Dew pond

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A pond with three mules in the Albanian Lunxhërisë Mountains

Dew ponds , also called celestial ponds , are used to supply fresh water in areas without usable springs, near-surface groundwater or other tributaries, such as. B. in coastal regions and karst areas.

Taut oak there is, among other things on the mounds of islets ( Fething ) in Schleswig-Holstein and in East Friesland and in England ( "dew pond", "cloud pond", "crap pond" ) and z. B. also in Albania .

In general, the functioning of a dew pond is based on the formation and collection of dew in areas or at times with constantly high humidity (and secondary to the collection of other precipitation). The surface of the pond and the surrounding plants (overhanging grass) cool down below the dew point of the air at night , which means that these surfaces act as heat sinks on which air humidity is condensed ( fog condensation ). If the air is heavily saturated with water vapor day and night (near the sea, on mountains "in the clouds" (in cloud and cloud forests ), in damp rainforests, during incline rain in monsoon climates ), then only little water can evaporate during the day and a lot of moisture can condense at night . In spite of the use of the pond as a drinking water reservoir or cattle trough, the water level fills up again and again without any apparent inflow. The mode of action can be studied in dew ponds from historical times that still exist today or in plants with " cistern water " in leaf rosettes in a damp climate.

In-situ measurements of the evaporation and condensation were carried out at the Tauteich Helmfleeth in the municipality of Poppenbüll (peninsula Eiderstedt in Schleswig-Holstein, Germany). Meteorological measuring devices and a floating evaporation boiler according to Brockamp & Werner (1970) were used. The measurements provided clear evidence of dew formation due to the temperature differences and the weather conditions. The Tauteich Helmfleeth is part of the water supply for marshland areas and is still used today.

Meteorological measuring instruments for measuring evaporation in Tauteich Helmfleeth

Historical ponds

The historical dew-ponds mostly have a semi- elliptical longitudinal cross-section with a flat bottom and a steep slope. After the hollow had been excavated, it was covered with a sufficiently thick, water-impermeable layer of clay. This was followed by a thermal insulation layer made of reeds, straw, moss or peat and a second layer of clay for the top seal. Occasionally a layer of stone was added to stabilize the clay.

The air humidity is (apart from air convection) in equilibrium with the soil moisture , i. H. moisture constantly evaporates from the soil and moisture from the air constantly condenses into the soil. The thermal insulation made of plant-based material against the soil causes the pond water to cool down faster on clear nights through heat radiation than the soil surrounding the pond, which provides humidity. As a result of the nightly cooling, the (soil) moisture contained in the air condenses on the water level or at the edge of the water and flows towards it. Bank vegetation can increase dew formation (so-called mist catchers ). If the thermal insulation layer got damp because the upper or lower clay layer was leaking, the insulating effect decreased. The technique was used as early as the Neolithic .

Replicas of historical ponds

In 2014, the traditional technology was verified using modern building materials on replicas of ponds in East Frisia. For this purpose, different techniques were tried out in two earth hollows. Commercially available PVC foils were used for sealing, and foam glass gravel was used for insulation . The construction was carried out by volunteer craftsmen, the climatological studies by Werner and Coldewey.

Schematic section of a Tauteich replica with modern materials

General requirements for a pond

A seal of the insulation layer up and down z. B. clay or foil is necessary to ensure the insulating effect and to prevent the leakage of water into the ground. A border made of wood, which offers a wind protection and also causes little shade, is useful. Furthermore, a depth of at least 1.5 m and a total area of ​​more than 500 m² is recommended. Direct use of the pond by large grazing animals should be avoided because of the sediment turbulence and the associated increase in temperature due to the action of the sun. In addition, grazing animals' hooves can damage the waterproofing.

Web links

Individual evidence

  1. ↑ B. Brockamp, ​​J. Werner: A further developed evaporation meter for small bodies of water (as a contribution to the hydrological decade of UNESCO) . In: Meteorol. Round band 23 , no. 2 . Borntraeger, Stuttgart 1970, p. 53 - 56 .
  2. ^ WG Coldewey, J. Werner, C. Wallmeyer, G. Fischer: The secret of the heavenly ponds - physical basis of a historical water supply in the coastal area . In: Christoph Ohlig (Ed.): Writings of the DWhG . tape 20 , no. 2 . Siegburg 2012, p. 315-329 .
  3. ↑ J. Werner, WG Coldewey, C. Wallmeyer, G. Fischer: Der Tauteich Helmfleeth in St. Johannis-Koog, Poppenbüll community - measurements and calculations of the water balance 2010 . In: Jb. "Between Eider and Wiedau" (home calendar North Friesland) . Husum 2013, p. 1 - 10 .
  4. ↑ D. Meier, WG Coldewey: Water supply in the North Sea marshes from the Roman Empire to the early modern period . In: Christoph Ohlig (Ed.): 10 years DWhG, DWhG publications . tape 20 , no. 1 . Siegburg 2012, p. 249-260 .
  5. ↑ Stefan Siegert: Good for the soul , broadcast script at dradio.de
  6. ^ Arthur John Hubbard, George Hubbard: Neolithic Dew-ponds and Cattle-ways , American Anthropologist, July – September 1907, Volume 7, Issue 3, pp. 529-531.
  7. ↑ J. Werner, WG Coldewey, D. Wesche, H. Schütte, F. Schütte, H. Fähnders, R. Neumann: Studies of the water balance on two modern replicas of historical ponds on the North Sea coast . In: Christoph Ohlig (Ed.): Writings of the DWhG . tape 25 . Siegburg 2016, p. 349-369 .

See also