Socio-hydrologic perspectives of the co-evolution of humans and groundwater in Cangzhou , North China Plain

This paper presents a historical analysis from socio-hydrologic perspectives of the coupled human–groundwater system of the Cangzhou region in the North China Plain. The history of the “pendulum swing” for water allocation between the economic development and aquifer environmental health of the system is divided into five eras (i.e., natural, exploitation, degradation and restoration, drought-triggered deterioration, and returning to the balance). The system evolution was 15 interpreted using the Taiji-Tire model. Over-exploitation was considered as the main cause of aquifer depletion and the groundwater utilization pattern was affected by the varying groundwater table. The aquifer depletion enhanced the community sensitivity of humans toward environmental issues, and upgraded the social productive force for restoration. The evolution of the system was substantially impacted by two droughts. The drought in 1965 induced the system from natural condition to groundwater exploiting. The drought from 1997 to 2002 resulted a pulse in further groundwater abstraction and 20 dramatic aquifer deterioration, and the community sensitivity increased rapidly and induced the social productive force to a tipping point. From then on, the system is returning the balance through new policies and water-saving technologies. Along with the establishment of a strict water resource management strategy and the launch of the South-to-North Water Diversion Project, further restorations of groundwater environment would be implemented. However, a comprehensive and coordinated drought management plan should be devised to avoid the irreversible change of the system. 25


Introduction
Through adaption processes, humans co-evolve with the hydrological system, resulting in a "pendulum swing" in the balance point of the human-water system (Kandasamy et al., 2014;Sivapalan, 2015).Kandasamy et al. (2014) first characterized the concept of the "pendulum swing" by tracing 100 years history of the competition for water between agricultural development and environmental health in the Murrumbidgee River Basin.Similar dynamics were also found in the human-water system in the arid Tarim River Basin (Liu et al., 2014) and in human-flood interactions (Baldassarre et al., 2015).A pendulum swing can be divided into four typical stages: (1) the initial exploitation stage, which is focused exclusively on economic development; (2) the onset environmental degradation stage, which is accompanied by the introduction of remedial infrastructure; (3) the widespread environmental degradation stage, which leads to the necessity of mitigation measures; and (4) the recovery stage, at which ultimate solutions are implemented.The pendulum swing of a human-water system can be clarified by considering all interactions within a universal socio-hydrologic framework (Kandasamy et al., 2014;Liu et al., 2014).The importance of socio-hydrology has been recognized by the International Association of Hydrological Sciences through their new "Scientific Decade" (2013)(2014)(2015)(2016)(2017)(2018)(2019)(2020)(2021)(2022) entitled "Panta Rhei (Everything flows)" (Montanari et al., 2013), which aims "to reach an improved interpretation of the processes governing the water cycle by focusing on their changing dynamics in connection with rapidly changing human systems." A socio-hydrological system contains human, hydrological, and environmental sub-systems.The Taiji-Tire model proposed by Liu et al. (2014) was first used as a framework for elucidating the complexities of socio-hydrological systems that co-evolve with direct or indirect interactions between factors from both human and water perspectives.In the model, a Taiji wheel, which is a term originating from a special concept in Chinese philosophy, is used to describe the direct humanwater relationship in a specific socio-hydrological system, whereas a human-water tire is used to represent the indirect effect of external natural and social factors affecting the system.The evolution of a socio-hydrological system is driven by the interactions between two main factors, namely, natural variability and social productive force (Liu et al., 2014).Social productive force refers to the combination of all factors that enable humans to utilize resources and create better material and spiritual products.To apply the Taiji-Tire model in interpreting the drivers of water reallocation from the economy to the environment during the recovery stage of the pendulum swing, the concept of "environmental restorative force" was defined as the opposing factor of social productive force (van Emmerik et al., 2014).During the recovery stage, environmental protection actions are usually conducted because of a high social awareness for environmental risk or welfare (Di Baldassarre et al., 2013).Elshafei et al. (2014) proposed a new concept of community sensitivity to signify the social awareness for environmental welfare.Community sensitivity refers to the sensitivity of humans to the changing environment.
A high community sensitivity implies that humans feel the pressure of environmental deterioration, motivating them to restrain human activities to restore environmental health.The concept of community sensitivity was also used to analyze the switching of the support between flood protection and wetland preservation in the Kissimmee River Basin, Florida (Chen et al., 2016).Above new concepts can be incorporated into the Taiji-Tire model to improve its explanatory power on a specific human-water system.
Being a ubiquitous source of high-quality fresh water, the groundwater system is closely connected with human.Driven by the increasing demand for water resources, improper exploitation of groundwater resources has resulted in serious environmental crises, particularly in regions with primarily groundwater-fed irrigation (Taylor et al., 2013) including North China Plain (NCP) (Liu et al., 2001;Chen et al., 2003;Zheng et al., 2010).In addition to the social productive force, natural variability is another external force.Climate variations influence groundwater systems both directly through replenishments by recharge and indirectly through variations in groundwater demand (Taylor et al., 2013).Unlike the inherent social forcing (i.e., increase in water demand for agricultural development), natural variability induces changes in human activities (such as drought-triggered pulse in water demand) that are extrinsic (Gurdak, 2012) and often perceived as shocks (e.g., droughts), which may push the system beyond its resilience thresholds (Sivapalan et al., 2012).In comparison, water resource management decisions could produce either positive or negative effects on both the environment and the society (Kandasamy et al., 2014).
North China Plain is an important agricultural region of China.The main crops in this region are wheat and maize.
Irrigation is necessary to maintain high levels of grain production (Liu et al., 2001), and 70% of the cultivated land is irrigated, consuming 70% of the total water supply.Given that over-exploitation causes the lower reaches of streams to dry up, groundwater has become the regular supply for irrigation since the 1970s.The density of pumped wells have led to severe groundwater depletion of both unconfined and confined aquifers (Zhang et al., 1997;Zheng et al., 2010).Quite a few studies have been carried out to better understand the spatiotemporal variations in groundwater depletion across the NCP and develop sustainable groundwater management options (Cao et al., 2013;Liu et al., 2008); in these works, the effects of human-induced change were evaluated by a scenario-based method (Wang et al., 2008;Liu et al., 2011;Liu et al., 2008).
However, two-way feedback should be considered for a better understanding of coupled human-groundwater systems (Montanari et al., 2013).
A representative example of the co-evolution of the human-groundwater system is in Cangzhou in the northeastern coastal plain of the NCP, which has the most serious depression cone since the 1970s (Liu et al., 2001).In recent years, aquifers have been recovered through the implementation of several measures (Han et al., 2013).The history of the coevolution of the human-groundwater system in Cangzhou seems like a "pendulum swing," particularly as to how the groundwater crisis unfolded and how it was addressed, which is selected as the study area in this paper.
The objectives of this paper are as follows: (1) to chart the history of the groundwater utilization in Cangzhou, focusing on the dynamics of the human-groundwater interactions that lead to the "pendulum swing" in the balance point in groundwater allocations between humans and aquifer ecosystems, as well as the natural variability and social factors that contributed to it; (2) to use the Taiji-Tire model to interpret the interactions and co-evolutions of the human-groundwater system in Cangzhou.The Taiji-Tire model will be specific to the groundwater system and will be incorporated with the concepts of restorative force and community sensitivity.

Study area, data, and methods
Groundwater pumping from aquifers has substantially increased since the mid-1960s.Thus, the NCP aquifer system has become one of the most over-exploited aquifers in the world (Kendy et al., 2007;Liu et al., 2008).Cangzhou region (total area: 14,056 km 2 ) is located in the east of the NCP and in the downstream of the Haihe River Basin (Fig. 1).Cangzhou is a prefecture-level city of Hebei Province consisting of 4 county-level cities and 10 counties.Cangzhou Municipal Government abides by the provincial and national policies of water resource management, and it devises policies for the entire region.
The Water Resources Bureau of Cangzhou, which is a department of Cangzhou Municipal Government, is responsible for water resource affairs and guides the Water Resources Bureau of the 10 counties and 4 county-level cities.In turn, this agency is guided by the Water Resources Department of Hebei, the Ministry of Water Resources, and the Haihe River Water Resources Commission.Cangzhou is characterized by strong inter-annual variable precipitation and occasional extreme droughts.In 2013, the total population of Cangzhou was 7.34×10 6 .The cultivated land area is 8,066 km 2 , of which 5,424 km 2 is irrigated.Given that surface water is intercepted by reservoirs in the upstream, natural streams have nearly dried up, leaving groundwater as the main water source for irrigation.

Figure 1. Location of Cangzhou in the North China Plain
The groundwater resides in aquifers of porous quaternary alluvial deposits, which can be divided into four major aquifer layers (I-IV).The thickness of each layer ranges between 20 m and 350 m.Aquifer Layer I is unconfined, and infiltration from precipitation is the main recharge source.The other three layers are confined.Saltwater accounts for 98% of Aquifer Layer II, with low exploitation capacity.Pumping wells mainly extract fresh groundwater from Aquifer Layers I and III.In this study, Aquifer Layers I and III are referred to as "shallow aquifer" and "deep aquifer," respectively.Groundwater levels have steadily declined because of over-pumping, resulting in saltwater intrusion and land surface subsiding (Kendy et al., 2003).In 2013, the groundwater withdrawal from the shallow and deep aquifers were 259×10 6 and 743×10 6 m 3 .In recent years, several measures have been implemented for sustainable groundwater management to mitigate aquifer depletion (Han et al., 2013).
The analyses were performed based on the data of annual precipitation, agricultural infrastructures and production, groundwater withdrawal from both the shallow and deep aquifers, and groundwater table depth.The data before 1985 was acquired from the Water Resource Annals of Cangzhou (Xue, 1994), and the data after 1985 was obtained from the Hebei Rural Statistics Yearbook, and the Hydrology and Water Resources Investigation Bureau of Cangzhou.The data on policies and initiatives were collected from the Water Resource Annals of Cangzhou (before 1985), the announcements and documents of the Ministry of Water Resources, the Government of Hebei, the Water Resources Department of Hebei, the Government of Cangzhou, and the Water Resources Bureau of Cangzhou.
The time series of the groundwater withdrawal from both the shallow and deep aquifers were analyzed to detect the behaviour of water users.The average water table depth of the shallow aquifer and the water table depth of the depletion cone of the deep aquifer were used as the main indices of the groundwater system.The time series of number of wells and irrigated area (including water-saving irrigated area) were analyzed to detect the changes in infrastructure, along with the social development which were revealed by the changes in population, and grain production.The break points of the time series of the water table depth and groundwater withdrawal were determined based on trend changes.The break points were also confirmed by examining the major policies and initiatives that facilitated developments or that resulted in turning points in groundwater water management.Then, the co-evolution of the human-groundwater system was classified into several eras, which signified the points at which a "pendulum swing" occurred.
Under the framework of the Taiji-Tire model, the Taiji represents the direct interactions between the groundwater utilization and the aquifers in Cangzhou, whereas the outer Tire represents all of the social and natural factors that indirectly influence the human-groundwater system.Precipitation and surface water change/variability were considered the natural variability in Cangzhou.The emphasis level of the social productive force is detected from the changes in the number of wells, in irrigated areas with groundwater, and in policies for groundwater exploitation.In this study, we believe that the concept of environment restorative force is misleading and should instead be regarded as a subtype of social productive force.
Social restorative force can be detected from the changes in water-saving irrigation areas and in policies for creating watersaving technologies.

Pendulum swing in groundwater utilization
The balance between groundwater extraction and efforts to mitigate and reverse the consequent degradation of the aquifer has evolved in Cangzhou since 1949 (Li et al., 2013).The evolutionary history of the human-groundwater system from 1949 to 2015 is presented in Fig. 2, and the major policies and initiatives are summarized in Table 1.The history is divided into five distinct eras: Era 1 (-1964): Natural variability dominates the human-groundwater system Era 2 (1965)(1966)(1967)(1968)(1969)(1970)(1971)(1972)(1973)(1974)(1975)(1976)(1977)(1978)(1979)(1980)(1981)(1982): Expansion of groundwater exploitation and onset of aquifer depletion Era 3 (1983)(1984)(1985)(1986)(1987)(1988)(1989)(1990)(1991)(1992)(1993)(1994)(1995)(1996) Historical records indicate that groundwater exploitation for irrigation in Cangzhou can be traced back to 1266, which was over 700 years ago.However, in 1949, the irrigated area with groundwater was only 74 km 2 , which was distributed in fewer places where shallow freshwater resources were abundant.From the early 1950s to the mid-1960s, the Haihe River basin was rich in surface water resources.The emergence of serious salinization problems necessitated the establishment of a drainage-oriented policy for low lands in Cangzhou.Many reservoirs and diversion projects were constructed, reducing the need for groundwater resources.Given that most wells were made of bricks and earth, groundwater utilization was restricted by the lack of infrastructure.By 1964, only a small portion of the wells (1,524) were pumped by motors, and the irrigated area with groundwater was 321 km 2 , which only accounted for 2.3% of the total area of Cangzhou.Therefore, this era was characterized by small-scale groundwater utilization because of less groundwater demand as well as technological incapability.The interaction between humans and the groundwater was weak.Humans were insensitive to groundwater change, and the groundwater sub-system was unaffected by humans at a large scale.

Era 2 (1965-1982): Expansion of groundwater exploitation and onset of aquifer depletion
In 1965, North China suffered from a catastrophic drought that threatened food production.In response, the Cangzhou government promoted the construction of motor-pumped wells for groundwater exploitation.In 1966, the State Council organized a conference to combat drought in North China, and they specified well drilling as an important measure.
Numerous well-drilling teams were organized after hydrogeological investigations were conducted on the agricultural water supply in the NCP.In 1970, the number of motor-pumped wells increased to 14,328 from 1,548 in 1965, and the irrigated area with groundwater increased to 920 km 2 .The rapid increase in groundwater exploitation drastically deepened the groundwater level of both the shallow and deep aquifers.The average annual shallow groundwater withdrawal from 1976 to 1982 was 372.4×10 6 m 3 (Table 2).In 1982, the regional average shallow groundwater table declined to 4.32 m beneath the ground, with an annual decline of 0.39 m from 1976 to 1982.As a result, many shallow wells were abandoned because of the significant decline in the groundwater table .Motor-pumped wells had to be drilled with increasing depth, signaling the start of a vicious cycle.Groundwater exploitation had to be drilled deeper than usual, and the deep groundwater withdrawal significantly increased, from 193×10 6 m 3 in 1976 to 357.7×10 6 m 3 in 1983, which exceeded the sustainable volume (292×10 6 m 3 according to Wang and Wang (2007)) since 1979.With increasing exploitation, the water table of the deep aquifer rapidly declined.The depression cone of the deep aquifer in Cangzhou first appeared in 1967, whereas the depth of the water table at the center of the depression cone was 22.5 m beneath the ground in June 1971.By 1983, the water level at the center of the depression cone was as deep as 72.9 m, with an annual decline of 3.82 m from 1973 to 1983.The drilling depth increased because of the drop in deep groundwater table, and the cost for both well drilling and installation of new motor-pumped wells rose.In addition, several environmental problems were triggered.Land subsidence occurred in Cangzhou City in 1970 because of the presence of the depression cone.The cumulative volume of subsidence in 1970 was 9 mm, and it increased to 744 mm in 1986.Because of the formation of the depression cone in Aquifer Layer III, the natural recharge-discharge balance was destroyed between the fresh water originally in Aquifer Layer III and the saltwater in the overlying aquifer layer II.The leakage recharge of saltwater from Aquifer Layer II to Layer III increased, leading to saltwater intrusion.

Era 3 (1983-1996): Awareness of environmental degradation and attempts for restoration
The intensification of the groundwater crisis raised the public clamor to address the water crisis.The Environmental Protection Law of the People's Republic of China (Trail), which was enacted in 1979, states that groundwater should be rationally exploited to prevent water resource exhaustion and land subsidence.In 1983, the Cangzhou Hydraulic Engineering Society submitted an appeal document entitled "Appeal for the Rational Exploitation of Water Resources" to government leaders.In this appeal, proposals for comprehensive water resource management were proposed.In 1984, the State Council appointed the Department of Water and Power as the general management department for water resources in China to unify water resource management efforts.In 1985, the Hebei Provincial Government enacted the Regulation of Water Resources, which stated that the exploitable volume of groundwater in urban areas should be strictly controlled, and the exploitation of groundwater in rural areas should be reasonably planned.In June 1985, the Water Conservancy Bureau in Cangzhou issued suggestions for strengthening the management of groundwater resources in which comprehensive water resource management measures were detailed.Specifically, shallow groundwater exploitation should be prioritized, deep groundwater exploitation should be restricted, and brackish water should be reasonably utilized.Furthermore, a licensing system for well drilling was established, and the planting of crops, such as rice, which consumes large amounts of water, was forbidden.
In 1988, the Water Law of China was released, which states that explorations should be strictly controlled in regions where groundwater resources have already been over-exploited.Moreover, measures should be taken to protect groundwater resources and prevent land subsidence.In 1993, the State Council issued the Implementation Measures of the Licensing System for Water Taking.As stipulated, groundwater exploitation should not exceed the annual exploitable volume of an administrative region.In regions where groundwater resources are over-exploited, including Cangzhou, groundwater exploitation should be strictly controlled, and the expansion of the exploitation is prohibited.Since the 1980s, well drilling was no longer subsidized by the central government, resulting in a sharp decrease in the number of wells.To fill the gaps, surface water was diverted from the Yellow River in 1993.
The aforementioned measures relied on comprehensive water resource management, by which deep, medium, and shallow groundwater exploitations were governed by unified planning, with shallow aquifer exploitations given priority.
Consequently, the increasing deterioration of groundwater resources in Cangzhou was halted.From 1984 to 1996, the shallow groundwater withdrawal slightly decreased (by approximately 2×10 6 m 3 per year), whereas the average shallow groundwater table rose at the rate of 0.02 m per year (Table 2).The increase in the deep groundwater withdrawal slowed down (by approximately 18.8×10 6 m 3 per year).Consequently, the water level at the center of the depression cone declined slowly at an annual rate of 1.63 m, and the subsidence at the center of the depression cone was also reduced.Unfortunately, the positive trend ceased after the outbreak of a serious drought in 1997.The precipitation in Cangzhou was only 296.3 mm (53.9% of the mean annual value) in 1997, and the average annual precipitation was only 391.7 mm from 1997 to 2002 (Table 3).However, because surface water has already been exhausted since the 1980s, well drilling seemed to be the immediate strategy for addressing the drought.During this period, the annual average shallow groundwater withdrawal was 485.9×10 6 m 3 , which represented an increase by 16.5% compared with 417.2×10 6 m 3 during 1984-1996, and the annual average deep groundwater withdrawal rapidly increased from 455.2×10 6 m 3 during 1984-1996 to 822.7×10 6 m 3 , which represented an increase by 80.7%.Accordingly, the shallow groundwater level rapidly declined again, from 4.03 m depth beneath the ground in 1996 to 8.69 m in 2002, with an annual decline of 0.75 m.At the same time, the water table at the center of the depression cone rapidly declined again from 90.4 m depth beneath the ground in 1996 to 111.1 m in 2001 (the deepest value), with an annual decline of 2.47 m.The area with the water table of the Aquifer Layer III deeper than 80 m beneath the ground dramatically increased from 157 km 2 in 1996 to 421 km 2 in 2002.In view of the sharp decline in the groundwater table, the environment deteriorated.The cumulative subsidence to 2001 was 2,236 mm, with a rate of 100.5 mm/a.The areas with subsidence larger than 500 and 800 mm are 9,717 and 3042 km 2 , respectively, which account for 92.9% and 29.1% of the total area of Cangzhou.Moreover, the interface of saltwater and fresh water declined at approximately 10 m, with a maximum depth of 30 m, threatening the fresh water in the deep aquifer (Han and Han, 2006).
During the drought in 1999, the Hebei Provincial Government issued the Management Method of License System for Water Taking.As stipulated, in over-exploited regions, groundwater exploitation should be strictly controlled, and the expansion of the exploitation scale is prohibited.In 2002, the Hebei Provincial Government specified the over-exploited regions and the severely over-exploited regions in the plain area.The entirety of Cangzhou was included in the list of regions with severely over-exploited deep aquifer.With respect to the shallow aquifer, the over-exploited and severely overexploited regions were 406 and 525 km 2 in area, jointly accounting for 6.6% of the total area of Cangzhou.To adopt the restrictions on groundwater exploitation, the Hebei Provincial Government formulated a program for developing watersaving technologies in 1998.Investments in water-saving projects were enhanced, and subsidies were provided.Accordingly, the irrigated area with water-saving technologies (mainly low-pressure pipeline irrigations and sprinkler irrigations) in Cangzhou rapidly increased from 96.4 km 2 in 1995 to 212.5 km 2 in 2002.

Era 5 (2003-present): Returning the balance
During the drought, the groundwater crisis in the NCP once again gained widespread concern, and several measures were implemented at the national, provincial, and regional levels.In 2003, the Ministry of Water Resources issued the Suggestions Concerning the Strengthening of the Management of Groundwater Over-Exploitation Regions, in which several targets and measures for groundwater management were proposed.In 2005, the Hebei Provincial Government published a notice stating that urban wells out of the management of the department for water resources should be shut down, and that In 2012, China launched the strictest water resource management strategy in which many strict groundwater management and protection measures were proposed.Subsequently, the Hebei Provincial Government and the Cangzhou Municipal Government published the implementation schemes.According to the implementation scheme, groundwater exploitation in Cangzhou will be strictly controlled.Except for the purpose of obtaining water for domestic use, the construction of new motor-pumped wells will not be approved.Groundwater exploitation is prohibited in urban regions already covered by a public water supply network.Since the South-to-North Water Diversion Project was established in 2015, which annually diverted 483×10 6 m 3 of water to Cangzhou, all wells in urban regions not controlled by the water resources administrators will be shut down and groundwater over-exploitation in rural regions will be gradually cut down.
According to the High-Efficiency Water-Saving and Irrigation Program in Cangzhou launched in 2013, overall water savings in agricultural production will be achieved by 2020.

Discussion: Interactions of the human-groundwater system
According to the preceding analysis on the co-evolution of the human-groundwater system in Cangzhou, the specific application of the Taiji-Tire model for the human-groundwater system is provided in Fig. 4(a).The changes in natural variability (precipitation and surface inflow), social productive force, and restorative force in Cangzhou through five eras are shown in Fig. 4(b).
Human groundwater utilization is the main cause of the varying groundwater table.With increasing groundwater exploitation, both the shallow and deep groundwater table declined, and groundwater depression cone extended in Cangzhou.
The contributions of increasing shallow groundwater utilization on the decline in the groundwater table is evident in the positive correlation between the depth fluctuations of the annual shallow water table (a negative change indicates a rise of the groundwater table) and the water withdrawal (Fig. 5(a)).The increasing deep groundwater utilization contributed to the decline in the water table depth of the depression core before 2002.The subsequent rapid reduction in rapid groundwater utilization caused the water table at the center of the depression cone of deep groundwater to quickly rise again (Fig. 5(b)).The groundwater sub-system also affects the behavior of water users.The significant decline in the shallow water table prevents the use of many shallow wells as before.By contrast, the water in the deep aquifer is guaranteed in terms of both the quantity and quality albeit with higher cost.Consequently, groundwater utilization was directed toward the deep aquifer.
In 1976, the average shallow water table depth was 2.59 m, and the water withdrawal from the deep aquifer was only 47.4% of that from the shallow aquifer.The ratio of deep water to shallow water withdrawal increased as the shallow water table declined from 1976 to 2002, especially during the drought in 1997-2002.In 2002, which had the deepest shallow water table, the water withdrawal from the deep aquifer was 2.82 times of that from the shallow aquifer.This feedback of the groundwater sub-system to the human groundwater utilization can be detected from the high correlations between the ratio of deep to shallow water withdrawal and the shallow water table depth before 2002 (Fig. 5(c)).
The co-evolution of the human-groundwater system in Cangzhou is driven by natural variability, particularly the change/variability of surface water and precipitation.With decreasing surface water resources, the recharge to the groundwater system decreased, and the demand for groundwater increased.Besides, the precipitation variability affected both the groundwater and human sub-systems.A larger precipitation implies a larger recharge to the shallow groundwater.
As shown in Fig. 6(a), the changes in shallow water table depth are highly negatively correlated with the annual precipitation.The drought years with low precipitation are characterized with not only small recharge to the shallow aquifer but also a large demand for groundwater.Thus, groundwater withdrawal is negatively correlated with annual precipitation (Fig. 6(b)).The co-evolution of the human-groundwater system in Cangzhou is also driven by the social productive force.Before 1965, groundwater utilization was not large scale because of technological limitations.The groundwater withdrawal was small compared with that of surface water, and the irrigated area with groundwater was constrained to a small fraction of Cangzhou.According to the Taiji-Tire model, the human-groundwater relationship was weak, lacking any kind of sophisticated interactions.Therefore, the human-groundwater system can be considered stationary (or a steady state) without significant external drivers.To overcome the drought in 1965, the Cangzhou Government proposed an expansive policy of groundwater utilization for the first time, and well drilling was rapidly accelerated.The relationship between the human and groundwater sub-systems was enhanced since then, and the stationary condition was broken.The groundwater exploitation was driven by the need to resist the drought up to the early 1970s and by the need to enlarge the irrigated area for food production.Since then, the number of motor-pumped wells increased rapidly.The growth of the social productive force has sustained a significant increase in groundwater consumption.The feedback of the outer Tire will educate society of the negative effect of over-exploitation and therefore promote the development of new responsive social behaviors to protect the environment and reduce the impulse of blind development.
Although the negative externality still gradually persists, it does so at low levels during a long period, preventing it from reversing this developing trajectory.Although the interactions of the inner Taiji leads to serious problems in ecosystems, the community sensitivity rises, and these environmental protection actions can be considered a new kind of social productive forces, which can be called social restorative forces.The social restorative force is not the same as the natural variations and is not an opposite force to social productive forces, which can also be restorative.The only difference is the social norms and values.In other words, humans themselves must decide how to use technology and devise policies.The social productive force only emphasizes the production but not the cost, including the direct production cost and the environmental externalities, whereas the restorative forces refer to the specific productive forces that further increase the production by lowering the environmental externalities.Therefore, the social restorative force can be regarded as a sub-branch of social productive force (with social destructive force being the other sub-branch).The social restorative force appears around the moment when the steady state of the natural system is broken.However, the social restorative force in Cangzhou was not strong enough during the drought of 1997-2002, while the productive force was dominated by that utilizes groundwater.
As a result, the groundwater table declined rapidly.When the social restorative force dominates the productive force, social productivity can be seen as green productivity (Tuttle and Heap, 2007;Mohanty and Deshmukh, 1998); that is, the societies can be seen as green (the other type is technological) (Baldassarre et al., 2015).
Until 2002, groundwater withdrawal increased with the population and GDP of Cangzhou, but it decreased after that (Fig. 7  Given that economic development has always been the primary target of the government and society of Cangzhou, the mitigation measures for reallocating water from the economic development to the environment, which have been implemented in Murrumbidgee River basin (Kandasamy et al., 2014), are unacceptable.Therefore, new policies are established to restrict groundwater exploitation.The development and application of water-saving technologies are strongly encouraged to protect the environment while ensuring economic development, even if such strategies are more costly than traditional groundwater exploitation.As shown in Fig. 2(f), the area of water-saving irrigation has increased rapidly.The social productive forces have therefore reached a critical point.These new improvements can be seen as an effort of humans to raise the social restorative force.Thus, some scholars believe that the human-water system is determined by the interactions between social productive and restorative forces (van Emmerik et al., 2014).Nonetheless, as we have mentioned, a more precise description is that the social restorative force is a new kind of social productive force.
The pattern of the human-groundwater system has significantly changed since the appearance of the social restorative force.For the inner Taiji, significant changes in water user behavior and human response to groundwater system can be observed if the dataset was divided into two periods (before and after 2002) based on the narrative of the different eras.In Cangzhou, the varying groundwater tables also influenced the community sensitivity of humans toward environment

Conclusions
The historical socio-hydrological analysis in Cangzhou enabled the recognition of the "pendulum swing" of the coevolution of the human-groundwater system.At the first era, the intensity of groundwater exploitation was low.The humangroundwater system was primarily dominated by natural factors.No records on groundwater crisis induced by human activities was found.At the second era, groundwater was first exploited to combat the drought.Thereafter, groundwater exploitation was driven by the need for grain yield.Moreover, with the decrease in surface water resources, the intensity of groundwater exploitation was elevated, and the human-groundwater system was driven by the social productive force.
Meanwhile, intensive human activities led to the deterioration of the aquifers environment, which drew considerable attention from society.At the third era, comprehensive management measures were implemented to address the groundwater crisis, and the deterioration was mitigated to a certain extent.At the fourth era, drought appeared as a shock, which terminated the mitigation because humans exert effort to enhance the productive force to meet the challenge by pumping more water.
Considering that comprehensive and coordinated drought management plans were lacking, people responded to ad hoc drought by drilling emergency wells or relying on unregulated groundwater withdrawal from existing wells.Consequently, the intensity of groundwater exploitation rapidly increased, leading to the dramatic deterioration of the aquifers environment.
Nevertheless, community sensitivity concerning water crisis was enhanced in the meantime.The increasing community sensitivity has also triggered the split of social productive forces and generated new restorative forces.At the fifth era, the drought was eased, and several measures were implemented to reduce groundwater exploitation.Water-saving technologies became acceptable economically and ideologically, and the aquifers environment began to be restored.The strictest water resource management scheme was launched in 2012, and the South-to-North Water Diversion Project was commenced in 2015.Further restoration of the aquifers environment is anticipated.However, the external environment variations, particularly drought, remain an unexplored variable.
The Taiji-Tire model was used to interpret the interaction and co-evolutionary dynamics of the coupled humangroundwater system in Cangzhou.The interaction between the human utilization and the groundwater are regarded as the inner Taiji.Over-exploitation is the major reason for the groundwater depletion.The decreasing shallow groundwater table also affected the groundwater utilization pattern, making the groundwater withdrawal turn to the deep aquifer.Precipitation variation directly affected the groundwater recharge as well as the demand for groundwater, which subsequently affected the human-groundwater system.Triggered by the drought, the social restorative force, as an upgrading of the social productive force, enhanced the ability to rebalance the human-groundwater system in Cangzhou with new water-saving technologies and corresponding management efforts.This feedback rebalanced the interactions of the inner Taiji that has intensified the human-water relations with the occurrence of the expected pendulum swing that has decreased the groundwater withdrawal since 2002.
: Awareness of environmental degradation and attempts for restoration Era 4 (1997-2002): Drought-triggered pulse in groundwater abstraction and aquifer depletion Era 5 (2003-present): Returning to the balance 3.1 Era 1 (Pre-1964): Natural variability dominates the human-groundwater system The exploitation of groundwater resources in Cangzhou has a long history.Archaeological discoveries show that Cangzhou residents drilled wells to obtain drinking water as early as the Han Dynasty (approximately 220 BC-220 AD).

Figure 2 .
Figure 2. Time series of (a) precipitation, (b) surface inflow (before 1980 the data is the average value), (c) population, (d) grain production, (e) well numbers (red line is referred the number of deep well), (f) irrigated area, (g) shallow groundwater withdrawal, (h) deep groundwater withdrawal, (i) shallow water table depth, and (j) water table depth of the depletion cone in Cangzhou during the study period (1949-2013)

Figure 3 .
Figure 3. Changes in the water table depth of the Aquifer Layer III along the cross-section from the west to the east.

Figure 4 .
Figure 4. (a) Taiji-Tire model representation of the interactions of the human-groundwater system; (b) Changes in natural variability (precipitation and surface inflow), social productive force, and restorative force in Cangzhou through five eras (Emphasis level used in relation to the vertical axis refers to the degree of increase in the variables described in the figure).

Figure 5 .
Figure 5. (a) Relationship between shallow water changes and withdrawal; (b) co-evolution of the depth of the depression cone center of deep aquifer with water withdraw from 1976 to 2013; (c) the ratio of deep to shallow water withdrawal against the shallow water table depth before and after 2002.

Figure 6 (
Figure 6 (a) Change in shallow water table depth; (b) total water withdrawal against annual precipitation.The interactions of the inner Taiji usually lead to over-exploitation of natural resources and additional human input on development.Without having full awareness of natural deterioration, human development patterns will remain uninformed.
(a)).The reason is that since 2002, after decades of accumulation, the community sensitivity toward aquifer environment has caused new actions toward environmental protection.In 2004, a leading group headed by the executive vice mayor was established to prevent and manage the subsidence in Cangzhou.Measures for sustainable groundwater management was even emphasized in the Cangzhou Government Work Report in 2004.More specifically, the broken stationary condition of natural systems suggests higher levels of uncertainty and risks of extreme hydrometeorological events, causing higher social awareness of risks and resultant actions.The drought during 1997-2002 reminded the public and policymakers about taking measures to modify the existing development pattern.

Figure 7 .
Figure 7. (a) Relation between annual groundwater withdrawal and the population and GDP; (b) Co-evolution of the total groundwater withdrawal against the annual grain production before and after 2002.
issues and motivated them to self-regulate their behaviors by establishing new policies (such as restricted groundwater exploitation policies) and developing new water-saving technologies.As community sensitivity continues to increase, new technologies and management efforts would enhance the restorative forces.Supported by theses productive forces, the grain production declined slightly during the drought during 1997-2002, but it grew rapidly again with the subsequent decline in groundwater utilization after 2002 (Fig.2(d)).Thus, the grain production was negatively correlated with the groundwater withdrawal during 1997-2013 (correlation coefficient of −0.87), which is different from that during 1976-1996 (correlation coefficient of 0.77) (Fig.7(b)).

Table 1 . Summary timeline of major policies and initiatives in Cangzhou
The benefits of the emergency wells drilled to combat the drought encouraged the continued construction of groundwater exploitation infrastructures after the drought.The primary goal of groundwater utilization was transformed to increase grain yield.In August 1970, the state government held a conference on agricultural production in Northern China and decided to accelerate the agricultural development in regions with food shortage, including Cangzhou.Specifically, the construction of motor-pumped wells was included in the national plan.From 1970 to the early 1980s, motor-pumped well constructions were generally supported by national special funds.In 1973, the head quarters for the construction of motorpumped wells were established in Cangzhou.Several specialized construction management departments were also set up in subsidiary cities and counties.After 1975, when the irrigated area with surface water reached a maximum of 4,084 km 2 (the area irrigated with groundwater was 2,401 km 2 in the same year), surface water resources were gradually exhausted (which can be detected from the changes in runoff of two rivers shown in Fig.2(b)).Groundwater then became an important water resource for agricultural irrigation.In 1982, the number of motor-pumped wells reached 51,611, and the irrigated area with groundwater reached 2,086 km 2 (Figure2(e)), which is 41.3% of the total irrigated area.Benefit from the groundwater utilization, the grain yield in this region increased from 0.79×10 6 tons in 1965 to 1.54×10 6 tons in 1982.

Table 2 . Changes in shallow and deep groundwater withdrawal and table depth from Era 2 to Era 5
The ratio of deep water to shallow water withdrawal was negatively correlated with the shallow water table depth after 2002, which is absolutely different from that before (Fig.5(c)).However, the correlation of the negative relationship is considerably weaker at a starting point before 2002 (for example, the coefficient of determination (R 2 ) is only 0.11 according to the data from 2002 to 2013).As shown in Figs.2(g) and 2(h), the deep groundwater withdrawal began to decrease slowly since 2002, whereas the shallow groundwater withdrawal continued to decrease rapidly as before.This finding indicates that people did not turn to shallow water as the shallow water table increased, because the infrastructure of deep water with highquantity exploitation already existed.