Diagnosis of Key Ecological Problems and Layout of Ecological Restoration Projects- A Case Study of the Aksu River Basin, China


 The sustainable development of ecological environments is the basis of the sustainable development of inland river basins in arid areas with fragile ecological environments. The aim of our study is to diagnose the ecological problems of the Aksu River basin (ARB) and explore the ecological security pattern (ESP) and the layout and implementation path of ecological protection projects for mountains, rivers, forests, farmland, lakes, grasslands and deserts (MRFFLGD) in arid areas under different security levels. The Aksu River, the largest source of the Tarim River, was selected as the case study. The equivalent factor evaluation method (EFEM) was used to estimate the ecological service value (ESV), and the InVEST model was employed to quantify the ecological function indicators. By constructing the richness index, the ESV of the study area was measured spatially, and the ecological problems in the ARB were identified and diagnosed. Using cold/hot spot detection analysis and the minimum cumulative resistance (MCR) model, the ESPs at three security levels were constructed. The layout of ecological protection measures and the implementation path for MRFFLGD were studied. The results showed that the ESV of ARB showed an upwards trend from 1990 to 2018, and the three services of water conservation, waste treatment and soil formation and retention grew fastest. The value of ecosystem services in the study area in 2018 ranged from 5,390 yuan to 131,080 yuan/hm2. The high-value areas were mainly located in the oasis and the northern mountainous area, and the low-value areas were mainly located in the desert area. The ecological source areas of high, medium and low safety levels were 1806.3 km2, 3416.8 km2 and 4804.32 km2, respectively. Based on the spatial distribution of ESV in the ARB and the preliminary diagnosis of the basic ecological problems in the study area, three kinds of ecological protection engineering models, namely, conservation, natural restoration and assisted regeneration, were proposed for the four types of ecological protection zones. To provide a decision-making basis for ecological environmental management and regional sustainable development in the ARB, the optimization patterns of points (6 key ecological protection projects), lines (ecological corridors) and polygons (ecological source areas) were constructed. The results of this study can also serve as a reference for ecological environmental protection in other arid inland river basins.

environmental management and regional sustainable development in the ARB, the optimization patterns of points (6 key ecological protection projects), lines (ecological corridors) and polygons (ecological source areas) were constructed. The results of this study can also serve as a reference for ecological environmental protection in other arid inland river basins.

Background
Ecological security guarantees that the ecological environment realizes regional security and social stability 1 . In 2012, the 18th National Congress of the Communist Party of China (CPC) prioritized eco-environmental progress and pursued green development under the guidance of Xi Jinping's thought on eco-civilization. Xi noted that "building the pattern of national ecological security" is an important approach for the construction of an ecological civilization 2 . Ecological security is of vital importance in national security 3 . The state attaches great importance to national spatial planning and ecological protection and restoration projects for MRFFLGD 4,5,6 . The implementation of the construction of territorial ESPs involves the identi cation of key areas, such as ecological sources and corridors, that play a key role in the protection and maintenance of the ecosystem in a region 7 . The spatial allocation of key ecological protection and restoration projects based on ecological security issues is also an important way to "optimize the ecological security barrier system, build ecological corridors and biodiversity protection network" according to the 19th National Congress of the Communist Party of China. The health degree of MRFFLGD is the external expression of the soundness of the ecological security barrier system. Therefore, the optimization of ESPs and comprehensive ecological protection engineering measures have become an important means to solve regional ecological environmental problems. It is also the premise of orderly ecological environmental protection work 8 . The optimization of ESPs is to maintain the integrity of ecosystem structure and function and to provide comprehensive ecosystem services for a region 9 . The optimization of the ESP of an inland river basin in an arid area is the key to the sustainable development of oases and the national ecological security strategy 10 . Based on the evaluation of ecosystem service value (ESV) in the ARB, the richness index was used to measure the ESV in space and then identify and diagnose ecological environmental problems in the study area. It is of great practical signi cance to study the optimization of the ESP and the layout of ecological protection/restoration projects in the ARB according to the objectives of ecological protection planning.
There are several studies on ESP optimization based on the pattern of "ecological source identi cation -resistance surface construction -ecological corridor construction". The MCR model 9,11,12,13,14,15,16 , circuit network model 17,18,19,20,21,22,23 , and multiple combination model 24,25,26 are widely used. Related studies on the construction and optimization of ESPs are aimed at cities and urban areas 16,22,27,28 , and more research is focused on natural ecological units such as the Loess Plateau 29 and Qinghai-Tibet Plateau 30 , Shule River basin 31 , Poyang Lake 24 , and Chao Lake 25 . Furthermore, at the practical level, the demarcation of ecological protection red lines 9,32 , ecological environment restoration 23 and the construction of ESPs of nature reserves 30 are common in the study of ecological environments. As the natural environment and utility on which human beings depend for survival, it is of great signi cance to scienti cally evaluate the spatial and temporal patterns and interaction mechanisms of ecosystem services to determine the basis of the regional ecological environment and construct regional ESPs. In recent years, there has been an increasing number of studies based on ecosystem service assessments 33,34,28 .
Therefore, most studies have used the equivalent factor method (ESV unit area of Chinese terrestrial ecosystems), which was improved by Xie 35 based on Costanza's 36 research, to estimate the ESV. However, the estimated value of ecosystem services by Xie 35 was static rather than dynamic, and the estimated results did not re ect the changes in ESV with social, economic and natural environmental conditions well. Although research on ecological security patterns has made much progress, there are still some de ciencies. Building an ESP and guiding regional land and space development is an important technique to ensure regional ecological security and stability to achieve sustainable development 9 . Improving and stabilizing ecosystems plays a fundamental and strategic role in ensuring national ecological security. Hence, it is necessary to carry out a dynamic assessment of ESV at the grid scale to identify ecological environmental problems. Thus, promoting the optimization of ESPs and the layout of ecological protection/restoration projects according to ecological protection planning objectives are essential.
In the zones oriented toward the National Plan for Major Function, "two barriers and three belts" are the main parts of the national ecological security strategy. On the basis of this strategic pattern, ecological protection and restoration projects of MRFFLGD can promote the maintenance and restoration of ecological space and ecological function. The ARB belongs to the important northern sand control zone in the "Three Belts", and it is also the water conservation area of the Tianshan Southern vein and the agricultural product supply function area of the Aksu Oasis. Additionally, the "Ecological Protection and Restoration Project Guide of Mountains, Waters, Forests, Fields, Lakes and Grass" noted that "the systematic integrity of the natural ecosystem should be comprehensively considered, and relatively complete natural geographical units such as rivers, lakes, basins and mountains" should be the focus of ecological protection and restoration. The ARB has a unique mountain-oasis-desert ecosystem, forming a unique landscape and spatial pattern in arid areas 37 . The Aksu River provides 74% of the water of the Tarim River and occupies an important position in the ecological security pattern of the Tarim Basin 38 . In recent years, with the continuous expansion of the Aksu arti cial oasis area, the fragile ecological environment has been affected to some extent, and soil deserti cation, grassland impoverishment, soil and water loss and other ecological security problems have become worrying.
Thus, the main objectives of this paper were to (1) measure the dynamic ESV at the grid scale; (2) identify and diagnose the ecological problems in the ARB; and (3) construct a network ESP that varies by point (ecological sources) -line (ecological corridors) -polygon shape to high -middle -low levels (different security levels). The results provide a reference for optimizing the ecological spatial structure, ensuring regional ecological security and promoting regional sustainable development in ARBs.

Materials And Methods
Study area.
The ARB is located in the alluvial plain of the Northwest Tarim Basin (Figure 1). It is the main stream of Tarim river.
The terrain features high values in the north and low values in the south and tilts from northwest to southeast. The Aksu Oasis is an oasis developed by the Aksu River with a typical temperate continental climate, drought and little rain, intense evaporation, long sunshine duration and large temperature difference between day and night. In this region, agriculture is a typical oasis irrigated agriculture that completely relies on surface irrigation projects and groundwater extraction. It is an important production base for grain, cotton, melon, and fruit crops in Xinjiang . In recent years, economic development in the Aksu area has led to a signi cant increase in water consumption and water demand in the basin and a continuous decline in groundwater level, which has affected agricultural production and ecosystem security, resulting in the gradual degradation of important ecosystems .
Data resources.
The data in this paper are mainly divided into the following categories: The statistical data: The statistical data used in this study are mainly from the statistical yearbook of Aksu, the statistical yearbook of Kizilsu Kirgiz Autonomous Prefecture.
The land use data (1990,2005,2018), net primary productivity (NPP) data, digital elevation map data, climatic (temperature, precipitation) raster data, and normalized vegetation coverage degree data (NDVI) used in this study are mainly from the Resource and Environmental Science and Data Center of the Chinese Academy of Sciences (available at http://www.resdc.cn). The main types of land use are cultivated land, forestland, grassland, water area, unused land, construction land and corresponding to "China's terrestrial ecosystems services equivalent unit area" of farmland, forest, grassland, water, and unused land, with 5 kinds of ecosystem types, construction land neglected, and the value of ecosystem services set to 0.
The soil thematic data: The soil project data used in this study are mainly from the Chinese Soil Science Database (available at http://vdb3.soil.csdb.cn/). The spatial resolution of the above data is 1 km*1 km.
Ecosystem service value calculation.
Ecosystem services (ESs) refer to the bene ts obtained directly or indirectly from ecosystems for human survival or improvement of quality of life , and the quantitative evaluation of their value is of great signi cance to maintaining regional ecological security and promoting the coordinated development of the regional economy and environment 10  1) Modi cation of the biomass factor based on regional differences using the value of 0.58 in Xinjiang .
2) Revision of the coe cient of social development. An ESV is closely related to social and economic development.
Each function of human well-being that ESs ow from natural capital is not immutable; it functions through interaction with human capital, social capital and built capital 36 . Considering the regional social and economic development level of the ARB, the ESV coe cient was adjusted a second time. The S curve model was used to characterize its development trend , as shown in Equations (2) and (3).
where l represents the social development coe cient corresponding to the willingness to pay for functions provided by the real ecosystem; L represents the willingness to pay for ecosystem functions under a state of abundant wealth, herein regarded as 1; e is the natural constant; and E n is the Engel coe cient.
where l' and l represent the average willingness to pay for functions provided by the real ecosystem of Xinjiang and the ARB, respectively.
According to Equations (2) and (3), the revised standard equivalent factor value is shown in Equation (4): The ESV coe cients of different land use types are calculated as follows (Equation 5): where VC ki represents the coe cient of ecosystem service type k of land use type i. EC ki is the equivalent factor of ecosystem service type k of land use type i. The sum of ESV of n types of land use is shown in Formula (6) where A k is the area of land use type i. Using the above formula and the area of 5 land use types in the ARB in 1990, 2005 and 2018, the ESVs for 5 land use types could be calculated.
To verify the rationality of ecosystem service value estimation, it is necessary to examine the sensitivity of ESV to changes in VC. Therefore, the coe cient of sensitivity (CS) was used to quantify the degree of in uence of ESV on changes in VC ,28 . The CS is shown in Formula (7): where ESV i and ESV j are the values of ecosystem services before and after adjustment, respectively, and VC ik and VC jk denote the value coe cients for land use type k before and after adjustment, respectively. When CS>1, it indicates that ESV is elastic relative to VC; when CS<1, it indicates that ESV is inelastic relative to VC, and the estimation result of ecosystem service value is credible.
Spatial measurement of ecosystem service value.
In this paper, we propose the spatial importance index Ri of ESs, which avoids the traditional method of estimating ESV based on the homogeneity distribution of various ESs on the same land use type and protects the sensitive function of ESV. Therefore, this paper used 2018 as an example to measure the spatial distribution of ESV in the study area.
The spatial accuracy of the ecological service index data in the study area was set as a 1 km×1 km grid, and the indices of each ecosystem service function indicator were quanti ed in grid units to construct the spatial importance index of ecosystem services, as shown in Table 1. Standardizing the importance index of each service function. The larger the value is, the more important the ecological service space value referred to by this index is (Equation 8): where Ri represents the spatial importance index of the i-th service in the k-th grid, C ki is the spatial aggregation degree of the i-th service in the k-th grid, Ci is the mean spatial aggregation value of the i-th service, and n is the number of grids.
After adjusting the importance index and combining it with the total area of the study area, the revised spatial grid map of each single ESV was obtained, and the spatial distribution map of each single ESV was superimposed to obtain the total spatial distribution map of ecosystem service value in ARB in 2018 Equation 9.
where ESV is the revised total ESV, ESV i is the ESV of item i, and A is the total area of the study area. Table 1 Key ecosystem service indicators and their quanti cation in the ARB Construction of ecological security pattern.
The construction of ecological security patterns is an important research topic in landscape ecology 45 , guiding the practice of ecological civilization. The construction of ESPs is based on the diagnosis of ecological problems, guided by ecological protection objectives, and the use of scienti c methods to identify ecological sources and ecological corridors. Key ecological protection projects should be planned and laid out in view of ecological problems, which is the perfect combination of theoretical and practical aspects of ecological security barrier system construction. This paper is based on the spatial measurement of ESV in the ARB. The cold-hot analysis method was employed to identify ecological source areas under high, medium and low ESPs, and the corresponding ecological corridors of high, medium and low ecological source areas were calculated by the cumulative minimum resistance model (MCR) 32,11,12,13,14,15,16 . Considering the community with a shared future of MRFFLGD, ecological problems were diagnosed in the low-value area of a single ESV. The ESP of the ARB was obtained by studying the layout of key ecological protection projects combined with ecological protection zones.
Identi cation of ecological source.
Ecological sources are the source points of species diffusion and maintenance, and they can provide key ecological services, provide continuity and integrity of landscape patterns, and prevent ecosystem degradation 29,32 . Based on ArcGIS cold/hot spot analysis tools, we extracted ARB ecosystem service values of 99%, 95%, and 90% of the hot spots in the space as low-, medium-and high-level ecological security sources. The connotation of ecological source areas is the continuity and integrity of the landscape, which must reach a certain scale to effectively isolate the external interference to the source area 11,12,13,14,15 . In this study, patches less than 0.5 km 2 were removed as ecological sources 10 .  49 . USLE = R * L * S * C * K P, RKLS = R * L * K S. R is rainfall erosivity factor, K is soil erodibility factor; LS is slope length & slope factor; C is coverage and management factor; P is the factor of soil and water conservation engineering measures. where MCR is the value of minimum cumulative resistance, f min represents the positive correlation between the minimum cumulative resistance and the ecological process, D ij is the spatial distance between ecological source unit j and landscape unit i, and R i denotes the resistance coe cient of landscape unit i to species movement.

Results
Analysis of ecosystem service value change.
The economic value of natural grain yield per unit farmland in 1990, 2005 and 2018 in the ARB was 973.73 yuan/hm 2 , 1473.78 yuan/hm 2 and 2588.42 yuan/hm 2 , respectively. The annual average economic value was 1678.64 yuan/hm 2 . After two coe cient modi cations (biomass factor adjustment and social development coe cient adjustment), the ecological service value coe cient applicable to the study area was obtained, and the results are shown in Table 2. Table 2 Ecosystem service value (ESV) coe cients per unit area of different land use and land cover change types in the ARB, 2018 According to the area of each type of land use in the study area, the value of each ecological service and its change during the study period were obtained ( From 1990 to 2018, the value of water conservation grew the fastest, with an increase of 4.7 billion yuan. The values of food production, raw materials and entertainment and culture ecosystem services were the lowest, and the total value of these three ecosystem services was approximately equal to the value of biodiversity conservation. From 1990 to 2018, the average annual change rate of each ESV in the ARB was 31.84%, showing a fast growth rate.
In 1990, 2005 and 2018, the change rate of ESV increased year by year. Signi cant differences in value were identi ed among the four functions. From 1990 to 2018, the growth rate of ESV of water conservation, waste treatment, and soil formation increased by approximately 160%, while the growth rate of raw material and food production remained at approximately 40%. During the study period, the change rate of water conservation ESV was the fastest, with a growth rate of 251%, while the raw material value was the lowest, with a growth rate of 17.29%.
Overall, the growth rate of ESV from 2005 to 2018 was more than three times that from 1990 to 2005.  As shown in Table 4, the coe cients of sensitivity of ecosystem services in the ARB in 1990, 2005 and 2018 were all less than 1, indicating that the calculated results of ecosystem services were inelastic and that the research results were reliable. Table 4 Results of the sensitivity coe cient in the ARB Priority index of ESV.
The priority coe cients (Ri) of ESs in the ARB are shown in Table 5. The Ri values of water conservation, waste treatment, soil formation and biodiversity conservation were above 1.3, which showed the rapid rate change of the Ri. The rate of change of Ri showed that food production and raw materials were relatively weak. The importance coe cient of food production and raw materials was less than 1/3 of the water conservation function.   and Gobi in eastern Aksu Oasis. It is worth noting that scattered low-value areas of ecosystem service value existed in oases, such as urban and rural residential land, and industrial and mining land were identi ed as low ESV areas. Due to its large population density per unit area, land use intensity and ecological environment pressure are relatively high, so the ESV is low.
Ecological security pattern construction based on ESV.
The cold/hot spot detection method was used to extract alternative ecological source areas of low, medium and high safety levels in the ARB. The corresponding ecological corridors of low, medium and high ecological security sources were obtained by the MCR model, as shown in Figure 4. The three levels of ESPs are the key to guaranteeing the ecological security of the ARB. This work can provide some reference for ecological environmental protection in the ARB, e.g., the alluvial fan plain between the Telan River and Kalayurkun River in the eastern part of the oasis. Due to its large evaporation and strong in ltration of surface water, the phenomenon of river ow interruption is serious in the dry season, and the river in this region should be considered in the dry season. Therefore, ecological corridors should be constructed to realize the effective connection of ecological sources and improve the ecological security of the whole region.
The ecological problems were diagnosed in the ARB.
Based on constructing the ESP and the value of each ESV indicator, the main ecological environmental problems, such as the degradation of vegetation, water and soil loss, degradation of soil quality, and environmental degradation, were diagnosed. We decomposed the ecological issues to the community with a shared future of MRFFLGD in each factor. In this paper, 8 kinds of ecological issues were selected to construct the diagnostic index system of ecological problems in the ARB (Table 6). According to the results of our quantitative assessment of ESs, the value of ecosystem functions was divided into 3 grades from high to low, which indicate the risk level of ecological problems. From the perspective of ecological protection and restoration of MRFFLGD, according to the number of restoration objects of MRFFLGD involved in each ecological index, the index weights were determined, and the corresponding ecological problems of each type of land were taken into account to divide the risk level. Ecological protection zoning and key engineering measures.
We started with the quanti able indicators corresponding to ecosystem service functions, the ecological problems in the ARB were identi ed, and the ecological risk levels of the corresponding regions were divided. The ecological protection zones of the ARB were divided into the following four categories: windbreak sand xation zones, water and soil conservation zones, water resource conservation zones and agricultural product ecological security zones ( Figure 5). The ecological protection zoning delineated in this paper was basically consistent with the ecological environment protection planning of the Aksu River basin (2015-2025), indicating that the ecological protection zoning delineated in this study was reasonable. Key project of river regulation in irrigation area The quantitative indices in Figure 2 and Table 5 were multiplied and superimposed to obtain the main ecological high-risk distribution areas of various land uses in the study area ( Figure 6). The key ecological engineering layout of the study area was carried out on the basis of the distribution area of soil and water loss area, insu cient water resource conservation area, insu cient water resource carrying capacity area, vegetation degradation area, barren grassland area, poor distribution area of cultivated land quality (soil organic matter content <0.5 g/kg), habitat quality <0.1, and deserti cation high-risk area.
To guide the ecological protection and restoration of MRFFLGD, we suggest that each type of ecological protection and restoration unit in the study area should adopt the protection and restoration mode of conservation, natural restoration and auxiliary regeneration 51 . Therefore, in view of the ecological environmental problems and the spatial distribution of high-risk areas in the ARB, we further put forward suggestions on the spatial distribution of key engineering measures in the area of ecological protection (Figure 7, Table 7). Table 7 Ecological protection zoning and key project measures in the ARB Key project of water and soil loss prevention and control: In soil and water conservation areas and areas with serious soil erosion, digging desert vegetation and destroying forests and meadows are strictly prohibited. The ecological environment should be protected around river banks, reservoirs and permanent snow areas. Overloading of grassland is strictly prohibited, and the project to return grazing land to grassland should be vigorously promoted.  13 . It is noteworthy that value input is not considered in most studies on ecosystem service valuation. We pay too much attention to the human welfare provided by ecosystem services, and the input part is seriously neglected. For example, the material and human cost of planting should be excluded from food production, which is also the main reason for the high estimated value of ecosystem services.
The effectiveness of constructed ESPs.
The construction of ESPs has a certain reference value in regional overall ecological security and ecological environment protection. The ecological restoration measures proposed by us can actively restrain local ecological degradation. However, the investment and scale of each ecological restoration project need further research to ensure the landing and implementation of each ecological restoration and protection project 4 . Landscape connectivity should also be considered to explore the multifunctional potential of local landscapes and enrich the elements and functions of ecological security barriers on the basis of ecological protection. It not only improves the supply capacity of ecosystem services but also promotes the coordinated development of ecological protection and the regional economy 49 . Limitations and future work.
The value evaluation method based on the value equivalent factor of unit area has the advantage of facilitating the comparison and analysis of the dynamic change in value among different ecological service functions. In this paper, we adjusted the service value of the ecosystem per unit area and effectively improved the value evaluation results based on the equivalent factor method. In addition, this paper considered only the evaluation of land use classi cation results 17 . A more detailed classi cation of land types would be more conducive to improving value assessment and deepening our understanding of the value of different ESs 34,48 . Furthermore, human activities are becoming increasingly diverse and disorderly, especially in arid areas, which affect ecosystem services more obviously. Spatial quantitative analysis and index re nement research on different human activities and ESs should be strengthened in future studies 37 .

Conclusion
Taking the ARB as the research object, this paper estimated the value of ecosystem services by using the equivalent factor method, constructed the priority index, quanti ed the ecosystem services function index based on the InVEST model, and measured the spatial value of ESs. Based on the reconstruction results, the ESP at three safety levels was established by using cold-hot spot detection analysis to identify the ecological sources and the MCR model to calculate ecological corridors, and the ecological environmental problems diagnosed in the ARB were studied to carry out the layout of ecological protection/restoration engineering measures. The main conclusions were as follows: (1) the ESV of the ARB increased from 1990 to 2018, and the following three ecosystem service functions increased the fastest: water conservation, waste treatment and soil formation. (2) The reconstructed ESV ranged from 5390 yuan/hm 2 to 131,080 yuan/hm 2 , and the high-ESV area was mainly concentrated in the oasis and northern mountainous areas, while the low-ESV area was mainly concentrated in the desert area.