sandy desertifieation

1. Distribution characteristics of desertification

Sandy desertification refers to a series of processes and phenomena, such as soil wind erosion, wind-blown sand deposition, sand dune moving forward, dust blowing and so on, which are also called desertification. As a result, land degradation, decreased biological yield, loss of available land resources and deterioration of ecological environment have seriously interfered with human normal life and economic activities.

The existing desert and desertified land in China are mainly distributed in inland basins and plateaus between 35 and 50 north latitude, forming a desert belt with a length of 4500km from east to west and a width of about 600km from north to south. Desertification involves Inner Mongolia, Xinjiang, Qinghai, Gansu, Ningxia, Shaanxi, Shanxi, Hebei, Liaoning, Jilin, Tibet and other provinces 12. The existing desertified land is 33.76× 104km2, accounting for 3.5% of the total land area in China and 7.2% of the total area of cultivated land and grassland, of which the desertified land is about 17.96× 104km2 (Table 97) and the potential desertified cultivated land is 15.8. In addition, there are 3.7× 104km2 island sandy desertification land scattered in humid and semi-humid areas of China (Zhang Weimin et al., 1994).

Table 9-7 Distribution of Sandy Desertification Land in China a

(According to Atlas of Geological Disaster Prevention and Control in China, quoted from Lanzhou Desert Research Institute of China Academy of Sciences)

According to the distribution characteristics of desertified land, desertified land in China can be divided into arid area, semi-arid area and semi-humid area (Zhu Zhenda, 1997).

(1) Sandy desertification area in arid area

Desertification areas in arid areas are mainly distributed near oases on the edge of some deserts and along the middle and lower reaches of inland rivers. The former is related to the destruction of vegetation on semi-fixed and fixed sand dunes on the edge of desert by firewood harvesting activities in oasis area, and the latter is related to the excessive utilization of water and soil resources in the middle and upper reaches of rivers. Its distribution is mostly unconnected small pieces, such as sandy desertification near the oasis on the southwest edge of Taklimakan Desert and oasis in Hexi Corridor.

(2) Desertification areas in semi-arid areas

Desertification areas in semi-arid areas are mainly distributed in central and eastern Inner Mongolia and Hebei, Shanxi and northern Shaanxi, and are common in the peripheral areas of grasslands and fixed sandy land, which is the most serious desertification area in China. Sandy desertification around grassland is caused by excessive firewood cutting, grazing or reclamation, such as Bashang in Hebei, Houshan in Inner Mongolia and Horqin grassland. Fixed sandy land or sand dunes activated by unreasonable development of water resources are the main reasons for desertification in its surrounding areas, such as Horqin, Mu Us and Hulunbeier.

(3) Desertification areas in semi-humid areas

Desertification areas in semi-humid areas are dotted in Nenjiang Plain, Huanghuai Plain and the lower reaches of Luanhe River, all of which are caused by excessive firewood cutting and vegetation destruction.

From the nature of sandy desertification, 42.2% of sandy desertification land belongs to non-desert land with similar sandy desert landscape, which has nothing to do with the original sandy desert, such as Wulanchabu grassland and Bashang sandy desertification land in northern Hebei; Another 52.3% sandy desertification land is due to the activation of sand dunes caused by overgrazing and firewood harvesting, such as Hunshandake Sandy Land, Horqin Sandy Land and Mu Us Sandy Land. The remaining 5.5% is caused by the sand dunes on the edge of the original desert moving forward and invading under the action of wind, such as the southwest (Pishan) and southeast (Qiemo) edge of the Taklimakan desert (Zhu Zhenda, 1997).

In addition, there are gravelly desertification and rocky desertification similar to sandy desertification in South China and Southwest China, such as rocky desertification formed by water erosion of loose weathering crust on limestone mountain surface in Guangxi, and gravelly desertification in granite area and red sandstone area in Jiangxi.

2. Causes of desertification

Desertification is the product of the interaction between human strong economic activities and fragile ecological environment. Climate change and human activities are two important factors affecting desertification (Ci Long Jun, 1998).

Among the natural factors that produce sandy desertification, drought and lack of rain are the basic conditions, the surface morphology and loose sandy sediments are the material basis, and the blowing of strong winds is the power source. Human activities such as overgrazing, reclamation, logging, industry and mining, urban construction and irrational use of water resources have stimulated and accelerated the process of desertification (Table 9-8). Man-made factors are superimposed on the fragile ecological environment, which makes vegetation destruction aggravate sandstorm activities and leads to the rapid formation and development of sandy desertification landscape (Figure 9- 1).

Table 9-8 Man-made Genetic Types of Desertification Land in Northern China

Grassland reclamation is one of the important reasons for land desertification in northern China. Taking the southeast area of Horqin Grassland adjacent to Kulun Banner and Kezuohou Banner as an example, the original undulating sparse grassland landscape is mainly low-lying land and gentle slope land. In the past hundred years, more than 30 square kilometers of land of1/kloc-0 has been reclaimed, and some land has completely become farmland; At the end of 1950s, quicksand accounted for about 14% of China's land area, 32% in the end of 1970s, 4 1.2% in the end of 1980s and 54% in the 1990s. The environment of sparse forest grassland degenerated into a desert landscape with staggered distribution of quicksand and semi-fixed sand dunes (Zhu Zhenda, 1997).

Cutting wood is another important factor affecting desertification. The development of desertification in the edge of Tarim Basin, the periphery of Hexi Corridor Oasis, Yanchi in southeastern Ningxia and the adjacent areas of Otog Banner in Inner Mongolia are all related to firewood cutting and medicinal materials mining in sandy areas.

Overgrazing and improper use of water resources are also the main factors of desertification. Overgrazing caused by the simple pursuit of increasing the number of livestock increases the load of grassland and leads to grassland desertification. The unreasonable utilization of water resources makes the groundwater level along the inland river drop, the growth of natural vegetation decreases, a large number of shrubs die, resulting in the bare surface and the gradual expansion of desertification area.

Figure 9- Figure 1 Formation process of desertification

3. Hazards of desertification

The harm caused by desertification is various, involving agriculture, animal husbandry, water conservancy facilities, traffic roads, industrial and mining construction and ecological environment. In essence, desertification disaster mainly destroys soil fertility and makes human beings lose the land resources on which they depend.

(1) embezzle farmland and pasture, and lose available land resources.

The harm of desertification is mainly to destroy land resources and reduce the land area that can be used for agriculture and animal husbandry production; The degradation of land fertility leads to the reduction of agricultural production capacity and biological yield. According to the comparative analysis of aerial photographs and aerial topographic maps in 1950s and 1970s, the sandy desertification land in northern China increased by 3.9× 104km2 in 25 years, with an average annual spread of 1560km2. By 1980s, the average annual increase was about 2 100km2. In the early 1990s, about 39,254.7 square kilometers of dry land, 49,234.7 square kilometers of grassland and 2,000 kilometers of railways and highways were threatened by desertification. Desertification causes the loss of 10% ~ 30% of fine materials in the plough layer, which leads to the coarsening of the surface and the accumulation of sand dunes, and the loss of available land resources.

(2) Land quality declines, and the biological yield of agriculture and animal husbandry decreases.

Desertification disaster, on the one hand, leads to the shrinking of available land area, on the other hand, leads to the gradual decline of land quality. Due to wind erosion, the organic matter and nutrients on the surface of cultivated land are eroded a lot, and the soil fertility is decreasing. The annual loss of soil organic matter, nitrogen and phosphorus in 4× 104km2 farmland and 4.67× 104km2 grassland in China is about 3542× 104t2, which is equivalent to the total amount of various fertilizers, with a total value of17047×/kloc. Due to the harm of desertification, grassland degradation in China has reached 137725.4km2, so more than 50 million sheep are raised every year.

Table 9-9 Desertification harms farmland, grassland and soil fertility loss.

(According to Zhang Weimin 1994)

Due to the sandy desertification disaster, the organic matter, nutrient elements, water and other substances in the dry farming ecosystem are seriously lost without compensation, which leads to the decline of farmland yield per unit area. Compared with the initial stage of reclamation, the average yield of dry farmland in farming-pastoral ecotone is reduced by 50% ~ 60%.

Desertification disaster has also greatly affected the animal husbandry production of five natural grasslands in China. The sandy desertification area in Wulanchabu grassland and Bashang area of Hebei Province accounts for 33.0% of the whole grassland area. After grassland desertification, the average yield per unit area generally decreased by 20% ~ 60%, and even decreased by 70% ~ 80% in some areas. The annual biomass loss of sandy desertification land in China is (632.2 ~ 992.4) × 104t2 (Table 9- 10).

Table 9- 10 Loss of land biological yield caused by desertification

(According to Zhang Weimin 1994)

It can be seen that the essence of desertification disaster is soil wind erosion, which fundamentally damages soil fertility, makes soil tillage layer thinner, soil coarsening, nutrient loss, fertility decline and land biological yield decline. Desertification is a long-term potential disaster. Once the soil is eroded by the wind, it usually takes several years or even longer to restore its original fertility even under the conditions of artificial measures.

(three) the destruction of construction projects and production facilities

1) Hazard of industrial and mining construction: Dongsheng Coalfield, Zhungeer Coalfield, Shenfu Coalfield, Ciyaobao Coalfield and Pingshuo Coalfield, which are located in Mu Us Sandy Land and its surrounding areas, are important high-quality coal bases under construction in China and are deeply endangered by sandstorms. After large-scale coal mining, the area of man-made sandy desertification is 1.26 times larger than that of natural sandy desertification, and it transports sand to the Yellow River every year 1. 19× 108t, accounting for more than 70% of the total sand transport in the triangle of Shanxi, Shaanxi and Mongolia, and it is the main sand-producing area of the Yellow River. The annual development cost caused by desertification is about 90 million yuan.

2) Harm to transportation industry: It is estimated that the existing 1500km railway and 3000km highway in China have been damaged to varying degrees due to sandstorms. Desertification has seriously affected the normal operation of traffic arteries in border areas and inland areas.

3) Harm to water conservancy facilities and rivers: Sandstorms mainly deposit various water conservancy projects and rivers, making it difficult for water conservancy facilities to play their normal benefits. There are about 50× 104km2 catchments in China suffering from sand damage, and the problem of reservoir siltation is more serious. For example, about (120 ~ 380) × 104m3 quicksand enters the reservoir area every year in Longyangxia Reservoir, Qinghai Province. With the increase of sediment deposition, the storage capacity gradually decreases, and the benefits of power generation, flood control and irrigation are seriously affected. A large number of sandstorms entered the river, which aggravated the siltation of the river bed and even caused serious blockage, leading to the breach of the river bank.

(4) pollute the environment and destroy the ecological balance.

Desertification has aggravated the deterioration of the whole ecological environment. In arid, semi-arid and even partially semi-humid areas, due to the thermal effect of weather process and the invasion of cold front, a series of sandstorm processes such as soil erosion, quicksand moving forward and sand blowing up are caused under windy weather conditions. Sandstorm is not only a disastrous weather process, but also a manifestation of desertification disaster. It has a wide range of influence and serious harm, and has become an important environmental problem that seriously threatens the production and life of people in northern China.

1On May 5, 1993, serious sandstorms occurred in Xinjiang, Gansu, Ningxia and Inner Mongolia, causing more than 200 casualties, 42,000 head (only) animals died and thousands of houses were damaged. The depth of soil wind erosion 10 ~ 50 cm and the depth of sand burial (20 ~ 10~50cm) caused extensive farmland destruction, with an economic forest area of 37.33km2 and an economic loss of 560 million yuan.

Sandstorms not only directly cause serious economic losses, but also make the atmosphere turbid, hinder people's normal activities and do harm to people's physical and mental health. Pervasive dust makes people feel obviously difficult to breathe outdoors, and tiny dust enters people's nose and mouth, which is easy to cause respiratory diseases such as cough and asthma. People who live in areas with high dust storms for a long time have a much higher chance of suffering from trachoma, respiratory tract and gastrointestinal diseases than people in other areas.

4. Remote sensing monitoring of desertification

Remote sensing technology has the advantages of large amount of information, wide observation range, high precision and high speed, and its strong real-time and dynamic nature is incomparable to traditional resource and environment monitoring and prediction. Especially, the combination of remote sensing (RS) and geographic information system (GIS) can realize dynamic monitoring and simulation analysis, which is an effective way to carry out desertification research. In recent 20 years, remote sensing technology has been widely used to study the formation mechanism, development process, distribution law and evolution trend of desertification in northern China (Wang Tao et al., 1998). For example, in the early 1980s, Zhu Zhenda and others made use of the comparative analysis of aerial photography in the late 1950s and the mid-1970s and field investigation, and proposed that the grassland reclamation area with the most obvious desertification spread in the past half century started from Horqin grassland in the east, passed through Zhangjiakou paddock, northern Fengning and Bashang area, went to Nansanqi of Xilin Gol League in Inner Mongolia, went to Shangdu, Siziwangqi, Wuchuan and Damaoqi of Wulanchabu League, and then went to northern Guyang.

In the process of desertification monitoring, a systematic, scientific and practical comprehensive evaluation index system should be established. The following indicators can be obtained by using RS and GIS: ① The percentage of wind erosion land or quicksand land in the total area of the study area; (2) The percentage of wind erosion land or quicksand area in the total area of the study area; ③ Vegetation coverage on the surface; ④ The land biomass in the study area. The technical route of dynamic monitoring of desertification land by remote sensing is shown in Figure 92.

Figure 9-2 Roadmap of Remote Sensing Dynamic Monitoring Technology for Desertification Land

5. Desertification control

The fundamental way is to protect natural vegetation, establish artificial vegetation, adhere to the correct policy of production and management, rationally adjust the structure and layout of agricultural production, strengthen the infrastructure of agriculture and animal husbandry, improve the management level, gradually establish a modern agricultural ecosystem, strengthen the construction of artificial grassland ecosystem, and rationally develop and utilize water resources. Effective measures should be taken to control the desertified land and prevent it from spreading.

China has always attached great importance to the prevention and control of desertification. By the end of 1998, 7× 104km2 of desertified land had been controlled, which significantly improved the local environment. However, due to the influence of many factors, the overall situation of land desertification in China is still deteriorating, and the situation of "sand entering and retreating" has not been fundamentally reversed, and the situation is still very grim.

(1) Forest and grass measures

Forest and grass measures include building farmland shelterbelts and sand belts, closing hillsides and planting grass, planting trees and fixing sand, and returning farmland to forests and grass. Different methods should be adopted for desert control in desert, semi-desert and grassland areas, and reasonable and efficient biological measures should be selected according to the characteristics of topography and wind.

In irrigated oases and dry farming areas in northern China, it is an important measure to build farmland shelterbelts and sand shelterbelts to prevent land desertification. According to the ventilation situation, there are three types of shelter forests: tight structure forest belt, sparse structure forest belt and ventilation structure forest belt. It is better to have a compact structure for the sand prevention forest belt at the edge of the oasis. The composite forest composed of trees and shrubs can reduce the wind speed, prevent large-scale quicksand and sandstorm from invading the oasis, and protect farmland from sand damage. The forest network for protecting oasis farmland can be composed of tall trees. Narrow forest belt with ventilation structure and sparse structure should be built in the northern dry farming area (Yang Ruizhen, 1996).

It is the fundamental way to prevent and control quicksand to take measures such as closing sand, planting grass, afforestation and sand fixation in large areas of sandy areas. Practice has proved that planting Populus euphratica forest in desert is very effective for wind prevention and sand fixation.

For cultivated land with serious desertification, it is necessary to change the land management mode, return farmland to forests and grasslands, plant forage grass under the protection of forest nets or introduce shrubs to restore vegetation, gradually control desertification and restore the productive potential of land.

(2) Agricultural farming measures

Agricultural cultivation measures include covering cultivation, combining grain and grass cultivation, adjusting agricultural structure and intercropping different crops.

1) Covering tillage: refers to the agricultural technical measures to enhance the anti-erosion ability of the surface by increasing the ground coverage, mainly including the methods of retaining crop stubble coverage, straw crushing and ground coverage, planting leguminous crops in bare orchards and tea gardens, and covering the ground with plastic films. Mulching tillage has a positive effect on maintaining nutrients and fine-grained substances in the topsoil and increasing soil erosion resistance.

2) Grain-grass farming: refers to taking different measures such as rotation, intercropping, interplanting and multiple cropping between grain crops and leguminous forage to improve soil structure, increase soil organic matter and enhance soil erosion resistance.

3) Adjust the agricultural structure and intercrop different crops: If tall crops such as corn, sorghum and sunflower are planted at intervals between the vertical main wind direction and the peripheral edge of the oasis, the purpose of reducing wind speed and consolidating soil can be achieved, and the spread of desertification can be effectively prevented.

(3) Water conservancy measures and engineering sand fixation

1) water conservancy measures: developing water conservancy, building basic farmland and completely changing the farming method of extensive cultivation and thin harvest are one of the main measures to prevent and control the harm of desertification. Using irrigation water to increase soil moisture and enhance the cohesion of soil particles can reduce the harm of sandstorms. The fertile soil in the valley can store water, divert water and irrigate by itself. Beach, Dianzi, ditch and other soil layers are thick and rich in groundwater, so well irrigation can be carried out.

2) Engineering sand fixation: Engineering sand fixation is a measure to set sand barriers to prevent quicksand, and snakes are an indispensable auxiliary measure for biological sand control in arid sand areas. For mobile sand dunes, clay or Artemisia arenaria sand barriers should be set on their windward slopes, and sand-fixing plants should be planted on the sand dunes protected by engineering sand barriers to prevent fast-moving sand dunes from burying plant sand barriers that have not yet formed sand-fixing ability.

(4) Improve policies and measures and strengthen scientific research.

Strengthen publicity and education, put an end to overgrazing and weeding, and do a good job in prevention; Increase investment in promoting desertification control projects and strengthen the construction of scientific research and technology promotion system; Popularize comprehensive management technology in desertification areas. Construction of eco-agricultural demonstration projects in desertification areas, exploration of comprehensive prevention and control methods of desertification, and realization of social and economic sustainable development in arid and semi-arid areas.