Newsletter and Technical Publications
of Alternative Technologies for Freshwater Augumentation
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3.11 Fog, Dew and Snow Harvesting
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
No cost data were available, but costs may be assumed to be negligible
as the technologies make use of commonly available materials and
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.
These technologies are generally suitable for use in mountainous
regions, where fogs are common and snowfalls occur.
These traditional technologies are inexpensive and simple to implement.
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).
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
Schemenaur, R.S. and P. Cerceda 1994. Fog Collections Role in Water
Planning for Developing Countries, Natural Resources Forum, Vol.
UNEP (United Nations Environment Programme) 1982. Rain and Storm
water Harvesting in Rural Areas, Tycooly International Publishing Ltd.