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of Alternative Technologies for Freshwater Augumentation
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PART C - CASE STUDIES
5.2 Fog Harvesting in Chile
The far north of Chile, between the cities of Arica (latitude 18oS)
and La Serena (latitude 29oS), is classified as an arid or
semi-arid zone, depending on the rainfall. The Antofagasta area (latitude
23oS), on the eastern edge of the Pacific Anticline, is a
desert climate with virtually no rainfall. In these areas, natural
watercourses are few and highly seasonal. Hence, alternative sources of
freshwater are required.
Special atmospheric conditions occur along the arid coast of Chile and
southern Peru, where clouds settling on the Andean slopes produce what is
known locally as camanchacas (thick fog). The clouds that touch
the land surface can be "milked" or "harvested" to
obtain water. This technology is described in Part B, Chapter 1, "Freshwater
To capture the water from fog, rectangular obstacles constructed of
polypropylene mesh are employed. These are usually placed perpendicular to
the prevailing flow of the clouds. The "fog harvesters" are
positioned 1.5 m above the ground, and are supported on vertical posts.
The size of the harvesters depends on the topographical conditions and the
purpose for which the water is to be used.
Drops of water collect on the mesh, coalesce, and flow by gravity along
a plastic conduit at the bottom of the mesh to a receptacle for later
treatment (if required) and distribution.
This technology is being used in the area of Paposo (latitude 25oS),
180 km south of the city of Antofagasta, in the Paposo Protected Forest
Area administered by the National Forestal Corporation (CONAF). The site
is 750 m above sea level. In this area, CONAF is operating a Research and
Development Center for the Study of Flora, Fauna, and Human Activities. A
research facility has been built, housing two park rangers. This facility
is supplied with water by a fog harvester, which is described in detail
Type and design of harvester. The harvester is a
multiple (three) screen type, forming a single structure with a useful
surface area of 144 m2 (see Figure 6). It is composed of two
independent structures, one holding the posts upright and the other
supporting the mesh. Each of the structures has its own separate anchoring
system. The mesh employed is the Raschel 35% shade-type.
Post supports. The structure is supported by eucalyptus posts
impregnated with copper sulfate and creosote, 7 m long, with diameters
tapering from 30 cm to 15 cm. The base of each post rests in a hole 0.80 m
x 0.80 m x 1.0 m filled with rounded stones of approximately 20 cm
diameter and sand. The posts are further supported by a system of cables
held in place by cone-shaped anchors. Four posts supported by ten anchors
are provided for the installation. The holes for the post anchors are
excavated at a linear distance of 5.75 m from the base of the posts, or at
the end of a cable describing an angle of 45o relative to the
posts. Galvanized steel cables connect the anchors and the posts (all
cables are 6 x 7, 3/16-inch k-stem steel, shark type). The cables are
attached to the buried cone-shaped anchors by means of a 1.8 m, 5/8-inch
diameter bar extending from the anchor to the point of cable attachment
immediately above the soil surface. The posts are installed 10 m apart,
and are also interconnected with the 3/16-inch cable. These cables are
attached to the posts by means of 5/8-inch diameter, 7-inch-long eye bolts
using 5/8-inch coupling flanges.
Mesh supports. The mesh is supported by cables at the
top and bottom of each panel. These cables are fixed to two cone-shaped
anchors which are independent of those supporting the posts. Two
intermediate 1/8-inch-diameter plastic-coated cables pass through the
center and are interwoven with the mesh thread.
Mesh attachment. The mesh is attached to the posts with
two moisture-treated smooth-planed boards, 4.3 m long x 7 cm x 3.5 cm. The
mesh is wrapped very tightly between the two boards and held with
galvanized bolts, 5/8-inch in diameter and 15 inches long, which pass
through the post, the boards and the mesh. The cable that supports the
mesh from the top passes through two pulleys mounted on the end posts,
which provide the structure with a measure of flexibility vis-à-vis
the force of the wind. For its entire length, the lower cable is encased
in a high-density polyethylene tube which passes through a gap between the
two boards holding the mesh onto the posts. This cable is extremely
important as it supports not only the mesh but also the channel that
collects the water.
Water channel attachment. The channel is made of 110 mm
diameter PVC pipe, from which one-quarter of the circumference has been
removed along the entire length. The tube is suspended, cut side
uppermost, from the lower cable using 2.16 mm galvanized wire, attached at
various points to provide increased strength. At each end, the PVC tube is
fitted with a 110 mm x 40 mm cap. The water flows out of the tube, via a
T-junction and a 3/4-inch polyethylene pipe, to a storage tank (cistern).
Storage tank. The storage tank used with this system is
a 30 m3 closed cistern, built of waterproofed reinforced
concrete, and equipped with flow control and cleaning valves. The cistern
also has a hermetically sealed inspection hatch, and is built entirely
Extent of Use
This technology is of relatively limited applicability. While it lends
itself to use along the coastal zone of northern Chile and southern Peru,
wherever the hills are higher than the base of the cloud layer, it
requires a specific combination of climatic and topographical conditions
for best results. Such combinations of climate and topography are
uncommon, but do exist outside of this region, as is shown in Figure 7.
Operation and Maintenance
Operation is simple, requiring only periodic inspection of the
collection channels and the water supply lines to prevent blockages. Few
other difficulties are experienced in the operation of this technology,
the most common being that strong winds may cause the mesh to come loose.
This problem can be easily resolved provided it is detected in a timely
manner. Problems with the support structure are unlikely if it is properly
constructed. There is generally no difficulty in obtaining replacement
parts if needed. The operation and maintenance of this technology do not
require any specific level of training unless it is necessary to purify
the water, but even then this is usually a simple process.
Level of Involvement
Depending on the proposed use of the water, government organizations may
be directly involved in implementation and maintenance of the technology.
Nevertheless, this technology may be easily constructed and installed by
individuals using readily available materials.
The cost of the fog harvesting system was as follows:
- Post support structure $3 020
- Mesh support structure $2 089
- Storage tank $5 710
For purely reference purposes, the initial capital cost per m2
of mesh installed was $90, with maintenance and operation costing
approximately $600/year. The resulting unit cost of production is $1.41/m3.
Effectiveness of the Technology
The average annual volume of water harvested was 2.5 l/m2/day
in the Antofagasta area.
- The system requires a low level of investment, and is inexpensive to
operate and maintain;
- It is modular in construction, allowing production to be increased
incrementally as funds become available or as demand grows.
- It has no significant impact on the environment.
- The availability of sites at which to install the fog harvesting
system is limited.
- While the technology has few environmental impacts, the harvesting
structures may be visually intrusive.
Future Development of the Technology
While the technology meets the need for small volumes of water, future
development work should be directed toward increasing the yield from the
harvesters for larger-scale applications. In particular, if this goal is
to be achieved, studies need to be aimed at designing spatial distribution
systems that will increase the flow of fog into the collection area. Also,
it is important to bear in mind that, while the technology has proved
satisfactory, its successful implementation depends on the existence of
the correct combination of geographical and meteorological conditions.
Thus, a study of ambient meteorological parameters must precede any
proposed application of this technology, not only to determine if the
correct combination of topography and climate exists but also to
contribute to the understanding of these factors so that their occurrence
may be predicted. A sociocultural development project should also be
conducted at the same time to ensure that an appropriate organization
exists to manage the system in an appropriate and efficient manner.
Roberto Espejo Guasp, Universidad Católica del
Norte, Facultad Ciencias, Departamento Física, Avenida Angamos
0610, Casilla de Correo 1280, Antofagasta, Chile. Tel. (56-55)24-1148,
ext. 211, 312, 287. Fax (56-55)24-1724 / 24-1756. E-mail: respejo @
Luis Martínez, Regional Director, Corporación
Nacional Forestal (CONAF), Avenida Argentina 2510, Antofagasta, Chile.
Espejo G., R. 1992. Comportamiento de los Estratocúmulos en
Antofagasta (Chile) y su Potencial como Recurso Hidrico. In: Primer
Encuentro Iberoamericano de Meteorología, Salamanca, España,
vol. 2. pp. 157-160.
-, et al. 1992. Variabilidad de la Inversión de
Subsidencia en Antofagasta. In: Segundo Encuentro de Fisica Regional
Norte. La Serena, Chile, Universidad de La Serena. p. 210-215.
-, et al. 1993. Calidad del Agua de los Estratocúmulos
Costeros. In:Tercer Encuentro de Fisica Regional Norte, I Reunion
Internacional Andina de Fisica, Arica, Universidad de Tarapacá.
-, et al. 1994. Arbres Fontaines, Eau de Brouillard et Forêts
de Nuages, Sécheresse, 5(4).
Gioda, A., et al. 1992. Cloud Forest, Fog Precipitation and
Digital Elevation Model in Tropics, Annales Geophysicae, 10,
Suppl. II, p. 273.
-, R. Espejo G., and A. Acosta B. 1993. "Fog Collectors in
Tropical Areas." In:Proceedings of the International Symposium on
Precipitation and Evaporation, Bratislava, vol. 3, pp. 273-278.