3.11 Fog, dew and snow harvesting

Technical description

Fog and dew are forms of precipitation, and, by helping to maintain high humidity, limit evaporation from the soil and transpiration from vegetation. Due to fine size of fog droplets (diameters range from about 1 Tm to 40 Tm), and their low velocity of descent (ranging from 1 cm/s to approximately 5 cm/s), moisture is carried readily by breezes of even low velocity. Hence, fog harvesting requires a nearly vertical surface as catchment area for its collection. In contrast, dew harvesting requires an horizontal surface. A gravel layer is commonly used in agricultural areas as a means of maintaining soil moisture by dew harvesting, while minimizing evaporative losses and increasing soil temperature. In the evening, the gravel layer cools and remains cool in the early morning, when water vapour condenses onto the gravel creating droplets which pass between the gravel particles and reach the soil surface, moistening the soil.

Snow, being another form of precipitation, can also be harvested to provide an alternative supply of freshwater. Snow harvesting requires the construction of a pit, generally ranging in size from about 6 to 8 metres in diameter and about 10 metres in depth. The pit is heavily compacted and the collected snow is dumped into the pit to a depth of 2 to 3 metres. The compacted snow is covered with earth, which acts as an insulator, and a bamboo tube is placed about 50 cm above the base of the pit to serve as an outlet. As the snow melts around the bamboo pipe, water trickles along the bamboo and into a pot placed beneath the outlet. The water collected in the pot may be used for household drinking water and can supply water to up to 14 families (UNEP, 1982).

Extent of Use

Fog and dew harvesting is practised in Gansu Province, in northwest China, where melons are cultivated with water supplied using dew harvesting techniques. The melons are cultivated in soil beds covered with a 10 cm to 15 cm thick layer of gravel. The pieces of gravel range from 2 cm to 5 cm in diameter, and have proven to be a satisfactory growing medium for melons. These farms are well known as the 'gravel fields for melons' in China (UNEP, 1982).

Applications of the traditional snow harvesting technology to augment drinking water supplies can be found in Takhar Province, Afghanistan.

Operation and Maintenance

The technologies for harvesting alternative forms of atmospheric moisture tend to be based upon simple precepts and are traditional freshwater augmentation techniques.

Level of Involvement

These technologies are generally implemented at the household and small community levels.

Costs

No cost data were available, but costs may be assumed to be negligible as the technologies make use of commonly available materials and implements.

Effectiveness of the Technology

Studies have shown that about 10 l/m²/day of fog-derived water can be collected from the vertical section of a tree. In the Dhofar region of southern Oman, water from fog was collected for 79 days at an average rate of 860 l/day. Thus, fog and dew harvesting technologies may be effective in supplying small volumes of water for specific, supplemental uses.

Suitability

These technologies are generally suitable for use in mountainous regions, where fogs are common and snowfalls occur.

Advantages

These traditional technologies are inexpensive and simple to implement.

Disadvantages

Fog and dew harvesting technologies do not utilize a reliable source of water; the occurrence of fogs, especially, is uncertain, although certain areas do have a known propensity for fog development (particularly, mountainous coastal areas on the western continental margin). Further, calculation of even an approximate quantity of water that can be obtained at a particular location is difficult (Schemenauer and Cereceda, 1994).

Cultural Acceptability

There are no known problems associated with the use of water harvested from fog, dew and snow.

Further Development of the Technology

A great deal of further research and experience with pilot scale projects is needed before these technologies can be considered to be fully developed.

Information Sources

Schemenaur, R.S. and P. Cerceda 1994. Fog Collections Role in Water Planning for Developing Countries, Natural Resources Forum, Vol. 18.

UNEP (United Nations Environment Programme) 1982. Rain and Storm water Harvesting in Rural Areas, Tycooly International Publishing Ltd. Dublin.