The 300 years cropland changes reflecting climate impacts and social resilience at the Yellow River–Huangshui River Valley, China

The YHV locates at the northeast edge of the Qinghai–Tibet Plateau (Zhang et al 2014) (figure 1), which is in the agro-pastoral belt of China. It is composed of a section of the upper reaches of the Yellow River and two secondary tributaries. The total area of the basin is approximately 3.3 × 10⁴ km², and the altitude ranges from 1689 m to 5218 m above sea level. The region has a significant geographical feature of transitions between wet and dry zones and between low mountains and the high plateau. These two basic features further make this area a remarkable region for east–west transitions from the pacific monsoon climate to non-monsoon climate, from forest to grassland, from agriculture to animal husbandry, from outflow zone to inflow zone, from the Loess Plateau to the Qinghai–Tibet Plateau. Therefore, social–economic system experiences more diverse and complex impacts from the climate and environmental conditions.

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High mountains (over 3500 m above sea level) account for 72.4% of the whole YHV area. The rest are mostly, hills, terraces and alluvial plains that are the main areas of croplands. The limited cultivable lands contribute to a cradle of the earliest human civilization in China, and the agricultural activities at the YHV can be traced back to the Majiayao Culture period around 5.9 ka BP (Liu et al 2010). Approximately around the 4.2 ka BP, the agriculture changed significantly due to the 'Meghalayan' global cold event. Since then, the proportion of animal husbandry was kept increasing, and eventually, animal husbandry was separated from agriculture (Liu et al 2010). As a typical agro-pastoral zone, the area with its cropland reclamations and changes are highly sensitive to and very pronounced of the combined climate impacts and human activities.

Historic cropland data of Qing Dynasty are collected from various sources, which covers:

• 'Xining Fu Zhi' (西宁府新志) (Yang 1968), compiled in the 12th year of the Qianlong period in Qing Dynasty (AD 1747).
• 'Xining Fu Zhi Continued' (西宁续志) (Deng 1938), compiled in the 9th year of Guangxu period (光绪) (AD 1883),
• 'Gansu Tongzhi' (甘肃通志), recompiled in the 34th year of Guangxu period (AD 1908) and the 18th year of the Republic of China (AD 1929),
• 'Gansu Tongzhi Manuscript' (甘肃通志稿), pressed in 1936 (Liu 1964),
• 'Xunhua Ting Zhi' (循化厅志) (Gong 1970),
• 'Dan ge er Ting zhi' (丹噶尔厅志) (Yang 1989),
• 'Compilation of local Chronicles of Qinghai' (青海省地方志) (Wang 1987), and a few other local history materials.

Natural disasters and their impacts, social unrest, local conflicts, wars and other social data come from Yuan's book 'History of Disasters in Northwest China' (Yuan 1994). The administrative boundary of the study area in the Qing Dynasty was different from the present. We adopted the 'Historical Atlas of China' (中国历史地图集) edited by Tan (1996) and the 'Comprehensive Table of Administrative Districts Evolution in Qing Dynasty' (Niu 1999) to keep consistency accuracy with the cropland reconstruction data.

The agricultural system at the YHV area has been well developed by the early Qing Dynasty (in early 17th century), and data of croplands is relatively easy to access in various survey materials and statistical documents. Generally, the YHV area has six types of cropland: Tun cropland (屯垦), Ke cropland (科垦), Qiu cropland (秋垦), Zhan cropland (站垦), Ken cropland (垦田) and Fan cropland (番垦). The Tun type is a kind of state-owned land but was cultivated by either military or farmers. Some of the Tun croplands were converted to private farmlands since the late Ming Dynasty (1386–1644 AD). Ke cropland is a privately owned farmland that was registered and taxed by the government. Qiu cropland has relatively less fertile soil, where only one season crops can be planted (sow in summer and harvest in autumn). The Zhan type is the cultivated land that were originally owned by local relay/post stations but were then converted for civilian cultivations. Ken cropland is the officially reclaimed land, and the Fan cropland is the type of cropland owned by the ethnic minority groups in some marginal areas.

We obtained the cultivated land quantity of every county at the YHV in seven historical periods by consulting a variety of historical documents (Shao 1901, Deng 1938, Yang 1968, Gong 1970, Liang 1987). On account of the measurement unit of cropland area, 'Qing Mu', a unit of area in the Qing Dynasty, an area measurement transformation was needed. And a 'Qing Mu' is equal to 0.9216 mu or 0.0614 ha in modern times. Then, a grid with a size of 1 km × 1 km is masked over the research area. The factors, affecting the distribution of cultivated land, are spatially transformed into each grid. Meanwhile, the reclamation rate of each grid was obtained according to the reclamation rate model. Finally, the reclamation rate of the grid was optimized and adjusted under the restriction of cultivated land quantity, so as to realize the spatial reconstruction of cultivated land in the historical period.

Considering the complex natural environment of the Qinghai–Tibet Plateau and its various effects on farmland distribution, two types of constraint factors and suitability factors were divided. The former, expressed numerically, would control the spatial distribution range of cultivated land. For example, if the elevation is more than 3500 m it means no cultivated land could be distributed. In this method, constraint factors were used for excluding areas impossible for farmland. It only had two values of 0 and 1, with 0 means there was no cultivated land and 1 means there had. The suitability factors determined the possibility of the distribution of cultivated land according to its value. It was the important basis for the quantity allocation of farmland in this method. We selected seven factors to be used in model of cropland reconstruction. Among these, elevation, slope and soil type were constraint factors. Suitability factors included aspect, climatic production potential, organic matter and settlement distribution.

Cropland area data were collected from historical documents, which recorded only its amount. In order to reasonably allocate it into the spatial distribution, a composite cropland reconstruction model was constructed. Firstly, we divided the research area into 1 km × 1 km grid cell. It meant that all subsequent calculations were performed for each grid cell. Secondly, the constraint factors were entered into the model, including of soil type, altitude and slope, represented by 0 and 1. And only those cells whose results were equal to 1 were retained. It implied the identification of spatial distribution range of the possible cropland. The suitability factors were then input into the model, consist of aspect, soil organic matter, climate production potential and settlement distribution (figure 2). So far, the G(i, v_n) in formula (1) could be obtained. Then, we continued to apply the formula (2), (3) and (4) to obtained the degree of reclamation, cultivated area ratio, and reclamation rate for each grid cell

$$\begin{equation}G\left( {i,{v_{\text{n}}}} \right) = \prod\limits_{j = 1}^H {{R_j}} \sum\limits_{c = 1}^L {{S_c}} {A_c}.\end{equation} \tag{ 1 }$$

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where G(i,v_n ) is the degree of reclamation for i grid in year v_n, H is the number of constraint factors, R_j is the value including of 0 and 1 which 0 is impossible for farmland while 1 is possible for farmland, L is the number of suitability factors, S_c is the value of suitability factor c, A_c is the weight of factor c

$$\begin{equation}B\left( {i,{v_n}} \right) = \frac{{G\left( {i,{v_n}} \right)}}{{\mathop \sum \limits_{i = 1}^n G\left( {i,{v_n}} \right)}}.\end{equation} \tag{ 2 }$$

$$\begin{equation}S\left( {i,{v_n}} \right) = B\left( {i,{v_n}} \right)D\left( {{w_m},{v_n}} \right)\end{equation} \tag{ 3 }$$

$$\begin{equation}KZ\left( {i,{v_n}} \right) = \frac{{S\left( {i,{v_n}} \right)}}{{{\text{area}}\left( i \right)}}\end{equation} \tag{ 4 }$$

where B(i,v_n ), S(i,v_n ) and KZ(i,v_n ) are the cultivated area ratio, cultivated area and reclamation rate respectively for grid i in year v_n. D(w_m,v_n ) is the total cropland area for county w_m in year v_n , area(i) is the area of grid i.

According to the historical literature (Shao 1901, Deng 1938, Yang 1968, Gong 1970, Liang 1987), the reliable cropland statistic date of seven periods (including 1650, 1726, 1772, 1806, 1845, 1878 and 1934, respectively) were collected to calculate cultivated land area over the past 300 years, mainly in the Qing Dynasty (1636–1912). As depicted by figure 3 shapely increases and decreases occurred in the early and late Qing dynasties, respectively. In the middle period of 1726–1845, the amount of cultivated land remained a relatively stable and high degree of reclamation, with a peak of 1504.72 km² in 1845. Previous researches also provided lots of reliable evidence that this fluctuation is indeed consistent with the actual change. For instance, in the Datong River Valley, which is a large tributary in the HuangShui River Basin, some woodland and grassland had been reclaimed for cropland in 1728. What is more, there is no suitable land for the increase of cultivated land in 1738, and people had to reclaim farmland in smaller valleys such as BaYanGou (巴燕沟) and AngSiDuoGou (昂思多沟) where are far away from Xining city. Those human activities made the cultivated land area of the YHV increased from 1426.68 km² to 14 870.04 km² in this stage.

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Some social activities had profound impact on agricultural reclamtion. At the beginning of Qing Dynasty, the government adopted a migration policy to promote the reclamation of marginal land, and large numbers of troops and poor people moved to this area to support agricultural production. Agriculture at the YHV had been greatly developed. It contributed to the first rapid increase occurred between 1650 and 1726. The amount of cultivated land increased from 406.63 km² to 1426.68 km². However, the YHV is a region where many ethnic groups are gathered. In 1862, a great uprising of Hui nationality in northwest China broke out. A large number of people died in the war and the farmland were abandoned. A sharp decline in farmland occurred. The amount of cultivated land decreased rapidly from 1504.72 km² to 1106.19 km². Thus it can be seen that agriculture activity at the YHV, located in the transition area of agriculture and animal husbandry, was significantly influenced by social factors, such as population size, social institution, and stability.

Furthermore, we highly concerned about the intensive reclamation of farmland at the YHV since 1726. Fan cropland is the type of cropland owned by the ethnic minority groups on the fringes of agriculture. Through sorting out the historical documents, we further found that the Fan cropland reclaimed by ethnic minorities, were dominated by Tibetans which occupied an important part (figure 4). More than 80% of Fan cropland is dry land. The specific distribution were mainly on the upper slopes on both sides of the valley, shown in figure 5 of 2—Guide, 4—Jishi, 5—Datong, 6—Xunhua, 7—Bayanrongge, 8—Dangaer, 9—Menyuan, 11—Huangyuan, 13—Hualong.

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The spatial distribution of cultivated land in seven historical periods showed apparent aggregation on both sides of rivers (figure 5). The YHV has a large scope. However, less area is suitable for cultivated land. Farmland are mainly distributed near the Yellow River, Huangshui River, and their tributaries, including the Beichuan River, Datong River. Among them, the valley of Yellow River and Huangshui River have the highest density distribution of cropland. With the increase in altitude and slope, the distribution of cultivated land has decreased significantly. Thus, the spatial pattern of cultivated land distribution is limited by natural environment conditions.

We choose three periods of 1650, 1772 and 1934 to analyze the change of spatial distribution (figure 6). In 1650, the Huangshui Valley experienced war. There were a few people and less cultivated land which were concentrated in the valley with good water and soil resources. The government actively adopted the policy of immigration to develop agriculture, and the intensity of cultivated land increased. By 1772, the reclamation rate of cropland at the YHV had risen significantly. Later, to the end of Qing Dynasty, the society was in turmoil coupled with a cold climate. A large number of people fled to other places. As a result, a lot of cultivated land at the YHV was abandoned, which led to a remarkable decrease in the reclamation rate of cultivated land at the YHV.

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The natural disaster at the YHV had a great destructive effect on agriculture in the Qing dynasty. For example, drought, hail, flood and frost were the top four agricultural meteorological disasters in this region (Zhu 2012). The frequency of drought was high which led to a massive loss of crop and famine. Hail directly caused crop losses and failures. Floods washed away low-lying farmland. Frost prevent crops from growing properly and reduce production in this alpine zone (Deng 1938, Yang 1968). These extreme agro-meteorological events greatly increased the vulnerability of agricultural system at the YHV.

4.1.1. Climate change significantly affects agricultural distribution

4.1.2. Natural disasters do more direct damage to croplands

To better understanding the relationship between the change of farmland area and the natural system in the Qing Dynasty, natural disaster records of the YHV were extracted from the historical documents of this period (Gong 1970, Liang 1987, Wang 1987, Yuan 1994). Natural disasters, such as drought, flood, landslide, debris flow, frost and hail, always happen locally and have a certain causal relationship with the change of cropland. During the period of 1650–1900, 238 records of natural disasters were extracted at the YHV, and then drew a frequency histogram (figure 7) with every 10 years as a unit. As shown in figure 7, the frequencies of natural disasters that occurred between 1650–1720 and 1780–1800 were low. However, during 1720–1790 and 1800–1900, the-high frequency of natural disasters, reached 101(1.44 times/year) and 136 times (1.36 times/year) respectively. The results showed that the increase of this natural disaster was consistent with the expansion of farmland acreage. Although it was not completely certain that the expansion of cropland acreage was caused by the increase in natural disasters, this positive relationship should be concerned.

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The further analyzed for nature disasters found that torrential flood was the main natural disasters during 1720–1780, accounting for about 40% of all disasters. To some extent, this information illustrates that more farmlands were reclaimed from plenty of native vegetation (forest or grassland). In another research work, the results proved that more farmland were reclaimed from shrub grassland (Wu et al 2016). Due to large-scale destruction of vegetation for the reclamation, water and soil conservation capacity was weaken and floods occured frequency. This further led to that a large amount of land was washed by water and sand (Deng 1938, Yang 1968). High frequent natural disasters was a key factor for the reduction of cropland acreage in this region. While, human-land conflict was an indirect factor. For example in 1645, the population at the YHV was 3.6 × 10⁴ (Cui 1998), but in 1746, the population increased to 26.4 × 10⁴ (Cui 1998), with an increase of 733% in a short period of 100 years. The rapid increase in population led to environment deterioration and frequent disasters.

There is no doubt that the warm and humid climate promoted the development of agriculture. In the Qing Dynasty, the short warm and humid period make the saturation of cultivated land reclamation, while natural disasters destroyed the cultivated land. In face of an unfavorable natural environment for crop planting, management and policies from the social system were promising to enhance the resilience of agricultural system.

4.2.1. Land reclamation policies

4.2.2. The construction of water conservancy facilities

4.2.3. Tibetans actively transferred into agriculture development

4.2.4. Special flexibilities of the agro-pastoral system

From the above analysis, we found that the change of cropland in this region was affected by natural factors (e.g. temperature rising and natural disaster etc) and human factors (such as policy and population growth etc). The YHV, where condenses the case of changes and conflicts in agriculture and pasture husbandry, is in a diverse-transition zone in China. Hence, this region provides us an easier way to understand the dialectic of human-natural systems and reveal the impacts of human and natural factors on the cropland. In the previous of article, we explained some mutual-feedback phenomena of cropland change. However, we still had a cautious attitude towards climate change, and further analyses are following: China's northern agro-pastoral transition zones is extremely sensitive to climate change. This phenomenon has already been studied in many cases, but the research on this area of the YHV is still relatively rare. At present, we called the period from 1400 to 1900 the little ice age period of the Ming and Qing Dynasties. The frequent low-temperature disaster happened in China, and the temperature curves of the three reconstructions we selected (figure 3) also conform to this trends. There was a period of lower temperature at the YHV before 1650, described 'the land is cold and cannot grain', 'the mountains are high and cold, the land is poor' by historica materials. Because of the cold climate, the area of land reclamation was limited and grew slowly. However, the human system never passively accepts all. The agricultural technological innovation always had been promoted and active management systems had been implemented in face of the harsh natural environment. The agricultural production at the YHV had been struggling to develop in the interaction of the pressure of the natural system and the support of the social system.

The cropland reclamation at the YHV had obviously suffered from great pressure from natural conditions that are not conducive to agricultural production. When the temperature in the northern hemisphere increased from 1720 to 1770, the temperature in this region also risen. It significantly promotes the rapid development of cultivated land. The scope of cropland was rapidly expanded. Our investigation in this area for nearly 20 years' works shows that the agricultural planting area here responds quickly to the temperature change, and it was also shown in other research work (Chen 2018, Hou 2018, Li et al 2019). During the period from 1770 to 1800, the temperature dropped again. The number of natural disasters caused by the increase of cultivated-land in the previous stage also increased, and the area of cultivated land decreased by about 320 km². During the period from 1800 to 1850, the temperature showed a gradual increase in the curve (figure 8). Driven by both policies and warm climate, the area of cultivated land had increased again. However, the cooling temperature in 1850 did not only lead to a reduction in cultivated land, but also caused ethnic conflicts within the region. Although the extent of the cooling was much lower than the event in 1750, the consequences were much more severe than 1750. The main reason might be that the population is much larger than the previous period of cooling, and the social contradictions are more profound.

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The development of agriculture at the YHV in the Qing Dynasty was limited by the natural environment (temperature, precipitation and natural disasters), but benefited from social factors (population and agricultural measures). The animal husbandry economy of 'living in pursuit of water and grass' is characterized by strong mobility, a wide utilization of land resources, which has strong adaptability to the harsh natural environment. However, the animal husbandry economy is also greatly affected by the changes in the natural environment. Its disadvantages manifest in terms of food storage, vulnerability, instability and heavy dependence on water and grass. Though the cold, drought and high altitude are the main themes of the vast Qinghai–Tibet Plateau region, there are always some rivers, lakes and valleys with fertile soil, abundant water and convenient irrigation, which are suitable for the development of the agricultural economy. One such place is the YHV, which has been a nomadic region of the Qiang (an ancient minority) people for thousands of years. When the population, agricultural technology and agricultural reclamation policies arrived in this region in the Qing Dynasty, water conservancy facilities were improved, which greatly enhanced the ability of agricultural system to resist drought. The implementation of the immigration policy has resulted in a massive increase in population, which promoted agricultural productivity. The reduction and exemption of various agricultural taxes greatly stimulated farmers' land reclamation activities. Ultimately, the production capacity of the agricultural social system is enhanced.

In summary, agriculture in the farming-pastoral zone is strongly limited by the natural conditions of temperature and precipitation. But it can also stimulate more feedback from the social system, which makes agriculture well developed at the YHV and thus enhance social resilience.

Limited to the map technology, the cultivated land in historical period cannot be expressed in an intuitive space. Fortunately, China has enough historical documents to provide a large amount of records and digital statistics that can be used to reconstruct historical farmland. Based on the information recorded in the literature, we reconstructed the cultivated land of a small area in the eastern edge of the Qinghai–Tibet Plateau. Around the cultivated land changes, the interaction among agriculture, the natural system and social support system were analyzed. Some conclusions are found.

• (a)  
  Cultivated land of the YHV, restricted by altitude, slope, soil, etc, was mainly distributed in the narrow river valley areas where population and settlements were concentrated. The cultivated land fluctuated under the influence of both natural and social factors at different stages. The changes in the reclaimed land between Tun, Ke, Ken, Qiu, Zhan and Fan cropland presented obviously the ebb and flow trend.
• (b)  
  Climate change has a significant impact on agriculture at the upper altitude. During the warm period, People living in high-altitude region had significantly expanded to a larger scape, which had promoted the increase of the cultivated land. On the contrary, in the cold period, cultivated land shrinked, high-altitude residents increased livestock supplementation to sustain livelihood.
• (c)  
  The cultivated land at the YHV appeared fragile and changeable. In order to adapted to this change, various measures have been adopted by the local government, farmers and herdsmen to cope with it. The government encouraged land reclamation, built water conservancy facilities, improved farming techniques to increase food production. At the same time, the complementarity of agriculture and animal husbandry enhanced the flexibility and adaptability of the agricultural system, and greatly improved the resilience of agricultural production in the edge of the plateau.