Appropriateness Evaluation and Impact Factor Identification of Dongting Lake Under the Concept of "three living"

. The "three living" space is an important guideline for the rational optimization of national land space in China. In the process of implementing the project of returning fields to lakes to solve the shrinking surface of Dongting Lake and the various ecological problems arising from it, livelihood issues such as land use conflicts have emerged. In order to balance the relationship between people and lake under the policy, the paper starts from the "three living" space, and evaluates the appropriateness of the Dongting Lake by constructing the index system of the "three living" functional space of the Dongting Lake and combining Arcgis 10.7 software to superimpose the index factors to evaluate the spatial function of the Dongting Lake. The spatial variability of the index factors is also analyzed by geographic probes to reveal the suitability characteristics of the returned land to the lake. Based on the results of the study, it can reduce the conflict of land use in the return of Dongting Lake, avoid the loss of production and living caused by the return of land to the lake to the greatest extent, and enhance the ecological benefits of the return of land to the lake, provide a reference for the implementation of the Dongting Lake return of land to the lake project.


1.Introduction
The current ecological problems such as spatial rupture of ecological environment and degradation of ecosystem function in the context of urban expansion have attracted great attention from the state.For this reason, China puts forward the important ecological protection strategy of building a living community of mountains, water, forests, fields, lakes and grasses, while ecological, production and living spaces together constitute a multifunctional spatial complex，which is also known as the "three lives" space [1] .Several Opinions of the Central Committee of the Communist Party of China and the State Council on Establishing a Territorial Spatial Planning System and Supervising its Implementation, issued in 2019, emphasize the scientific layout of production space, living space and ecological space, and strengthen the guiding and constraining role of territorial spatial planning [2] , under the policy guidance, China has started ecological restoration projects such as returning fields to lakes and wetlands, however, the limited and competitive nature of spatial resources and the spillover of spatial functions have caused land use conflicts [3] .It has become a very important proposition to balance the relationship between people and land, to reasonably return the land to the lake, to minimize the land conflicts caused by the encroachment of production and living land, to coordinate the development of the "three living" spaces, to optimize the pattern of spatial development and protection of the national land, and to improve the quality of spatial development and land use efficiency.
At present, scholars at home and abroad focus on theoretical framework [4] , policy interpretation [5] , land use function relationship [6] , land use suitability evaluation [7] , land carrying capacity [8] , and remediation of hollowing out [9] , including national, provincial, municipal, county, and village scales.In regard to the issue of returning land to lakes, scholars at home and abroad have conducted studies on policy interpretation [10] , ecological compensations [11] , and impact on biodiversity [12] , but there is still a lack of comprehensive studies on returning land to lakes and the "three living" space.The "three living" space is a national classification of land space in terms of land attributes and land functions, which guides the development and planning of the land, while the essence of returning land to lakes is the re-division of land.Therefore, this paper takes the concept of "three lives" as the guide, constructs the evaluation system of the suitability of the land returned to the lake, analyzes its driving factors and its influence mechanism, and provides a theoretical basis for the formulation of important ecological planning such as the ecological protection project of the Dongting Lake.

Study Area Overview
Dongting Lake, formerly known as Yunmeng, is the third largest lake in China and the second largest freshwater lake in China, located at the junction of Hunan Province and Hubei Province.It spans 28°59′-29°38′ N latitude and 112°43′-113°15′ E longitude, with a surface area of 190,000 hectares, east of Yueyang City and south of Changsha City.It was approved as a national nature reserve by the State Council in 1994, and is also one of the internationally important wetland reserves approved by the state.
As the main flood storage lake in the middle reaches of the Yangtze River, Dongting Lake has lost its flood storage function due to the massive encroachment of farmland on the lake surface and the accumulation of sand against the shore year by year, resulting in the phenomenon that the lake outside the lake is higher than the farmland inside the lake.The shrinkage of Dongting Lake is obvious due to the perennial compression of living and production space, and three main lakes are gradually formed: East, South and West.Among them, East Dongting Lake is the largest and most important part of the three lakes, and the interconnected small rivers and drainage and irrigation ditches form a vast lake area.Therefore, this study selects East Dongting Lake as the research object (all references to Dongting Lake in the following refer to East Dongting Lake) and constructs an evaluation system for the suitability of the "three living" functional space for returning fields to lakes, so as to provide a theoretical reference for the local land return projects.In order to explore the development of the land around it, a rectangle of 7.5KM east-west and 10km north-south around Dongting Lake is used as the study area in this study.

Data source and processing
In this study, the 2021 land use data of Dongting Lake were obtained from the official website of the United States Geological Survey (USGS) by parsing the 2021 Landsat8 remote sensing images; the digital elevation data of Dongting Lake were obtained from the 30M resolution DEM data in the geospatial data cloud platform; Soil type data were obtained from the Global Soil Database at 30M resolution; population density data were obtained from the World Population 2000-2020 national population dataset; traffic data were downloaded from the Open Street Map (OSM); and other data were obtained from official government websites.All data were processed and analyzed based on the AreGIS10.7 platform.

Indicator system construction
Following the principles of "systematic integration, regional differences and precise evaluation", the land was divided into three types of land use: production, living and ecological, based on the concept of "three lives".The suitability of production land is mainly measured by two indicators: suitability of agricultural production and convenience of agricultural production, and the suitability of agricultural production is evaluated by five indicators: soil organic content, soil sand content, slope, slope direction and elevation [13] , taking population density and distance from construction land as the evaluation indicators of convenience of agricultural production, in conjunction with the "double evaluation" (evaluation of resource and environment carrying capacity and evaluation of suitability of national land spatial development) indicator system and the cultivation of reed and rice, the main crops in Dongting Lake; The suitability of living space mainly includes two factors of living convenience and living safety, based on which the distance from the town, distance from the traffic artery, slope and elevation are selected as evaluation indexes respectively; the suitability of ecological land selects two factors of ecological safety and ecological vulnerability, and in order to alleviate the ecological problems of Dongting Lake, such as flooding and soil erosion, the risk of flooding and soil erosion are selected as evaluation indexes of ecological safety (Table 1).In addition, ecological vulnerability indicates the degree of ecosystem response to environmental changes due to the combined effect of internal and external factors, and can directly reflect the vulnerability of the ecological environment to external disturbances.

Indicator normalization and weighting
As each indicator has different attributes and units, it is necessary to normalize each indicator by using extreme difference standardization so as to eliminate the influence of inter-indicator dimension and make it comparable, and to classify and assign values to each indicator according to existing studies.See formula (1)： In the formula，  for normalized data， 0  1， X i is the original data，X max and X min are the maximum and minimum values of this indicator, respectively.
The hierarchical analysis method is used to determine the weights of each indicator, construct the interaction matrix between each layer of indicators, invite 22 experts in multiple fields such as landscape ecology, land use and spatial geography to score the evaluation hierarchical model of indicators using the 1-9 scoring method, and finally use YAAHP software to determine the weights of each indicator and pass the consistency test, and finally get the evaluation indicator weights that satisfy the consistency test.See equation (2) (3): In equation (2) (3)， denotes the nth root of the product of the elements of each row of the discriminant matrix, and n is the number of evaluation indicators. is the scalar value obtained by comparing the relative importance of the i-th factor with the j-th factor, and W j1 is the calculated factor weight.

Geographical probe analysis
Geographic probes are statistical models for analyzing spatial differentiation and exploring the drivers of factors [15] , While traditional models are based on the assumption of linearity and covariance of variables to detect direct factor interactions, geographic probes use the sum of variance of sub-regions to explore whether there is spatial heterogeneity in the total variance of the region.The geographic detectors mainly include four detectors: divergence and factor detection, interaction detection, risk area detection and ecological detection, and this paper mainly uses divergence and factor detection and interaction detection for analysis.

Divergence and factor detection
Divergence and factor detection are used to identify the driving force of each indicator on the suitability factor of the retired land to lake, see equation (4 Where, q is the influence value of each indicator factor on the suitability of land returned to lakes; N is the number of samples in the detection area; N h is the number of stratified samples; L is the number of driving factor samples; σ 2 is the variance of the study area as a whole; σ h 2 is the variance of stratum h; the range of q is [0,1].SSW and SST are Within Sum of Squares and the Total Sum of Squares, respectively.

Interaction Detection
Interaction detection is the q-value when two driver indicators act simultaneously, and is used to determine whether the strength of the interaction between two drivers is enhanced or weakened.5 cases of nonlinear weakening, single-factor nonlinear weakening, two-factor enhancement, mutual independent action and nonlinear enhancement may occur between the indicators, and interaction detection between multiple factors is an important complement to single-factor detection.

Results and Analysis
4.1 Analysis of the suitability of the spatial function of the "three lives" of the retired fields of Dongting Lake to the lake From the results of agricultural production suitability evaluation (Figure 1-a、b、c), although the eastern part of Dongting Lake has a higher degree of urban and rural development and more convenient agricultural production, the soil organic content is low and the soil is infertile, so the spatial suitability of comprehensive production in this area is low.However, this part of the site is at a low altitude and is always at the junction of land and water, with infertile soil and high fine sand content, this part of the site is considered unsuitable for production activities.The production space level 4 and 5 suitable areas are mainly distributed in the northwest side of Dongting Lake and the east side of Yueyang City.The tertiary suitable area for production space is scattered on the west and south sides of Dongting Lake, where the soil has a high chalk content and is suitable for reed and rice cultivation.The areas of production space I and II are mainly distributed along the shoreline of Dongting Lake, which are affected by water scouring and seasonal changes all year round leading to the low suitability of production space in this part of the lake.
The overall life security within the study area is relatively high (Figure 1-d、e、f), mainly due to the relatively flat terrain of the whole land with little elevation difference.However, the life safety of some mountainous and forested land on the north and northeast sides of Dongting Lake is not enough to guarantee.In addition, it can be seen from the figure that the areas with the highest spatial suitability for living are located near the towns and in areas with well-developed road networks.The areas with medium suitability for living land are mainly located in areas with flat terrain and low urbanization level, and this part of the area has good development potential.The areas with medium suitability for living land are mainly located in areas with flat terrain and low urbanization level.
Rainwater and flood problems in Dongting Lake are mainly concentrated in the west, south and southwest of the lake.Especially on the south side of Dongting Lake, when encountering 10-year, 20-year and 100-year floods, there will be different degrees of inundation, and the whole area will be flooded when the 100-year rainfall comes, so this part can be considered for priority return of fields to the lake.From Figure 1-g、 h、 i, it can be seen that the ecological security of the western and northern sides of Dongting Lake is low, and it can be seen that the ecological vulnerability of the plain area on the eastern side of Dongting Lake is higher.From the suitability of the overall ecological space, we can see that the suitability of the north and south sides of Dongting Lake is poor, and there are some areas with poor ecological suitability on the west side of the lake, which are mainly caused by rain and flooding.

Analysis of the Suitability Drivers for
Returning Land to Lake in the "Three Life" Functional Space of Dongting Lake

Key Driving Factors
The p-values of population density index and distance to construction land index are greater than 0.05, indicating that these two indexes are not representative.Its The influence (q) values of other factors are, in descending order, rainfall flood risk > soil erodibility > soil chalk content > soil organic content > elevation > slope > distance from town > vegetation cover > distance from road > land use > slope direction (Table 2).Among them, the indicator of rainfall and flood risk has the largest q value of 0.172, which is consistent with the main purpose of returning the fields to the lake, i.e., to solve the ecological problems.In addition, it was found that the driving factors of ecological indicators were generally higher than those of production and construction indicators, indicating that the main influencing factor of the suitability of returning land to lakes is the comprehensive ecological environment, which is not significantly influenced by the artificial built environment, guided by the "three living" concept.

4.2.2Interaction of driving factors
The interactions between the 11 effective driver indicator factors were examined using the exchange detector, and as shown in Table 3, the q-values of the interactions of any 2 indicator factors were greater than the q-values of a single indicator factor, indicating that the interaction detection results between each indicator factor were enhanced.The superposition of any two indicators will enhance their explanatory power on the suitability of the "three living" space of Dongting Lake, which proves that the suitability of the land returned to the lake based on the "three living" spatial function is subject to the combined effect of multiple factors (Table 3).The interaction between the risk of rainfall flooding and soil erodibility was the strongest (0.317), and the risk of rainfall flooding and soil chalk content was the second strongest (0.312), but the interaction between soil chalk content, soil organic content, and soil erodibility was not significant and not much different from the single factor detection, The interaction enhancement of soil erodibility and soil chalk content was the weakest, with single factor q-values of 0.153 and 1.150, respectively, while the q-value for interaction detection was 0.154.The main reason for this is that all three elemental attributes are determined by soil properties, and the gap between the same soil properties is not obvious, However, the interaction of elevation factor with soil organic content and soil chalk content is greater than its interaction with the risk of rainfall and flooding because the site elevation is closely related to the risk of rainfall and flooding, while the association between it and soil properties is not significant, which leads to its interaction with the risk of rainfall and flooding will be more significant.The results of interaction detection further prove the dominance of ecological index factors in the evaluation of the suitability of receding fields to lakes based on the "three living" space, and also indicate that the risk of rainfall and flooding, soil erodibility, soil chalk content and soil organic content are the main influencing factors of its spatial variability.By constructing the evaluation system of the suitability of the "three living" space of Dongting Lake and using the methods of hierarchical analysis and geographic probe, we can conduct a comprehensive evaluation of the index of the "three living" space of Dongting Lake and analyze its intrinsic driving factors, and draw the following conclusions: (1) 5.54% of the area is suitable for level 5 and 12.29% is suitable for level 4, which is the area with the highest priority for returning fields to the lake (Figure 2).The ecological suitability of Dongting Lake can be improved to the greatest extent after returning fields to the lake, and in addition, it will not affect the social development and agricultural production.It is mainly located in the north and south sides of Dongting Lake, and most of the land in these two areas are mudflats and paddy fields formed by the impact of water flow, which are threatened by the risk of rainfall and flooding all the year round.The area of Level 3 Suitable Area is 1,065.97KM2,accounting for 20.92%.This area generally has high economic and production value, and although returning fields to the lake can effectively improve its ecological environment, it is not conducive to the comprehensive and sustainable development of the society.Level 1 suitable area and Level 2 suitable area account for 36.46% and 24.79% respectively.The ecological value of this area is low, and the production and living value is high, so it is the most unsuitable area for returning land to the lake.The ecological value of this area is low, and the production and living value is high, so it is the most unsuitable area for returning land to the lake.(2) The ecological function of the land use space plays a key role in the suitability of the "three living" space for returning land to the lake in Dongting Lake.Among them, the risk of rainfall and flooding and soil erodibility are the strongest explanatory factors for the suitability of returning land to the lake, with 0.172 and 0.153 respectively, and these two indicators can be taken into account when constructing the relevant evaluation system in the future.The development level of artificial construction has a complementary effect on the overall suitability evaluation, but there is no decisive influence.The least influential factor is population density, with an influence factor of only 0.001.(3) The relationship between ecological factors, production factors, and living factors will have a joint promotion effect on the suitability of the land for returning Dongting Lake to the lake.Among them, the strongest interaction effect is between rainfall and flooding risk and soil erodibility, which is 0.317.Therefore, the evaluation of the suitability of Dongting Lake for "three lives" space to return land to the lake is based on the results of the joint action of multiple aspects and multiple influencing factors.

Discussion
This paper proposes the development direction of the Dongting Lake land return project based on the suitability of the "three living" space, and analyzes the driving factors affecting its suitability to provide theoretical value for future research.However, the factors affecting the return of land to the lake are complex and diverse, including policy influence, local culture, residents' interests, economic development, cost value, implementation feasibility, etc.This paper is an analysis of the suitability of Dongting Lake based on land use.In the future, we can combine land use with socio-economic, political and cultural aspects to study the implementation plan of returning the land to the lake in a comprehensive manner.

Figure. 1
Figure. 1 Evaluation index of suitability of returning farmland to lake in Dongting Lake

Figure. 2
Figure. 2 Suitability evaluation of the retired fields of DongtingLake to the lake Suitability evaluation results

Table 2
Results of the suitability index factors of the "three living" space of Dongting Lake for returning fields to the lake

Table 3
Interaction detection results of the suitability index factors of the "three living" space of Dongting Lake for returning fields