Little Drops of Water An Ocean Maketh

A look into various micro-water harvesting systems

The other day, my children and I tried to measure how much water got wasted through the dripping of a tap. We set the tap to drip in two speeds; first speed set to almost a drop every two seconds and the second speed set to a little more than that. We collected the droplets into a measuring flask for a set time and estimated water loss for a day from the data. It shocked the kids that they could lose as much as 10 to 40 litres a day from an innocuous dripping of tap! Another day when we were watering plants just before sun rise, we observed tiny dew drops on plants...pretty little pearls waiting to bounce off. One of my children wondered how many litres of water one could salvage from all the dew drops in the plants. That made us look up what people had done about “harvesting” dew. I got hold of a book by Girija Sharan titled “Dew Harvesting”, and what I discovered was astounding!

Dew, fog and mist harvesting are ancient skills people have used down the ages. Dew ponds still exist in southern England on slightly higher altitudes that retain water during summer too. The basic idea is to condense water from water vapour in the surrounding atmosphere through radiative cooling. The ancient dew ponds were excavated ponds which were then lined with a layer of dry straw and puddled clay. Straw cools faster and helps in quicker condensation. These ponds are usually shallow having depths of 3-4 feet and could have a diameter of about 30-70 feet. These dew ponds provided good sources of water for wandering shepherds and their sheep. Favourable conditions for condensation were of course a clear sky, as surfaces cooled much faster without clouds reflecting heat back to earth.

It’s possible to harvest dew from rooftops or other surfaces too. A typical dew harvesting system would consist of:

- a condenser, made of thin metal sheet or plastic film insulated on the underside;

- a water coolection and conveyance system

- and a storage system

The cooling surfaces should be made of materials with high emmissivity and be light in weight.

Fog as we know is a mass of water droplets suspended in air. When these droplets come in contact with an object they precipitate as water. In dry, coastal regions of Chile the camanchacas or fog even provide an effective alternative source for fresh water. Fog collectors are simple, flat rectangular nets of nylon supported by a post at both ends and arranged perpendicular o wind’s direction. If you visited Chungungo in Atacama Desert, you will be shown “the water net” by villagers with great pride. To harvest fog, 82 large square nets trap water that then drips into a tube. Water passes through an attached hose and pipelines to a holding tank.

The people of Chungungo, no longer have to depend on the water trucks that would make sporadic visits to their community. A villager, boasts of her new shower and shows off her garden. She says people can now shower every day and water their plants regularly. Another resident said she can now wash her clothes when she needs to, rather than having to wait for long periods of time. Chile’s fog-catching project, one of the first in the world, started with funding from the Canadian government.

It was an eye-opener for us...tiny airborne drops of water can go a long way if only you thought a little and devised a means of collecting them.

-Radha Eswar