Freshwater resources in China.

(Former Vice Minister of Geology and Mineral Resources and President of the International Geoscience Union)

Fresh water is a renewable resource, supplemented by atmospheric precipitation. Static storage capacity is only a regulating space, and it cannot solve the long-term water supply demand. There are many fresh water resources per unit area in China, which is close to the global average. However, because the population density is three times higher than the world average, the per capita fresh water resources are only weaker than the world average 1/3. The spatial and temporal distribution of atmospheric precipitation in China is extremely uneven, which increases evaporation. Agriculture is inseparable from irrigation, which further aggravates the contradiction between supply and demand of fresh water resources. Fresh water is one of the few resources that cannot be imported. In the future, we can only solve the water problem in China by saving water and scientifically adjusting the existing fresh water resources. Both surface water and groundwater are components of fresh water resources. The surface water body has strong water conductivity, but small water storage capacity; Groundwater aquifers have weak water conductivity, but large water storage capacity. Combining the two can realize the timely regulation of fresh water resources more effectively. There are more fresh water resources in the south of China, but less in the north. The population distribution is generally suitable for the distribution of fresh water resources, and it is necessary to avoid excessive population flowing to water-deficient areas. The South-to-North Water Transfer Project can alleviate the situation of water shortage in the north, but the cost of water transfer is high, so it should be mainly used in special drought years, and the basic water supply source should still be adapted to local conditions. There is abundant precipitation in the south, and water shortage is mainly caused by pollution, so we should concentrate on pollution control. In order to rationally allocate freshwater resources and effectively control water pollution, it is necessary to strengthen the unified management of river basins. Set the total limit of water intake and sewage discharge in each area. Deep confined groundwater resources have little potential, and long-term large-scale development will lead to serious consequences such as land subsidence, so it should be used with caution and less.

Fresh water is a resource that people can't live without at any time. Things that seem to belong to common sense. However, whenever I hear the argument about which is more important, surface water or groundwater, the warning of well-intentioned people about "the last drop of water on the earth" and the discovery of a large underground reservoir in a certain place, while exclaiming that "the largest funnel in the world" has appeared in China, I feel that people's understanding of fresh water resources is not consistent. Knowing whether it conforms to the objective law of fresh water resources has great influence on correct decision-making.

First, the main body of fresh water resources depends on natural regeneration, and the function of static storage capacity is to make up for it with abundance.

Various natural resources can be divided into two categories: renewable resources and non-renewable resources. The significance of this division can be vividly illustrated by a family's economic balance account.

Most families have a fixed income every month. If the expenditure is less than the income, deposit the extra money in the bank. If the expenditure exceeds the income, the bank deposit will be used. It can be simply expressed by the following formula:

Bank deposit increment = income-expenditure

In order to ensure the sustainable life of the family, we always try our best to live within our means and make a slight savings every month. So bank deposits increase month by month. However, once there is additional consumer demand, such as buying big items, holiday travel and so on. You can use the savings accumulated for months or even years. For such families, their daily life mainly depends on fixed wage income, and the role of bank deposits is only to adjust the surplus and deficiency. We can say that the economic source on which such families depend is renewable.

However, there are still very few people in the society who have no fixed wage income, but their ancestors left them a large legacy in the bank, which will last for 100 years if they are not profligate. For these people, the income item in the above formula is equal to zero, and the bank deposit is negative every month. Until it's all spent. These people's financial resources are obviously non-renewable. Use a little and use less.

Mineral resources are obviously non-renewable resources. Take coal as an example, it was formed in a long geological history. At present, the process of turning plants into coal continues in some parts of the earth, but it is very slow. Compared with the coal consumed all over the world every year, the newly generated coal is negligible. The coal we mine today is essentially a legacy left by nature to mankind. Because the coal reserves on the earth are large enough to meet the demand for hundreds of years, there is no need to worry about the future at present. In 100 years, mankind will always find other alternative energy sources.

Forest resources belong to renewable resources. Because trees are constantly updated, as long as reasonable planning, we can ensure that the annual felling is not greater than the renewal. If the cutting volume is greater than the regeneration volume, the forest volume will decrease, and vice versa.

The situation of fresh water resources is a little more complicated, and its main body belongs to renewable resources. However, under very special circumstances, out of helplessness, there are also examples of using water stored in underground geological history as non-renewable resources.

The earth's atmosphere is a huge distilled water factory, powered by solar energy. It continuously evaporates water from the ocean and the ground, and then provides fresh water to the land where people live in the form of precipitation. The average annual atmospheric precipitation on land is about 1 19 trillion cubic meters. After deducting the loss of evaporation and transpiration, it can still be converted into 42.7 trillion cubic meters of fresh water resources that human beings may use every year. Far more than the current water consumption of about 4 trillion cubic meters per year for all mankind. In the foreseeable future, fresh water resources will be sustainable. There can be no "last drop of water" crisis. Therefore, in general, human beings can completely rely on renewable fresh water resources to meet the needs of sustainable development.

However, the distribution of fresh water resources on the earth is very uneven. In some arid areas, rainfall is extremely low, and renewable fresh water resources are close to zero. If these areas are vast and sparsely populated, there is groundwater buried in geological history, the water quality meets the needs, and the storage capacity can meet the needs of the local small population for hundreds of years, they can live on their laurels for a long time. For example, the Sahara desert in northern Africa has been buried with fresh water in geological history, which has provided fresh water for the local population of Egypt, Libya and other countries for hundreds of years. A similar situation is extremely rare in other parts of the earth.

Groundwater stored in underground aquifers can be compared with bank deposits. It can cope with the emergency of short-term expenditure, but it cannot meet the needs of long-term sustainable development. In addition, the storage capacity of groundwater is different from that of sediments. There is no difference in procedure between withdrawing the first deposit and withdrawing the next deposit. With the decrease of groundwater reserves, the groundwater level drops. Beyond a certain limit, even if there is water, it is difficult to use it.

Second, China's per capita fresh water resources are not bad. The per capita fresh water resources are the result of a large population.

The contradiction between supply and demand of fresh water resources in some areas of China is becoming increasingly serious, giving people the impression that China is a country with particularly poor fresh water resources. This impression is not in line with reality. The richness of fresh water resources in a region can be evaluated by the average fresh water resources per unit area. The world's renewable freshwater resources are 42.7 trillion cubic meters per year, and the global land area is 654.38+34 billion square kilometers, that is, 654.38+034 trillion square meters. So: global freshwater resources per unit area = global freshwater resources/global land area = 42.7 trillion cubic meters134 trillion square meters/year = 3 19 mm/year.

China's renewable freshwater resources are 2.8 trillion cubic meters per year, with a land area of 9.6 million square kilometers and freshwater resources per unit area of 292 mm per year. It is equivalent to 9 1.5% of the global average. This shows that China is not a country with particularly poor fresh water resources. In the United States, whose land area is similar to that of China, the freshwater resources per unit area is 3 1.7mm/ year, which is not much different. However, due to the large population in China, the population density per unit area is three times the world average. Therefore, per capita freshwater resources are only weaker than the world 1/3. At present, the widely quoted number is 1/4, which is actually closer to 1/3. The population density of the United States is only nearly 1/5 that of China, so the per capita freshwater resources are about five times that of China. In a word, the shortage of fresh water resources in China is not because of insufficient resources, but because of too many people.

Three, fresh water resources can not rely on imports, can only be based on domestic.

Among all kinds of natural resources, fresh water is the most used resource. The sum of all other natural resources is not equal to a small part of fresh water resources. Fresh water is the cheapest resource and can't stand a lot of long-distance transportation. China's terrain is relatively high, and most of the international rivers are outbound rivers. Only Xinjiang has a small number of inbound rivers. This situation excludes the possibility that fresh water resources depend on imports. Unless the global climate changes significantly, the total amount of fresh water resources is not expected to change substantially in the future, but the population will increase. No matter how the national economy develops in the future and how the economic scale doubles, it can only be based on the existing 2.8 trillion cubic meters of fresh water resources every year. At this point, some people still have the hope of opening up new sources of goods. The following article will prove that although open source has certain prospects, it is impossible to have a significant impact on the total fresh water resources in China.

Fourth, the temporal and spatial distribution of fresh water resources in China is extremely uneven, which intensifies the contradiction between supply and demand.

Although China's per capita fresh water resources are only 1/3 of the global average, there are still 2300 cubic meters per capita every year. According to the current consumption level, it is enough. However, the distribution of fresh water resources in China is extremely uneven in space and time. This further aggravated the contradiction between supply and demand.

The renewal of fresh water resources mainly depends on atmospheric precipitation. Most of the land in China is located in the mid-latitude arid zone in the northern hemisphere, which should be relatively dry. Fortunately, the southeast Asian monsoon from the Pacific Ocean and the Indian Ocean brought water vapor. But it also leads to the extremely uneven distribution of precipitation. There is more precipitation in the south and east of China, while the northwest is dry. Generally speaking, there is no shortage of water in the south of Kunlun Mountain, Qinling Mountain and Huaihe River. If there is water shortage, it is generally mainly caused by pollution. However, the northwest region is dry and rainy, and the fresh water resources are relatively poor.

It should be pointed out that China has been dominated by agriculture for thousands of years. Agriculture is closely related to fresh water resources. Due to the random flow for thousands of years, the population distribution in China is generally adapted to the distribution of fresh water resources. It is generally not easy to change. With the development of China's productive forces, the proportion of agricultural population attached to cultivated land will gradually decrease. The premise of population distribution according to cultivated land will gradually weaken. It is no longer necessary to move to dry areas. Any migration to arid and semi-arid areas will inevitably increase the demand for fresh water in this area and further expand the contradiction between supply and demand of fresh water resources. We must be careful.

The uneven distribution of fresh water resources in time is an important reason for the shortage of supply and demand in northern China. Judging from the average precipitation for many years, many places in North China are not very rich, but they are not too little. Taking Beijing as an example, the average annual precipitation is 630mm, which is similar to that of Paris, Moscow, Vienna, Austria and Budapest, Hungary, but slightly higher than that of London, England and Berlin, Germany. So why is Europe wet and North China dry? This is due to the distribution of precipitation over time in many parts of Europe, which is surprisingly uniform in both years. This is unimaginable for people who have been living in the Asian continent.

In most parts of Europe, the air humidity is high, and the evaporation is far less than the precipitation. The contrast between northern China and Europe is extremely strong. Taking Beijing as an example, the monthly precipitation in June, July and August accounts for more than 3/4 of the total annual precipitation, while the semi-annual precipitation from 1 1 to April of the following year is less than110 of the annual precipitation. Due to the long duration of dry season, the annual evaporation is mostly above 1000 mm, far exceeding the annual precipitation. Not only during the year, but also the annual precipitation has changed greatly, and droughts have occurred from time to time for three consecutive years. Only a small part of atmospheric precipitation can be converted into effective fresh water resources, and most of it is re-evaporated. In addition, because the precipitation in rainy season is too concentrated, the precipitation that some reservoirs can't accommodate often enters the sea in the form of floods, which can't be used and sometimes even causes floods. Another advantage of uniform precipitation in Europe is that there is not much water for irrigation in farmland. Atmospheric precipitation can meet most of the water demand for crop growth, and many places don't even need irrigation at all. There are more water resources for industry and life. However, in China, especially in the north, agriculture is inseparable from irrigation. Farmland irrigation water occupies the vast majority of fresh water resources, and the water resources left for life and industrial production are very limited. In short, the uneven distribution of precipitation with time, on the one hand, reduces the effective fresh water resources; On the other hand, it has increased agricultural water consumption. This has greatly aggravated the contradiction between supply and demand of fresh water resources.

V prospecting and "water"

Whether the fresh water resources in China still have undiscovered potential. Some people pin their hopes on "finding water".

There is no problem of "finding" surface water, and everything is clear in broad daylight. "Looking for water" actually refers to looking for groundwater. The proposal of "looking for water" is obviously influenced by "looking for ore", especially "looking for oil". Oil is the fluid in the oil layer, and groundwater is also the fluid in the formation. If you can find oil, why can't you find water? Of course, the flow of oil, gas and groundwater follows the basic laws of seepage mechanics. We can learn a lot from each other. But there is a fundamental difference: oil and gas are non-renewable resources, and the main body of groundwater can only be renewable resources.

As a non-renewable resource, if you exploit a little, the proven resources will be less and will be exhausted sooner or later. In order to ensure sustainable development, we must strive to find alternative resources. In most cases, mines were actually found. Because, due to the limitations of people's understanding, far from all minerals have been identified. The whole prospecting history can be summarized as follows: (1) After finding the outcrop ore, find the concealed ore; After finding the shallow ore, look for the deep ore. This experience can be extended to the field of fresh water resources: there is not enough surface water to find groundwater; Shallow groundwater is not enough. Looking for deep groundwater.

However, groundwater is quite another matter. As mentioned above, the storage of groundwater can only be used to regulate the abundance and dryness, and it cannot be relied on for a long time. What human beings can rely on is mainly renewable fresh water resources. And this kind of resource is right under our noses, and we don't need to "find" it specially. From the macro-strategic point of view, "finding water" cannot solve the problem of "open source" of fresh water resources.

However, in some arid areas with scarce surface water and high salt content in shallow groundwater, there are aquifers with good water quality in some places. So the problem of "finding water" in deep aquifer is put forward. The vast majority of deep aquifers belong to closed confined aquifers, and because it is extremely difficult to be supplemented by atmospheric precipitation, the freshwater resources contained in them belong to non-renewable resources. Long-term exploitation of this deep groundwater will lead to rapid decline of groundwater level and land subsidence. Only under the conditions of a large population and a small amount of water per unit area, such as drinking water for people and animals in border posts and pastoral areas, or short-term water use in extremely dry years, can this resource be moderately exploited.

Another situation can be called "looking for water", that is, in areas lacking effective aquifers, such as large areas of granite or metamorphic rocks. In these areas, it is necessary to use geological and geophysical methods to find hidden structural fracture zones. Because only in the area where the rock is broken, there are enough pores to store and conduct groundwater. Simply put, only in this case can water be drained from wells or other water collection projects.

In any case, "looking for water" can't solve the strategic account of fresh water resources, but mainly focuses on the water use of people and animals in water-deficient settlements with small population.

So, what is the potential of fresh water resources for open source? The potential of fresh water resources does not lie in "finding water", but it is not without potential to be tapped. We can find ways to tap the potential by reducing the losses caused by the uneven distribution of atmospheric precipitation in China. There are two main aspects: one is to seize evaporation. In China, especially in arid areas, most of the atmospheric precipitation evaporates into the sky. There is great potential in this. The second is to seize the sea and abandon the water. Due to the concentrated rainfall in the flood season, the surface reservoir did not have enough storage capacity to block the flood, and some water ran to the sea in vain. If this part of the water is impounded, the amount of water will be considerable.

However, these two articles are easier said than done. If we want to catch evaporation, we must find ways to make atmospheric precipitation penetrate more underground and reduce sunlight exposure. In order to catch the waste water flowing into the sea, we must try to store the flood in flood season. Therefore, it is necessary to have sufficient regulating storage capacity, and the surface reservoir is built for this purpose. Miyun Reservoir on the Chaobai River in Beijing has an average inflow of more than 654.38 billion cubic meters and a storage capacity of 4 billion cubic meters. It is a good reservoir that can be adjusted for many years. Unfortunately, in most other river basins, the total storage capacity of the existing surface reservoirs and the surface reservoirs expected to be built together is still far from meeting this requirement. The regulation capacity of groundwater aquifer is much larger than that of surface reservoir.

Therefore, groundwater aquifers play an extremely important role in collecting evaporated or abandoned water.

VI. Relationship between Surface Water and Groundwater Aquifer

In many people's minds, surface water and groundwater are two different water sources. This is a one-sided view, which is not conducive to the scientific and rational utilization of the whole fresh water resources. From the point of view of being a renewable resource, both surface water and groundwater come from atmospheric precipitation, and they are also mutually transformed. Take the Neiliu River Basin in Xinjiang and Gansu as an example. Almost all the little precipitation at the bottom of the basin evaporated, which could not form effective fresh water resources. The local fresh water resources mainly come from the precipitation in the mountains around the basin and the subsequent melting of snow. These waters are collected in rivers in mountainous areas and flow to the foothills, and a large part of them penetrate into the piedmont diluvial fan composed of gravel and coarse sand and are converted into groundwater. The debris of alluvial fan gradually becomes thinner from upstream to downstream, and the ability to transport groundwater gradually weakens. Finally, the groundwater is blocked at the edge of the diluvial fan, overflows the surface in the form of spring water, and then turns into surface water. In those areas, it is meaningless to artificially divide surface water and groundwater resources.

In a broader sense, the flow of rivers depends on groundwater for a long time in a year. Rivers have a strong ability to transport surface water. After the rainy season's atmospheric precipitation flows into the river, it will be discharged into the sea in a short time. After the rainy season, the continuous flow of many rivers is attributed to underground aquifers. Groundwater aquifers can store fresh water formed by precipitation infiltration. Because the water transfer capacity of groundwater aquifer is much lower than that of surface water body, the groundwater stored in aquifer can only be released slowly in rainy season. All these trickles finally converge into a river, forming a considerable flow, making the river flow continuously. The river flow after flood season is called "base flow". Base flow is the most valuable part of fresh water resources, which comes from groundwater aquifer.

Both surface water bodies and underground aquifers are carriers of natural fresh water resources, but they have different characteristics.

As a container of water, the surface water body has low friction resistance, so it has high water transport capacity. In addition, the area of fresh water on land accounts for less than 1% of the land area, so the water storage capacity is very small. On the other hand, the underground aquifer is subjected to great frictional resistance, because water flows in the pores of rocks. Under the same hydraulic gradient, the velocity of groundwater is several orders of magnitude smaller than that of surface water. However, groundwater aquifers are widely distributed and almost everywhere, and their water storage capacity is much larger than that of surface water bodies. DC resistor and capacitor circuits can be used to compare the above two situations. The surface water body is like a circuit with small resistance and capacitance, that is, a circuit with small time constant, while the groundwater aquifer is like a circuit with large resistance and capacitance, that is, a circuit with large time constant. The water in the surface water body comes and goes quickly. The water in the groundwater aquifer comes and goes slowly, which can filter short peak pulses and level off extremely uneven precipitation. This is exactly what is needed in areas with extremely uneven precipitation.

Groundwater aquifer can not only adjust the wet year and dry year, but also greatly reduce evaporation. Once the atmospheric precipitation seeps into the ground, the evaporation will decrease sharply. If the groundwater level is below 1 m, the evaporation is actually close to zero.

There are two rivers in Shenmu coalfield in northern Shaanxi, one is Kuye River and the other is Tuwei River. The two rivers flow into the Yellow River in parallel from northwest to southeast, only a few tens of kilometers apart. The rocks in the area where the Kuye River passes are exposed. Whenever the rainy season, floods carry a large amount of sediment into the Yellow River quickly, while the dry season lasts a long time. However, a large area of the upper reaches of the Tuwei River is covered by the edge of the Mu Us Desert. During the rainy season, rainwater is absorbed by the desert and rarely overflows. After the rainy season, groundwater slowly seeps out of the desert, making Tumen River maintain a relatively uniform flow all the year round. Due to the protection of desert, the evaporation in Tuwei River basin is greatly reduced. More than half of the atmospheric precipitation can be converted into effective fresh water resources. This is extremely valuable in the Loess Plateau.

Groundwater aquifer has the characteristics of "big belly and small throat", and it is slow to receive atmospheric precipitation recharge. This makes it more difficult for us to use it. The Beijing area is a good example. The alluvial fan of Yongding River has a huge aquifer. This used to be the main water supply source in Beijing. Its huge water storage capacity has helped the capital survive the water shortage year after year. After years of over-exploitation of groundwater, the groundwater level has dropped significantly, forming a large underground storage capacity. This should be an excellent place to regulate water resources. Feasibility study was carried out in 1970s, and it was found that the flood period of Yongding River was only ten days a year. The annual water intake in Beijing is several billion cubic meters. Even if 6.5438+million cubic meters of water is injected manually every year, it will not solve many problems. However, in order to complete this humble task within 10 days, it is necessary to irrigate10 billion cubic meters every day in flood season, which requires the construction of a huge recharge project. Moreover, the high sediment concentration in the river during the flood season will quickly block the infiltration surface of the groundwater aquifer. Xihuangcun artificial recharge test site was built at that time. Although the geological and geographical conditions are superior, it can only be used for artificial recharge in non-flood season.

Practice has taught us a very important lesson. To achieve greater recharge effect, it is not enough to rely solely on manual measures. According to the specific situation in Beijing, we put forward the method of "virtual recharge". The existing facilities for pumping groundwater in Beijing area have a large capacity. No matter how large the capacity of recharge facilities is, it is impossible to exceed the pumping capacity. Under the condition of maintaining the operation of existing pumping facilities, it is physically equivalent to irrigating 1 100 million cubic meters of water each time, but reducing pumping 1 100 million cubic meters without reinjection. Therefore, reducing pumping is equal to increasing recharge, which is a kind of "virtual" recharge. This kind of recharge does not need special recharge facilities, but it needs other water sources to replace the reduced pumping capacity. This water diversion can come from the excess atmospheric precipitation in the wet year. If Beijing builds two sets of water supply facilities, one using surface water and the other using groundwater, then each set can independently meet the water supply demand of the whole city. If the facilities for pumping groundwater are stopped in wet years, it means that 654.38 billion cubic meters of water will be pumped back and stored underground in a year without any recharge facilities. In dry years, we can use less surface water and draw underground stock to tide over the difficulty of water shortage. In this way, fresh water resources can also be adjusted every year. Before the flood season comes, we should make use of the "abandoned" water in the reservoir as much as possible to "empty the reservoir to meet the flood season", instead of pumping groundwater and using less underground inventory.

The above scheme requires unified dispatching of surface and underground reservoirs, giving full play to their respective "strengths" and avoiding their respective "weaknesses". Combined with the characteristics of each region, other schemes can be designed.

In a word, surface water and groundwater are not two different water sources. If we are good at learning from each other's strengths and giving full play to our respective advantages, we can make better use of the limited fresh water resources. It is a narrow and one-sided view that surface water and groundwater are artificially separated and each holds one end.

VII. Open Aquifers and Closed Aquifers

Groundwater is water that exists in underground rocks. All rocks contain groundwater to some extent, but not all strata are aquifers. Only those strata that contain a certain amount of water and allow groundwater to flow are called aquifers, otherwise they are aquicludes. Of course, this division is only relative.

According to the relationship between groundwater and aquifer and aquifuge, groundwater aquifer can be divided into "phreatic aquifer" and "confined aquifer". The translation of these two introduced terms is really puzzling. Not only laymen don't understand, but also experts are often confused. The author thinks that the use of "open aquifer" and "closed aquifer" can better reflect the essential difference between the two aquifers.

Can be compared with the familiar surface water. Rivers and lakes belong to open water bodies, and tap water pipelines belong to closed water bodies. The amount of fresh water in the river increases with the rise of water level, and the flow rate also increases. The amount of fresh water in the tap water pipeline changes little with the head, which is almost negligible. The discharge is only related to the hydraulic gradient, and almost has nothing to do with the head.

If the groundwater in the aquifer does not fill the whole aquifer, the situation is similar to open water bodies such as rivers, lakes and reservoirs. When the volume of groundwater in the aquifer increases or decreases, the groundwater level will rise or fall. This aquifer should be called "open aquifer". But the current term is "phreatic aquifer". Because of its openness, the open aquifer is easy to be replenished directly from atmospheric precipitation or surface water, so the fresh water resources in it are easier to be regenerated to ensure the needs of sustainable development. At present, most of the groundwater pumped in the world comes from open aquifers.

If the aquifer is covered by an aquifuge and the groundwater fills the aquifer, it will become similar to a closed tap water pipeline. When the groundwater level changes, the aquifer volume is limited by the overlying water-resisting layer and cannot change freely like an open aquifer. This kind of aquifer should be called "closed aquifer", and the scientific term in textbooks is "confined aquifer".

In fact, neither the tap water pipeline nor the confined aquifer is absolutely rigid. They are both elastic and compressible. When the water level rises, the volume will expand, otherwise it will be compressed and reduced. This kind of expansion and contraction is negligible for tap water pipes and is usually ignored. However, for confined aquifers, this compressibility and elasticity cannot be ignored, and the reasons will be discussed below.

First of all, the water conductivity of a closed aquifer is many orders of magnitude smaller than that of a tap water pipeline. Any faucet at the end can be replenished from the water storage container of the waterworks almost immediately. The head loss is relatively small. However, the frictional resistance of aquifer to water is large, and the distance from the lateral recharge source of closed aquifer to the position of water intake and well drilling is generally very far. During this period, there is a great head loss to reach a steady state. In fact, before reaching the steady state, the water pumped from the well is not from the distant lateral recharge source, but from the compression of the aquifer around the well. The pumping of the well lowers the groundwater level and forms a descending funnel around the well. For a closed aquifer, it is like using a hydraulic jack to drain oil, deflate automobile tires and compress the aquifer. This will squeeze some groundwater out of the aquifer. The groundwater pumped by the well is actually the part of water compressed from the aquifer. In the early days, the groundwater pumped by the well was mainly compressed from the aquifer near the well. With the expansion of the descending funnel, the pumped water gradually comes from the aquifer farther away. It takes a long time from the beginning of pumping to the point where most of the well water comes from the recharge boundary instead of the aquifer compression. If the recharge boundary is far away from the pumping well, even for decades, the water level of the pumping well will drop deeply and even the pumping cost will be unacceptably high. In addition, every time the groundwater level in the closed aquifer drops 1 m, the water that can be given by compression is very small, only a few thousandths to a few ten thousandths of that in the open aquifer. Therefore, under the same water yield, the volume of the groundwater level drop funnel is several thousand times larger than that of the open aquifer.

According to the principle of conservation of mass, groundwater extracted from aquifer can't be produced out of thin air, it always comes from a source. Open aquifers are easy to understand. Some of the water extracted from the aquifer comes from the drainage of the aquifer and some comes from the recharge of the surface water body. Confined aquifers are somewhat puzzling. The latter is neither discharged nor replenished from surface water. So where does the water pumped from the well come from? An important progress in groundwater hydraulics in the early 20th century was the discovery that groundwater extracted from a closed aquifer was compressed by the volume of the aquifer. Finally, it is manifested in land subsidence. According to the results of long-term observation in Cangzhou and Tianjin, Hebei Province, the total amount of groundwater extracted from closed aquifers for many years is roughly equivalent to the total amount of land subsidence, and the lateral recharge can be ignored.

Extracting groundwater from confined aquifer will lead to land subsidence! This is a serious problem. So far, we have many negative cases. As early as 1960s, Shanghai suffered irreparable losses due to land subsidence. Because the early stage of land subsidence is difficult to find intuitively, the lessons of Shanghai have not been absorbed by other places in time, and similar problems have appeared in Tianjin. Suzhou, Wuxi and Changzhou in the Yangtze River Delta, because the aquifer is not as wide as that in Shanghai, are restricted by local small fault basins, resulting in uneven settlement and ground fissures. The ground fissure in Xi 'an is also the result of pumping water from confined aquifer for a long time.

It can be seen that pumping groundwater from a closed aquifer for a long time does more harm than good, and often does more harm than good. Not pumping much water will cause the groundwater level to drop sharply, which will often lead to serious land subsidence.