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Mohammad Rahimi 1, Reza Baradaran Rad 2 1 Meteorological Organization, Tehran, I. R. of Iran 2 Mahab Ghodss Consulting Engineering Company, Tehran, I.R. of Iran Abstract In some regions because of special meteorological and topographical characteristics fog occurs regularly. The leaves of trees collect the droplets in fog. Recently technology helps man to get more water by artificial fog collectors. Many of Arid and Semi Arid countries supply their water for domestic, agriculture and forestry uses from fog collection projects. Since Iran is one of these countries, fog collection has been noticed and it takes initial steps. The fog collection concepts and also theoretical analysis of fog collection in Iran has been discussed in this paper. Introduction It is generally accepted that there is an ever-going need to identify new source and methods for collecting fresh water in both the developed and developing countries specially Arid and Semi Arid regions. With and ever increasing population and the intensified use of land for agriculture purpose it is clear that serious consideration needs to be given to unconventional water supplies (1). Trees or other tall vegetation collect the droplets from fog. They can also be collected by appropriately designed man made collectors, to provide large volumes of water for domestic, agriculture or forestry uses (2). Fog Collection : The potential for extracting water from fog is a function of the amount of water it contains, how often it occurs, and wind speed. The fog water content depends upon its altitude. Its frequency of occurrence depends on regional atmospheric circulation, the temperature of the ocean water and the stability and intensity of the thermal inversion processes (2). If the climatic phenomenon that produces the fog is stable, fog will form regularly; however, its behavior may vary from one area to another and specific seasonal variations may occur. On South America's Southwest coast, especially in Chile, the fog-producing condition are constant throughout the year, although more intense during the spring and summer months and less during fall and winter. One of the largest projects has provided, since March 1992, an average of 11000 litters of water per day to a village of 330 people in arid coast desert of North Chile (2). There were 22 countries on six continents where literature references to the collection of fog by trees or small collectors would support and evaluation of the amount of water that could be produced by operational fog collection arrays. A broader look at the meteorological and oceanographic condition on a worldwide basis, as well as the topography, will lead to the conclusion that many other countries may have the potential to benefit from fog collection programmers (2). Geographical Considerations for Choosing Fog Collection Site : Since the clouds are carried to the site by the wind, and the fog in then moved through the collectors by the wind, the interaction of the large-and small- scale topographical features with the wind will enlarge part determine the success of the site chosen. A number of the most important geographical factors will be briefly reviewed here. Persistent winds from one direction are ideal for fog collection.
· The region of the stratocumulus cloud, which normally has the highest liquid content, is between 400 m to the direction of the wind bringing cloud and fog from the ocean. · One should try and work as close to the coast as possible, ideally within 5 km but possibilities exist up to at lest 25 km inland. · It is very important that there be no major obstacle to the wind within a few kilometers of the site. · An upwind ridgeline that there be no major obstacle to the wind within a few kilometers of the site. · The presence of an inland depression or basin that will heat up during the day will cause a local low-pressure area. This will enhance the sea breeze and increase the wind speed with which marine cloud decks flow over the terrain (2). Collector : The fog collectors are flat rectangular nets supported by a post at either and arranged perpendicular to the direction of the prevailing wind. They can be simple units that made up of a series of collection panels joined together. The collection surface is a fine-mesh net made from a nylon material (1). As water collects on the net, droplets join to form larger drops that fall under gravity toward a trough or gutter located at the bottom of the panel. The collector itself is completely passive and water is pulled by gravity through the system to the site where it will be used (1). Quantity of Collected Water : Amount of collected water is different. For example the average water collection rates during the fog collection seasons in Chile, Peru, and Oman were 3, 9, 30 Liter/m2/Day respectively(2). Quality of Collected Water : Water in the incoming fog and from the fog collectors can be expected to be of good quality. It will contain some marine salts and soil dust but little contamination from anthropogenic sources given the remote locations of most proposed sites. The ion and trace element concentration in the fog water at the El Tofo collection site in Chile have been studied in details and found to meet Chilean and WHO drinking water standards (3). Materials and Methods In current investigation data of more than 115 stations are used to define the length of fog season. Data included foggy days in each month throughout the year. The length of fog season in 42 stations is 50 or more. Most frequently fog collection projects are not successful in those regions that length of fog season is less than 50 days. However the length of fog season in highlands and mountainous areas is more than synoptic and airport stations. The list of these stations is shown in T 1. Also the distribution of these stations in the map of Iran has shown in F 1. Results As it is seen in Table 1 and Figure 1 there are found three different categories: - Coastal regions such as Astara, Bandar Abbas, and Abadan. Most of fogs that form in these areas are advection fogs. - Islands include Kish and Siri islands. Advection fogs mostly form in islands especially in nights because lands coming to be cold sooner than sea weather and condensation occur sooner. - Highlands such as Parsabad and Ardabil. These areas experience radiation fogs frequently due to low temperature and high altitudes and consequently Orographical fogs are formed.  In some regions all kinds of fog form together and increase the length of fog season. Astara is a good example for this case. Astara is located in the Northwest highlands in the West Coast of Caspian Sea. The average length of fog season in Astara is 220 days in year. Figure 2 shows distribution of foggy days in Astara throughout the year. Fog forms all months of year but because of advection and orographycal fogs in cold season we have more foggy days in these months. The regions shown in T1, are suitable places for performance evaluation experiment by standard 1m2 fog collectors. We are going to start this step in Iran and we hope that we can define suitable places for water collection projects from fog. In islands fog occurs at warm months. F 3 shows the foggy days throughout year in Kish island. In Northern coastline of Iran, fog mostly occurs in cold months. The distribution of fog season length in Anzali shown in F 4. In Southern coast of Iran, fog occurs mostly in warm months. F 5 shows the fog season length in Bandar Abbas. In some regions fog forms equally in all months. F 6 shows the foggy days in Qaem shahr. Conclusion Fog collection by man-made collectors may be a non-conventional source of water, but it is not unproven. Applications exist in many countries where conventional methods can not provide an adequate supply of water. It has been shown in the literature that the water can be delivered in large quantities, that it is potable and that the cost of other potable water systems in rural arid regions. The cloud deck bring an essentially unlimited amount of water to the mountain sites, so in principle the amount of water that can be collected is limited only by the number of collectors that one chooses to install. The water source is sustainable over periods of hundreds and probably thousands of year because the driving forces for the formation of the cloud decks are global in nature and will change only slowly. The collectors themselves are simple, require no energy other than wind and deliver their water by gravity flow. Countries located in arid and semi arid zones, such as Iran and Turkey can use this divine gift, fog, for water collection to be supplied for irrigation and forestry project. The authors would like to recommend the water resources planners and managers to take the issue in more consideration and hopefully initiate water collection project from fog in different locations.  References . Mukerji S. at all, 1993, Fogwater collection system, IDRC library documentation . Schemnauer R. & Cereceda P., 1994, Fog collection's role in water planning for developing countries, Natural Resources Forum. 18(2), PP, 91-100. . Schemnauer R. & Cereceda P., "the quality of fog water collected for domestic and agriculture use in Chile", journal of Applied Meteorology, Vol. 31, 1992, PP275-290 Acknowledgment Hereby we would like to thank Dr. Robert Schemnauer from environment Canada as great pioneer of such projects for his kind support and cooperation.  |