South-to-North Water Diversion Project, why not transfer the flood from south to north in rainy season?
First, the movement of rain belt in China.
Because the rainfall in our country is mainly brought by monsoon, the rain belt advances from south to north. The north-south movement of rain belt is deeply influenced by the intensity of subtropical high. Whether in the south or the north, precipitation is concentrated in summer. At the beginning of June every year, warm air from the south and cold air from the north meet in South China, forming a quasi-static front in South China. By mid-June, the influence of warm air was enhanced, and the quasi-static front moved to the middle and lower reaches of the Yangtze River in China, forming a quasi-static front in Jianghuai. At this time, the rainy weather continued in the middle and lower reaches of the Yangtze River, which coincided with the ripening season of plums in the south of the Yangtze River. Therefore, the rainfall brought by the quasi-static front of the Yangtze-Huaihe River is also called "Meiyu". In mid-July, the middle and lower reaches of the Yangtze River "bloomed". At this time, the middle and lower reaches of the Yangtze River are controlled by subtropical high, and the downward flow prevails. The temperature rises, the sunshine is strong, and sometimes it even forms summer drought weather. Therefore, after the rainy weather, the middle and lower reaches of the Yangtze River may immediately enter the summer drought weather, which lasts from mid-July to mid-August.
When the middle and lower reaches of the Yangtze River are controlled by subtropical high, warm air pushes the rain belt northward to North China and Northeast China. At this time, the rivers in North China and Northeast China will enter the flood season. In summer, although the rainfall in the north of China is obviously less than that in the south, floods may also occur. 70% of the rainfall in the Yellow River basin is concentrated in summer and autumn, with frequent rainstorms, and the rivers show a trend of skyrocketing and plunging.
About the change of the Yellow River runoff, Chinese ancient history books describe it vividly. For example, Yuan Shi and Hequ Zhi recorded: "Generally speaking, the Yellow River is a place where the water potential is slow, and it is harmful not to observe it. When it rains, the waves are urgent and fierce. " As early as the Spring and Autumn Period and the Warring States Period, Zhuangzi said: "When autumn comes, the rivers will be full; The river is so big that there is no dispute between the two cliffs. " Zhuangzi's "hundred rivers irrigate the river" refers to the situation that a lot of water flows into the Yellow River during the rainstorm in summer and autumn in North China Plain. Zhuangzi's description is a literary language. There is no dispute between the two cliffs, but it is exaggerated. However, it truly reflects the state of the Yellow River during the rainstorm.
So there are floods in northern China in summer, but the time of floods is a little later than that in the south. If the flood in the south can be transported to the north during the flood season, it may aggravate the flood disaster in the north.
In summer, the river runoff in the Yangtze River basin and the Yellow River basin will increase. However, the increase in runoff does not mean that floods will definitely occur. Floods only occur in rainy years.
The second is the water storage function of natural lakes in the Yangtze River basin and the Yellow River basin.
In fact, we have a better way to store water, which is endowed by nature, but sometimes, human beings don't know how to cherish it.
During the Warring States Period, the Yellow River frequently oscillated in the North China Plain. At that time, the population was relatively small, and there were a lot of virgin forests in the North China Plain, so this oscillation did not affect human activities. By the middle of the Warring States period, with the increase of population, in order not to divert the Yellow River and affect agricultural production, large-scale dikes were built in the lower reaches of the Yellow River, and the Yellow River channel was gradually fixed. During the long geological period, the swing of the Yellow River caused many lakes to appear in the North China Plain. The North China Plain itself is flat, with many depressions and depressions, so there are many lakes. However, since the Qin and Han Dynasties, the lakes in the middle and lower reaches of the Yellow River have been shrinking day by day, and many lakes have been deposited into the ground. The famous Yingze, Leixiaze and Heze in history have all disappeared. The water margin in Water Margin was deposited into the ground during Kangxi period of Qing Dynasty, and now Dongping Lake is the remnant of Water Margin in 800 Li.
Of course, in the process of diverting water from the Yellow River, some lakes will be silted up and some lakes will increase in area. However, on the whole, the area of lakes in the Yellow River Basin in China is shrinking.
The climate in the Yangtze River basin is warmer and wetter than that in the Yellow River basin. However, in the past 5000 years, the lakes in the Yangtze River basin have been shrinking day by day, but the shrinking speed is slower than that in the Yellow River basin. Poyang Lake is the largest freshwater lake in China and Dongting Lake is the second largest freshwater lake in China. In ancient times, the water surfaces of Poyang Lake and Dongting Lake were very vast. However, since the middle of19th century, Poyang Lake and Dongting Lake have been shrinking day by day, with serious siltation, and some places even silted up as land, and the area of Taihu Lake is also shrinking. During the Ming and Qing Dynasties, many small lakes around Taihu Lake were cultivated into farmland. In recent decades, China has strengthened the management of Poyang Lake, Dongting Lake and Taihu Lake, so that Poyang Lake, Dongting Lake and Taihu Lake will no longer shrink. However, compared with the Song and Yuan Dynasties, the area of lakes in the Yangtze River valley in China is still greatly reduced.
Poyang Lake, Dongting Lake and Taihu Lake have obviously regulated the runoff of the Yangtze River. When the flood season comes, they can store some water. For example, Poyang Lake is a typical huff and puff lake. In the summer flood season, the water storage capacity is large and the lake is vast. In winter and dry season, the water level of the lake drops sharply and the beach is exposed.
There are a large number of natural lakes in both the Yangtze River Basin and the Yellow River Basin in China, which can be used by human beings to regulate and store floods.
In recent years, a large number of artificial reservoirs have been built in the Yangtze River basin in China, which can also regulate the runoff of the Yangtze River well. Water can be replenished in dry season, and part of flood water can be intercepted in flood season.
Third: the topography of North China Plain.
Why is the South-to-North Water Transfer so difficult? China's North China Plain is very flat, with a slight inclination from west to east as a whole. Because the North China Plain is very flat, the Yellow River siltation is very serious. In the middle of the Warring States period, with the increase of population, in order not to divert the Yellow River and affect agricultural production, a large-scale dike was built in the lower reaches of the Yellow River, and the channel of the Yellow River was gradually fixed. As a result, the riverbed of the Yellow River gradually rises. Although the Yellow River is a river, the area where the Yellow River flows is relatively high. The South-to-North Water Transfer Project must first consider how to cross the Yellow River.
The North China Plain is flat and slightly inclined from west to east, so the main rivers in the North China Plain flow from west to east, not from south to north. Artificial water transfer must give artificial power to make water flow from south to north. In addition, the rivers in the south of China have a very large flood volume in the rainy season, and the artificially built canals have a very limited water flow, so they can't transport that much water at all.
Every rainy season, there will be continuous rainy weather in southern China, and the water level of rivers will rise. Does this mean that these floods will be wasted? In fact, there are many lakes in the Yangtze River basin itself that can store water. The middle and lower reaches of the Yangtze River include Dongting Lake, Poyang Lake and Taihu Lake.
During the Ming and Qing Dynasties, a large number of mountainous hills in the upper reaches of the Yangtze River were reclaimed, forests were destroyed, and soil erosion became more and more serious, resulting in serious sediment deposition in the middle and lower reaches of the Yangtze River. Due to sediment deposition, the riverbed of the Yangtze River rises and the water level of the Yangtze River changes greatly. In the rainy season, floods are prone to occur.
The forest itself also has the function of storing water. The forest is like a natural reservoir, which can store water. However, during the Ming and Qing dynasties, a large number of virgin forests in southern China were destroyed, which actually reduced the water storage function of forests.
Compared with the cement floor, the natural vegetation surface obviously has a strong water retention capacity. In addition, the surface of natural vegetation can also make a lot of rainwater seep under the soil, and once the cement pavement is not well drained, water will accumulate.
The climate of China is obviously monsoon climate, with rainfall decreasing from the southeast coast to the northwest inland, and concentrated in summer. Therefore, the water resources in China are unbalanced in space and time. The South-to-North Water Diversion Project in China, and a large number of artificial reservoirs and water conservancy projects in the Yangtze River and Yellow River basins can, to some extent, make the water resources in China more balanced in space and time. This shows that with the improvement of science and technology, human beings can control nature to a certain extent, but they should also respect the laws of nature and not destroy the natural environment. Rivers, lakes and wetlands in nature all have their own functions of regulating water resources. Only by respecting the laws of nature and giving full play to the role of science and technology can we create a better life.