Page 217 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

now
geosystems
Getting Water from the Air in Arid Climates
Many people live in arid and semi- arid regions of earth where local amounts of annual precipitation are insufficient to provide enough water to sustain life. The two most common re- sponses to limited annual precipitation are drawing water from rivers flowing from areas with higher precipitation and draw- ing water from wells tapping into ground- water supplies. Another source, used for centuries in coastal villages in the deserts of Oman, is collection of water drips de- posited on trees by coastal fogs.
Sand beetles in the Namib Desert in extreme southwestern Africa har- vest water from fog by holding up their wings so condensation collects and runs down to their mouths. As the day’s heat arrives, they burrow into the sand, not emerging until the next night or morning when the advection fog brings in more water for harvesting.
Perhaps taking a cue from the remark- able adaptations of insects, residents of Peru are harvesting fog. in lima (12° 3’ S, 77° 2’ W), the capital of Peru, with 8 mil- lion inhabitants, average annual precipi- tation is less than 50 mm. Although the city is situated at sea level on the coast of the Pacific Ocean (Figure GN 7.1), prevail- ing southeasterly trade winds blow off- shore and the city lies in the rain shadow of the Andes Mountains. Temperatures are moderated by the cold Humboldt current, with mean daily values ranging between 17°C and 24°C.
Many Andean communities rely on rivers fed by glaciers and seasonal snowpacks for water supplies. Three of these rivers run through lima, but with a warming climate, tropical glaciers are shrinking rapidly and these supplies are diminishing. Studies of the economic and social impact of these changes pre- dict severe consequences for Peru, one of the countries to be most affected.
Humidity is often high in lima, in the form of fog, a topic described in this chapter.
in the Atacama Desert of Chile and Peru, residents stretch large nets to in- tercept advection fog: moisture con- denses on the netting, drips into trays, and then flows through pipes to a 100 000-l reservoir. Harvesting fog with nets (Figure GN 7.2) is becoming a more common solution to developing a sus- tainable water supply given the right local conditions. Fog water droplets catch on the net, coalesce, and drip down to a trough which directs the water to a storage tank.
At a project in Antofagasta, Chile, fog nets collected 3.0 litres of water per square metre of mesh per day. Vol- umes are not large, but the alternative in many Andean locations is to buy water from a truck passing through the neigh- bourhood. A small amount of water can improve the lives of the people (Figure GN 7.3).
A Canadian nonprofit charity, Fog- Quest, has been using fog collectors since 1987 with current projects through- out latin America and Africa (see www .fogquest.org). FogQuest states that a village project supplying an average of 200 litres per day can be constructed for around CAD $16000. large sheets of plastic mesh along a ridge of the el Tofo Mountains harvest water from advection fog (Figure GN 7.4). Chun- gungo, Chile, receives 10 000 l of water from 80 fog-harvesting collectors in a project developed by Canadian (inter- national Development Research Cen- ter) and Chilean interests and made operational in 1993.
Fog harvesting is proving to be an economically viable alternative for sup- plying water on small scales in arid
▲Figure GN 7.2 Fog nets in Bellavista near Lima, Peru. [MARiANA BAZO/Reuters/ landov.]
▲Figure GN 7.3 Water collected from fog nets is used to maintain a small garden. [MARiANA BAZO/Reuters/landov.]
PERU
Lima
0 1000 KILOMETRES
▲Figure GN 7.1 Location of Lima, Peru.
▲Figure GN 7.4 Fog harvesting at Chungungo, Chile. [Robert S. Schemenauer.]
regions. At least 30 countries across the globe experience conditions suitable for this water resource technology. For a full analysis about techniques of col- lecting water from fog, see www.unep .or.jp/ietc/Publications/techpublications/ TechPub-8c/fog.asp.
geosystems now online Go to Chapter 7 on the MasteringGeography website (www .masteringgeography.com) for more in- formation about Pacific coastal fog and its environmental effects.
181
80°
60° 50° 40°
10°
20° 30°
0° 10°
0°
20°
30°