Zhang Hongren
(Former Vice-Minister of Geology and Minerals and President of the International Union of Geological Sciences)
Freshwater is a renewable resource, which is recharged by atmospheric precipitation. The static reservoir capacity is only regulating space and cannot solve the long-term water supply needs. China's freshwater resources per unit area is not a lot, close to the global average, but due to the population density is three times higher than the world average, the per capita freshwater resources is only 1/3 of the world's average value of the people weak. China's atmospheric precipitation in time and space distribution is extremely uneven, so that the evaporation increased. Agriculture is inseparable from irrigation, exacerbating the contradiction between supply and demand of freshwater resources. Freshwater is one of the few resources that cannot be relied upon for import. In the future, we can only rely on water conservation and scientific regulation of existing freshwater resources to solve our water problems. Both surface water and groundwater are components of freshwater resources. Surface water bodies have a strong ability to conduct water, but the volume of stored water is small; groundwater aquifers have a weak ability to conduct water, but the volume of stored water is large. Combining the two for joint scheduling can more effectively realize the regulation of freshwater resources in time. China's freshwater resources in the south and north less, the distribution of population and freshwater resources distribution is generally compatible, should avoid excessive population flow to water-scarce areas. South-to-north water transfer can alleviate the situation of water shortage in the north, but the cost of water transfer is high, should be used mainly for special drought years, the basic source of water supply, should still be based on local. In the south of China, where there is abundant precipitation, water shortages are mainly caused by pollution, and efforts should be concentrated on combating pollution. In order to rationally allocate freshwater resources, effective treatment of water pollution, it is necessary to strengthen the unified management of the basin. Limits on the amount of water to be withdrawn and the total amount of sewage to be discharged in each area should be set. Deep confined groundwater resources have little potential, and long-term large-scale development will lead to serious consequences such as ground subsidence, so they should be used carefully and sparingly.
Fresh water is a resource that people can't live without at any time. It seems to be a matter of common sense. However, whenever you hear about "surface water, groundwater which is more important" debate, the good people on the "last drop of water on earth" warning, a certain place, "the discovery of large underground reservoirs "On the one hand, China has the "largest funnel in the world", and on the other hand, it is pinning its hopes on the search for new sources of deep groundwater, we feel that people's understanding of freshwater resources is not consistent. And whether the understanding is consistent with the objective law of freshwater resources, the correct decision-making has a great impact.
One, the main body of freshwater resources by natural regeneration, the role of the static reservoir is to make up for the failure of abundance
All kinds of natural resources can be divided into two categories: renewable resources and non-renewable resources. The meaning of this division can be graphically illustrated by a family's economic balance sheet.
Most families have a recurring monthly income. If expenses are less than income then the excess is banked. If expenses are greater than income, bank deposits are used. This can be expressed simply by the formula:
Increase in bank deposits = Income - Expenditures
In order to ensure that the family can live sustainably, it always tries to live within its means, and makes a small saving each month. Thus bank deposits increase from month to month. However, once there is an additional consumer demand, such as buying big-ticket items, holiday travel, etc., you can use the savings that you usually save for many months or even years. For this type of family, ordinary life mainly depends on regular salary income, the function of bank deposits is only to regulate the balance. We can say that the financial resources on which such families rely are renewable.
However, there are still a very small number of people in the society who do not have regular salary income, but their ancestors have left them a large amount of inheritance in the bank, which is enough for 100 years if they do not squander it arbitrarily. For these people, the income term in the above formula equals zero, and the bank balance grows negatively every month. Until it is spent. The financial resources of these people are obviously non-renewable. Use a little and you get less.
Mineral resources are clearly non-renewable. Coal, for example, was formed over a long geologic history. The process of turning plants into coal is still going on in some parts of the planet today, but it is going on extremely slowly. The amount of new coal produced each year is negligible compared to the amount of coal consumed globally each year. The coal we mine today is essentially nature's legacy to mankind. Since the Earth's coal reserves are large enough to meet needs for hundreds of years, there is no need to worry about the future at the present time. In another 100 years, mankind will always be able to find other alternative sources of energy.
Forest resources, on the other hand, are renewable. Because forests are constantly being renewed, as long as they are properly planned, it is possible to ensure that the amount of trees cut down each year is not greater than the amount of trees regenerated. If the amount of trees cut is greater than the amount of regeneration, the amount of trees stored in the forest will decrease, and vice versa.
The case of freshwater resources is slightly more complicated, as the bulk of it is renewable. However, in very special cases, out of necessity, there are examples of using water stored in the underground geological history as a non-renewable resource.
The Earth's atmosphere is a vast, solar-powered distillation plant for water. It constantly evaporates water from the oceans and the ground up into the sky, which in turn provides fresh water in the form of precipitation to the land where people live. On average, about 119 trillion cubic meters of atmospheric precipitation falls on land each year. After deducting evaporation and transpiration losses, there are still 42.7 trillion cubic meters per year that can be converted into freshwater resources that humans can potentially utilize. This far exceeds the current annual water consumption of all mankind, which is about 4 trillion cubic meters. In the foreseeable future, freshwater resources are sustainable. There is no possibility of a "last drop" crisis. Therefore, on the whole, mankind can fully rely on renewable freshwater resources to meet the needs of sustainable development.
However, freshwater resources are unevenly distributed on Earth. There are some arid regions that receive very little rainfall and have close to zero renewable freshwater resources. If these areas are sparsely populated, and there is a geological history of buried, the quality of water to meet the needs of the local population and storage can meet the needs of a small number of hundreds of years of groundwater, you can rely on a long period of time, "eat the old money" to live. For example, in the Sahara desert in northern Africa, there are geologically historical deposits of fresh water that have provided countries such as Egypt and Libya with fresh water for hundreds of years to meet the needs of their populations. Similar conditions are extremely rare elsewhere on the planet.
Groundwater stored in underground aquifers can be compared to bank deposits. It can cope with short-term emergencies, but it cannot meet the needs of long-term sustainable development. In addition, the amount of groundwater stored is also different from a deposit. There is no difference in procedure between withdrawing the first deposit and withdrawing the subsequent deposit. Instead, as the amount of groundwater stored decreases, the water table falls. Beyond a certain limit, it is difficult to utilize the water even if it is available.
Two, China's average freshwater resources are not poor, per capita freshwater resources is the result of a large population
Some areas of freshwater resources in China, the growing contradiction between supply and demand, giving people an impression: as if China is a particularly poor country of freshwater resources. This impression is not consistent with reality. The abundance of freshwater resources in a region can be used to evaluate the average freshwater resources per unit area. The world's renewable freshwater resources are 42.7 trillion cubic meters per year, and the global land area is 134 million square kilometers, or 134 trillion square meters. Thus: global freshwater resources per unit area = global freshwater resources ÷ global land area = 42.7 trillion cubic meters ÷ 134 trillion square meters per year = 319 mm/year
China's renewable freshwater resources per year is 2.8 trillion cubic meters, with a land area of 9.6 million square kilometers, and freshwater resources per unit area of 292 mm/year. Equivalent to 91.5% of the global average. This shows that our country is not particularly poor in freshwater resources. The United States, which has a land area similar to ours, has a freshwater resource per unit area of 317 mm/year, which is not a big difference. However, due to our large population, the population density per unit area is three times the world average. As a result, per capita freshwater resources are only 1/3 as weak as those in the world. At present, the widely quoted figure is 1/4, in fact, closer to 1/3. The United States, due to the population density is only China's nearly 1/5, per capita freshwater resources are therefore equivalent to about 5 times our country. In short, China's freshwater resources, not due to resource poverty, but due to the large population.
Three, fresh water resources can not rely on imports, can only be based on domestic
In a variety of natural resources, fresh water resources is the largest amount of resources. The sum of all other natural resources is not worth a fraction of freshwater resources. Fresh water is also the cheapest resource and cannot withstand a lot of long-distance transportation. China's terrain is high, and most international rivers are outbound, with only a few inbound rivers in Xinjiang. This situation precludes any possibility of dependence on imports for freshwater resources. Unless there are major changes in the global climate, the total amount of freshwater resources is not expected to change substantially in the future, while the population will still grow, and no matter how the national economy develops in the future, and how many times the size of the economy is doubled, it will only be able to stand on the basis of the existing 2.8 trillion cubic meters of freshwater resources per year. At this point, some people still hold the hope of opening up new sources. The following article will prove: open source, although there is some prospect, but it is unlikely to have a significant impact on the total amount of freshwater resources in China.
Four, China's spatial and temporal distribution of freshwater resources is extremely uneven, exacerbating the contradiction between supply and demand
Although China's per capita freshwater resources for the global average is only 1/3 of the weakness of each person per year there are still 2300 cubic meters. According to the current level of consumption is enough. However, China's freshwater resources whether in space or in time the distribution is extremely uneven. This further exacerbates the contradiction between supply and demand.
The renewal of freshwater resources depends mainly on atmospheric precipitation. Most of our country is in the mid-latitude arid zone of the northern hemisphere, which is supposed to be relatively dry. Fortunately, the Southeast Asian monsoons from the Pacific and Indian Oceans bring water vapor. However, this has also led to extreme heterogeneity in the distribution of precipitation. The south and east of the country receive more precipitation, while the northwest is arid. By and large, south of the line of the Kunlun Mountains, the Qinling Mountains, and the Huaihe River, there is generally no water shortage. If there are water shortages, they are generally caused mainly by pollution. Northwest China, on the other hand, is arid and has little rain, and freshwater resources are thus relatively poor.
It should be noted that our country has been dominated by agriculture for thousands of years. Agriculture, in turn, is closely linked to freshwater resources. As a result of thousands of years of random flow, the distribution of our population is largely compatible with the distribution of freshwater resources. It is generally inappropriate to change it easily. With the development of China's productivity, the proportion of the agricultural population dependent on arable land will gradually decrease. The premise of population distribution according to arable land will gradually weaken. Migration of population to arid areas will no longer be a great necessity, any migration to arid and semi-arid areas, is bound to increase the demand for fresh water in the region, further expanding the contradiction between the supply and demand of fresh water resources, it must be prudent and cautious.
The unevenness of the distribution of freshwater resources in time is an important reason for the tension between supply and demand in the north of China. From the multi-year average precipitation, many parts of northern China, although not very rich, but also can not be considered too little. Beijing, for example, the average annual precipitation of 630 millimeters, and Paris, France, Moscow, Russia, Austria, Vienna, Hungary, Budapest and so on about the same, than the United Kingdom, London, Germany, Berlin, but also slightly more. So why is Europe more humid, while China's North China is more arid? This is due to the distribution of precipitation over time in many parts of Europe, both intra- and inter-annually are surprisingly even. This is unimaginable to people who have been living on the Asian continent.
Much of Europe has high air humidity, and evaporation is much less than precipitation. The contrast between our northern regions and Europe is extremely strong. Beijing, for example, June, July and August months of precipitation, accounting for more than 3/4 of the total annual precipitation, and from November to April of the following year, half a year of precipitation less than 1/10 of the annual precipitation. due to the dry season extends for a long time, most of the annual evaporation of more than 1,000 millimeters, far more than the annual precipitation. Not only intra-annual, but also inter-annual precipitation varies greatly, and droughts of three consecutive years occur from time to time. Only a small portion of the atmospheric precipitation is converted into an effective freshwater resource, and most of it is reevaporated into the sky. In addition, due to the over-concentration of precipitation during the rainy season, a portion of the precipitation that cannot be contained in reservoirs often enters the sea in the form of floods, which cannot be utilized and sometimes even cause floods. Another advantage of uniform precipitation in Europe is that not much water for irrigation, atmospheric precipitation can meet most of the water needs of crop growth, and many places do not even need to be irrigated; water resources left for industrial and domestic use is relatively large. In our country, especially in the northern region, agriculture can not be separated from irrigation. Agricultural irrigation water takes up the vast majority of freshwater resources, can be left to life and industrial production with very limited water resources. In short, the uneven distribution of precipitation over time has reduced effective freshwater resources on the one hand; on the other hand, it has increased water consumption in agriculture. This greatly exacerbates the contradiction between the supply and demand of freshwater resources.
Fifth, looking for minerals and "looking for water"
China's freshwater resources are still undiscovered potential. Some people put their hopes on "finding water".
Surface water does not exist "to find" the problem, everything is laid out in broad daylight, clearer. "Finding water" actually means finding groundwater. "Looking for water" is obviously influenced by "looking for", especially "looking for oil". Oil is the fluid in the oil-bearing layer, and groundwater is also the fluid in the stratum. If we can look for oil, why can we not look for water. Of course, the flow of oil and gas and groundwater follow the basic laws of seepage mechanics. There are many things that can be learned from each other. But there is one fundamental difference: oil and gas is a non-renewable resource, the main body of groundwater can only be renewable resources.
As a non-renewable resource of minerals, mining a little, the amount of proven resources will be a little less, sooner or later will be depleted. In order to ensure sustainable development, efforts must be made to find replacement resources. And in most cases there are indeed mines to be found. For, due to the limitations of people's understanding, far from all minerals have been identified. The entire history of the search for minerals can be summarized as follows: after the outcrops are found, the hidden minerals are found; after the shallow minerals are found, the deep minerals are found. This experience has been extended to freshwater resources: if surface water is not enough, look for groundwater; if shallow groundwater is not enough, look for deep groundwater.
However, groundwater is a completely different matter. As discussed earlier, groundwater storage can only be used to regulate abundance and depletion, and cannot be relied upon for long-term living. What humans can rely on is primarily a renewable freshwater resource that is constantly renewed. This resource is right under our noses and we do not need to "search" for it. From the macro-strategic point of view, "find water" does not solve the problem of "open source" of freshwater resources.
But in certain arid areas where there is a lack of surface water and high salinity in shallow groundwater, there are places where there are aquifers with good water quality deep underground. So the deep aquifer "find water" problem. The vast majority of deep aquifers are closed pressurized aquifers, and since it is extremely difficult to obtain recharge from atmospheric precipitation, the freshwater resources they contain are non-renewable. Large-scale and long-term exploitation of such deep groundwater will lead to a rapid decline in the water table and ground subsidence. Only under the conditions of a small number of people and a wide area and a small amount of water per unit area, such as the water supply for human and animal consumption in border guard posts and pastoral areas, or short-term water use in extreme drought years, this resource can be exploited moderately.
There is another situation that can be called "finding water", that is, in the lack of effective aquifers, such as large areas of granite or metamorphic distribution of the region. In these areas, geological and geophysical methods are needed to find hidden tectonic fracture zones. This is because only in the fractured sections of rock are there sufficient pore spaces to store and conduct groundwater, in short, only in this case can water come out of the wells or other water harvesting works.
In either case, the "water search" does not solve the strategic debt of freshwater resources, but focuses mainly on the problem of water for humans and animals in water-scarce settlements with small populations.
So what is the potential of freshwater resources? The potential of freshwater resources does not lie in the "search for water", but it is not without potential to dig. Can be from the reduction of our atmospheric precipitation in the time distribution of unevenness caused by the loss, find ways to tap the potential. There are two main aspects: one is to capture the evaporation. Our country, especially in arid areas, most of the atmospheric precipitation is evaporated to the sky. There is a lot of potential here. The second is to capture the abandoned water into the sea. Due to the concentrated rainfall during the flood season, surface reservoirs do not have enough capacity to store floodwaters, and some of the water goes to the sea for nothing. If this part of the water is stored, the amount of water is also very considerable.
However, these two articles are easier said than done. To capture evaporation, we need to find ways to allow more atmospheric precipitation to seep into the ground and less sun exposure. To capture the abandoned water into the sea, we must try to store the flood water during the flood season. To this end, there is a need for adequate regulation of the reservoir capacity, surface reservoirs are built for this purpose. The Miyun Reservoir on the Chaobai River in Beijing has a multi-year average inflow of more than 1 billion cubic meters of water and a capacity of 4 billion cubic meters, making it a good reservoir for multi-year regulation. Unfortunately, in most other river basins, the existing surface reservoirs and the projected surface reservoirs that can be built together, the total reservoir capacity is still far from enough to meet this requirement. Groundwater aquifers, on the other hand, have a much larger regulating capacity than surface reservoirs.
Therefore, groundwater aquifers play an extremely important role in capturing both evapotranspiration and water disposal.
Sixth, the relationship between surface water bodies and groundwater aquifers
In the minds of many people, 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 freshwater resources as a whole. From the point of view of renewable resources, surface water and groundwater both come from atmospheric precipitation, and they also transform each other. Take the inland river basin in Xinjiang and Gansu as an example. The very small amount of precipitation at the bottom of the basin is almost completely evaporated and does not form any effective freshwater resource. Local freshwater resources come mainly from precipitation and subsequent snowmelt in the mountains surrounding the basin. This water collects in mountain streams and runs to the foothills, where a large portion of it infiltrates into the premontane flood fans, which are composed of gravel and coarse sand, and is transformed into groundwater. The gravelly material of the floodplain fan gradually becomes thinner from upstream to downstream, and its ability to transmit groundwater gradually decreases. In the end, groundwater is bloated at the edge of the floodplain fan and overflows to the surface in the form of springs, which is transformed into surface water again. In those areas, the artificial division of surface water and groundwater resources has no substantial meaning.
In a broader sense, the flow of rivers is sustained by groundwater for a good part of the year. Rivers have a high capacity to transmit surface water. The atmospheric precipitation of the rainy season is discharged into the sea within a very short time after it reaches the rivers. The constant flow of water in many rivers after the rainy season is due to groundwater aquifers. Groundwater aquifers are capable of storing large quantities of freshwater formed by infiltration of precipitation. Because groundwater aquifers have a much lower capacity to transport water than surface water bodies, groundwater stored in aquifers during the rainy season can only be released slowly. All of this trickle of water is eventually brought together in the river to form a sizable flow that keeps the river flowing. The flow of the river after the flood season is called "base flow". Base flow is the most valuable part of freshwater resources, and it comes from groundwater aquifers.
Surface water bodies and groundwater aquifers are both carriers of natural freshwater resources, but they each have different characteristics.
Surface water bodies have low frictional resistance as water containers and thus have a high capacity to transmit water. In addition, the area of freshwater bodies on land accounts for less than 1% of the land area and thus has a small capacity to store water. Groundwater aquifers, on the contrary, are subject to high frictional resistance as water flows through the pores of the rock. At the same hydraulic gradient, groundwater flows several orders of magnitude smaller than surface water. However, groundwater aquifers are widely distributed and virtually ubiquitous and have a much greater capacity to store water than surface water bodies. An analogy for both of these situations can be made with DC resistance and capacitive circuits. A surface water body is like a circuit with low resistance and low capacitance, i.e., a circuit with a small time constant, while a groundwater aquifer is like a circuit with high resistance and high capacitance, i.e., a circuit with a large time constant. Water in a surface water body comes and goes quickly. Water in groundwater aquifers comes and goes slowly, and is able to filter brief flood pulses and level out highly uneven precipitation. And that's exactly what is needed in areas with very uneven precipitation.
Groundwater aquifers not only regulate water years and dry years, but also greatly reduce evaporation. Once atmospheric precipitation seeps into the ground, evaporation is drastically reduced. If the groundwater level is below 1 meter above ground, evaporation is practically close to zero.
There are two rivers in the area where the Shenmu Coalfield is located in northern Shaanxi, one is the Cave Wild River and the other is the Bald Tail River. Both rivers flow into the Yellow River parallel to each other from northwest to southeast, only a few dozen kilometers apart. The area through which the Cuyeo River flows is rocky and bare, and during the rainy season, flood water carrying a lot of sediment is quickly discharged into the Yellow River, while during the dry season, it dries up for a long time and lacks water. But the upper reaches of the Baldy River has a large area covered by the edge of the Mao Wusu Desert, rainwater is absorbed by the desert during the rainy season, rarely forming floods, after the rainy season, the groundwater slowly seeps out from the desert, keeping the Baldy River with a relatively even flow all year round. Due to the protection of the desert, evaporation in the Baldy River basin is greatly reduced. More than half of the atmospheric precipitation can be converted into effective freshwater resources. This is extremely valuable in the Loess Plateau.
The groundwater aquifer is characterized by a "big stomach, small throat", receiving atmospheric precipitation recharge is relatively slow. This gives us to utilize it increases the difficulty. The Beijing area is a good example. The Yongding River alluvial and floodplain fan has a huge thick aquifer. It used to be the main source of water supply for Beijing. Its huge reservoir capacity used to help the capital survive one water shortage year after another. After years of over-extraction of groundwater, the groundwater level has dropped considerably, resulting in a large underground reservoir capacity. This should have been a perfect place to store water resources. 1970's feasibility study, found that the Yongding River annual flooding period is only a dozen days. And Beijing's annual water intake of billions of cubic meters. Even if the annual artificial recharge of 10 million cubic meters, but also can not solve much of the problem. However, to complete even this inconspicuous task in 10 days, the flood season every day to recharge 1 million cubic meters, which requires the construction of a huge recharge project. Moreover, the high sediment content of the river water during the flood season would quickly silt up the infiltration surface of the groundwater aquifer. At that time the construction of Xihuangcun artificial recharge test site, although the geological and geographical conditions are very favorable, but only in the non-flood season with the reservoir abandoned water for artificial recharge.
Practice gives us a very important lesson. To achieve a large recharge effect, artificial measures alone are not enough. For the specific situation of Beijing, we proposed a "virtual recharge" approach. The existing groundwater pumping facilities in Beijing are already very large. It is unlikely that the capacity of a larger recharge facility will exceed the pumping capacity. In the case of maintaining the operation of existing pumping facilities, every 100 million cubic meters of water recharged is physically equivalent to not recharging, but reducing pumping by 100 million cubic meters. Therefore, a reduction in pumping is equivalent to an increase in recharge, a kind of "virtual" recharge. This type of recharge does not require specialized recharge facilities, but it does require an alternative source of water to replace the reduced pumping. This water can come from excess atmospheric precipitation in years of abundant water. If two sets of water supply facilities were constructed in Beijing, one using surface water and the other using groundwater, then each set would be able to meet the city's water supply needs on its own. If the groundwater pumping facilities are stopped in the year of abundant water, it is the same as not using any recharge facilities, a year on the recharge of 1 billion cubic meters of water stored in the ground. To the dry water year can be less surface water, and extract the underground stock to get through the water shortage. This approach can also be used for annual regulation of freshwater resources, before the flood season, as much as possible with the reservoir to prepare for the "empty reservoir to meet the flood" of the "abandoned" water, instead of extracting groundwater, less use of underground stocks.
The above program requires the unified scheduling of surface and underground two reservoirs, full of their respective "long", to avoid their "short". Combined with the characteristics of each region, may also design other programs.
In short, surface water and groundwater are not two different sources of water, if good at complementing each other's strengths, to play their respective advantages, we can better utilize the limited freshwater resources. The kind of surface water and groundwater artificially divided, each arguing one end of the argument, is narrow, one-sided view of the gateway.
Seven, open aquifer and closed aquifer
Groundwater is endowed with water in the underground rock. All rocks contain groundwater to a greater or lesser extent, but not all strata are aquifers. Only those strata that both contain a certain amount of water and allow groundwater to flow are called aquifers, otherwise they are aquifers. Of course, this division is only relative.
According to the relationship between groundwater and aquifers and aquifers, groundwater aquifers can be divided into "submersible aquifers" and "pressurized aquifers". These two introduced terms are really puzzling to translate. Not only do laymen not understand them, but also insiders are often confused. The author believes that the use of "open aquifer" and "confined aquifer" can better reflect the essential difference between the two types of aquifers.
The more familiar surface water can be used as an analogy. A river or lake is an open body of water and a water pipeline is a closed body of water. The volume of fresh water in a river increases as the river level rises, and the flow rate increases. The volume of fresh water in a water main has only an extremely small change with head, so small as to be almost negligible, and the flow rate is only related to the hydraulic gradient, which is almost independent of head.
If the groundwater in the aquifer does not fill the entire aquifer, the situation is similar to that of open bodies of water such as rivers, lakes, and reservoirs. As the volume of groundwater in the aquifer increases or decreases, the water table rises or falls. Such aquifers should be called "open aquifers". However, the prevailing terminology is "submerged aquifer". Because of their openness, open aquifers are easily recharged directly from atmospheric precipitation or surface water bodies, and the freshwater resources in them are therefore easier to regenerate in order to ensure the needs of sustainable development. At present, the vast majority of groundwater pumped worldwide comes from open aquifers.
If the aquifer is covered by a water barrier and filled with groundwater, it becomes similar to a closed water pipe. When the water table rises and falls, the volume of the aquifer is limited by the overlying aquifer and is not free to change as in the case of an open aquifer. Such aquifers should be called "confined aquifers", while the scientific term used in textbooks is "pressurized aquifers".
In reality, neither water pipes nor confined aquifers are absolutely rigid. They are both elastic and compressible. When the water level rises, the volume is expanded, and vice versa, it is compressed and made smaller. This expansion and contraction is negligible for water pipes and is usually ignored. For confined aquifers, however, this compressibility and elasticity cannot be disregarded for reasons that will be discussed below.
First, the ability of a confined aquifer to conduct water is many orders of magnitude smaller than a water main. Any end tap that releases water can be recharged almost immediately from the water storage containers at the waterworks. The head loss in between is relatively small. In contrast, the frictional resistance of the aquifer to water is high, and the distance from the lateral recharge source of the confined aquifer to the location of the well to draw water is generally very long. A large head loss is required to reach a steady state in the interim. In fact, before reaching a steady state, the water pumped from the well does not come from a distant lateral recharge source, but from the compression of the aquifer around the well. Pumping from the wells lowers the water table and creates a landfall funnel around the wells. For confined aquifers, this compresses the aquifer like a hydraulic jack draining oil or a car tire deflating. This squeezes some groundwater out of the aquifer. The groundwater pumped by wells actually comes from the part of the aquifer that is compressed. In the early days, wells pumped mainly groundwater compressed from aquifers close to the wells, and as the landfall funnel expanded, the water pumped gradually came more from more distant aquifers. It takes a long time from the start of pumping until most of the water in the well no longer comes from compression of the aquifer but from the recharge boundary, or even decades if the recharge boundary is far away from the pumping wells, by which time the water level in the pumping wells will have fallen very y, to the extent that pumping costs are unacceptably high. In addition, for every 1 meter drop in the groundwater level in a confined aquifer, the amount of water that can be given by compression is very little, only a few thousandths to ten thousandths of that in an open aquifer. As a result, the volume of the descending funnel of the water table is thousands of times larger than that of an open aquifer for the same amount of water produced.
According to the principle of conservation of mass, groundwater pumped from an aquifer cannot be produced out of thin air; it always has to come from a source. Open aquifers are better understood, as the water pumped from them comes partly from draining the aquifer and partly from recharging surface water bodies. Closed aquifers, on the other hand, are a bit of a puzzle. The latter is neither drained nor recharged from surface waters. So where does the water pumped from the well come from?One of the major advances in groundwater hydraulics at the beginning of the 20th century was the discovery that groundwater pumped from confined aquifers is compressed by the volume of the aquifer. This eventually manifested itself in ground subsidence. According to the results of previous long-term observations in Cangzhou, Hebei and Tianjin, the total volume of groundwater extracted from the confined aquifer over the years is roughly equal to the total volume of ground subsidence, and lateral recharge is negligible.
Pumping groundwater from confined aquifers causes ground subsidence! This is a serious problem. We've had tons of negative cases so far. Back in the 1960s, Shanghai suffered irreparable damage from ground subsidence. Since ground subsidence is difficult to detect intuitively in its early stages, the lessons of Shanghai were not learned in time by other places, and similar problems ensued in Tianjin. Suzhou, Wuxi and Changzhou in the Yangtze River Delta, where the aquifer is not as broad as Shanghai's, were constrained by small basins of localized fractured subsidence and suffered from uneven subsidence, resulting in ground cracks. Ground cracks in Xi'an are also a consequence of long-term pumping from confined aquifers.
It can be seen that the long-term large-scale pumping of groundwater from the closed aquifer, the harm outweighs the good, and the loss often outweighs the gain. Not much water can be pumped to cause a significant drop in the water table, and often lead to serious consequences of ground subsidence.