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 was 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 of dew-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 Flemings. This, however, is purely conjectural.
Originally published as No 64 in the School Nature Study Union series.