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.
What is Fog Harvesting & How does Fog Harvesting Work
Fog harvesting provides an alternative source of freshwater through a technique used to capture water from wind-driven fog. Fog harvesting systems are typically installed in areas where the presence of fog is naturally high, typically coastal and mountainous regions. The systems are usually constructed in the form of a mesh net, stabilized between two posts that are spread out at an angle perpendicular to the prevailing wind carrying the fog. As the wind passes through the mesh, drops of freshwater form and drip into an underlying gutter, from which pipes lead the water into a storage tank.
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.
Table 1: Water collection rates from fog collectors
|Project||Total collecting surface (m2)||Water collected (liters/day)|
|University of South Africa||70||3,800|
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):
- Inspection of mesh nets and cable tensions to prevent loss in water harvesting efficiency and avoid structural damage
- Maintenance of nets, drains and pipelines to include removal of dust, debris and algae
- Maintenance of the storage tank or cistern to prevent accumulation of fungi and bacteria
- Where spare parts are not available locally, it is recommended that a stock of mesh and other components be kept in reserve as local supply might be restricted, especially in remote mountainous regions.
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.
Fog Harvesting System Implementation
Fog harvesting systems are best installed in open locations with a fairly high elevation that are exposed to wind flow. Meteorological and climatic information such as predominant wind-flow direction might have to be gathered to identify optimal placement. After technical setup, training may also be necessary to introduce the system and its maintenance requirements to the local community. Thick fog, high wind speeds, and tighter mesh material can all improve the efficiency of the harvesting system. The water-harvesting rate ranges between 5.3 litres per m2/day and 13.4 litres per m2/day depending on the season, location and type (material used) of the harvesting system (Organization of American States, n.d.). Water collected from fog harvesters can be used for a wide range of purposes, including potable water, irrigation, and other domestic applications. The mesh, typically nylon, polyethylene or polypropylene netting, is tightly spread between two firmly planted posts, usually wooden poles. The size of a fog harvesting system can vary greatly, with the smallest being around 1 m2 unit, and the largest spanning up to 1600 m2 (Dar SI Hmad, n.d.). An underlying gutter collects the water drops that fall from the mesh. The drops are diverted to a separate water storage tank where the water can be collected and used. Maintenance includes routine check-ups and cleaning of mesh nets, pipes and tanks to remove dust, algae, bacteria, etc. to ensure maximum efficiency and maintain water quality.
Advantages of the Fog Harvesting System Technology
- Does not require energy to operate.
- Decreases pressure on local freshwater reservoirs in low water availability periods.
- 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).
- Provides an additional source of freshwater in dry coastal and mountainous regions, thus increasing quality of life in communities.
- Provides generally clean water that can be used immediately after harvesting.
- Minimizes costs and the need to transport freshwater into the area, which is difficult to reach.
Disadvantages of the Fog Harvesting System Technology
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.
Financial requirements and costs
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.
Institutional and organisational requirements of 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).
Box 1: Key information requirements for assessing fog harvesting suitability
|“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.
Barriers to fog water harvesting implementation
Several challenges and issues have emerged from fog harvesting projects implemented to date:
- Where fog is a seasonal source, water has to be stored in large quantities for dry season use (WaterAId, no date)
- If not properly maintained, water quality becomes an issue during low-flow periods
- Fog water collection requires specific environmental and topographical conditions, limiting its application to specific regions
- Procurement and transportation of materials is hindered by remote locations and steep terrain
- Strong winds and snow fall can result in structural failure during the winter season
- Water yield is difficult to predict, requiring feasibility studies prior to large scale implementation
- For harvesting to be effective, frequent fogs are needed and sufficient water collected for the investment to be cost-effective. This limits the technologies to areas with specific conditions.
- There are few commercial producers of mesh currently in operation, with main suppliers located in the Chile. There is none in Africa, North America or Asia (FogQuest). Therefore implementation and maintenance can be costly due to import or transportation.
- Harvested volume can be difficult to predict, particularly in long term, as it depends on the presence of fog, wind speed, etc.
- In some coastal regions the quality harvested fog water for drinking is inferior due to high concentrations of chlorine, nitrate and minerals
- Large fog harvesting constructions may damage or impede flora and fauna
- Harsh weather conditions such as very strong winds and snowfall can damage harvesting systems
Opportunities for for harvesting technology implementation
- Relatively simple technology. Once established, it can be run by the community and little maintenance is required
- Cheaper source of freshwater compared to some other non-conventional sources of water supply, such as desalination
- Diversification of freshwater resources in areas where freshwater access is limited increases climate resilience
- Materials for system components can often be sourced locally, creating local business opportunities
- 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.
Technological maturity: 4-5
Initial investment: 1-3
Operational costs: 1-2
Implementation timeframe: 1-2
* This adaptation technology brief includes a general assessment of four dimensions relating to implementation of the technology. It represents an indicative assessment scale of 1-5 as follows:
Technological maturity: 1 - in early stages of research and development, to 5 – fully mature and widely used
Initial investment: 1 – very low cost, to 5 – very high cost investment needed to implement technology
Operational costs: 1 – very low/no cost, to 5 – very high costs of operation and maintenance
Implementation timeframe: 1 – very quick to implement and reach desired capacity, to 5 – significant time investments needed to establish and/or reach full capacity
This assessment is to be used as an indication only and is to be seen as relative to the other technologies included in this guide. More specific costs and timelines are to be identified as relevant for the specific technology and geography.
- UNEP-DHI Partnership- Fog harvesting
- IDRC (International Development Research Centre) (1995) Reading Clouds in Chile, IDRC Reports, Ontario.
- Schemenauer, R.S., P. Osses, and M. Leibbrand (2004) Fog collection evaluation and operational projects in the Hajja Governorate, Yemen. In: Proceedings of the 3rd International Conference on Fog, Fog Collection and Dew, Cape Town, South Africa, 38.
- Schemenauer, R.S. and P. Cereceda (1994). Fog collection's role in water planning for developing countries. Natural Resources Forum, 18, 91-100, United Nations, New York.
- UNEP (1997) Sourcebook of Alternative Technologies for Freshwater Augmentation in Some Countries in Asia, UNEP, Unit of Sustainable Development and Environment General Secretariat, Organisation of American States, Washington, D.C.
- NISA (University of South Africa) (2008) Research Report, UNISA. Cape Town.
- WaterAid, Technical Brief: Rainwater Harvesting, no date
- Cho, R. (2011). The Fog Collectors: Harvesting Water From Thin Air. Earth Institute, Columbia University. Available at: http://blogs.ei.columbia.edu/2011/03/07/the-fog-collectors-harvesting-w…
- Dar Si Hmad (n.d.) Fog-Harvesting. Darsihmad.org. Available at: http://www.darsihmad.org/fog-harvesting/
- Gur, E. and Spuhler, D. (n.d.) Fog Drip. Sustainable Sanitation and Water Management (SSWM). Available at: http://www.sswm.info/content/fog-drip
- OAS (n.d.) 1.3 Fog Harvesting. Organization of American States. Available at: http://www.oas.org/dsd/publications/unit/oea59e/ch12.htm
- Quezada, A., Haggar, J., Torres, J. and Clements, R. (n.d.) Fog Harvesting. ClimateTechWiki. Available at: http://www.climatetechwiki.org/content/fog-harvesting
Wikipedia (2016). Fog Collection. Wikimedia Foundation. Available at: https://en.wikipedia.org/wiki/Fog_collection