Spatial-Temporal Evolution Characteristics and Driving Factors of Agricultural Space in Jianghan Plain from the Perspective of “Three-Zone Space”

Studying the agricultural spatial distribution characteristics of Jianghan Plain from 2000 to 2018 and its spatial-temporal transformation with ecological space and urban space is of great significance for optimizing the spatial structure of the country, enhancing the supply capacity of agricultural products, and ensuring food security. The research results show that during the period from 2000 to 2018, the agricultural spatial concentration in the area along the “Xiaogan-Jingzhou” link was relatively high, and the areas with large declines were mainly distributed in the flat areas between the Yangtze River and Han River; the space is continuously reduced by the occupation of ecological space and urban space, and ecological space and urban space have been improved respectively; agricultural space has been transformed into ecological space, and the largest urban space has been transformed. Ecological space has been transformed into agricultural space and urban space. Less urban space has been transformed into agricultural space and ecological space.


Introduction
Agricultural space resources are the basic natural conditions for human beings to survive and develop. They are responsible for meeting people's food needs and are also the basic driving force for urban development [1] [2] [3]. From the perspective of human survival and sustainable development, a reasonable agri-cultural spatial pattern is an important guarantee for national food security. It is of great significance for optimizing the national spatial pattern, enhancing the supply of agricultural products, and promoting the healthy development of cities [4] [5] [6].
The current literature on agricultural space is mainly focused on three aspects: first, focusing on rural residents' villages, focusing on describing and analyzing the evolution of agricultural village space, and the evolution of the spatial pattern of rural settlements [7] [8] [9] [10]; second, focusing on the development of agricultural modernization, and studying the spatial distribution, types, measures and constraints of agricultural modernization development [11] [12] [13] [14] [15]; third, exploring the relationship between urbanization and cultivated land protection, and studying the root causes of the contradiction between urbanization and cultivated land protection, in order to achieve the protection of cultivated land and comprehensive coordination of urbanization and all land use [16] [17] [18] [19].
This article analyzes the agricultural spatial pattern distribution and spatial-temporal evolution characteristics of the Jianghan Plain in different periods, and quantifies the mutual transformation between agricultural space, ecological space, and urban space in combination with the "three-zone space" background of the current land space planning, and analyzes and studies the driving force behind it.

Study Area
The Jianghan Plain is located in the middle reaches of the Yangtze River and the central and southern parts of Hubei Province. It has an excellent climate, rich products, and developed agriculture. It is an important food producing area in China. A total of 21 counties (districts) including Jingzhou 2 districts, 3 cities and 3 counties, Yichang 2 cities, Jingmen 2 cities and 1 county, Xiaogan 2 cities and 1 county, Xianning 1 city and 1 county, and 3 cities directly managed by the province As the research object (see Figure 1), the total land area is 41,213.03 km 2 , and the total population at the end of 2018 reached 15,128,800.

Data Sources
The land use and "three-zone space" quantitative information involved in this article comes from Data Center for Resources and Environmental Sciences, Chinese Academy of Sciences (RESDC) (http://www.resdc.cn) [20], and is obtained through factor extraction and overlay analysis. The demographic and economic data involved are from the Statistical Yearbook of Hubei Province (2000-2018), statistical bulletins of counties (districts) and cities and government work reports.

Research Method
The ArcGIS technology was used to extract remote sensing data for land cover in  According to the land use attributes, it was divided into six categories: cultivated land, forest land, grassland, waters, construction land and unused land. According to the "three-zone space" in the land space plan, the six types of space are integrated into three categories: agricultural space, ecological space, and urban space. Among them: agricultural space includes cultivated land, ecological space includes forest land, grassland, and water. Urban space is dominated by construction land. Unused land is included in urban space.

Total Change in Different Years
It can be seen from Figure 2

Spatial Distribution Map and Area Statistics of Agriculture in Different Regions and Years
It can be seen from Figure 3 that the agricultural space in the Jianghan Plain is mainly concentrated and continuously distributed in Jingzhou District, Jiangling County, Jianli County, Shayang County, and Qianjiang City between the Yangtze River and Hanjiang River, and Gong'an County and Shishou south of the Yangtze River, as well as Tianmen, Hanchuan, Yingcheng and Yunmeng counties north of the Han River. Counties and cities with relatively sparse agricultural spatial distribution, such as Jingshan County and Honghu City, have a large area of non-agricultural space. As can be seen from Table 1, the proportion of agricultural space in the Jianghan Plain has been decreasing year by year, and the proportion of agricultural space in each county (district) has also shown a gradual decline. Yunmeng, Jiangling, and Shayang counted as three agricultural counties with over 80% of agriculture in 2000. By 2018, only Yunmeng County has more than 80%, and Jiangling and Shayang counties have fallen to 79.49% and 79.26%, respectively. In 2000, 10 counties (districts) and cities accounted for more than 60% of agriculture, including Yingcheng City, Tianmen City, Gongan County, Qianjiang City, Jianli County, Jingzhou District, Hanchuan City, Shishou City, and Xiantao City. And Shacheng District, the Shacheng District will fall to 55.52% by 2018, and other counties (districts) and cities will also show varying degrees of decline.
It can be seen from Figure 4 that from 2000 to 2018, the largest change in agricultural space was in Honghu City, which reached 177.76 km 2 ; followed by Xiantao City, which fell by 143.95 km 2 ; Jianli County, Qianjiang City, and Tianmen City with a change of more than 50 km 2 They are 90.13 km 2 , 55.11 km 2 , and 50.04 km 2 respectively; the smallest change is in Yingcheng, which has only decreased by 10.9 km 2 .

Location Quotient of Agricultural Space in Each County and City
Using the concept of location quotient to analyze the degree of agricultural spatial Open Journal of Ecology  agglomeration and professionalization [21] [22], the calculation formula is: In the formula, Q represents the location quotient, a represents the agricultural space area of a county (district) city, A represents the total land area of the county (district) city, b represents the agricultural space area of the whole region, and B represents the total land area of the whole region.
It can be seen from Figure 5 and Figure 6 that from 2000 to 2018, Yunmeng County, Jiangling County, and Shayang County have always been in the top three agricultural space quotients, indicating that Yunmeng, Jiangling, and Shayang have a high degree of agricultural spatial concentration. Compared with other counties (districts) and cities in the Jianghan Plain, it has a comparative advantage; in 2000, there were 9 regions with a location quotient greater than 1, and they were Tianmen City, Yingcheng City, Gong'an County, Qianjiang City, Jianli County, Jingzhou District, Hanchuan City, Shishou City, and Xiantao City    units with a higher degree of agricultural spatial concentration are distributed in a centralized and continuous pattern. The spatial pattern is belt-shaped, mainly located between the Yangtze River and the Han River and in areas extending northeast and spreading southward, that is, the direction of the "Xiaogan-Jingzhou" connection has the largest agricultural spatial density, and decreases with the spread to both sides small.

"Three-Zone Space" Distribution Characteristics
The land space is divided into three categories: agricultural space, ecological space, and urban space. GIS is used to draw the spatial distribution maps of the three districts in different years in the Jianghan Plain, and the area of the three types of space is quantified to obtain Figure 8, Table 2. The analysis shows that in the past 18 years, agricultural space has been continuously occupied by ecological space and urban space and has declined; although ecological space has also been continuously occupied by urban space, it has increased due to the simultaneous occupation of agricultural space; urban space has shown an upward trend year by year.
On the whole, the "three-zone space" in the Jianghan Plain has not risen or fallen violently. After nearly 20 years of rapid economic development, the proportion of various types of space has not changed significantly, and the territorial space structure remains relatively stable.

"Three-Zone Space" Mutual Transformation Characteristics
Based on the "three-zone space" distribution map of the Jianghan Plain, Arcgis was used to superimpose land properties, and the conversion relationship between the "three-zone space" in each county (district) and city from 2000 to 2018 was obtained and quantified ( Figure 9). Open Journal of Ecology   The analysis shows that: 1) The transformation of agricultural space into the largest ecological space, especially in Honghu City, Xiantao City, Chibi City and Jianli County, the scale of transformation is the most prominent, and has become the largest administrative unit of spatial function transformation in 18 years. These areas are rich in Water resources; 2) The conversion of agricultural space into urban space appears to varying degrees in counties (districts) and cities, and has become the mainstream trend of spatial conversion in most administrative units; 3) The conversion of ecological space into agricultural space Chibi City is the most obvious, followed by Jiayu County and Honghu City; 4) The conversion of ecological space into urban space is also more significant, of which Zhijiang City, Jiayu County, Honghu City, and Jingshan County have a larger increase; 5) The scale of transformation into agricultural space is relatively small, mainly distributed in Hanchuan City, Yunmeng County, Honghu City and other places; 6) Urban space is transformed into the least ecological space, mainly concentrated in Chibi City, Jiayu County, Hanchuan City and other places.

Physical Geography
The natural geographical environment is the foundation that affects the composition and use of land and space, and often combined with factors such as location conditions and human history to determine the distribution characteristics of the "three-zone space" in the Jianghan Plain: 1) Jingzhou has been the main production area of grain, cotton, and aquatic products. 2) Zhongxiang City, Jingshan County in the north of the Jianghan Plain, and the western and southeastern areas are mostly mountainous and hilly, its functions are more significant; 3) Hanchuan City, Xiantao City, and Jiayu County in the eastern part of the Jianghan Plain are located in the Wuhan City Circle, which is driven and affected by the regional growth poles; Zhijiang City and Dangyang City are located in Yichang Provincial Deputy Center has unique advantages in infrastructure construction and industrial structure optimization. Urban spatial expansion trend is more pronounced.

Population Migration and Accelerated Urbanization
According to the statistics of the population of Jianghan Plain from 2000 to 2018, we obtained the population change trend chart ( Figure 10). The analysis shows that the total population of Jianghan Plain has been decreasing year by year, a decrease of 10.47%, from 29.93% of the total population of Hubei Province in 2000 to 25.57% in 2018, but the number of urban population increased by 3,292,100, an increase of 59%, exceeding the 56% increase in urban population in Hubei Province during the same period, and the urbanization rate also increased from 31.62% in 2000 to 56.00% in 2018. Therefore, during this period, a large number of people flooded into towns, the urban construction land continued to expand, and the increase in urban space was a significant phenomenon. At the same time, the rural population in the Jianghan Plain fell by 39.27%, far exceeding the 30.11% decline in rural population in Hubei Province during the same period. The outward migration of the population has led to the emergence of wasteland and wasteland in many places, coupled with the increase in agricultural output and production efficiency, leading to the constant shrinking of agricultural space.

Economic Development and Industrial Structure Transformation
Statistics on the industrial development of the Jianghan Plain from 2000 to 2018 can be found in Figure 11. The analysis shows that: 1) In terms of industrial structure changes, the growth rates of the primary, secondary, and tertiary industries have reached 368.90%, 894.14%, and 738.14%, respectively. The development is more than twice that of the primary industry, and the proportion of tertiary industry has also changed from 26.

Conclusions
Taking W. Wei, H. Xing the spatial and temporal evolution of agricultural space, ecological space, and urban space were analyzed from the perspective of "three-zone space". The relationship between the three regions is analyzed from the aspects of physical geography, policy promotion, population migration and acceleration of urbanization, economic development and industrial structure transformation. The main conclusions are as follows: 1) The regions with a high degree of agricultural spatial concentration are distributed in the direction of the "Xiaogan-Jingzhou" connection; the areas with relatively small changes in agricultural space, that is, the regions with a large decline, are mainly distributed in the relatively flat terrain between the Yangtze and Han; 2) Agricultural space continues to be occupied by ecological space and urban space and declines. Although ecological space is also continuously occupied by urban space, it is increasing due to the simultaneous occupation of agricultural space, and urban space continues to expand and shows a rising trend year by year; 3) The transformation of agricultural space into ecological space has the largest scale; the transformation of agricultural space into urban space has appeared to varying degrees in each administrative unit and has become the mainstream trend of spatial transformation in most administrative units; the conversion of ecological space into agricultural space and towns space is also significant; less urban space is transformed into agricultural space and ecological space; 4) Natural geography conditions, location conditions, human history, and other factors work together to determine the distribution characteristics of the "three-zone space" in the Jianghan Plain; the national and provincial governments have issued a series of policies to provide for the expansion of the ecological space and urban space in the Jianghan Plain. But institutional safeguards and management methods still need to be further explored in terms of bottom-line control; the sharp increase in urban and rural population has become a powerful driving force for urban space expansion and agricultural space compression; changes in the industrial structure. The correlation mechanism between the change of industrial structure and the spatial structural adjustment it brings is very obvious. The adjustment of the proportion of the three industries provides a strong guide for the expansion of urban space and ecological space.