Why is there a three-year drought in Yunnan Province?

1 The history of severe drought in Yunnan

Looking at the history of Yunnan, you will find that the three consecutive droughts in Yunnan in recent years are not an accidental event, but starting from the Ming and Qing Dynasties. Droughts have increased year by year, especially at the beginning of the 21st century, when droughts accelerated, eventually leading to an unprecedented three consecutive years of drought.

1.1 Ancient Yunnan Climate Historically, the climate of Yunnan was mild and humid. The average annual rainfall in most areas of the Yunnan-Guizhou Plateau was around 1 100 mm, with abundant rainfall. “There was no shortage of springs and abundant rainfall” [2]. In southern Yunnan, the It has a tropical rainforest climate and is more humid. The soil in Yunnan is mainly red soil and mountain red soil. Because it is located in a warm and humid environment, strong weathering and leaching, and high iron dissociation, the soil appears red [3]. Although there are distinct rainy and dry seasons every year, serious droughts rarely occur. There was a nationwide large-scale drought in the late Eastern Han Dynasty, but there was no record of the disaster in Yizhou (Yunnan, Guizhou, Sichuan) [4].

1.2 The development of droughts in Yunnan since the 14th century

The occurrence of obvious droughts in Yunnan began in the Ming and Qing Dynasties. [4] (P25-29)》 (The disaster data in the table is as of 1990.) It can be seen from the table that the number of floods and droughts has increased since the 14th century. Floods have increased from 7 to 57 times in a century, and droughts have increased from 15 to 57. It has developed to 61 times, of which the number of droughts per century is 15 times in the 14th century, 15 times in the 15th century, 24 times in the 16th century, 32 times in the 17th century, 29 times in the 18th century, 35 times in the 19th century, and 61 times in the 20th century as of 1990. ; The frequency of floods and droughts has increased from once every 4.55 years to once every 0.77 years. "The most floods and droughts occurred in the 20th century." As time goes by, the number of disasters is increasing day by day, and their frequency is also trending [1].

Since 1961, when there were meteorological records in Yunnan, the annual precipitation in Yunnan has shown a decreasing trend. In the past half century, the annual precipitation has decreased by 39mm, with a reduction rate of -8mm/10a, of which summer and The decreasing trend in autumn is more obvious than that in spring and winter. The most significant example is that the annual precipitation days in Xishuangbanna have dropped sharply from 270 days in the 1950s to the current 150 days; the annual fog days have dropped from 180 to 30 days. The climate of the formerly humid tropical rainforest has undergone significant changes [5] .

In the 21st century, droughts in Yunnan have become more frequent and severe:

2001: It encountered close to the "worst drought" in history.

2005: Suffered the worst drought in the past 50 years.

2006: Suffered the worst drought in 20 years.

2009: A severe drought occurred once in 50 years.

2010: A once-in-a-century drought scorched Yunnan.

2011: Yunnan suffered from a sustained local drought, with 25 counties experiencing the lowest rainfall in history.

2012: The drought in Yunnan continues to worsen.

2 Cause analysis: What caused the drought in Yunnan?

2.1 Abnormal atmospheric circulation? The main cause of abnormal atmospheric return is the El Ni?o phenomenon, also known as El Ni?o current. It is a climate anomaly caused by the loss of balance between the ocean and the atmosphere in the equatorial zone of the Pacific Ocean. It generally occurs every 2-7 years. El Ni?o The whole process is divided into the occurrence period, the development period, the maintenance period and the decay period, which generally lasts about a year, causing the wind direction and ocean current to reverse. Therefore, it cannot explain the phenomenon of drought in Yunnan increasing year by year. It cannot explain the three consecutive years of drought, because the El Ni?o phenomenon cannot occur for three consecutive years.

2.2 Is the foundation of water conservancy facilities weak? From a historical perspective: the increasing number of droughts in Yunnan since the Ming and Qing Dynasties is directly proportional to the increasing number of water conservancy facility construction since the Ming and Qing Dynasties. Obviously, the weak foundation of water conservancy facilities is not the cause of droughts. On the contrary, water conservancy facilities The more years there are, the more droughts there are. In addition, vigorous development and utilization of groundwater to combat drought is not a long-term solution. Excessive over-exploitation will also lead to continued decline in groundwater levels and drought and desertification of the surface. Once the groundwater is exhausted and the land becomes desertified, it will be too late [6].

2.3 Global warming? Data show that since 1961, when there were meteorological records in Yunnan, the annual average temperature in Yunnan has been on a rising trend. As of 2010, the average annual temperature in Yunnan has increased by 0.74°C in 49 years, with the annual warming rate being 0.015°C/year [5] . Although there is a certain connection, from a historical perspective, for example, the temperature during the Jurassic period was about 3°C ??higher than today. However, the Jurassic period was one of the wettest periods in the history of the earth [7]. Therefore, global warming is not the main cause of drought in Yunnan.

2.4 What are the effects of special geographical location and complex geological structures? Yunnan is located in the subtropical high pressure zone around the Tropic of Cancer. Where the Tropic of Cancer runs through, there are mostly vast deserts: the Sahara Desert in North Africa, the Rub al-Khali Desert in the Arabian Peninsula, the Lesser Nett Desert, and the Tower of the Indian Peninsula. Er desert... This special geographical location may mean that the climate of Yunnan is destined to be arid. However, in ancient times, Yunnan was famous for its warm and humid climate. It is only in the past hundred years, especially in the past ten years, that it has gradually turned to drought. What is it? What caused the dramatic changes in Yunnan's climate?

2.5 forest coverage? The forest coverage rate in Yunnan has been extremely high in history. Before the Song Dynasty, the forest coverage rate in northwest Yunnan was 70%, and in southern Yunnan it was 90%. Since the Ming and Qing Dynasties, the forest coverage rate has been extremely high. The forest coverage rate declined slowly, but it was still about 70% before the mid-Qing Dynasty [1]. By 1935, the forest coverage rate dropped to 50%, 47% in the early 1950s, and 24% in the 1970s [8]. It is obvious that the increasing number of droughts in Yunnan in history is obviously related to the decreasing forest coverage rate. The lower the forest coverage rate, the more droughts there are. Climate drought is not the cause, but the result. Because there is no vegetation, the surface temperature rises sharply during the day, the air rises upward, and clouds and moist air cannot enter and are swept out of the area [9].

However, after the reform and opening up, the officially announced forest coverage figure increased from 40.8% in 2005 to 47.5% in 2010[10], and recovered to 49.9% in 2009. In January 2012, it was said to have reached 52.93%[11], (There are also false coverage numbers due to changes in forest coverage calculation standards that began in 2003[12]. The official coverage rate is 52.93%, but scholars found that Google Maps shows less than 30%[12]) . However, droughts are still increasing year by year, and even accelerating to become more frequent and severe. From this, we can conclude that droughts have increased rapidly with the rapid increase in forest coverage after reform and development?

So, what kind of forests are newly added? Since 2000, 3 million acres of new rubber forests have been planted in Xishuangbanna Prefecture, and the rubber forest area has increased from 1.16 million acres in 1988 to 6.15 million acres. Along the Lancang River bordering Myanmar and Laos, the original forests on the Chinese side have almost disappeared, and rubber trees or other economic trees have been planted regardless of the conditions. "China Science News" mentioned in a February 2012 report titled "Crisis in Rubber Forest Plantings": Nowadays, in Xishuangbanna, which accounts for half of China's rubber planting areas, many villages have experienced running water cuts and well water drying up. "A scholar from the Xishuangbanna Tropical Botanical Garden of the Chinese Academy of Sciences said that studying the runoff data of Xishuangbanna found that even in years with very low rainfall, there is obvious continuous surface runoff in the tropical rain forest, while frequent cessation of flow occurs in the rubber forest [13]. The Group's APP (China) Company entered Yunnan Province in September 2002, planning to introduce 27.5 million acres of fast-growing and high-yielding forests mainly composed of eucalyptus, and build the largest forestry industry project in Yunnan's history - a forest-pulp-paper integrated project [14] Scholars in the forestry community have long reached a consensus that artificial pure forests can easily lead to a series of problems such as soil erosion, intensification of pests and diseases, loss of biodiversity, and increase in forest fires. The large-scale planting of artificial eucalyptus forests in Yunnan has worried scientists [ 15].

In fact, rubber trees and eucalyptus trees do not harm the ecology in the natural forests of their origin. What really causes soil erosion is the logging of native mountain forests to plant artificial economic forests such as rubber forests and eucalyptus forests. completely, and “cut off all shrubs, weeds, ferns, etc. in the forest, fertilize the land, and use chemical herbicides” to completely remove the vegetation on the surface [16], and even “make mountains” (making mountains means artificial control Fire burning is a silviculture measure in which people use fire to clear forest land for afforestation on logging sites or land suitable for forestation [17]). It completely destroys the natural mountain forest ecosystem in the area. The surface vegetation is eradicated and the land is plowed. It has also caused serious water and soil erosion [18], because “places with relatively barren land are not suitable for planting eucalyptus [20]”. Due to long-term water and soil erosion, the barren mountains where the forest has long been destroyed have become barren and have thin soil layers. Planting eucalyptus and other economic forests in forestless barren hills grows slowly and has low economic benefits. Therefore, investors have extended their clutches to forested lands and proposed the so-called "medium-low-yield forest transformation [21]". In fact, it is to transform species-rich primary secondary forests with so-called low economic benefits. The natural mountain vegetation has been wiped out [16] and replaced by fast-growing and high-yield forests and economic forests with single species of eucalyptus, rubber, walnut, etc. This excuse for so-called "afforestation" and "medium-low-yield forest transformation" has damaged the original mountain ecosystem. The damage is even worse than simply cutting down natural forests. After cutting down natural forests, shrubs, herbs and tree seedlings are still retained. As long as the natural secondary forests are protected and closed for afforestation, the natural secondary forests can be gradually restored. However, planting fast-growing and high-yield forests has wiped out all natural forest species in the area [16 ][17][18]! After a few years, repeated cutting will deplete the land and turn it into a truly barren mountain, exacerbating the serious water and soil erosion in Yunnan this century, and the ecological deterioration in Yunnan has become more and more serious in recent years. One of the reasons is that the rapidly increasing area of ??artificial forests and the area of ??"medium-low-yield forest transformation" is actually the area that has completely destroyed the natural forest ecosystem in Yunnan's mountainous areas. Therefore, this is why Yunnan's "forest coverage" has grown rapidly in recent years and droughts have Also the reason for rapid growth.

Dr. Xie Yan of the Chinese Academy of Sciences introduced that the reason why artificial pure forests are called "green deserts" is because the plant species in such forests are extremely single and cannot provide food or suitable habitats for most animals. , so there are very few animals in the forest; secondly, the surface vegetation of such woods is very poor, the ability to retain water is very poor, dry and prone to fire; thirdly, the level of biodiversity in such woods is extremely low, so the ecology is very fragile and lacks natural resources. If you do not control the enemy pests, it is easy to be infected by pests and cause large-scale damage. Afforestation once became the spiritual sustenance and excuse for deforestation in my country. The so-called "cutting one tree to create ten trees" has actually caused huge damage and degradation of my country's forest resources to a certain extent. Afforestation reflects serious problems in natural resource management in our country, such as the management department being both the manager and the biggest beneficiary [22].

In fact, the area of ??natural primeval forests and primeval secondary forests is constantly shrinking. Except for some inaccessible and steep mountain forests and protected areas, Yunnan’s primeval forests have been cut down almost completely, and now have reached a historical low. At present, the entire existing virgin forest in Yunnan may only cover 30,000 square kilometers at most! That is less than 8% of the total area of ??Yunnan Province[23]! It is exactly inversely proportional to the trend of drought growth, indicating that the more fundamental reason for the increase in droughts is the deforestation of virgin forests.

Former Professor Long Yongcheng of the Chinese Academy of Sciences reported: "One day in late March 2010, I walked through the virgin forest in the Ailao Mountains of Yunnan. The Ailao Mountains are the most important virgin forests in southwest China. It was one of the most beautiful areas in China. It was during the severe drought of the century in southwest China. However, when I walked into the forest, I saw a completely different scene: gurgling mountain streams, echoing apes in the forest, and flowers blooming everywhere. Full of life. Where is the drought? Is it really a once-in-a-hundred-year drought? Under the same blue sky and no rain for several months, why is there such a big contrast between Ailao Mountain and the disaster-stricken areas? After more than half a year of hard work, the research results show that the higher the percentage of nature reserve area in each cell, the fewer the number of drought-stricken locations. When the percentage of natural reserve area reaches 20%, drought locations are almost zero [24].

Primary forests (so-called overmature forests) are one of the most important ecosystems on earth. Primeval forests maintain the natural environment, regulate the weather through the control of rainfall and evaporation, and maintain the ecological balance of the earth [25]. The forest will release a large amount of condensation nuclei (ions, especially negative ions) into the air to create clouds (there is a lot of water vapor in the air, and only the water vapor attached to the condensation nuclei can become the water vapor clouds we can see). In order to achieve this under gravity (water vapor is attached to the condensation nuclei) more and more) and it will rain when it gets cold. The humidity above the forest vegetation is close to the humidity above the ocean, especially when the coastal forest vegetation is very good (above 50%) and there are enough forests along the water vapor transport route (above 30%), it can easily transport a large amount of ocean water vapor to deep inland deserts such as zone! The original forest is not just tall trees, but a comprehensive ecosystem, including the food chain relationship between animals and plants. In the original forest, the decrease of one species can affect the survival of other species. This situation is especially true in tropical rainforests. The performance is more prominent. After people destroy the original forest, even if they replant a large number of forests artificially, they cannot make up for the ecological impact of the forest destruction [26]. On the other hand, there is a thick humus layer and soil layer under the original forest, but when it rains heavily, the humus layer absorbs a large amount of rainwater like a sponge, and then slowly penetrates into the soil and underground to become groundwater. During the dry season, the water is slowly released to prevent floods. Extremely drought resistant.

The fragility of this type of ecosystem is that it must maintain a high level of species diversity, even without cutting down the jungle, or reducing its level of biodiversity, or reducing the number of vines and palms. This will cause the jungle to die due to lack of water and induce ecological degradation in local areas.

After the original forest is cut down, the humus layer is easier to be washed away by wind and rain and gradually disappears, and the water retention capacity becomes weaker. If it is further used to grow crops and economic forests, the remaining shrubs and herbs will be cleared. Clean, "through deep excavation and reclamation, dig out all shrubs and shrubs, and inhibit the growth of weeds [18]", leaving the soil exposed, further accelerating soil erosion, the soil layer continues to become thinner, gradually becomes rocky desert, and the water retention capacity becomes worse It gets worse and worse and eventually it is completely lost. This creates the tragedy of floods when there is rain and drought when there is no rain.

Yunnan is one of the provinces and regions in China that suffers the most serious harm from rocky desertification in karst areas. The formation of rocky desertification often goes through three stages: deforestation for land reclamation - water and soil erosion - rock exposure, which is rocky desertification. Unreasonable human activities are the main causes of rocky desertification. If the rocky desertification land in southwest China is not treated in time, its scale will double in 25 years at the current pace. The area of ??water and soil erosion in Yunnan Province has reached more than 140,000 square kilometers, accounting for 37% of the province's total land area, and more than 500 million tons of soil are lost every year. Analysis by the Chinese Academy of Engineering believes that in the three provinces of Guizhou, Yunnan, and Guangxi, where rocky desertification is the most serious in my country, according to the current progress, its scale will double in 25 years, and if it is not controlled in time within a hundred years, it will be half The land area has become rocky desertification [27].