The cause of drought really lies in climate change, which is driven by human hands. Burning a lot of fossil energy, such as coal and oil, is the cause of the spring drought.
The emission of greenhouse gases such as carbon dioxide into the atmosphere is the fundamental driving force of global warming. Insufficient rainfall is one of the reasons for the serious spring drought. In some places, it rains alternately. The precipitation in most parts of central and eastern Chongqing is more than 25mm, and the total precipitation in most parts of western Chongqing is less than 25mm. The rainfall in Tongnan, Dazu, Rongchang, Yongchuan, Tongliang and Jiangjin is 30% to 80% less than that in the same period of last year. Another important reason is that severe drought has seriously affected the water storage capacity of reservoirs and rice fields. For example, some land is seriously cracked, and it doesn't matter if it rains lightly. The storage capacity of the reservoir is about165438+700 million cubic meters, which is less than 50% of the storage capacity in the same period in history. The temperature in northern China rises rapidly in spring and evaporates strongly. The summer monsoon is weak, the rainy season has not arrived, and the precipitation is less; The water demand for spring ploughing is large, but the rainy season has not arrived and the groundwater level is low; Less precipitation than at the same latitude, explained by geographical reasons, mainly due to mountain barriers or monsoon leeward slopes, and less precipitation due to the "foehn effect". In some areas, the bay mouth effect enables the winter wind to go straight down. Thirdly, the height field from Balkhash Lake to Baikal Lake and the northeast of China is abnormally high compared with the same period of normal years. Warm high-pressure ridges frequently appear and develop in this area, and the dominant circulation system is high-pressure ridges, which is the main reason for the spring drought in; The low SST and abnormal solar activity in the equatorial eastern Pacific Ocean are indirect causes of spring drought.
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Midsummer drought: summer drought (sweating again ~ ~)
Generally from mid-July to mid-August. At this time, the quiet front of Meiyu has passed the summer drought in early July.
To the middle and lower reaches of the Yellow River and Northeast China, the middle and lower reaches of the Yangtze River are controlled by the "subtropical high", forming an anti-cyclone weather, with the main downdraft, long sunshine, strong solar radiation, high temperature and strong evaporation. Crops grow fast, and farmland needs a lot of water. However, due to the single air mass, except for thunderstorms in some areas, there is no big rain area, and generally there is dry and hot summer weather, so it is called "summer drought". In this season, the afternoon temperature in the middle and lower reaches of the Yangtze River generally reaches 33 ~ 35℃, and the high temperature as high as 43 ~ 45℃ appears in some places. Generally, when the subtropical high in the western Pacific is controlled and typhoon activity is low, severe drought is prone to occur. It mainly occurs in the Yangtze River basin and the south of the Yangtze River, especially in Hubei, Hunan, Jiangxi, Jiangsu, Anhui and other provinces. When the subtropical high in the western Pacific is controlled and typhoon activity is low, severe drought is prone to occur, and the precipitation is obviously less than the multi-year average. Generally from mid-July to mid-August. (The dog days begin in summer and last for about 10 days after July 12 every year. )
———————————— The dividing line between small ⊙⊙⊙ B. B. Khan—
Typhoon: Typhoon is a kind of tropical cyclone. In meteorology, according to the definition of the World Meteorological Organization, a tropical cyclone with a sustained wind speed of 12 (that is, more than 32.7 meters per second) is called a hurricane or a local synonym, and the name of a hurricane is used in the North Atlantic and the East Pacific; Typhoon is a synonym for the western North Pacific (north of the equator, west of international date line, east of east longitude 100).
Conditions: Direct sunlight on the tropical sea surface raises the temperature of seawater, and seawater evaporation provides sufficient water vapor. The water vapor condenses in the process of uplift, releasing a lot of latent heat, promoting the further development of convection, reducing the sea level pressure, making the surrounding warm and humid air flow in and replenish, and then rising again. This cycle forms a positive feedback, that is, the second conditional instability (CISK) mechanism. On the vast sea surface with suitable conditions, the influence range of circulation will continue to expand, reaching hundreds to thousands of kilometers. Due to the high-speed rotation of the earth from west to east, there is friction between the air column and the surface. The closer to the equator, the greater the friction, which causes the steering column to rotate counterclockwise (clockwise in the southern hemisphere). Because of the fast rotation of the earth, the air column can't keep up with the speed of the earth's rotation, forming a westward sense and forming what we now call typhoons and typhoon paths. The following is the description in the first volume of geography book for senior one in People's Education Edition: In tropical or subtropical oceans where the sea surface temperature exceeds 26℃, a large amount of air expands and rises due to the high sea surface temperature, which reduces the sea surface pressure, and the peripheral air continuously flows in and rises. Affected by the geostrophic force, the inflowing air rotates. The rising air expands and cools, and when the water vapor in it cools and condenses into water droplets, it will release heat and promote the rising of the lower air. In this way, the air pressure near the ocean surface drops lower, and the air rotates more violently, eventually forming a typhoon. the typhoon structure
Judging from the typhoon structure, such a huge behemoth must have unique conditions: a vast high-temperature ocean. The seawater temperature should be higher than 26.5℃ and the depth should be 60m. Typhoon is a very violent weather system, with an average daily consumption of 3100 ~ 4,000 calories/cm? Energy, this huge energy can only be supplied by the latent heat released by the vast tropical ocean. In addition, the strong winds around the typhoon will cause the sea water 60 meters deep near the center to churn. In order to ensure that the sea surface temperature is always higher than 26.5℃ during tumbling, the thickness of warm water layer must reach 60 meters. There should be a suitable flow field. The formation of a typhoon requires a strong upward movement. Suitable flow fields (such as East Storm and equatorial convergence zone) are easy to produce tropical weak cyclones, and the pressure of tropical weak cyclones is low in the middle and high in the periphery, which makes the airflow converge to the cyclone center and move upward. In the process of rising, water vapor condensation releases huge latent heat, forming a warm heart to supply typhoon energy, making the rising movement stronger and stronger. There should be enough geostrophic deflection. If the converging airflow reaches the cyclone center and the air accumulation block, a typhoon cannot be formed. The large enough geostrophic deflection force makes it difficult for the convergent airflow to flow directly into the low-pressure center, but rotates along the center, which strengthens the cyclonic circulation. The geostrophic skewness of the equator is zero, and gradually increases towards the poles, so typhoons occur at places more than 5 latitudes from the equator, ranging from 5 degrees to 20 degrees. The vertical shear of airflow should be small. That is, there is little difference between high and low wind directions and wind speeds. If the air speed difference is too large, the latent heat will flow out quickly, which is not conducive to the formation and maintenance of typhoon warm heart. In areas with latitude greater than 20 degrees, there is strong wind at the top, which is not conducive to warming up and typhoons are not easy to appear.
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Red tide: Red tide, also known as red tide, is a harmful ecological phenomenon that some phytoplankton, protozoa or bacteria in seawater proliferate or explosively gather under certain environmental conditions, causing discoloration of water. Red tide is a historical name. It is not always red.
Red tide is produced under specific environmental conditions, and there are many related factors, but marine pollution is one of them. A large amount of wastewater containing various nitrogen-containing organic substances is discharged into seawater, which promotes the eutrophication of seawater and is an important material basis for the reproduction of red tide algae. A large number of studies at home and abroad show that marine phytoplankton are the main organisms that cause red tides. There are more than 260 species of marine phytoplankton in the world that can form red tides, and more than 70 of them can produce toxins. Some toxins secreted by them can directly lead to a large number of deaths of marine life, and some can even spread through the food chain, causing human food poisoning.
+———————————————— The dividing line of red water.
Salt tide (also known as the upper reaches of salt tide and salt water intrusion) is a natural hydrological phenomenon, which is caused by the attraction of the sun and the moon (mainly the moon) to the surface seawater. When the flow of Danshui River is insufficient, seawater flows backwards, and the mixing of salt and fresh water leads to the salinization of the upstream water body, that is, the salt tide is formed. Salt tides generally occur in winter or dry season, that is, from 10 to March of the following year, at the intersection of rivers and seas, such as the Yangtze River Delta and the Pearl River Delta. The main factors that affect the salty tide are the weather change and the ebb and flow of the tide. Especially in the astronomical tide, the salt tide is more serious. In addition, the process of sea level rise caused by global climate change makes the salt tide increase very slowly, but the long-term accumulation gradually appears.
Less precipitation is the main reason.
In 2003, the rainfall in the whole basin decreased by more than 20% on average. The amount of water in Xijiang River basin in the upper reaches of Shawan Waterway in Panyu District, Guangzhou has decreased by 60%, and the rainfall has dropped sharply since the winter of 2004, resulting in a sharp drop in the water levels of rivers, lakes and reservoirs in southern Guangdong. At present, the total storage capacity of 30 large reservoirs in Guangdong Province is 1 10484 million cubic meters, a decrease of 336.36 million cubic meters or 23% compared with the same period in 2003. The decrease of rainfall leads to a serious decrease of river flow. At the beginning of 2005, the water level of Gaoyao Station in Xijiang River was -0.06 meters. There is little rain in the upper reaches of the Pearl River, and the source water is reduced, and the downstream is affected by seawater tides, forming a salty tide. The direct cause of the salt tide is the drought in South Guangdong for years.
The disorderly dredging of the Pearl River
The average annual sediment transport in the whole Pearl River Estuary is 80 million tons, which is a salty tide including suspended sediment.
However, only 5% ~ 6% of the coarse sand is sunk at the bottom of the river, and the dredged sediment is coarse sand. The continuous exploitation for 15 years exceeds 80 million tons, and the river sand accumulated in history is also dredged, and the upstream sediment is not enough to supplement the dredged river sand. At present, there are more than 100 illegal sand mining vessels in the whole Pearl River Delta, resulting in no river sand. No river or sandy reach is from bottom to top along the river; Over-exploitation of river sand caused serious downcutting of river bed, which led to upward movement of salt tide.
Sea level rise has intensified the spread of salt tide.
The relationship between sea level rise and salt tide is remarkable. Recently, a study completed by China Academy of Sciences, Guangdong Academy of Sciences and other 13 units and 100 scientific and technical personnel over 8 years shows that by 2030, the sea level in the Pearl River Delta region may rise by 30 cm. A researcher of the research team said that if preventive measures are ignored, the most developed Pearl River Delta region in China will be hit by more serious floods, storm surges, waterlogging and salt tides, and it is in danger of being submerged.
The increase of production and domestic water consumption aggravated the salt tide.
With the rapid economic development, the expansion of industrial production scale, the growth of permanent population and the sharp increase of production and domestic water consumption in the southern coastal areas of China, the river flow decreases, leading to the increasing intrusion of local salt tide.
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Ice flood, commonly known as ice platoon, is a hydrological phenomenon that the river water level rises obviously due to the resistance of ice to water flow. Ice sometimes converges into an integrated ice plug or dam, which leads to a sharp rise in water level and eventually floods the beach or bursts its banks. This is the so-called Hong Ling. Ice flood may occur during the interception period in winter and the river opening period in spring. Generally speaking, there is ice in the water meter, and the ice is broken into pieces, and there is water flow under the ice, which drives the ice to move downstream. When the river bank is narrow, the ice keeps piling up, resulting in excessive dam pressure, which is ice flood.
The natural conditions of ice flood depend on the geographical location and channel shape of the river. In the alpine region, the river flows from low latitude to high latitude, and the shape of the river channel is wide and narrow, so there are more opportunities for serious ice flood where the river channel bends into a ring. This is because this river closed the flood of the Yellow River.
The lower segment is earlier than the upper segment in freezing, and the upper segment is earlier than the lower segment in thawing. And the thickness of the ice sheet is thicker in the lower section and thinner in the upper section. When there is ice in the lower reaches of the river, part of the upstream water flow is blocked, which increases the water storage capacity of the river. When the ice melts to open the river, the water storage capacity of this river is released sharply, and the Ling Feng descends, and the ice water gathers more and more along the way, and the ice peak increases. When the ice water from the upstream spreads downstream, it will get stuck in a narrow reach or where the river turns, forming an ice dam, thus increasing the water level in the upstream.
The severity of ice flood depends on the influence of river ice on water level. Usually, the water level will rise sharply only after serious ice or ice dam appears in the river, causing serious ice flood. In short: 1, rivers with ice ages. 2. From the lower latitude to the higher latitude, there is an obvious north-south flow direction. The Yellow River in China has ice flood in Ningxia and Shandong, and the rivers in the northeast will also have ice flood when the above conditions are met.