desert

desert

This data is reviewed by the project of data compilation and application of Science Encyclopedia of science popularization in China.

Desert mainly refers to the barren area where the ground is completely covered by sand, plants are very rare, rain is scarce and air is dry. The desert is also called "sand curtain", where there is drought, water shortage and few plants. Most desert areas are sandy beaches or sand dunes, and rocks under the sand often appear. Some deserts are salt flats with no vegetation at all. Deserts are generally aeolian landforms. There are sometimes valuable deposits in the desert, and a lot of oil reserves have been found in modern times. There are few inhabitants in the desert, and the development of resources is relatively easy. The desert climate is dry. It is also a happy home for archaeologists. Many human relics and earlier fossils can be found.

Brief introduction to desert

It is also called "sand curtain". It refers to the area where the ground is completely covered by sand, drought and water shortage, and plants are scarce. Wei Ruanji of the Three Kingdoms wrote a note for Zheng Chong to persuade the king of Jin: "the former Ming Dynasty conquered Lingzhou in the West and faced the desert in the north." Yu Xin of the Northern Zhou Dynasty wrote that "Qilian is still far away, that is to say, it is the time for the champion; before the sand curtain opened, it was the place of Changping in the Yuan Dynasty." Li Bai of Tang Dynasty wrote a poem to judge Chang Hao: "I am ashamed to be born in Jinan, and I recite the ancient prose 90 times. If not, the desert will receive miracles. " "We should change the common people's interpretation of desert," he Yonghe said in his "prospecting team on the Gobi desert". The desert is not only a collection of sand particles, but also a storehouse of rich resources. "

Desert refers to sandy desertification land. One third of the earth's land is desert. Because there is little water, most people think that the desert is desolate and lifeless, so it is called "barren sand". Compared with other areas, there is not much life in the desert, but if you look carefully, you will find that there are many animals hidden in the desert, especially those that come out at night.

Most desert areas are sandy beaches or sand dunes, and rocks under the sand often appear. The soil is very thin and there are few plants. Some deserts are salt flats with no vegetation at all. Deserts are generally aeolian landforms.

There are sometimes valuable deposits in the desert, and a lot of oil reserves have been found in modern times. Because there are few inhabitants in the desert, it is easier to develop the resources. The desert climate is dry, but it is a happy place for archaeologists, because many human relics and earlier fossils can be found there.

Percentage of global deserts in land:

The land area of the world is 162 million square kilometers, accounting for 30.3% of the total area of the earth, of which about one third (48 million square kilometers) are arid and semi-arid desert areas, and it is expanding at the rate of 60000 square kilometers every year. The desert area has accounted for 20% of the total land area, and 43% of the land is facing the threat of desertification.

（1）“

Green desert

”

Refers to a large area of green forest, in which the tree species is single, the age and height are consistent, and very dense. The dense and single canopy completely blocked the sunlight, so that the lower vegetation could not grow, and there was no middle shrub layer and ground vegetation under the forest.

The reason why it is called "desert" is that the species of plants in such forests are very single, unable to provide food or suitable habitat for a variety of animals, so the species of animals are also very rare; the second is that the surface vegetation of such forests is very poor, so the ability to maintain water is very poor, generally relatively dry, and it is easy to form fire; the third is that the biodiversity level of such forests is extremely low Low, so the ecology is very fragile, lack of natural enemy control of pests, it is easy to infect pests, and once pests occur, it is easy to cause large-scale damage.

(2) Hot desert and cold desert

Hot desert is the desert in tropical area, which is mainly caused by strong solar radiation, intense evaporation and long-term dry monsoon control.

The cold desert is different from the cold desert, which is named because it is in the colder zone. The formation of cold desert is mainly due to the fact that the precipitation in the area controlled by the continental climate is very little, and it is controlled by the dry and cold monsoon for a long time.

The cold desert ground is full of large and small stones, which is due to the drastic temperature changes in the desert, and after strong weathering, the rocks change from large to small and become fragments.

origin

The so-called desertification refers to the process that after the vegetation is destroyed, the ground loses its cover, and under the action of arid climate and strong wind, the green field gradually becomes a desert like landscape. Land desertification mainly occurs in arid and semi-arid areas. Climate is the key factor for the formation of desert. However, in the edge of desert, the original vegetation may be grassland. Due to human factors, desertification occurs in the following aspects

（1） Over reclamation

From 1958 to 1962, agriculture was developed unilaterally. Due to the serious wind erosion, the yield per unit area in the first 1-2 years can still maintain 20-30 kg after reclamation in the desert area, and it is difficult to recover even the seeds after cultivation.

（2） Overgrazing

Due to overgrazing of local herdsmen and insufficient supply of grassland yield, many high-quality grass seeds are eaten before seed setting or seed maturity. In addition, goats, which account for more than half of the total number of livestock, act very quickly. They are good at stripping the stems and skins of sandy shrubs, digging the grass roots, and trampling on them. As a result, the grass yield of grassland is becoming less and less, forming a sandy land and creating a vicious circle.

（3） Unreasonable cutting

Taking YIKEZHAO League as an example, it is estimated that a family of five needs to burn more than 700 kg of firewood a year, and 5000 kg of Artemisia ordosica for each family, which is equivalent to most or all of the Artemisia ordosica produced by more than 3 hectares of fixed and semi fixed sand dunes.

climate

In desert areas, the climate is dry and the rainfall is scarce. The annual rainfall is less than 250 mm. In some desert areas, the annual rainfall is less than 10 mm (such as the Taklimakan Desert in Xinjiang, China), but occasionally there is a sudden heavy rain. The evaporation in desert area is large, far more than the local precipitation; the air humidity is low, and the relative humidity can be as low as 5%.

Climate change is quite large, the average annual temperature difference is generally more than 30 degrees Celsius; the absolute temperature difference is often more than 50 degrees; the daily temperature difference is extremely significant, the near surface temperature in summer and autumn can reach 60 to 80 degrees at noon, but it can drop to less than 10 degrees at night.

Desert areas often clear, cloudless, strong wind, the maximum wind up to Hurricane level. The causes of tropical desert are as follows: it is mainly shrouded by subtropical high, the air is mostly sinking and warming, and the surface convection is restrained, so it is difficult to cause rain.

If it is isolated by high mountains, located inland or on the west coast of tropical zone, desert can be formed. For example, the desert in the interior of the Australian continent is formed by the loss of all water vapor when the sea breeze arrives. Sometimes, desert forms on the leeward side of the mountain.

The surface material desert is not all sandy ground, but more common is stone or rock ground; there are lakes and oases on the ground.

classification

Most deserts are classified according to annual rainfall days, total rainfall, temperature and humidity. In 1953, Peveril Meigs divided the dry regions of the earth into three categories

Special dry area

It is a zone with no vegetation (annual precipitation is less than 100 mm, no rainfall and no periodicity), and its area accounts for 4.2% of the global land;

Dry area

It refers to the area where grass grows seasonally but trees do not grow (evaporation is greater than precipitation, annual precipitation is less than 250mm), and its area accounts for 14.6% of the global land;

Semi dry area

With 250-500mm rainfall, it is a place where grass and low trees can grow. Especially dry and dry areas are called deserts, and semi dry areas are called steppes.

But not all the areas that are only dry enough are deserts. For example, the north slope of Brooks Range in Alaska has less than 250 mm of rain a year, which is not usually considered a desert.

Desert wind trade

Trade wind (trade wind) is a kind of wind which radiates from subtropical high and converges to equatorial low pressure area. The trade wind from land is more and more hot. Dry trade winds blow away the clouds, making more sunlight heat the ground. The main reason for the formation of the world's largest desert, the Sahara desert, is the dry and hot trade wind (locally known as the Hamadan wind). The daytime temperature can reach 57 ℃.

Mid latitude desert

The middle latitude desert (or temperate desert, see temperate desert climate) is located between 30 ° and 50 ° latitude. The Sonoran desert in southwestern North America and the Tengger Desert in China are both middle latitude deserts.

Rain shadow desert

The rain shadow desert is a desert on the edge of a high mountain. Because the mountain is too high, it causes rain shadow effect and forms desert on the leeward side of the mountain, such as Judean desert in Israel and Palestine.

Coastal desert

The coastal desert is generally on the west coast of the continent near the Tropic of cancer and the Tropic of cancer. Due to the cold current, it cools down and reduces humidity. In winter, there is a lot of fog to cover the sun. The reasons for the formation of coastal desert are: land influence, ocean influence and weather system influence. Atacama Desert, the coastal desert of South America, is the driest desert in the world. It often rains more than 1 mm every 5-20 years. The Namib Desert in Africa has many crescent shaped sand dunes, which are often windy.

Cactus Garden

Due to the desert climate in Arizona, there are quite a lot of cactus, especially the huge tree cactus (EN: saguaro). Therefore, in 1994, the tree Cactus National Park (EN: Saguaro National Park) was established. There are more than 1000 cactus from different parts of the world in the park. In addition, there are cacti in Ryukyu, Japan

Chinese PinYin : Sha Mo