, many of which have stood for scores of years practically without repair, are still more watertight than most modern ones in which Portland cement has been employed.[4]
The initial supply of water after construction has to be provided by the builders, using artificial means. A preferred method was to arrange to finish the excavation in winter, so that any fallen snow could be collected and heaped into the centre of the pond to await melting.[10]
The discrete occupation of 'dew pond maker existed, but was rare in the UK, by the end ofWorld War II.[11]
The mystery of dew ponds has drawn the interest of many historians and scientists, but until recent times there has been little agreement on their early origins. It was widely believed that the technique for building dew ponds has been understood from the earliest times, asKipling tells us inPuck of Pook's Hill: "the Flint Men made the Dewpond under Chanctonbury Ring."[13] The twoChanctonbury Hill dew ponds were dated, fromflint tools excavated nearby and similarity to other dated earthworks, to theNeolithic period.Landscape archaeology too seemed to demonstrate that they were used by the inhabitants of the nearbyhill fort (probably from an earlier date than that of the survivinglate Bronze Age structure) for watering cattle.[14][15] A more prosaic assessment fromMaud Cunnington, an archaeologist from Wiltshire, while not ruling out a prehistoric origin, describes such positive interpretations of the available evidence as no more than “flights of fancy”.[16] A strong claim to antiquity may, however, be made for at least one Wiltshire dew pond: Aland deed dated 825CE mentions Oxenmere (51°22′33″N1°50′54″W / 51.375960°N 1.848221°W /51.375960; -1.848221) atMilk Hill, Wiltshire, showing that dew ponds were in use during theSaxon period.[16][17] Theparliamentary enclosures of the mid eighteenth to mid nineteenth centuries caused many new upland ponds to be made, as access to traditional sources of drinking water for livestock was cut off.[3] The suggestion has also been made that the nursery rhyme aboutJack and Jill may refer to collecting water from a dew pond at the top of a hill, rather than from a well.[18][19]
The naturalistGilbert White, writing in 1788, noted that during extended periods of summer drought the artificial ponds on thedowns above his nativeSelborne, Hampshire, retained their water, despite supplying flocks of sheep, while larger ponds in the valley below had dried up.[20] In 1877 H. P. Slade observed that this was because the lower ponds have debris washed into them from surface water drainage, making them shallow, but the higher ones do not: the smaller volume of water is depleted more rapidly.[21] Later observations demonstrated that during a night of favourable dew formation a typical increase in water level of some two or three inches was possible.[14] However, there remains controversy about the means of replenishment of dew ponds. Experiments conducted in 1885 to determine the origin of the water found that dew forms not from dampness in the air but from moisture in the ground directly beneath the site of the condensation: dew, therefore, was ruled out as a source of replenishment.[6] Other scientists have pointed out that the 1885 experiments failed to take into account the insulating effect of the straw and the cooling effect of the damp clay: the combined effect would be to keep the pond at a lower temperature than the surrounding earth and thus able to condense more moisture.[14]
In 1919 architect George Hubbard, in a lecture at theRoyal Institute of British Architects, described how he constructed a 100-foot (30-metre)-square dew pond to demonstrate how to capture at night the large volume of water vapour contained in air warmed during the previous day. The lining of the pond incorporated 2,500 slabs ofmica, 2-foot (0.61-metre)-square and of 2-inch (51-millimetre) thickness, set in sand,pitch andasphalt, for insulation from the warm earth. After a rainless autumn night he found that, above the insulated slabs only, a thick layer ofhoar frost had formed. As the morning progressed the frost melted to produce "hundreds, if not thousands, of gallons of water", but this rapidly dried out in the heat of the sun.[22]
In turn these conclusions were disproved in the 1930s, when it was pointed out that the heat-retaining quality of water (itsthermal capacity) was many times greater than that of earth, and therefore the air above a pond in summer would be the last place to attract condensation. The deciding factor, it was concluded, is the extent of the saucer-shaped basin extending beyond the pond itself: the large basin would collect more rainfall than a pond created without such a surrounding feature.[23]
In 1979 naturalistRalph Whitlock suggested an alternative origin of the name: after dismissing as "hopelessly wrong" the proposal that "dew" was a corruption of the French languaged'eau ("water"), he described how theroyal archives at Windsor Castle contained a reference to a certain "Mr Dew" as pond maker toKing George III in Hampshire and Surrey. Thus, Dew became aneponym for the artificial downland pond.[24]
The first scientific experiments to measure and correlate the rate of dew deposit with evaporation were made byHarry Pool Slade ofAston Upthorpe, Berkshire, between June 1876 and February 1877, at a dew pond on Aston Upthorpe Downs (51°32′58″N1°13′20″W / 51.54944°N 1.22222°W /51.54944; -1.22222). Slade measured overnight dew deposit (by weighingcotton wool when dry and after overnight exposure), evaporation from copper pans beside the pond, the depletion of the pond, andrelative humidity. He found that on days with heavy overnight dewfall the level of water in the pond was not replenished but invariably diminished.[21][25][26]
In situ measurements of evaporation and condensation were taken at the Helmfleeth dew pond inPoppenbüll municipality (Eiderstedt Peninsula in Schleswig-Holstein, Germany) using meteorological measuring instruments and a floating evaporation pan after Bernhard Brockamp & Julius Werner (1970).[27] These measurements proved the dew formation on the basis of temperature changes and the weather conditions.[28][29] The Helmfleeth dew pond is part of the water supply for a marsh area and is still in use today.[30]
In 2014, the traditional technique was verified by means of modern building material at reproductions of dew ponds inNorth Friesland. In this context, various techniques were tried in two terrestrial hollows. Commercially available PVC-film was used for the sealing and foam glass gravel for the insulation. The construction was carried out by craftsmen and the climatological analysis by Julius Werner and Wilhelm Coldewey.[31]
Dew ponds are still common on the downlands of southernEngland, theNorth Derbyshire andStaffordshire moorlands, and inNottinghamshire.
The idea that the so-called dew-ponds are really filled by dew has been satisfactorily disposed of by Mr. H. P. Slade.
[…] a thick mist arose from the pond […] and rolled away over the downs, leaving a strong dew deposition in its track.
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History, observation, and experiment.(2nd. ed. 1915)
Construction of dew ponds.
dew ponds.
