Fogs have the potential to provide an alternative source of fresh water in dry regions and can be harvested through the use of simple and low-cost collection systems. Captured water can then be used for agricultural irrigation and domestic use. Research suggests that fog collectors work best in locations with frequent fog periods, such as coastal areas where water can be harvested as fog moves inland driven by the wind. However, the technology could also potentially supply water in mountainous areas if the water is present in stratocumulus clouds, at altitudes of approximately 400 m to 1,200 m (UNEP, 1997b). According to the International Development Research Centre (1995), in addition to Chile, Peru, and Ecuador, the areas with the most potential to benefit include the Atlantic coast of southern Africa (Angola, Namibia), South Africa, Cape Verde, China, Eastern Yemen, Oman, Mexico, Kenya, and Sri Lanka.
Fog harvesting technology consists of a single or double layer mesh net supported by two posts rising from the ground. Mesh panels can vary in size. The ones used by the University of South Africa in a fog harvesting research project measured 70 m² (UNISA, 2008) whereas in the Yemen, a set of 26 small Standard Fog Collectors (SFC) of 1 m² were constructed (Schemenaur et al, no date). The material used for the mesh is usually nylon, polyethylene or polypropylene netting (also known as ‘shade cloth’) which can be produced to various densities capable of capturing different quantities of water from the fog that passes through it (UNEP, 1997b). The collectors are positioned on ridgelines perpendicular to prevailing wind and capture and collect water when fog sweeps through. The number and size of meshes chosen will depend on the local topography, demand for water, and availability of financial resources and materials. According to FogQuest the optimal allocation is single mesh units with spacing between them of at least 5 m with additional fog collectors placed upstream at a distance of at least ten times higher than the other fog collector. In South Africa, the University research project arranged several mesh panels together in order to expand the water catchment area and provide greater stability to the structure in windy conditions (UNISA, 2008).
The collector and conveyance system functions due to gravity. Water droplets that collect on the mesh run downwards and drip into a gutter at the bottom of the net from where they are channelled via pipes to a storage tank or cistern. Typical water production rates from a fog collector range from 200 to 1,000 litres per day, with variability occurring on a daily and seasonal basis (FogQuest). Efficiency of collection improves with larger fog droplets, higher wind speeds, and narrower collection fibres/mesh width. In addition, the mesh should have good drainage characteristics. Water collection rates from fog collectors are shown in Table 1 below.
| Project | Total collecting surface (m2) | Water collected (liters/day) |
|---|---|---|
| University of South Africa | 70 | 3,800 |
| Yemen | 40 | 4,500 |
| Cape Verde | 200 | 4,000 |
| Dominican Republic | 40 | 4,000 |
| Eritrea | 1,600 | 12,000 |
Sources: UNISA, 2008; Schemenauer et al, 2004; Washtechnology; FogQuest
The dimensions of the conveyance system and storage device will depend on the scale of the scheme. Storage facilities should be provided for at least 50 per cent of the expected maximum daily volume of water consumed. For agricultural purposes, water is collected in a regulating tank, transferred to a reservoir and then finally into an irrigation system that farmers can use to water their crops (UNEP, 1997b).
Operation and maintenance are relatively simple processes once the system has been properly installed. Nevertheless, an important factor in the sustainability of this technology is the establishment of a routine quality control programme which should include the following tasks (UNEP, 1997b):
Drought caused by climate change is leading to reductions in the availability of fresh water supplies in some regions. This is having an impact on agricultural production by limiting opportunities for planting and irrigation. Fog harvesting provides a way of capturing vital water supplies to support farming in these areas. Furthermore, when used for irrigation to increase forested areas or vegetation coverage, water supplies from fog harvesting can help to counteract the desertification process. If the higher hills in the area are planted with trees, they too will collect fog water and contribute to the aquifers. The forests can then sustain themselves and contribute water to the ecosystem helping to build resilience against drier conditions.
Atmospheric water is generally clean, does not contain harmful micro-organisms and is immediately suitable for irrigation purposes. In a number of cases, water collected with fog harvesting technology has been shown to meet World Health Organisation standards (UNISA, 2008; WaterAid, no date). The environmental impact of installing and maintaining the technology is minimal (WaterAid, no date). Once the component parts and technical supervision have been secured, construction of fog harvesting technology is relatively straightforward and can be undertaken on site. The construction process is not labour intensive, only basic skills are required and, once installed, the system does not require any energy for operation. Given that fog harvesting is particularly suitable for mountainous areas where communities often live in remote condition, capital investment and other costs are generally found to be low in comparison with conventional sources of water supply (UNEP, 1997b).
Fog harvesting technologies depend on a water source that is not always reliable, because the occurrence of fogs is uncertain. However, certain areas do have a propensity for fog development, particularly, mountainous coastal areas on the western continental margin of South America. Further, calculation of even an approximate quantity of water that can be obtained at a particular location is difficult (Schemenauer and Cereceda, 1994). This technology might represent an investment risk unless a pilot project is first carried out to quantify the potential water rate yield that can be anticipated in the area under consideration.
The costs vary depending on the size of the fog catchers, quality of and access to the materials, labour, and location of the site. Small fog collectors cost between $ 75 and $ 200 each to build. Large 40-m² fog collectors cost between $1,000 and $1,500 and can last for up to ten years. A village project producing about 2,000 litres of water per day will cost about $ 15,000 (FogQuest). Multiple-unit systems have the advantage of a lower cost per unit of water produced, and the number of panels in use can be changed as climatic conditions and demand for water vary (UNEP, 1997b). Community participation will help to reduce the labour cost of building the fog harvesting system.
It is generally recommended that the local population is involved in the construction of the project (UNEP, 1997; WaterAid, no date). Community participation helps to remove labour costs and also helps to ensure a sense of ownership by the community and a commitment to maintenance. A community management committee could be set up and consist of trained individuals responsible for repair and maintenance tasks, helping to ensure the long-term sustainability of the technology. In the initial stages, government subsidies may be required to buy raw materials and fund technical expertise.
A range of meteorological and geographic information is required for choosing a site to implement fog harvesting technology, including predominant wind direction and the potential for extracting water from fogs (such as frequency of fog occurrence and fog water content). A feasibility study and pilot-scale assessment should also be carried out to assess the magnitude and reliability of the fog water source. Some of this information can usually be gathered from government meteorological agencies but may require local meteorological stations and the use of a neblinometer (a device to measure the liquid water content) for collection of localised data (Box 1).
| “Global wind patterns: persistent winds from one direction are ideal for fog collection. The high-pressure area in the eastern part of the South Pacific Ocean produces onshore, south-west winds in northern Chile for most of the year and southerly winds along the coast of Peru.
Topography: it is necessary to have sufficient topographic relief to intercept the fogs/clouds. Examples on a continental scale, include the coastal mountains of Chile, Peru, and Ecuador, and, on a local scale, include isolated hills or coastal dunes. Relief in the surrounding areas: it is important that there are no major obstacle to the wind within a few kilometres upwind of the site. In arid coastal regions, the presence of an inland depression or basin that heats up during the day can be advantageous, as the localised low pressure area thus created can enhance the sea breeze and increase the wind speed at which marine cloud decks flow over the collection devices. Altitude: the thickness of the stratocumulus clouds and the height of their bases will vary with location. A desirable working altitude is at two-thirds of the cloud thickness above the base. This portion of the cloud will normally have the highest liquid water content. In Chile and Peru, the working altitudes range from 400 m to 1,000 m above sea level. Orientation of the topographic features: it is important that the longitudinal axis of the mountain range, hills, or dune system be approximately perpendicular to the direction of the wind bringing the clouds from the ocean. The clouds will flow over the ridge lines and through passes, with the fog often dissipating on the downwind side. Distance from the coastline: there are many high-elevation continental locations with frequent fog cover resulting from either the transport of upwind clouds or the formation of orographic clouds. In these cases, the distance to the coastline is irrelevant. However, areas of high relief near the coastline are generally preferred sites for fog harvesting. Space for collectors: ridge lines and the upwind edges of flat-topped mountains are good fog harvesting sites. When long fog water collectors are used, they should be placed at intervals of about 4.0 m to allow the wind to blow around the collectors. Crestline and upwind locations: slightly lower-altitude upwind locations are acceptable, as are constant-altitude locations on a flat terrain. But locations behind a ridge or hill, especially where the wind is blowing downslope, should be avoided.” Source: UNEP, 1997b |
Aside from hard data detailed in Box 1, expertise in the construction and maintenance of the fog harvesting technology is required and training should be provided to local communities to undertake regular quality control and equipment inspections.
Several challenges and issues have emerged from fog harvesting projects implemented to date:
Fog water collection has emerged as an innovative technology for mountainous communities without access to traditional sources of water. Still largely in a state of development, there is opportunity for research and development into fog harvesting technology and its potential to support agricultural production. Given the lack of mesh suppliers, using locally available materials for component parts presents an opportunity for local business development. This technology also provides an opportunity to restore natural vegetation and support agricultural practices through the sourcing of clear water for crops and livestock.
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