Man-Environment relationships in tropical desert

F. 6 geography

1. Climatic impact on man

• Affect human eyes

• Mirages ( 海市蜃樓 )may form on hot and windless days

• lower mental ability 減低思考能力

• upset metabolism 阻礙新陳代謝

• Sandstorm may reduce visibility 減低能見度

2. Use of underground• more reliable than surface water

• Known as artesian ( 自流水井 ) – the aquifer( 蓄水層 ) is trapped by impermeable beds above and below

蓄水層處於不透水層間

• The rock strata are arranged in a gentle downfold forming a shallow basin 岩層向下形成淺的盆地

• Aquifer outcropping on the magins 蓄水層在盆地外露

• Under pressure which cause the water to rise

The Sahara’s underground reservoirs There are seven basins with great capacities :

• The Great Western Erg ( 西部大沙漠 )• The Great Eastern Erg ( 東部大沙漠 )• Tanezrouft ( 塔尼茲魯夫詩 , 阿爾及利亞 )• Niger ( 尼日爾 )• Fezzan ( 費贊 , 利比亞 )• The Western Egyptian Desert ( 西部埃及沙漠 )• Chad ( 乍得 )

• The three southern ones : Niger ,Chad and the western Desert are the largest, containing 75% of total reserve

Salt content • Underground water have a high content of s

alt. It is because :

• Evaporation causes salts to concentrate in the upper soil layers when water is drawn upwards

• Endoric / inland drainage prevents salt from escaping out to sea 內陸河系防止鹽份消失

Atmospheric recycling cause wind to bring salt into the area from sea and other bodies of water 大氣循環將鹽份由海帶入內陸

• The salinity level is expressed in parts of dissolved salt per million parts of water (ppm) 鹽度以 ppm 作單位量度

• It is 2500ppm to 4000ppm in desert( humid region only have about 570ppm)

• The high ppm only suitable for livestock

Agricultural use • Foggaras ( 坎兒井） are subterranean channel

s dug nearly horizontally to reach the watertable at a slightly higher point 坎兒井是於水線稍高位置挖取地下水道取水

• These are common in Northern Saraha ,especially in Algeria阿爾及利亞

• Funnels ( 漏斗 )are used where the water table is just below the surface

• Farmers dig down to it and plant their trees at the

bottom 15-20 m down

• The root of the trees ,e.g date palms ( 棗椰樹 ) ,can draw water directly from the water table

• The Souf Oasis is one place where this method is used

• Wells 35-50 m deep are dug in Egypt, with shadufs( 汲水吊桿 ) used for drawing water

• Artesian wells are used to irrigate large gardens and parks across the country

• Water obtained from wells is purified and sent by pipeline to areas population

3.Other method used to increase water supply

• Use of surface water - Pipelines and canals are also used to bring water fr

om Hoggar or Atlas Mountain to cultivate the land 阿哈加爾山脈

- Building Dam such as Aswan Dam in Egypt to provide perennial irrigation ( 全年灌溉 )

- It should be noted that most stream in desert are intermittent （ 間歇） and ephemeral （短暫）

• Creation of inland sea

- this has been done by digging a canal from the Gulf of Gabes in Tunisia to Chott Melhrir to flood an area of 8000 sq km and create an inland lake which may cause dew and even rain 由突尼西亞的加貝斯灣至梅勒吉爾鹽沼 , 阿爾及利亞

Man made rain - Experiment have been carried out in the Hog

gar Mountains 阿哈加爾山脈

• Distillation of sea water

- sea water is used to supply communities along the coast with water, but this method is very costly as it requires desalination on a large scale

For reference of artesian well ,can see :

http://www.affa.gov.au

4. Nomadism • Over 60 % of the Saharan inhabitants were

pastoral farmers, but now only 30% • The true nomads ,in Hoggar and Tibesti hav

e to move continuously because grazing land is too poor and scattered

• Nomads trade in golds, camels, silver, weapons,cloth ,etc Semi-nomads,like those in the Atlas Mountains herb sheep,goats and cattle and grow barley, wheat and fruit

5. Cultivation

• Poor land because one-third is eroded rock and lava , one third is sand dune

• There are high mt like Hoggar and Tibesti Mountain .Difficult climatic conditions : extreme temp, intense insolation, insufficient rainfall

• The soil is thin and infertile , sometimes a layer of salt accumulate on the surface

• Locusts may destroy crop

• Other unfavourable conditions:- Primitive methods of farming- Increase in population

6. Mining • In the Sahara, there are rich deposit : oil ,nat

ural gas, iron ,manganese , phosphate, coal, tin, lead and salt

• Oil was discovered on the Libyan border at

Hassi R’Mel • Pipelines were constructed leading to the po

rt. The oil is pure, sulphur-free and occur at a shallow level

• Natural gas were found in Salah, Hassi R’Mel , Hassi Messaoud

• Pipelines were constructed to the ports where the gas is refined into methane and sent oversea by tankers or sent by pipeline to Spain, Italy and France

• Iron and copper were discovered at Fort Gourand (Mauritania )

7. Tourism • There is the existence of large variety of

amazing landscapes

• The emptiness of the Sahara attracts many tourists who feel like pioneers

• Oasis settlement and the nomad’s way of life are interesting to tourists

8. Industries • The industries are mostly primary

industries ,for example : mining for minerals

• The natural resources provide fuel and raw materials for industries such as food processing, textiles and electrical goods manufacture in Alexandria and Algiers

9. Communication

• Roads and railway construction faces the following problems

• the area is not well surveyed

• Roads or railways serving prospecting areas might not need to be permanent

• Settlements in desert are highly scattered

• The cost of transporting building materials is high

• Finding specialists and labour is difficult

• There are technical problems such as shifting sand dunes and stony desert

• Navigation is not possible cause the river in desert is shallow

• In addition, shifting dunes, dust storms, intense insolation, lack of water supply, mountain and long distance all make development difficult

Sparse population • Uneven distribution : over 1000 /sq km in

oasis , but less than one person /sq in the rest of Sahara

• Sparse population will make the natural resource under utilization

Climatic change causing desertification • Desertification means the desert

expanding ,slowly taking in land previously not a desert

• Since 5000 years ago , the aridity increase. Sand dune advance and invaded the Nile Valley cause the Nile flood decline . Famines are more common

• The climate( especially rainfall ) is unpredictable, with no pattern of drought and non- drought years.Prolonged dryness will extend the desert areas

The impact of man on the environment

1. Misuse of underground water

Underground water is lost in various way

- Over–pumping

Effects: decline in artesian pressure, lowering

of water table, increase in salinity,

artesian water become sub-artesian

- groundwater pollution

- inefficient irrigation

2.Nomadic herding • As population increase, the farmers enlarge

their herbs, and increase the range of their migration into land with fragile ecosystem

• The range of migration controlled by fences and borders, hence the nomads cannot “follow the rain” freely to allow for unpredictable rainfall

• This lead to to overgrazing and the trampling of soil by domestic animals

• Too many animals grazing may exceeding the carrying capacity of land which deteriorates. It will remove vegetation cover and exposing soil to wind and water erosion

3. The felling of trees

• Surface runoff will increase, becoming fast-flowing torrents, eroding deep gullies

• If the rate of the vegetation removal is greater than natural regrowth, the vegetation will be destroyed

4. Cultivation Irrigation may lead to various problems:• Salinization or alkalinization of soil

• Waterlogging sometimes happened in some place

• Irrigation systems trap silt

• Overcultivation on marginal land

• Mechanized farming also cause rapid soil erosion

• Dry-land farming, wrong approach of ploughing will easily lead to soil erosion as soil is easily broken structure into fine silt

• A short fallow or continuous cultivation may exhaust soil moisture and fertility

• Monoculture is dangerous cause pest can reproduce rapidly under high temperature

5. Increasing population • This produce a greater demand for water

• It leads to greater decline in hydraulic pressure

• A drop in water table

• An increase in salinity

• Artesian well may become subartesian

6. Changes brought by mining in the Sahara

• Mining lead to the development of towns and cities ,especially around oil field and mines

• It cause a decline in agricultural

• Mining may cause destruction of the soil structure ,exposing soil which is more easily eroded

Solutions to the problems of desertification • Increase the production of meat by keeping

camels instead of sheep, goats and cattle because camels can graze further from their source of water

• Plant prickly pear ( 霸王樹） as a source of animal feed and to reduce runoff from rainstorms, thereby preventing soil erosion

• Transfer people to other areas or activities, forcing emigration and providing education, as well as gaining some control over the use of land and water

• Provide a green belt across North Africa to consist of farms, woodlands, shelter belts ,etc designed to offer soil and dune conditions, moisture conservation and afforestation

• Reduced the salt content by soaking the ground through tubes

• Keeping the soil covered under a plastic sheet