Why are there these two extremes of “drought kills people, flood kills people”?

Audit expert: Zhan Mingjin

President and Professor of Jiangxi Vocational College of Information Application

PhD in Meteorology, University of Chinese Academy of Sciences

This summer, the Beijing-Tianjin-Hebei region was hit by heavy rains for several consecutive days due to the landing of Typhoon Dusurui, and suffered serious flood disasters. As the old saying goes, "long drought will lead to floods", although there are many complete versions handed down, the most widely circulated one is "severe drought will be followed by severe floods, and severe floods will be followed by severe epidemics."

Among them, "severe drought" refers to a natural disaster, usually referring to hot weather and no rain for a long time, resulting in dry air and water shortage in the soil; "severe flood" refers to the phenomenon of excessive rainfall causing flooding and waterlogging in low-lying areas; "severe epidemic" refers to the epidemic.

The widespread and rare droughts in the past few years and the heavy rains and floods this year seem to have perfectly verified this saying, but why is there such an empirical saying? Is it really scientifically based?

Why does heavy rain after a drought cause flash floods?

In order to explain why heavy rain after drought may lead to flash floods, Professor Robert Thompson from the University of Reading in the UK placed three cups of water upside down on three different types of grass with different humidity levels, and recorded the time it took for each piece of grass to absorb a full cup of water. The very wet grass on the left took only 9 seconds, the moderately wet grass in the middle took 52 seconds, and the dry grass on the right took about 15 minutes. The experiment shows that the drier the soil, the harder it is for water to penetrate it.

Source: News screenshot

Although the presence of grass will indeed have a certain impact on the experimental results during the comparison process, and it cannot guarantee that the three cups of water are perfectly attached to the ground, the huge gap in the results can still explain certain problems.

Relevant experts said that for areas that have been suffering from drought for a long time, low-intensity precipitation, especially gentle rain, is very beneficial for alleviating the drought. However, sudden high-intensity rainfall is likely to cause a sudden transition from drought to flood in areas that have been suffering from drought for a long time, and then cause flood disasters.

Source: News screenshot

When the soil is in a state of drought for a long time, the connection strength between soil particles will decrease, resulting in loose soil. At this time, if there is heavy rainfall, the rainwater will not be quickly absorbed by the soil, but will directly flow through the impermeable dry land, and then wash away everything on the ground with great kinetic energy, causing disasters such as mountain torrents, landslides and mud-rock flows.

This phenomenon is also very common in daily life. If you have a potted plant at home that has not been watered for a long time and the soil is just ordinary loess, then if you water it heavily one day, you will find that the water will splash directly out of the pot and basically cannot be absorbed by the soil.

Source: A social software

In order to timely prevent and respond to flood disasters that may be caused by heavy rainfall after drought, it is far from enough to rely solely on the soil's own water storage capacity. It is necessary to build enough water storage facilities, such as large reservoirs, so that while storing the required moisture for the soil, most of the precipitation can be reasonably stored as much as possible to avoid causing greater disasters.

In addition, the frequent occurrence of this sudden change from drought to flood also puts higher demands on the accuracy of weather forecasts.

Why are there these two extremes of “drought kills people, flood kills people”?

my country has a large north-south geographical span. The northern region is controlled by the subtropical high pressure belt, while the southern region is affected by the subtropical high pressure and low pressure over the ocean. Therefore, my country's precipitation is mainly related to the advance and retreat of the monsoon. Once the monsoon is abnormal, it will cause abnormal precipitation, and drought and flood disasters will occur alternately over a large area.

Under the influence of the summer monsoon, the rain belt gradually advances from the southeast coast to the northwest inland. When the summer monsoon is strong, the rain belt advances too fast, the rainy season in North China will arrive early, the drought will be lifted early, and even floods and waterlogging disasters will occur. In the south, there will be drought and little rain, and severe summer drought may occur.

The origin of drought-cracked soil丨pixabay

When the summer monsoon is weak, the rain belt advances slowly or even stops moving, and will stay south of the Yangtze River, causing prolonged rainfall in southern China. The drought in northern China will not be relieved, and a situation of "some people will die of drought, and some people will die of floods" may occur.

In addition, global climate change will also lead to more uneven temporal and spatial distribution of rainfall. From a spatial perspective, it may cause dry areas to become even drier and rainy areas to have more rain. From a temporal distribution perspective, with the total amount of rainfall remaining unchanged, the number of rainy days will decrease, precipitation will be more concentrated, and sudden changes from drought to flood may occur more frequently.

Flood source: pixabay

Why is it that “a major epidemic will follow a major flood”?

Floods caused by high-intensity continuous rainfall can easily lead to plagues. There are four main reasons:

Dead plants and animals and waste may lead to water pollution. Flood disasters will bring a large amount of waste, and may also bring dead animals, which are likely to become the source of bacteria, fungi, viruses and other pathogenic microorganisms. People are very likely to be infected with diseases after drinking or touching these contaminated water sources.

Damage to sewage treatment facilities results in the inability to treat sewage in a timely manner, increasing the risk of the spread of infectious diseases.

Floods may block the city’s sanitary drainage system, and poor drainage may lead to stagnant water and the breeding of mosquitoes and flies.

When a flood disaster occurs, a large number of people may be evacuated to temporary shelters, where there are dense crowds, overcrowding, and poor sanitary conditions, which may cause people's physical fitness to decline, making them more susceptible to infectious diseases and more likely to infect others.

After a rainstorm or flood, pay attention to the following:

After a flood, you should first ensure the safety of yourself and your family, and try to avoid wading and entering dangerous areas. If you are in dangerous waters, try to find a high place to wait for rescuers. If your vehicle is flooded, do not try to start it to avoid damaging the engine.

Keep a close eye on weather forecasts and warning information issued by local governments to keep abreast of flood disaster conditions and related response measures.

Maintain personal hygiene, develop the habit of washing hands frequently, and avoid using dirty water to wash food. Keep your body clean.

Clean the house, remove waste and sludge, and ensure that the house is dry and ventilated. Check the house structure for damage to ensure it is safe to live in.

Try to avoid drinking untreated tap water directly. You can drink it after boiling it, or treat it with disinfectant.

Check food for water damage and discard if in doubt. Avoid eating unprocessed wild and dead animals.

Prevent the spread of diseases and avoid contact with garbage, wastewater and corpses. If you have symptoms such as fever or diarrhea, please seek medical attention immediately.

Floods may cause mosquito breeding; use mosquito repellent and wear long-sleeved clothing and long pants to prevent mosquito bites.

If an electrical appliance gets damp, wait until it is completely dry before using it to avoid electric shock accidents.