Adaptive changes in traditional settlements in the Loess Plateau of the Yellow River Basin over 500 years

Basin settlements are social‐ecological systems that embody human–land relationships in river basins and face the challenge of adaptive development. The traditional human–land relationship in the Loess Plateau of the Yellow River Basin has changed under the combined effects of human activities and natural disturbances. Traditional settlements face the challenge of balancing conservation, development, ecology, and livelihood. To clarify the long‐time evolution process of settlements in this region, a framework of “population–production–ecology” coupled system in the Loess Plateau of the Yellow River Basin was proposed from the perspective of social‐ecological systems. Based on long‐term time‐series data, the evolution of the regional human–land relationship and the coupling coordination degree of the system were identified. Further, based on the differences in the levels of systems, the adaptation of basin settlements to the external environment was analyzed using water resources as an example. The results showed that the coupling coordination degree within the basin had been decreasing, suggesting the disintegration of the dynamic mechanism of regional development. Moreover, the changes in the development pattern of basin settlements were more delayed compared with those in the basin environment. Overall, this study provides suggestions for the development of sustainable integrated river basin management strategies.


| INTRODUCTION
Changes in the natural environment influence the behavior of settlements (Płaza et al., 2015). As a divisional unit of natural geography, a river basin is a relatively complete and independent area, which is the cradle and core of human civilization (Best, 2019). Early humans used the available water resources for drinking water, food (Tao et al., 2022), agricultural irrigation, fishing (Lu et al., 2019), and building materials. During the long evolutionary process, settlements in the basin gradually formed a mutual adaptation of population, production, and ecology, and started exhibiting a unique locality (Wang & Prominski, 2020). Furthermore, cultural exchanges and economic relations of settlements along the river (Lu et al., 2022) formed regional socioeconomic patterns. Thus, traditional settlements in river basins represent stable habitat units that maintain a positive cycle of their own systems and adapt to the whole basin under changes in natural and humanistic elements (Chen et al., 2020). These settlements are also an integral system that embodies the interaction between humans and the land in the basin (Zhang et al., 2021).
However, several challenges have been encountered in the river hydrology and settlements in the basin due to climate change and human activities since modern times (Montanari et al., 2013). Owing to differences in ecological endowments in the basin, different settlements face different degrees and dimensions of disturbance. The Loess Plateau of the Yellow River Basin is suitable for human settlements, as it is conducive to settlement evolution and cultural development . Moreover, it is a major birthplace of Chinese civilization. However, the Loess Plateau region of the Yellow River Basin is faced with natural constraints, such as fragile ecological environment (Fang et al., 2021) and water resource scarcity (Omer, Elagib, et al., 2020); additionally, potential conflicts exist between the natural and human ecosystems (Bai et al., 2019). In recent years, the increasing intensity of human activities in settlements has resulted in the destruction of vegetation on the Loess Plateau (Kong et al., 2018) and a series of ecological degradation problems (Jiang et al., 2018), such as frequent droughts , soil and water loss , and low land fertility (Cheng et al., 2018), which have restricted settlement development. However, the modern transportation network allows the flow of resources, culture, and other factors by breaking the limitations of physical geography (Du et al., 2022) and allows the reconstruction of the regional pattern of settlements in the basin. Therefore, under the dual effects of natural disturbances and human activities, the relationship between people and the land of traditional settlements in the Loess Plateau of the Yellow River Basin is changing (Xue et al., 2022).
Difficulty of the settlements in the basin to achieve a balance among protection, development, and ecological livability has attracted the attention of the government. Since December 2012, the Chinese government has been recording traditional villages in a directory and providing them financial and political support. High-quality development and rural revitalization in the Yellow River Basin have become major national strategies. Since the implementation of these policies, the protection of settlements in the basin has improved. However, some traditional villages are still affected by the natural environment, traffic location, and other factors, resulting in population loss and cultural decline . Thus, systematic protection of these settlements should be conducted from the perspective of the human-land relationship, considering the historical evolution law of settlements and modern development needs.
Presently, academic research and practical management of the adaptation of settlements often discuss each component of the complex social-ecological system separately and focus on certain elements, such as the morphological evolution of settlements (Chen et al., 2020), spatial reconstruction , factors influencing development , industrial transformation process (Gao & Wu, 2017), and the effects of protection policies (Yurui et al., 2019). Additionally, few studies have been conducted on the adaptive evolutionary mechanisms of human activities and environmental changes. The social-ecological system theory provides a systematic perspective for exploring the adaptation of settlements in the basin based on the human-land relationship (Turner et al., 2007). Unlike a single factor or perspective, the social-ecological system theory, based on the interaction between human and natural systems, can be used to analyze how the external environment of settlements and their own systems interact and evolve together (Zhu et al., 2021).
Population, production, and ecology are three typical subsystems of the social ecosystem in the Loess Plateau of the Yellow River Basin, and their coordinated development is important for the sustainable development of the basin (Li et al., 2021). Therefore, based on the theory of social-ecological system, this study proposed a coupling system framework of "population-production-ecology" for traditional settlements in the Loess Plateau of the Yellow River Basin based on the internal interaction and external connection of the social ecosystem of the settlements in the basin (Figure 1). Under the external "stimulation" of multiple adaptive natural and human elements, the social ecosystem of the settlement in the basin showed a dynamic response process.
Based on the analysis of the human-land relationship evolution stage of the settlements in the basin since the past 500 years, the changes in the population-production-ecology interaction between the entire basin and the settlements were quantified to identify the adaptation process of the traditional settlements to the basin environment. In this study, rivers were considered as a typical adaptive factor, and an adaptation model for settlements in the basin of the social ecosystem was discussed. From a systematic perspective, this study provides suggestions for constructing a sustainable integrated basin management model.

| Study area
The Loess Plateau of the Yellow River Basin was selected as the study area. This region has the most concentrated conflicts between humans, resources, and the environment in China. Additionally, this region has a fragile ecological environment that is sensitive to climate change. It is also an important source of water (owing to the presence of the Yellow River) and a source of most sediments, thus, indicating the relationship between water and sand and upstream and downstream coordination in the Yellow River (Yu et al., 2020). The total area of the Loess Plateau is approximately 573,000 km 2 . The historical towns and traditional villages on the List of Chinese Historical and Cultural Cities and List of Chinese Traditional Villages were considered as the study areas. In total, 679 traditional settlements exist in the study area ( Figure 2). This study was conducted over a period of nearly 500 years, from 1500 to 2020. The reason 1500 was chosen as the starting year was because around this year, after the Tumu Crisis (1449) and before Jiajing (1522), the frontier defense posture and policies of the Ming Dynasty underwent significant changes, and the construction of large-scale defense projects began, thus, changing the traffic, settlement, and population structure of this area and laying the basic framework of its social structure (Ge & Hua, 2002).

| Data sources
The data used in this study were a combination of nearly 500 years of historical records, historical period reconstruction data, and statistical observations. Data on the traditional settlements were extracted from the Digital Museum of Traditional Chinese Villages (http://www.dmctv. cn). Population and productive land data from 1500 to 1980 were extracted from the HYDE database (History Database of Global Environment, https://www.pbl.nl/en/image/links/ hyde). Historical ecological data were extracted from the KK10 data set (https://doi.org/10.1594/PANGAEA. 871369). Population, productive land, and ecological data for the 21st century were obtained from the Data Center for Resources and Environmental Sciences of the Chinese Academy of Sciences (https://www.resdc.cn/). Further, data of the Yellow River during the historical period were obtained from the electronic historical atlas of China, and data of the modern Yellow River were obtained from the Data Center of Resources and Environmental Sciences, Chinese Academy of Sciences. Basic map data were obtained from the Geographic Data Platform of the College of Urban and Environmental Sciences, Peking University (http://geodata.pku.edu.cn).
The Loess Plateau of the Yellow River Basin.

| Piecewise linear regression
The relationship between various components of the social-ecological system showed mutation and inflection points during the study period, which represented key points in the transition from one stable state to another. These turning points are important for understanding the evolutionary stage of the social-ecological system in the study region. Therefore, piecewise linear regression was used to identify these turning points in the interaction of the system components over the study period (Wu et al., 2020). The equation for the piecewise linear regression can be expressed as: where Y is the dependent variable, X is the independent variable, a 1 and a 2 are the intercepts of the linear segments, b 1 and b 2 are the slopes of the linear segments, and T 1 is the turning point. The following two criteria were used to select T 1 : (1) Using Equation (2), the minimum time point for the sum of the residual squares of the regression equation was calculated.
(2) Using Equation (3), the F test of the two regression lines before and after the breakpoint was performed, and the p value was <0.05. After identifying one turning point, the same method was used to identify other turning points, if any, until no other point met the criteria for turning point identification.

| Coupling and coordination degree analysis
For the Loess Plateau of the Yellow River Basin, the growing population with the following demand of food production had been a central issue. Livelihood strategies, such as the cultivation of cropland and expansion of pastureland, relied on deforestation, which leaded to a series of ecological issues that, in turn, constrain the region's development. Therefore, this study used population density, proportion of productive land and forest coverage rate as indicators to quantify the "population-production-ecology" subsystem of the socio-ecological system, respectively. Their interactions represent the relationship between social development (population growth) and its demand for food production and sustainable environment. To study the coordinated development level of "population-production-ecology" from the perspective of system, and judge whether the relationship between "population-production-ecology" showed positive development and orderly coordination, the coupling and coordination degree analysis was used. Coupling is a concept in physics, which refers to the phenomenon that two (or more) systems or motion forms affect each other through various interactions, and coordination is a benign relationship between two or more systems or elements . Coupling and coordination degree analysis has been widely applied to study the interaction of the society ecological system (Xing et al., 2019;Yang et al., 2020) The formula used was as follows: (4) where f(x), g(y), and h(z) are the standardized population density, proportion of productive land, and forest coverage rate, respectively. Further, a, b, and c are undetermined coefficients, and a + b + c = 1. As there is no essential difference in the interaction between the three subsystems in the coordinated development of population, production, and ecology, the values of a, b, and c were 1/3. C is the coupling degree, and the larger the value, the stronger the connection between "population, production, and ecology." D is the coupling coordination degree between social ecosystems, and 0 ≤ D ≤ 1, where the larger the value, the higher the coupling coordination degree.

| Evolution of the human-land relationship in basin settlements
Population and land were two key elements of the human-land relationship. Productive land (cropland and pastureland) affected both human society and ecosystems, and was closely related to river water resources and the aquatic environment. During the past 500 years, the population and productive land in the Loess Plateau of the Yellow River Basin increased gradually from 1500 to 1700, while after 1700, especially after the mid-20th century, rapid population growth and expansion of productive land was observed. After the implementation of the "grain for green" project in 1999, the proportion of productive land started decreasing ( Figure 3). This was inseparable from the phased evolution of human-land relations in the basin. The interaction between population and productive land was assumed to remain constant during a certain period of the stable evolutionary stage of the human-land relationship. Therefore, by detecting the changes in the interaction between total population and total productive land area through segmented linear regression (Figure 4), the human-land relationship in the Loess Plateau of the Yellow River Basin over the past 500 years can be approximately divided into five stages (Table 1).
Stage 1 (1500-1800): The social-ecological system of the basin was still in its traditional development model. The inhabitants cleared land by cutting forests, to support the increasing population. The Ming and Qing governments pursued a series of policies to encourage reclamation, and regional agricultural development reached its greatest extent in history. Limited by the technological level and soil conditions, farming methods at the time were primitive and brought about short-term agricultural profits. In the long term, an expanding population and an increase in cultivation intensity reduced soil fertility and increased water consumption. Soil erosion and other aquatic environmental problems were also observed.
Stage 2 (1800-1900): The introduction of high-yielding crops, such as corn and potatoes, during the mid-to-late Qing Dynasty increased grain production per unit of production area. Droughts and wars during the cold period led to a decrease in the population and production area of the basin.
Stage 3 : Frequent disasters, such as wars, earthquakes, droughts, and famines, affected the original development model. The development of hydraulic engineering and the emphasis on agriculture and livestock by the revolutionary government allowed the gradual progression of the original development model amidst many unfavorable conditions.    4 (1950-2000): Food security was prioritized, and accordingly, a large amount of land was reclaimed on the Loess Plateau to meet the increasing demands of the increasing population. Many erosion control measures, including damming, terracing, afforestation, and conservation tillage, were implemented to relieve ecological pressure. Additionally, technological advances increased agricultural productivity, resulting in a negative correlation between population and arable land area.
Stage 5 (2000-present): Owing to the floods of the 1990s, ecological security and sustainable development received considerable attention. The Loess Plateau pioneered the "grain for green" project to reduce the disaster risk by restoring forests and grasslands. It also changed the livelihood strategies of farmers and reduced poverty (Wu et al., 2019). The "grain for green" project has converted 16,000 km 2 of productive land into ecological land (Feng et al., 2016). In addition to land-use changes, the project has also significantly influenced ecological subsystem characteristics, such as vegetation status, river runoff, and sediment load in the Loess Plateau (Yu et al., 2020). Regarding the regional population, the "grain for green" project has promoted household migration (Treacy et al., 2018), leading to a change in the population distribution patterns. Thus, the "grain for green" project has exhibited an unprecedented impact on the coupled and coordinated relationships between the three subsystems of "population-production-ecology" in the basin. Basic farmland and ecological economic forest construction, as well as measures, such as facility agriculture, water-conserving agriculture, and intensive agriculture have enhanced agricultural production and water-use efficiency, improved soil quality, and effectively reduced soil erosion. Overall, ecological policies and ecological projects over the last 20 years have promoted not only ecological restoration, but also sustainable economic development and transformation of the agricultural structure, which has completely changed the basin development model . Thus, the correlation between population and productive land in Stage 5 was opposite to that in the first four stages.
3.2 | Changes in the coupling coordination degree of the social-ecological system in the basin over time Over the past 500 years, the degree of coupling between population, production, and ecology systems in the Loess Plateau of the Yellow River Basin has gradually decreased ( Table 2) from moderately coupled to decoupled, indicating that the interaction between these three subsystems has gradually weakened. This proved that the development model constructed by population, production, and ecology in the past was disintegrating. Simultaneously, the degree of coordination between the three subsystems has also decreased from weakly coordinated to moderately uncoordinated, implying that the benign interaction between the three subsystems is also weakening. In conclusion, for the basin as a whole, population, production, and ecology were mutually constrained at a low level, but the degree of mutual constraint is gradually weakening.
T A B L E 2 Overall coupling coordination in the Loess Plateau of the Yellow River Basin in the last 500 years.

| 191
Although the coupling between the subsystems gradually weakened, the level of coupling between production and ecology was relatively high among the three relationships in the same period. This was because under the traditional agricultural production model, production land and ecological land were directly in competition, which was indicated by the slash-and-burn reclamation method. This competitive relationship was also observed through the "grain for green" project. Contrastingly, the level of coupling between the population, production, and ecological subsystems was low and decoupled in the 19th and 20th centuries. This may be because the population subsystem was indirectly related to productive and ecological lands. Although agricultural production on productive land directly affected the number of people supported by the watershed, productive activities were not only influenced by productive land, but also related to complex factors, such as production efficiency, technological level, natural disasters, and wars.

| Differences in the coupling coordination degree between basin settlements and regional systems
The coupling coordination degrees can be calculated separately for individual basin settlements and the basin system as a whole. In the Loess Plateau region of the Yellow River Basin, the degree of coupling coordination of basin settlements and the overall basin value differed ( Figure 5). Between 1500 and 1900, the degree of coupling coordination for more than 300 settlements was higher than the overall basin value. Between 1950 and 2000, the coupling coordination degree of the river basin settlements decreased, as did the overall basin value. After 2000, most basin settlements were uncoupled, which was the same as the overall basin value. This suggested that in the early years, basin settlements, as the areas of concentrated human activity, were more dependent on the socialecological system (population, production, and ecology) than on the basin system as a whole. However, subsequently, the basin settlements gradually moved out of their original cycle along with the overall evolution of the basin.
For the coupling coordination degree, between 1500 and 1980, the number of settlements corresponding to the state where the basin's overall value was located increased in the next stage (the curve where the overall value was located increased in the next stage). This implied that the traditional settlements converged to the value of the overall basin after a certain time, indicating that the change in traditional settlements lagged slightly behind the overall basin. However, after 1980, the number of highly uncoordinated traditional settlements increased rapidly, but the overall basin did not deteriorate and became highly uncoordinated. The phenomenon that the change of traditional settlements preceded that of the overall basin may be because rapid urbanization had a more significant impact on the population-production-ecology relationship in rural areas, which made the change of traditional settlements more sensitive.
3.4 | Spatial differences in the coupling coordination degree of basin settlements Spatial differences in the degree of coupling coordination among population, production, and ecology were observed in the Loess Plateau of the Yellow River Basin (Figure 6). Before 1800, areas with high degrees of coupling were widely distributed in the eastern and southern parts of the Loess Plateau region; after 1900, they were concentrated in southern Shaanxi and southern Shanxi. After 2010, the areas with moderate and high levels of coupling disappeared, and a small number of areas that maintained a certain level of coupling were also located in southern Shaanxi and southern Shanxi. This indicated that among the early settlement sites, where conditions for population, production, and ecology were better, the original model of coupled development could continue longer.
Additionally, the coupling coordination degree along the main stream of the Yellow River and in the Fen River-Wei River Basin was higher than that in other areas of the basin at the same time, reflecting the important role of rivers in the development of social-ecological systems. Similar to the coupling degree, the coupling coordination degree exhibited spatial differences. From 1500 to 1800, the eastern and southern parts of the basin were mostly coordinated at the low-coordination level and above. The western and northern parts of the basin reached the peak level of coordination in 1800. However, after 1800, except F I G U R E 5 Number of settlements with different coupling degrees (left) and coupling coordination degrees (right). along the main stream of the Yellow River and in the Fen River-Wei River Basin, all other regions experienced a rapid decline in the level of coupling coordination. After 2000, most regions were either moderately or highly uncoordinated. Thus, rivers had an important influence on the development of the Fen River-Wei River Basin (also known as the Guan Zhong region). Particularly, this area had a dense network of rivers with good water and heat conditions suitable for agriculture, thus, giving birth to a large number of traditional settlements. Rivers also created good waterway transportation conditions for this region, providing an advantage for outward development.

| DISCUSSION
As mentioned above, water resources and aquatic environment are closely related to the production and ecology subsystem, and formed as important foundations for the development of basin settlements. The role of water resources in the development dynamics of basin settlements can reflect the population-production-ecology cycle model.
For example, the basin settlements in the study area were dependent on agriculture and water to meet the needs of the increasing population as showed in the highly positive correlation between population and production area in early stages. However, water resources in the Loess Plateau have long been scarce. Therefore, many basin settlements were located in areas with easy access to water, which accounted for the higher coupling coordination degree in some of the near-water areas mentioned above. Further analysis showed that, the basin settlements in the study area had, over the ages, been more commonly located within 10-30 km of the Yellow River (Figure 7), fed by the surrounding groundwater. The flooding of the Yellow River is closely related to the irrigation of the cropland of the surrounding settlements (Du et al., 2019); thus, despite the frequent flooding of the Yellow River, a significant proportion of the basin settlements were located within 1 km of the Yellow River. This proportion has F I G U R E 6 Coupling degree (left) and coupling coordination degree (right) among population, production, and ecology in the Loess Plateau of the Yellow River Basin.
F I G U R E 7 Distance between the settlement and the Yellow River.
WANG ET AL. | 193 increased since the 1950s compared to that in the Ming and Qing dynasties, suggesting that measures, such as the construction of hydraulic facilities, have improved the risk prediction and disaster prevention capabilities of basin settlements, which reflect changes in the productionecology coupling due to technological developments.
The results of this study showed that, the coupling between population, production, and ecology systems in the Loess Plateau of the Yellow River Basin has gradually become dysfunctional over the past 500 years. One of the main reasons for this is that, the large population, weak economic base, and flawed environmental protection facilities and mechanisms in the Loess Plateau of the Yellow River Basin have long led to overexploitation and poor water resource management. The disruption of the balance of the ecological subsystem has further led to the dysfunction of the whole system, resulting in water resource problems and water environment security issues, such as a sharp reduction in terrestrial water storage (Xie et al., 2019), water pollution (Xiao et al., 2019), groundwater arsenic enrichment , and soil salinization . The Yellow River Basin is an important food-producing area in China, and the water resources of the basin are strongly associated with the food production and security of the national population . In recent years, the national strategy for ecological protection, high-quality development of the Yellow River Basin, and the implementation of ecological measures, such as soil and water conservation measures, have achieved remarkable results. The change in the correlation between population and production area in Stage 5 indicated that the current uncoordinated coupling of the "population-production-ecology" system is expected to be reversed.
The present case study demonstrated that water resources and aquatic environment security issues were closely linked to the development of "population-production-ecology" systems in river basins. Moreover, river systems supported the sustainable development of ecosystems and human society. In recent years, the social-ecological cycles of river basins have changed dramatically under the influence of climate change and human activities, creating an urgent need for more sustainable models of integrated river management . The priority objectives for future river basin management are to reconcile ecological protection and social development, promote the coupling and coordination of the "population-production-ecology" system, and ensure the sustainable development of basin and settlements.

| CONCLUSIONS
From the perspective of social-ecological systems, this study focused on the stage differences in human-land relations in the Loess Plateau of the Yellow River Basin since 1500 along with the interaction between the basin environment and basin settlements in each stage. The differences in the human-land relationships in the study area in different periods and their causes were revealed, and the adaptive evolution of basin settlements were explored from the perspective of coupled coordination. The main findings of this study were as follows: (1) The dynamic mechanisms that have historically driven the development of the Loess Plateau of the Yellow River Basin are gradually disintegrating. The human-land relationship in the Loess Plateau of the Yellow River Basin since 1500 can be divided into five main stages: Stage 1 of rapid expansion of agriculture and livestock (1500-1800), Stage 2 of fluctuation and fallback in population and production area (1800-1900), Stage 3 of slow expansion of agriculture and livestock , Stage 4 of transitional exploration of cropland contraction , and Stage 5 of transformational development after the "grain for green" project (2000-present). The coupling degree between the three subsystems of the social-ecological system has gradually decreased over time, which implied that the interaction between "population-production-ecology" has gradually weakened, proving that the development model constructed by "population-production-ecology" is gradually weakening.
(2) Basin settlements were more dependent on the "population-production-ecology" social-ecological system, and the change in the development model was delayed.
The evolution of the social-ecological systems of the river basin settlements in the Loess Plateau of the Yellow River Basin lags behind that of the basin, probably because the settlements were more dependent on the "population-production-ecology" socialecological system. Basin settlements that had survived throughout history were more likely to be located in areas where the relationship between population, production, and ecology was better, which in turn ensured that the original model of settlement development could continue longer. However, changes in the basin environment could eventually push the basin settlements out of their original cycle. When the cumulative impact of either the natural environment or anthropogenic policies reached a threshold, it pushed the social-ecological system of the settlements in the basin out of its original homeostasis and into the next stage of a new adaptive cycle.
(3) A sustainable integrated basin management model is required from the perspective of settlements in the basin.
The evolution of human-land relationships in the basin settlements in the Loess Plateau of the Yellow River Basin was representative. "People" were the more active and proactive party in the human-land relationship, while "land" represented the natural resources and environment that changed the system by influencing, constraining, and even changing human activities. Therefore, future basin management strategies need to pay more attention to the relationship between water ecology and industrial development while helping reshape the dynamics of basin settlements. Therefore, a sustainable model for integrated basin management is necessary. On the one hand, the intelligence and technologies of river management should be enhanced, to improve the allocation and management of water resources. On the other hand, policy decisions should focus on local experiences from traditional water resource use and develop a sustainable nature-based ecological approach to improve the adaptability of basins and basin settlements.

ACKNOWLEDGMENTS
The study was supported by the Key Project of the National Social Science Fund of China (Grant No. 21AZD033) and the Key Project of the National Natural Science Foundation of China (Grant No. 52130804).

DATA AVAILABILITY STATEMENT
The data that support the findings of this study are available from the corresponding author upon reasonable request.
ETHICS STATEMENT None declared.