Audit expert: Zhan Mingjin
Dean and Professor of Jiangxi Provincial Information Application Vocational and Technical College
Ph.D. in Meteorology, University of Chinese Academy of Sciences
This summer, the Beijing-Tianjin-Hebei region was affected by the landfall of Typhoon Du Surui, and heavy rains fell for several days and suffered serious floods. As the old saying goes, "a long drought will lead to flooding", and although there are many complete versions that have been handed down, the most widely fluent is "a great drought will be followed by a great flood, and after a great flood there will be a great epidemic".
Among them, "major drought" refers to a natural disaster, usually refers to the phenomenon of hot weather and no rain for a long time, resulting in dry air and lack of soil water; "Flooding" refers to the phenomenon of flooding and water accumulation in low-lying areas caused by excessive rainfall; The "pandemic" refers to the plague epidemic.
Widespread rare droughts in previous years and torrential rains and floods this year seem to perfectly validate this saying, but why are there such empirical words? Is there really a scientific basis?
Why do heavy rains after droughts cause flash floods?
In order to explain the phenomenon that heavy rains after drought may cause flash floods, Professor Robert Thompson from the University of Reading in the United Kingdom poured three glasses of water on three grass with different humidity, and recorded the time it took each grass to absorb a full glass of water, the very wet grass on the left took only 9 seconds, the generally wet grass in the middle took 52 seconds, and the dry grass on the right took about 15 minutes. Experiments have shown that the drier the soil, the more difficult it is to be saturated with water.
Source丨News screenshot
Although the presence of grass in the comparison process does have a certain impact on the experimental results, and does not guarantee that all three glasses of water fit the ground perfectly, the huge gap in the results can still explain certain problems.
Relevant experts said that for arid areas, low-intensity precipitation, especially gentle wind and drizzle, is a very beneficial thing for alleviating drought, but sudden high-intensity rainfall is likely to lead to rapid changes in drought and flooding in arid areas, which in turn will cause floods.
Source丨News screenshot
In soils that continue to be in a state of drought for a long time, the connection strength between soil particles will be reduced, resulting in loose soil. At this time, once it encounters heavy rainfall, the rainwater will not be quickly absorbed by the soil, but will directly flow through the impervious dry land, and then wash away everything on the ground with great kinetic energy, causing disasters such as flash floods, landslides and mudslides.
This phenomenon is also very common in daily life, if there is a pot of potted plants that have not been watered for a long time at home, and the soil is only ordinary loess, then a large amount of watering one day will find that the water flow will directly splash out of the pot, basically can not be absorbed by the soil.
Source丨A social software
In order to prevent and respond to floods that may be caused by heavy rainfall after drought, it is not enough to rely on the soil's own water storage capacity, and it is necessary to build enough water storage facilities, such as large reservoirs, in order to store the required water for the soil while reasonably storing most of the precipitation as much as possible to avoid causing greater disasters.
In addition, the frequent occurrence of this phenomenon of rapid drought and flooding also puts forward higher requirements for the accuracy of meteorological forecasting.
Why are there two extremes of "drought death, waterlogging death"?
The northern region is controlled by the subtropical high pressure zone, while the southern region is affected by the combination of subtropical high pressure and low pressure on the ocean. Therefore, the precipitation situation of the mainland is mainly related to the advance and retreat of the monsoon, and once the monsoon is abnormal, it will cause abnormal precipitation, and alternately drought and flood disasters occur in a large range.
Under the influence of the summer wind, the rain belt gradually advances from the southeast coast to the northwest interior. When the summer wind is strong, the rain belt advances too fast, the rainy season in northern China will come early, the drought will be lifted early, and even floods will occur, while the south will be dry and rainless, and severe drought may occur.
Dry cracked soil Source丨Pixabay
When the wind is weak in summer, the rain belt advances slowly or even does not move, it will stay in the south of the Yangtze River, so that the southern region will rain for a long time, and the drought in the northern region will not be lifted, and there may be a situation of "drought death, waterlogging death".
In addition, global climate change will also lead to a more uneven spatial and temporal distribution of rainfall, which may cause drier places in arid places and more rain in rainy places; From the perspective of time distribution, under the condition that the total amount of rainfall remains unchanged, the number of rainfall days decreases, precipitation is more concentrated, and the phenomenon of rapid changes in drought and flooding may occur more frequently.
Flood Source丨Pixabay
Why is there a pandemic after a flood?
Floods caused by high-intensity continuous rainfall can easily cause plague for the following four main reasons:
- Animal and plant carcasses and waste can lead to contaminated water sources. Floods bring a lot of waste and may bring drowned animal carcasses, which are likely to become a source of pathogenic microorganisms such as bacteria, fungi, and viruses, and people are susceptible to disease after drinking or touching these contaminated water sources.
- Damage to sewage treatment facilities leads to untimely treatment of sewage, increasing the risk of infectious disease transmission.
- Flooding can hinder a city's sanitation drainage system, which can lead to stagnant water and breeding mosquitoes and flies.
- Floods can evacuate large numbers of people to temporary shelters, where dense, overcrowded and unhygienic conditions can make people less physically fit and more susceptible to infectious diseases as well as to others.
After a rainstorm and flood, pay attention to the following:
- After a flood, the first thing to do is to keep yourself and your family safe, and try to avoid wading and entering dangerous areas. If you are in dangerous waters, try to find a high ground and wait for rescuers to arrive. If the vehicle is flooded, do not try to start it to avoid damaging the engine.
- Keep an eye on weather forecasts and warnings issued by local governments, and keep abreast of flood disasters and related countermeasures.
- Practice personal hygiene, wash your hands frequently, and avoid using sewage to wash food. Keep your body clean and clean.
- Clean up the dwelling, remove waste and sludge, and ensure that the dwelling is dry and ventilated. Check the structure of the house for damage to ensure the safety of living.
- Try to avoid drinking untreated tap water directly. It can be drunk after boiling, or treated with a disinfectant.
- Check that food is not soaked in water and can choose to discard if in doubt. Avoid eating untreated wild and dead animals.
- Prevent the spread of diseases and avoid contact with garbage, wastewater and corpses. If you have fever, diarrhea and other symptoms, please seek medical attention promptly.
- Flooding can cause mosquito breeding, use mosquito repellent, and wear long-sleeved clothing and long pants to prevent mosquito bites.
- If the electrical appliance is damp, it should be used after it is completely dry to avoid electric shock accidents.