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  • 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.


  • 4 Comments sorted by
  • I'm wondering is using modern materials would make this easy. Put down straw or other insulation, put pond liner over it, and that might be all you need. Or maybe put a layer of clay over the pond liner to add some heat capacity to it, to allow for a cool surface for which dew can condense.
  • I got as far as "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." and "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." Then stopped reading.  It is quite likely I missed something stated later on, but how do dew-ponds differ from normal ponds?
  • First of all, they go on to say that other investigations found they did collect water from dew.

    Anyway, they're insulated from the earth, via a layer of straw. On top of the straw, a waterproof layer of clay is deposited. The ponds are quite wide and shallow. What happens is the water collects in the pond, and heats up from the sun during the day. The heat doesn't escape very quickly into the earth because it's pretty insulated. Then, at night, the shallow edges of the pond are cooled via evaporative cooling. So you've got a situation where there is a body of warmth in the center surrounded by cold clay. This causes water vapor to condense from the warm air above the pond onto the cold surface around the pond.

    That's the technical explanation. On the empirical side, why in the world would somebody make such a pond shape, put a layer of straw down, then spend a lot of time sealing the top with clay to make a waterproof bowl above the straw if it didn't work?
  • sounds like a fun day project.

    ps i lol'd when i saw "pond liner". it comes up in every discussion on here :)

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