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Dew Ponds to the Rescue

By Sandy Mitchell

Dew Ponds to the Rescue

Could there be a very simple answer to drought in England asks Sandy Mitchell.

Already, the search for remedies to what it is feared will be the direst water shortages in southern England since the summer of 1976 (despite May being unusually wet) has led to discussion in the House of Commons of scenarios that sound like science fiction, including icebergs towed from the Arctic Ocean, and a giant desalination plant on the Essex coast. Yet nothing could be simpler than a dew pond.

These small ponds can be found scattered across the downs, wherever sheep and cattle traditionally went for summer grazing, in Hampshire, Sussex, the Peak District and Yorkshire. Surviving ponds probably number at least 500 across the country, although they are often overgrown nowadays, their banks badly trampled by livestock, serving as little more than picturesque havens for butterflies or a romantic spot for picnicking ramblers.

But they still have a magical and highly significant property. 'People have noticed that they rarely run dry, even in the hottest summer, and it is apparent that, during the night, they receive a supply of water sufficient to counter-balance the great drags that are made upon them by cattle and evaporation,' notes Edward Martin, in a research paper entitled Dew Ponds: History, Observation and Experiment.

The great mystery is where the water that fills them up at night can come from. These ponds—also known as 'mist ponds' or 'fog ponds' lie on the downs far above the level at which streams begin to form, nor does any piped-water supply reach them. The name 'dew pond' is the clue. According to folklore, it is the overnight dew itself, falling on the round-backed downs and on the ponds themselves, that keeps them full, whatever the weather.

If that really is the case, then surely water companies and the government should be thinking not only of mega-projects such as a national water grid to bring down water from Scotland, but also of encouraging farmers in suitable areas to harvest the dew with new ponds. Dew ponds could even be something that a house owner, with a big enough garden and on high enough land, might see as a fashionable eco-friendly accessory to match his heat-exchanging borehole or roof-top windmill. Far better, after all, than relying on standpipes.

To create a dew pond is relatively simple. According to Jackson House, a Somerset-based pond builder with 50 years' experience in the business, 'the secret of making one is to insulate it so that the water remains colder than the earth beneath. That means that when the dew is falling, it hits the cold surface of the pond and drops its own moisture. In the old days, people used to put down layers of straw and layers of clay in the bottom, which worked the same as a thermos flask'. He estimates the cost of digging a typical 10yd by 10yd dew pond, and of lining it with a tough waterproof layer laid over an insulating geo-textile blanket, would work out at no more than £12,000.

Of course, it was a more romantic and much tougher task back at the turn of the century when the last specialist gangs were creating them by hand in the age-old way, as this description in the Wiltshire Gazette of December 29, 1922, goes to show: 'Up to ten years ago, the dew pond makers started upon their work in September, and they toured the country for a period of six or seven months, making in sequence from six to fifteen ponds in a season of winter and spring.

The dewpond maker, with three assistants, would require about four weeks to make a pond 22sq yd. The work commenced by the removal of the soil to the depth of eight feet. The laying of the floor is then proceeded with from the centre, called the crown, four or five yards in circumference, and to this each day a width of about two yards is added.

'Only so much work is undertaken in one day as can be finished at night, and this must be covered over with straw. No layering may be done in frosty or inclement weather. And this is the method of construction: 70 cart loads of clay are scattered over the area. The clay is thoroughly puddled, trodden and beaten in flat with beaters, a coat of lime is spread, slaked, and rightly beaten until the surface is as smooth as a table, and it shines like glass.'

Descriptions follow of yet more stages of laborious hammering of the ground, and wetting it, then coating it with further layers of lime, straw and earth. The cost of this Herculean labour was a meagre £40, the wages of three men included. 'There are ponds in good condition now which were made 36 years ago, and which have never been known to fail to yield an adequate supply of water even in this year of drought,' concluded the Gazette's correspondent.

One man who is currently on a quest to resolve the abiding mystery of dew ponds is Martin Snow, an IT consultant based in Worthing. In his spare time, he marches around the hills from East Sussex to Beachy Head as part of a university study, and the very first task he set himself was to locate remaining dew ponds.

'It is becoming like a treasure hunt,' says an eager Mr Snow. 'Occasionally you get a hint of a pond, then go back to the maps, and find, on different editions, that they appear or disappear.' By his calculation, there are as many as 100 to 200 in West Sussex alone, some of which may have begun as watering holes dug by Neolithic man for his livestock.

He goes on to point out that dew ponds were strategically positioned to make the most of mist and of rainclouds billowing up from the nearby coast to the chilly heights of the downs, where any water that collects is less likely to evaporate. 'Effectively you are often in the cloud up here, and, if it is chilled enough, it will condense. Some people say an overhanging tree will help a pond a lot and I can believe that because, if you go out walking when the mist on the downs is extremely thick, you will find trees dripping with moisture.'

So it seems that dew ponds are indeed fed by dew and are truly droughtproof. 'It seems magical, but when you start looking at the numbers, it starts to make sense. Dew ponds work,' he concludes.

who can build me a dew pond?

House Bros & Bailey: Hillside Mill, Yeovil, Somerset (01935 433358; www.lakesandgolfcourses.com)

Land & Water: Albury, Surrey (01483 202733, www.land-water.co.uk)
White Horse Contractors: Abingdon, Oxfordshire (01865 736272; www.whitehorsecontractors.co.uk)
Miles: Bury St Edmunds, Suffolk (01359 242 356, www.miles-water.com)

Comments

Henry Gibson

There is a new material that is available to make dew ponds.

Glass foam bricks can be made from most old glass. The foam glass itself is water proof. They can be machined into forms that could make pond bottoms. The insulating properties are never lost. Rock or concrete tile overlay could prevent damage from hoofs. The bricks may be resistant enough. The bricks could be keyed together to avoid displacement but remain flexible for ground movements. Two or more layers of bricks or tiles, layed with thin seams of bentonite, should be water tight. The glass foam bricks might have to be made extra heavy if not covered the appropriate puddle or they will float.

They should be layed on an appropriate puddle. Bentonite clay might be used in it for improved water tightness. Lime as used in old times may also improve tightness. Old concrete cracks can probably be made water tight with lime and bentonite clay puddle. This puddle could be high pressure injected through the cracks to behind the concrete. The swelling of the bentonite and the reaction of the lime to carbon dioxide in the air create a dynamic seal.

The insulated water of any dew pond is dark as is the bottom of the pond, and this is an excellent radiator of heat. The pond water is naturally cooler than the earth surrounding it because the sun cannot heat it up as quickly because of its high heat content.

Old writings on dew ponds indicate that they would not allow water to build up beyond a certain depth, but would refill to that depth if water were removed. There may be an optimum depth for dew ponds, and above that level, water should be drained into a tank. The reason for the puddle above the straw was to keep it dry so that it retained insulating properties. It may be that the operation of a dew pond relied upon the water changing temperature more rapidly than deep soil can.

There may also be a minimum size for dew ponds. I would recommend that a large shallow plastic tank be mounted on wheels and towed to various sites. The bottom and sides of the tank would be insulated with glass foam bricks. Measurments of accumulations or loss could be made by weighing.

One method of obtaining small amounts of water from a dry river bed is to dig a hole to the moist soil and place a collection container in the bottom. A plastic sheet is placed over the hole and sealed with dirt around the hole. A small rock is placed in the center of the plastic sheet to cause the vapour consensing on the sheet to run to that point and fall to the container below. Prior to placing the plastic sheet, a small plastic tube could have been inserted into the drip container to allow the removal of the water without distubing the installation.

Large piles of large rocks were used in the middle east in ancient times to collect water from the air.

Cool rocks that allow air to infiltrate under a Tarmac road surface will condense the water in the air, and this water has caused frost damage to the surface after freezing.

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