rexresearch.com
Dew Pond
Construction details
See also : Air Wells, Dew Ponds and
Fog Fences: Methods to Condense Atmospheric Humidity...
http://www.rexresearch.com/airwells/airwells.htm
http://www.rexresearch.com/airwells/klaphake,htm
http://www.rexresearch.com/airwell3/airwell3.htm
http://www.rexresearch.com/ellsworth/ellsworth.htm
&c...
Dew Ponds ~
The water collectors known as "dew ponds"
were invented in prehistoric times, but the technology is nearly
forgotten today. A few functional dew ponds can still be found
on the highest ridges of England's bleak Sussex Downs and on the
Marlborough and Wiltshire Hills, and connected to castle walls.
They always contain some water that apparently condenses from
the air during the night. Gilbert White described a dew pond at
Selbourne (south of London), only 3 feet deep and 30 feet in
diameter, that contained some 15,000 gallons of water which
supplied 300 sheep and cattle every day without fail.
Investigations by UNEP (1982) and by Pacey
and Cullis (1986) determined that the ponds do not catch
significant amounts of dew, but actually were filled mainly by
rainfall. Pacey and Cullis may, however, have confused dew
precipitation with rainfall --- two different processes. The
ponds may also collect fog. (Ref 18)
Edward A. Martin proved that dew ponds are
not filled by precipitated dew because the water usually is
warmer than the air, so no dew could be deposited. He concluded
that mist condenses on the water already in the pond, or else
the grass collects dew which gravitates to the bottom and forms
a pond. Both mechanisms probably are active.
The ancient question, "Does dew rises from
the soil by evaporation or precipitates by condensation from the
air?" first was posed by Aristotle. John Aitken proved in 1885
that dew rises or falls as conditions allow. He also determined
the favorable conditions for the formation of dew: (1) a
radiating surface, (2) still air, and (3) moist, warm earth. The
ability of materials to capture dew depends on their specific
heats. The best material is swan's down, followed by flax or
cotton, silk, paper, straw, wool, earth, charcoal, silica sand,
and powdered chalk.
Arthur J. Hubbard described a dew pond in
his book Neolithic Dew-Ponds and Cattleways (1907):
"There is [in England] at least one
wandering gang of men... who will construct for the modern
farmer a pond which, in any suitable situation in a sufficiently
dry soil, will always contains water. The water is not derived
from springs or rainfall, and is speedily lost if even the
smallest rivulet is allowed to flow into the pond.
"The gang of dew-pond makers commence
operations by hollowing out the earth for a space far in excess
of the apparent requirements of the proposed pond. They then
thickly cover the whole of the hollow with a coating of dry
straw. The straw in turn is covered by a layer of well-chosen,
finely puddled clay, and the upper surface of the clay is then
closely strewn with stones. Care has to be taken that the margin
of the straw is effectively protected by clay. The pond will
eventually become filled with water, the more rapidly the larger
it is, even though no rain may fall. If such a structure is
situated on the summit of a down, during the warmth of a summer
day the earth will have stored a considerable amount of heat,
while the pond, protected from this heat by the non-conductivity
of the straw, is at the same time chilled by the process of
evaporation from the puddled clay. The consequence is that
during the night the warm air is condensed on the surface of the
cold clay. As the condensation during the night is in excess of
the evaporation during the day, the pond becomes, night by
night, gradually filled. Theoretically, we may observe that
during the day, the air being comparatively charged with
moisture, evaporation is necessarily less than the precipitation
during the night. In practice it is found that the pond will
constantly yield a supply of the purest water.
"The dew pond will cease to attract the dew
if the layer of straw should get wet, as it then becomes of the
same temperature as the surrounding earth, and ceases to be a
non-conductor of heat. This practically always occurs if a
spring is allowed to flow into the pond, or if the layer of clay
(technically called the 'crust') is pierced."
Additional construction details were
explained in Scientific American (May 1934):
"An essential feature of the dew-pond is its
impervious bottom, enabling it to retain all the water it
gathers, except what is lost by evaporation, drunk by cattle, or
withdrawn by man. The mode of construction varies in some
details. The bottom commonly consists of a layer of puddled
chalk or clay, over which is strewn a layer of rubble to prevent
perforation by the hoofs of animals. A layer of straw is often
added, above or below the chalk or clay. The ponds may measure
from 30 to 70 feet across, and the depth does not exceed three
or four feet.” (Figures 39 & 40)(Ref. 19)
Figure 39 ~ Dew Pond:
Dew Pond (Oxteddle Bottom, Sussex):
( Photo: Chris Drury )
Edward A. Martin also described their
construction in his book
Dew Ponds (London, 1917). In
particular, he notes that in order to ram the clay and puddle the
surface, horses are driven round and through the pond for several
hours. The base of the pond is planted with grass; without grass,
the pond dries up. Trees and brush are planted around the pond to
provide shade.
The simplest form of dew pond is used in
Cornwall, where areas of about 40 square feet are prepared on
mountain slopes by coating the ground with clay and surrounding
it with a small wall. The clay is covered with a thick layer of
straw that collects dew during the night. Straw is said to be
more effective than grass for the purpose. Since the straw is
moist both day and night, it rots quickly and must be replaced
frequently.(Ref. 20)
In his book, The Naturalist on the
Thames, published circa 1900, C. J. Cornish gave a
description of British dew ponds, excerpted here:
“The dew ponds, so called because they are
believed to be fed by dew and vapours, and not by rain, have
kept their water, while the deeper ponds in the valleys have
often failed. The shepherds on the downs are careful observers
of these ponds, because if they run dry they have to take their
sheep to a distance or draw water for them from very deep wells.
They maintain that there are on the downs some dew ponds which
have never been known to run dry. Others which do run dry do so
because the bottom is injured by driving sheep into them and so
perforating the bed when the water is shallow, and not from the
failure of the invisible means of supply. There seem to be two
sources whence these ponds draw water, the dew and the fogs...
“The fogs will draw up the hollows towards
the ponds, and hang densely round them. Fog and dew may or may
not come together; but generally there is a heavy dew deposit on
the grass when a fog lies on the hills. After such fogs, though
rain may not have fallen for a month, and there is no water
channel or spring near the dew pond, the water in it rises
prodigiously...
“The shepherds say that it is always well to
have one or two trees hanging over the pond, for that these
distil the water from the fog. This is certainly the case. The
drops may be heard raining on to the surface in heavy mists.”
Cornish quoted Gilbert White’s Journal
of May, 1775:“’[I]t appears that the small and even the
considerable ponds in the vales are now dried up, but the small
ponds on the very tops of the hills are but little affected’.
Can this difference be accounted for by evaporation alone, which
is certainly more prevalent in the bottoms? Or, rather, have not
these elevated pools some unnoticed recruits, which in the night
time counterbalance the waste of the day? " These unnoticed
recruits, though it is now certain that they come in the form of
those swimming vapours from which little moisture seems to fall,
are enlisted by means still not certainly known. The common
explanation was that the cool surface of the water condensed the
dew, just as the surface of a glass of iced water condenses
moisture. The ponds are always made artificially in the first
instance, and puddled with clay and chalk.
“Mr. Clement Reid… notes his own experiences
of the best sites for dew ponds. They should, he thinks, be
sheltered on the south-west by an overhanging tree. In those he
is acquainted with the tree is often only a stunted, ivy-covered
thorn or oak, or a bush of holly, or else the southern bank is
high enough to give shadow. ‘When one of these ponds is examined
in the middle of a hot summer's day’, he adds, ‘it would appear
that the few inches of water in it could only last a week. But
in early morning, or towards evening, or whenever a sea-mist
drifts in, there is a continuous drip from the smooth leaves of
the overhanging tree. There appears also to be a considerable
amount of condensation on the surface of the water itself,
though the roads may be quite dry and dusty. In fact, whenever
there is dew on the grass the pond is receiving moisture’.
“Though this is evidently the case, no one
has explained how it comes about that the pond surface receives
so very much more moisture than the grass. The heaviest dew or
fog would not deposit an inch, or even two inches, of water over
an area of grass equal to that of the pond. None of the current
theories of dew deposits quite explain this very interesting
question. Two lines of inquiry seem to be suggested, which might
be pursued side by side. These are the quantities distilled or
condensed on the ponds, and the means by which it is done; and
secondly, the kind of tree which, in Gilbert White's phrase,
forms the best "alembic" for distilling water from fog at all
times of the year. It seems certain that the tree is an
important piece of machinery in aid of such ponds, though many
remain well supplied without one.”
An improved form of traditional dew pond was
invneteded by A.J. Hubbard, et al., and granted British
Patent # 13,039 (1 March 1905), "Improvements in Reservoirs for
Collecting Dew". The complete specification is as follows:
"For the purpose of securing water supply in
the absence of rainfall, springs, or streams, reservoirs adapted
to collect dew have been constructed by excavating the ground
over an area and to a depth sufficient to form a basin of
adequate capacity and providing the same with a compound lining
comprising a bed of straw as a non-conductor and a superimposed
water-tight layer of clay.
"The action of such a reservoir depends on
the fact that the non-conducting layer of straw prevents the
tendency to the equalization of temperature of the clay layer
and water to that of the earth. The water, cooled by radiation,
consequently retains the coldness so caused, and the temperature
of the air in contact with it is reduced below the dew point.
Consequently the water vapour in the air condenses and collects
in the reservoir. Such reservoirs as heretofore constructed,
however, have been comparatively inefficient, and, unless on
chalk, liable to destruction by moisture, worms, or other
causes, disadvantages which it is an object of this invention to
obviate, for which purpose, according thereto the basin is
provided with a lining possessing both impermeable and
non-conducting properties. In practice this can be
advantageously effected by the use of a compound lining
comprising two layers of impermeable material with interposed
material that is non-conducting or is a bad conductor of heat.
In order that the non-conducting property of the lining be not
impaired it is necessary that the lining should be so formed
that the non-conducting material cannot become wet. For this
reason when absorbent non-conducting material is interposed
between two layers of impermeable material, the two layers of
impermeable material are continuously joined a the edges in a
watertight manner. For the purpose of increasing the evaporation
and consequently reducing the temperature of the surface upon
which the water vapour is to be condensed, a water-retaining
cover is preferably superimposed upon the upper layer of
impermeable material. Where necessary owing to the character of
the soil, a solid foundation is formed in the basin upon which
the lower layer of impermeable material is laid.
"The drawing shows, diagrammatically, in
vertical section, by way of example, a reservoir for dew
according to this invention, in which a is a concrete
foundation, b a layer of asphalt, c a layer of
asbestos, d a second layer of asphalt having its edge
joined to that of the lower layer b as at e so
as to completely enclose the asbestos c, and thus
prevent it becoming wet and its non-conducting property becoming
impaired; f is a layer of bricks of a porous nature
which are rapidly cooled to a low temperature owing to the
evaporation of the water absorbed by them; g is a stone
curb which serves to prevent the edges of the lining being
damaged.
"In some cases the concrete foundation a may
be dispensed with, as also the porous bricks f."
Another form of dew pond was invented by
S.B. Russell in the 1920s. According to the description in Popular
Science (September 1922), "A dew reservoir 30 feet square
will collect 24,000 gallons of water in a year, or an average of
120 gallons daily during the hot summer months and 50 gallons
daily for the remainder of the year...
"The Russell reservoir consists of a
concrete cistern about 5 feet deep, with sloping concrete roof,
above which is a protective fence of corrugated iron which aids
in collecting and condensing vapor on the roof and prevents
evaporation by the wind. The floor of the cistern is flush with
the ground, while sloping banks of earth around the sides lead
up to the roof.
"Moisture draining into the reservoir from
the low side of the roof maintains the roof at a lower
temperature than the atmosphere, thus assuring continuous
condensation.
"At one side of the reservoir is a concrete
basin set in the ground. By means of a ball valve, this basin is
automatically kept full of water drawn from the reservoir."
(Figure 41)(Ref 21)
Russell's Dew Pond:
http://www.countrylife.co.uk
02 June 2006
Dew
Ponds
to
the
Rescue
by
Sandy Mitchell
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 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; <a href="http://www.lakesandgolfcourses.com"
target="_blank")
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
BTCV Handbooks Online
handbooks.btcv.org.uk
Waterways & Wetlands
Chapter 387
A look at ponds and waterways
New ponds
... 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
penetratinDesign</h3> 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,
howeveg 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.
Wiltshire Council
http://www.wiltshire.gov.uk
Wiltshire
History
: Dew Ponds
Question Date :3rd January 2003
Question:
Are the dew ponds on the Wiltshire downs fed by dew and how old
are they?
Answer:
Dew ponds are
actually fed by rainwater and they are normally situated in a
slight depression so that there is a reasonable sized catchment
area for rain. The amount of dew falling in a year is around half
an inch whereas the rainfall will be between 30 and 40 inches a
year. The water retention properties of the dew ponds lay in their
making and the appreciation of the margins of the pond as a
catchment area. The bottom layer of the pond was puddled chalk or
clay, which was normally covered by straw, laid as it would have
been on a thatched roof. This was then covered by a mixture of
loose materials such as chalk rubble, sand, flints or gravel.
Most existing dew ponds date from the 19th or early 20th
centuries, although a few may be 18th century. The only apparent
ancient one is Oxenmere on Milk Hill on the downs to the north of
the Vale of Pewsey. A Saxon charter of 825 refers to this pond as
marking the boundary of Alton Priors, which it still does. It is
possible that a pond has been here since that date but only if it
has been cleaned out and its lining renewed every 100 to 200 years
for Ralph Whitlock estimated that the life of a dew pond is 100 to
150 years.
The ponds were about eight feet deep and would often be fenced,
with a small gap that nothing bigger than a sheep could get
through. This saved the bottom of the pond being damaged by
cattle. Dew pond makers tended to tour the country between
September and April making up to 15 ponds, depending on their
size. It would take four men about four weeks to construct a fair
sized pond. Well known dew pond makers at the end of the 19th
century were Charles White, Joel Cruse, Jabez Earley and Daniel
Pearce of Imber. The decline of sheep on Salisbury Plain and the
downs lessened the need for these ponds and it is believed that
the last one in Wiltshire was made by the Smith family of West
Lavington in 1938.
http://www.jstor.org/stable/1777822
DEW PONDS
By
Edward A. Martin, F.G.S
Author of "Dew Ponds: History,
Observation, and Experiment."
What is a Dew Pond ?
The subject of the dew
pond is one of perennial interest, and questions as to how it
works and where it is to be found, and what is its secret, are
frequently asked.
The ponds to which the name has been applied are found as a rule
on high chalk country, and hence they have been closely connected
with the chalk downs which extend from Beachy Head to the west of
Sussex.
People have noticed that they rarely run dry, even in the hottest
summer, and it is apparent that they receive during the night a
supply of water sufficient to counter-balance the great draughts
that are made upon them by cattle and evaporation during the day.
It has been assumed, therefore, that the dew which falls so
heavily on the grass of the downs during the hot summers and
autumns also falls into the ponds, and thus makes up for any loss
during the' day-time, whether by cattle or by evaporation.
That this is pure assumption has, I think, been satisfactorily
shown, but the idea is a fascinating one, and the mysterious
filling of the dew pond will no doubt for a long time still
exercise the minds of the curious.
Owing to the bringing of water by pipes and bore-holes to many
lonely downland farms, it is sad to notice that many of the ponds
have been allowed for want of a little attention to leak their
contents away.
When once they are made they require so little attention, and they
seem to be so perennial, that successive farmers neglect them. The
growth of water-weeds and rushes is rapid, and unless these are
cleared away from time to time the ponds are bound to leak.
The roots of such vegetation find their way through the
foundations and provide many channels for the water to pass
through. They may, too, be repaired carelessly. One fine pond on
the Sussex Downs that I knew was not only cleared of its weed, but
all the valuable chalky puddle, which formed its waterproof bed,
was cleared away also. The result has been that it will no longer
hold water. Neglect and carelessness has been the ruin of most of
the Sussex dew ponds, and really fine ones are now few and far
between.
I have used and still use the term " dew pond "
for these upland ponds, but it may be startling to some to be told
that there is no such thing as a dew pond. You may ask a farmer
where he gets his water from, and he will answer you from the dew.
If you ask him what becomes of the rain, or the mists that roll
over the downs, he will say, well, that is just the same as dew ;
everything that comes out of the air is the same as dew. As a
matter of fact, the name " dew pond " was not
known much more than a century ago. They were known as "
mist ponds." This name has been met with in Surrey, Kent,
and Wiltshire, and at Worms Heath, and the name of "fog pond" or
"cloud pond" was at one time well known at Hampstead.
Gilbert White noticed the phenomenon of high-lying ponds and wrote
about them in 1776.
Miall wrote, " It is plain that the water in such ponds
is not drawn from springs, nor from surface drainage, nor wholly
from rain.
Being a native of the south-down country I have always been
interested in these ponds, although for some years my interest did
not extend beyond the newts and the pond-snails that they
contained. But some time ago I was awarded a grant by the Royal
Society to make actual observation and experiments into the
working of dew ponds. I occupied a disused windmill on Clayton
Down, so as to be on the spot day and night, and so catch the dew
in the act, so to speak, if perchance it had anything to do with
the filling of ponds. My experiments extended over a period of
three years, during which I observed the habits of a good many
ponds in the central area of the South Downs between the coast and
the Weald.
Dew in the strict meaning of the word can never feed a pond. It is
formed from the moisture in the air being in contact with the
cooled earth when this has radiated its heat after nightfall.
Formation of dew on grass is, of course, a very common phenomenon.
But in three months' observations on a pond there were but five
occasions when the water was found to be below dew-point. Four of
these were between 7 a.m. and 8 a.m., and the rising heat of the
sun would have soon done away with this state of affairs. But even
where most likely to be formed, it has been estimated that the
annual dew-fall does not exceed 1.5 inches.
Night after night I have watched for the semblance of dew being
formed on the banks of a pond, but without success. The grass on
the down may be wringing wet with dew, but it was impossible for
this to trickle down into the pond. A few grasses with shallow
roots growing in a pond might cause the formation of some dew, but
in the best -kept dew ponds the grasses are not allowed to grow.
Only on one occasion out of many hundreds of observations did
there appear in one pond to be a slight increase of water during a
clear night when no rain fell. I was forced to conclude that the
name "dew pond" was a misnomer, and nothing has
appeared since the period of my observations to shake the
stability of my conclusions.
All my observations, went to show, as was to be expected, that the
temperature of water of a pond rarely went down below dew-point
during the night. In hot summer weather, when with the
accumulation of heat the temperature of the water gradually rose
in the day-time, there was insufficient time during the short
summer nights for radiation to proceed to such a degree that its
temperature went down below dew-point. The specific heat of water
is, of course, very great, and its changes of temperature are very
gradual, whereas changes on the grass, on the dry bank of a pond,
and in the atmosphere, are more rapid.
These changes are shown in the diagram, from which it will be seen
that in a short summer night the temperature on the grass went
down to 54° F., whilst that of the water, although sympathetically
following the former, went down only to 66° F. If dew can ever be
deposited on the water a much longer period of radiation is
necessary than can as a rule be found in a summer night. Of course
the water appears to be cool to the hand, but that is only because
the hand is of a much greater temperature.
The construction of the bottom of a pond must be such as to ensure
that it be watertight. In Sussex, finely-powdered chalk is almost
invariably used, and this is worked into a puddle, giving a whitey
colour to the water when disturbed by cattle. Clay is used in some
parts, and this is of course more easily worked into a puddled
condition. In western Sussex clay is found here and there in
pockets on the Downs, and where this is the case it is brought
into requisition. But if it has to be brought up from the gault
clay below the hills, the cost of making a pond is considerably
augmented, and chalk puddle is used. In the remaking of the
Chanctonbury ponds a few years ago sufficient clay was found near
at hand for the purpose. The whole secret of getting the bottom to
be waterproof lies in the finely-divided condition to which the
chalk or clay is reduced. This is frequently done by driving a
team of horses and a broad-wheeled cart round and round the pond
for an hour or more each day, so as to reduce to powder any lumps
that remain. An old labourer told me that when he was a boy he was
employed for this purpose. After the broad-wheeled cart had done
its work, the puddle was flattened out with a spade, until it was
quite smooth. The margin as treated in the same way, and thus
nearly all the rain that fell ran down into the pond. When the
bottom is made of clay it is the practice to mix the puddle with a
certain amount of lime, and this prevents the working of worms.
These creatures can be very destructive to the waterproof bottom
of a pond.
Although straw is never used, so far as could be ascertained, in
making ponds in Sussex, it is used considerably in Wiltshire and
Yorkshire. But there is a considerable division of opinion as to
why it is used. One on Thorpe Downs, near Lough-borough, was
stated by Mr. Slade to be laid down as follows: First, about 12
inches of clay, mixed with some lime, then a layer of straw, to
prevent the sun cracking the clay, and then a layer of loose
rubble.
During 400 years it only leaked in one year, and this was caused
by the roots of rushes which penetrated the clay. In Yorkshire,
Mr. Mortimer said that there were very many ponds in the Midwolds.
In constructing them, straw is placed on the impervious bed of
beaten clay, to prevent the broken chalk, which is strewed on the
clay, from being trodden into the clay. On the Wiltshire Downs,
straw is cut into short lengths and mixed with the clay to prevent
it cracking and letting the water through. But some of the ponds
there are of more complicated construction, consisting of three
layers of clay alternating with three layers of straw. The straw
would prevent a good deal of slipping and cracking of the clay,
but it would, of course, be thoroughly compressed by the weight of
the clay, and would also be in so thoroughly a water-logged
condition that it would be useless as an encouragement to
precipitation, and its non-conducting power would be lost.
Diagrammatically, the basis of some of the ponds that have come
under notice are shown here, and it will be seen that there is
considerable variety in
construction.
Gilbert White noticed the contrast between those ponds which were
situated at an elevation above the surrounding country and those
which were situated at the lower levels. Many of the latter are,
of course, fed by brooks, and when these dry up in the course of a
hot summer the ponds also suffer. No one questions the source of
their water. It is quite apparent. But it only occurs to an
observant mind to ask the question why water is still found in the
higher ponds when the lower ones are dried up. If the rainfall
were spread equally over all months of the year then ponds would
never run dry. But it is not so, and those months which are most
liable to drought are just those months when the higher ponds
furnish a supply of water in spite of the drought below. Further,
there is evaporation from the surface of a pond to consider. This
is very considerable, and it is, of course, only the difference
between the two that will be of service in feeding a pond. During
four years (1909 to 1912 inclusive) I found the average rainfall
to be 43-61 inches on Clayton Down, considerably more than had
been anticipated. The most reliable experiments of evaporation
that I have been able to find, extending over thirty years, gave,
at Croydon, an average of 18-14 inches. The difference between the
two, namely, 25-47 inches, would be the amount of rain that would
go to feed the pond, supposing that the surface-area of the pond
was the only collecting ground. But the area of the banks around
the pond form a collecting area at least as much again as the
pond-surface, and sometimes twice or more than that area, so that
the total of rain feeding the pond must be multiplied accordingly.
But it must also be remembered that a good deal of rain falling on
the bank percolates into the soil. Some banks are of loose
material and others are found to have been rammed hard. With the
best of ponds perhaps not ,more than a half of the rainfall flows
down into the pond.
Thus we have a total as follows, when the banks are twice the area
of the pond.
On the pond surface 25-47 inches, on the banks half of twice 43-61
inches, or a total of 69-08 inches. A wide bank twice the area of
the pond is quite a common occurrence. Unfortunately there is
least rainfall in the months when there is the greatest
evaporation. Nine-tenths of the total evaporation occurs in the
six summer months, and only about a third of the rainfall, so that
in order to account for the filling of the ponds we must look to
some other recruit. This is to be found in mists and fogs.
No one can be on the watch on the Downs for many weeks together
without being struck by the frequency and density of the mists.
Rolling up from the coast they fill up all hollows, before
apparently jumping off at the escarpment facing the Weald.
Sometimes when they disappear masses are still seen filling up the
hollows of the ponds. During my experiments I distinctly found
occasions when there was a slight rise in the surface of a pond,
when rain was but a slight factor. These were always cloudy or
foggy nights or days, or so windy that the deposition of dew was
out of the question. One has but to walk on the downs in a thick
mist to experience the quantity of water that they give out. Not
only is the grass wetter than even after the heaviest dew, but
one's clothing becomes reeking with moisture. Bushes can be heard
dripping their loads on to the grass or fallen leaves beneath. If
such bushes are' on the edge of a pond their moisture will in part
trickle down into it. It has, therefore, been advocated that if
possible trees or bushes should be planted at the edge of, or
overhanging, the pond. Such would undoubtedly add to the water in
it, but this would not be dew. And the difficulty would remain of
getting trees or bushes to grow in such exposed positions. But it
must be confessed that all the best ponds have no trees or bushes
on or near them. By a gradual process of elimination I was forced
to the conclusion that there was no source of moisture but mist or
low clouds to account for the fact that well-made ponds do not dry
up in the summer.
Real dew, that is, dew formed out of the low-lying layer resting
immediately upon the soil, is almost pure water. It occurred to me
to ascertain how the water from ponds compared with pure water. To
do so I obtained specimens of water from eighteen ponds, and these
were analysed. The quantity of chlorine found in them was
noticeable, and this was probably brought in by winds from the
sea, or by mists blown in from the same source. One specimen was
obtained soon after a pond had dried up, -but had been partly
filled again by rain. It contained the lowest proportion of
chlorine of the whole series. But the mists that blew in from the
sea were probably condensed around finely-divided salt nuclei, and
when these fell into pond-areas they would gradually increase the
salinity of the water, owing to the process of evaporation which
is always going on. Thus those ponds that had been in existence
for the longest time would have the most chlorine, and this was
fully borne out by the analyses of three pond-waters which have
never been known to fail. As a contrast, the water of a
pond was analysed into which there fell a good deal of animal
pollution. It was a clay-puddled pond. The amount of chlorine
therein found was com-parable to that contained in sewage, and
this was but to be expected. The total hardness of all chalk
-puddled ponds was naturally great. When soils from the Downs have
been analysed these have always been found to contain chloride of
sodium, and this is gradually washed downwards into the sub-soil.
But in carefully-prepared ponds this remains, and accumulates, and
hence their increasing salinity as time goes on. As the salt-laden
mists roll up from the sea their particles are, as I hold,
deposited by gravity wherever arrested by a depression. As the
clouds which give rise to rain are formed in the same manner,
these must contain a good deal of salt also, and when it rains
this will also fall. But in the summer months it is the deficiency
of rain that has given rise to the phenomena ofdew-ponds ; hence I
think we must look entirely to mists for the explanation of a
constant supply of water when there is a deficiency of rain. Most
authorities have observed that when a pond is first dug out it is
advisable to give it some artificial assistance, and this is done
by pouring water over the puddle or by heaping snow around it when
that is possible. A water surface thus appears to favour the
further deposition of water out of the mists. My own observations
did not go to show that after misty nights there occurred any of
those great rises in the surfaces of ponds which have been from
time to time recorded, and I am of opinion that these increases
have been exaggerated, but herein lies ground for further
experimental work.
Details of experiments which I carried out on the South Downs,
together with thermometric tables of many observations which I
made, will be found in "The Geographical
journal," for August, 1909, and October, 1910, and can
there be referred to, if desired.
As regards the age of the ponds. As the earliest men in our
country probably dwelt on the Downs they must have in some way
provided themselves with a water supply. They must have made ponds
for this purpose, having noticed that when chalk had been
well-trampled out by cattle it became watertight. Such ponds may
thus have been made by Early Man, but it requires a keen
imagination to say that the existing ponds date from the time of
neolithic man. As man took to the lower lying country he probably
neglected his upland ponds. Many new ones may have been brought
into use when the downs once more came to be used as sheep-walks,
during the wool-growing times of the mediaeval sheep-masters, and
it is possible that some of the present-day ponds date from the
times of the immigrant Fleming. This, however, is purely
conjectural.
Originally published as No 64 in the School Nature Study Union
series. Copyright © 2002
<martin(at)dewponds.info>Martin Snow and contributors as
noted. All rights reserved.
http://www.wiltshire.gov.uk/community/getprinted.php?id=156
Wiltshire Community History
Title :
The Dew Pond Makers of Imber
Author :
Edgar
Glanfield
Date : 1922
WAM Vol. 42, pages 73-5
Full Text : A valuable
article by the Rev. Edgar Glanfield, Vicar of Imber,
appeared in the Wiltshire Gazette, Dec. 29th, 1922, in which he
sets down information as to the method of making these ponds,
gained directly from living parishioners of Imber, who in past
years carried on a regular and hereditary business of dew pond
making -Charles Wise, aged 81, Joel Cruse, aged 79, both
master dew pond makers, and Jabez Earley and Daniel Pearce, both
nearly 80 years old, their assistants. A great deal has been
written on the subject of the way in which dew ponds gain their
water supply, but it is generally believed now that they are
chiefly dependent on rain. Mr Glanfield however, is concerned only
with their formation.
“Up to ten years ago the dew pond makers started upon their work
about the 12th of September, and they toured the country for a
period of six or seven months, making in sequence from six to
fifteen ponds, according to size and conveniences, in a season of
winter and spring..... They travelled throughout Wiltshire and
Hampshire, and occasionally into Somersetshire arid Berkshire, and
even into Kent.” The dew pond maker with three assistants at 18s.
a week, would require about four weeks to make a pond 22 yards, or
one chain, square. Providing all his own tools and
appliances he would charge about £40 for the work. “ 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, and
continued, course by course until the sides of the basin attain to
the normal level of the site. Only so much work with the
layers of materials set in order, is undertaken in one day as can
be finished at night, and this must be covered over with straw and
steined. No layering may be done in frosty or inclement
weather. And this is the method of construction:- seventy
cart loads of clay are scattered over the area, suggested
above. 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. After it has been hammered in twice, a second
coat of lime is applied, to the thickness of half-an-inch, which
is wetted and faced to save the under face. A waggon load of
straw is arranged and the final surface is covered with rough
earth to the thickness of nine inches. The pond when
finished affords a depth of water of seven feet." It is then
fenced round to keep off cattle and horses, whose hoofs, would
break through the bed, and admit sheep only, for whose use the
ponds are made. The durability of the dew pond is put at
“perhaps 20 years, though “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 (1921). The decay of the industry is attributed
partly to the greatly increased cost of the making of the ponds,
and partly to the fact that they have been superseded by the
windmill pumping water from wells.
Mr. Edward Coward, of Devizes, had an excellent letter in the
Spectator, January 14th, 1922, p. 47, on the method of making Dew
Ponds in Wiltshire. He says “the site is first excavated,
and the soil taken out thrown up as a bank so as to lengthen the
shore of the pond. A start is made from the centre. A
layer of clay about three inches thick when loose, is strenuously
and methodically rammed. Then lime is spread, and it is
rammed again. Two more layers of clay and lime are treated
in the same way. The work is built up from the centre, not
sectionally up the sides. Each day's work is carefully
covered with straw; this, for the moment, is to prevent the puddle
from drying and cracking. When the whole area is treated it is
covered with a layer of straw more than a foot thick. This
in turn is covered with nine inches of chalk
rubble. The object of the straw is to protect the puddle
from indentations which might be made by the rubble until it is
properly set. A pond made in this way, thirty feet square at
the edge of the puddle area, took seventy small cartloads of clay
and about twelve tons of lime. I have heard, of course, of the
straw being put under the clay, and am aware of the insulating
theory involved. I cannot conceive, however, how a puddle
could be made good on the top of a springy substance like
straw. Firm ground to ram upon is the very essence of this
method of construction.” He regards rain as the most important
factor in the filling of the ponds. “In my opinion the whole
surface of the hollow in a pond which is used daily by sheep
becomes puddled by the action of their hoofs, and with the
exception of the first rainfall after a drought, practically the
whole of the rain which falls finds its way to the water.
Abbreviations used: WAM Wiltshire Archaeological & Natural
History Magazine
http://www.buildagardenpond.com/build/gardenponddesigns.html
More
Pond Designs
Clay puddling, I am glad to report, is an art in pond construction
that went out with handlebar mustaches and high-wheeled bicycles.
It never was much of an art, in my opinion, but I don't think any
discussion of pond building would be complete without some mention
of it, for it was one of the pioneer techniques of water
gardening.
Puddling with Clay
The builder of a clay-puddled pond first made a concave
excavation, sloping the sides carefully at an angle of not more
than 40 to 45 degrees. He then lined the concavity with a 6- to
8-inch layer of the coarsest straw he could find. On top of the
straw he put a 6-inch layer of clay, which he moistened and tamped
down into the straw as tightly as possible.
All the clay-puddled ponds I have seen required a constant trickle
of water into them to make up for that lost by seepage. Spring or
stream water, always 15 to 20 degrees colder than still water
warmed by the sun, maintained a pond temperature too cold for good
(frequently too cold for any) water-lily growth. A trickle piped
into the pond from a household water system would be even colder,
and expensive to boot.
The clay linings of these ponds attracted crawfish as spilled
molasses attracts flies. After a couple of them had burrowed
through the pond's lining, many a pond enthusiast in the old days
discovered that overnight his beautiful water garden had become
just a mud hole, so to remain until he patched the holes and
refilled the pond.
Clay-puddled ponds are now blessedly of the past, and good
riddance. Any structure that will enable a couple of crawfish to
ruin a man's whole summer just isn't practical enough to bother
building.
DEW PONDS
Natural dew ponds, also known as cloud ponds and mist ponds, are
near-miraculous works of nature. They just happen, sometimes, in
shady places that remain constantly damp, even in hot, dry
weather. The mystery of them is their principal fascination. They
have no apparent source of water, and yet they thrive, their cold,
clear waters never seeming to diminish.
A few English water gardeners with plenty of time and even more
patience have been able to construct them and make them work. An
artificial dew pond is built, much as a puddled-clay pond,
although on a much smaller scale. The layer of straw that goes
into the excavation is much thicker, perhaps a foot or more of it.
The clay lining is tamped in upon the straw in the conventional
way.
Nature does the rest, sometimes with a little help in the
beginning. In time, the pond fills with rain water. The nest of
straw serves as insulation, so that the clay shell and the water
in it remain somewhat cooler at all times than the surrounding
air. Dew condenses nightly on the cool clay banks, runs down into
the little pond, and replaces moisture lost by seepage and
evaporation.
An amateur should not attempt to build a dew pond, because few of
them work out. As only the very hardiest of aquatic growths can be
coaxed to live in one, even if it does turn out satisfactorily,
the water gardener receives little reward for his labors.
Dewponds - Gazetteer - Brighton
Area
Brighton and Hove City Council own most if not all the farmland
withinand adjacent to their borders.
Much is on lease to tenant farmers, who have to pay a rent to the
finance department, it might be thought that this arrangement
could be at variance to any desire for conservation that the
ratepayers may have.
All the council owned farmland is administered by the Estates
Section of the Performance and Resources Department of the City
Council. I was pleased to note that there is a Countryside
Service, which has been involved in a number of restorations of
Dewponds, which lie on land owned by the city, though
not necessarily within it's administrative borders. They are also
responsible, among other duties, for the ongoing management of the
ponds for wildlife and amenity.
I am pleased to feature this contribution from one of their
rangers :-
From David Larkin (Countryside Ranger)
Having worked on the restoration of several dew ponds for Brighton
& Hove Council.
The modern method is:
Re-profile the site (silt builds up in the centre more than the
edges) Remove large flints (to prevent liner being damaged )
Spread Teram over the area ( a tough protective "cloth")
Spread liner, we were using butyl but are now using heavy gauge,
virtually tear proof 0.5mm PVC (which can be welded into 20x40m
strips in the factory) as it is a lot cheaper
Spread another layer of Terram
Spread a layer of clay about 1ft thick
A liner is necessary in most ponds now as we no longer have the
patter of sheep's feet to re-puddle the clay as the pond refills
over the winter.
Straw was used to protect the liner and should not rot too quickly
in anaerobic conditions but Terram is stronger and should not rot
at all. The clay can be quite a problem, quarry bottoms are
cheapest, but the wrong sort of clay slumps and exposes the liner.
Unfortunately this method is not very successful where there are
cows present as they puncture the liner.
I have seen an example of a stone pitched pond in the Chilterns (I
think) but would be interested in any ideas on how to construct a
cow proof dew pond.
Excavation
Prior to restoration I have had some trial excavations done, there
was no evidence of any clay in the pond Bevendean. I assume this
was puddled chalk of which there are references in the literature
(the base for new coast road at Black Rock was created by spraying
and rolling the chalk).
The pond on Hollingbury Hill shown on the 1970 map was originally
built of local clay, then concreted over, before being backfilled
with the remains of an adjacent barn to make a green for the golf
course in the 1930's. This subsequently went out of use and the
whole area was densely scrubbed over prior to restoration.
Hopefully the drawings from this excavation will get published
eventually.
Puddling
I seem to recall Oxen were used for puddling, pulling a wide
wheeled cart full of stones around, this would have been during
relatively dry weather, possibly laying a layer, wetting it,
puddling it, then laying another layer, etc, I think this is how
the chalk ponds were done. Quite different from having them in the
pond all year.
I remember reading of experiments with mica at the turn of the
century into the heat effects around dew ponds, I think they were
inconclusive as vandals ended the experiment prematurely,
From memory Brighton Council/Brighton & Hove Council have
restored the following dew ponds
Ditchling Beacon (following clearance of munitions by army)
Lotts Pond, Stanmer Woods (concrete, 1980's MSC team)
Housdean Farm
Bevendean Down
Varncoombe Hill, Waterhall Farm
Sweet Hill, Waterhall Farm
Piddingworth, Stanmer Park
Rock Pond, Standean Farm
Tanners Pond, Standean Farm
Falmer Hill, Falmer Court Farm (Removal of silt)
and created dew ponds at Waterhall East Brighton Golf Course
http://www.jstor.org/stable/659048
PDF, 7 MB