Evaluation of the Importance of Ecological Service Function and Analysis of Influencing Factors in the Hexi Corridor from 2000 to 2020

Abstract

The Hexi Corridor plays a pivotal role in safeguarding China’s ecological security, functioning as a crucial conduit between economic and ecological systems. This study employs a multidisciplinary approach, integrating climate, evapotranspiration, and other variables, to analyze the trend in factors influencing ecological function and to evaluate the import of ecological service functions in the Hexi Corridor from 2000 to 2020. The findings reveal a distinctive spatial distribution pattern for regulating functions, with higher concentrations observed in the southern regions and lower concentrations in the northern regions. These functions include the storage of carbon, quality of habitat, the conservation of water, and soil and water conservation. It can be observed that the areas of general importance for ecosystem services are predominantly distributed across the northern and western sections of the Hexi Corridor, collectively representing 76.96% of the total area. Conversely, areas of general importance for ecosystem services are situated in regions characterized by a high altitude, intricate topography, and extensive glaciers as well as permafrost.

1. Introduction

Ecosystem services (ESs) are defined as the direct or indirect beneficial outcomes produced by natural ecosystems that satisfy human needs and contribute to human well-being [1]. In recent years, a number of scholars have assessed the ecosystem environment. André proposed biophysical quantities to evaluate the ecosystem services of forests in Europe [2], Talbot examined the functioning of aquatic ecosystem services to discern the impacts of flooding on the region [3], and Ilse identified ecosystem services based on the concept of global sustainability [4]. It is essential that the study of the importance of ecosystem services takes into account the specificities of different regions and is adjusted in order to make the evaluation more realistic and reliable.

The Hexi Corridor has been significantly impacted by the utilization of natural resources and the advancement of agricultural techniques, which have collectively shaped the region’s ecological landscape [5]. In particular, the consequences of population growth and the expansion of cultivated areas in the Hexi Corridor have resulted in significant changes to the region’s ecosystem in recent years [6]. Accordingly, a series of inter-related factors, including land, water resources, climate, and other environmental elements, were the primary focus of this study. Additionally, the three primary aspects of integrated water conservation, soil and water conservation, and wind and sand control were used to assess the significance of ecosystem services. Utilizing a long-term data series from 2000 to 2020 as a reference, the influence of natural resources on the regional ecological environment was elucidated to ascertain the rationality as well as sustainability of ecological protection and development strategies [7]. This offers a theoretical foundation for optimizing the configuration of major functional regions and identifying ecological protection zones [8].

2. Data Sources and Research Methodology

2.1. Study Area

The Hexi Corridor (37°01′~42°50′ N, 93°21′~104°05′ E) is located within a region of Northwestern China that is characterized by aridity and semi-aridity. The area in question extends from the fringes of the Loess Plateau, located in the east, to the north, encompassing the Inner Mongolian Plateau. The region is bordered to the south by the Qinghai–Tibetan Plateau and extends west of the Yellow River mainstream (Figure 1). The landmass in question spans roughly 1200 km from east to west and 50 km from north to south, encompassing an area of around 24.78 × 104 km². The Qilian Mountains, which run through the Hexi Corridor, are the largest marginal mountain system in the northeastern portion of the Tibetan Plateau, including a dozen east–west-trending mountains of various sizes and consisting of folded and broken peaks with an elevation of 4000–5000 m. The entire mountain system is broad in both the east and west [9]. Around the Hexi Corridor mountains, interconnected pre-mountain sloping plains are formed by the accumulation of materials carried down by mountain rivers [10]. The Hexi Corridor is of great significance for opening up international corridors to the west, enhancing the level of opening up to the outside world, and leading to the rise of Central and Western China.

2.2. Data Sources

In this study, a range of ecological resource data were selected for analysis, including meteorological data, precipitation, vegetation, soil, and others. The meteorological information was obtained from China’s 1 km resolution annual mean temperature and annual precipitation data from the “National Earth System Science Data Centre”. Land-use type data were obtained from the “Resource and Environment Science Data Centre of the Chinese Academy of Sciences”. The DEM data were obtained from the “Geospatial Data Cloud Platform”. The data are employed principally for the examination of the inter-relationship between disparate elevations and slopes and ecosystem service functions. The soil data primarily encompass soil texture and soil depth data, which are derived from Chinese soil information in the HWSD.

2.3. Method

This study is focused on examining the impact of various environmental elements on the ecological service functions of the Hexi Corridor, with an emphasis on the three principal regulatory roles of water conservation, wind and sand control, and soil and water conservation [11]. The subsequent section outlines the particular research methodologies utilized for each regulatory function and presents the final weighting of the significance assigned to the provision of these ecological services [12].

2.3.1. Water Conservation

The assessment of water yield is founded upon the coupled hydrothermal equilibrium hypothesis, as initially postulated by Budyko in 1974 [13]. The evaluation of this indicator can determine the current and future key areas that are capable of performing the function of water conservation. The formula is as follows:

$$Y_{xj}=(1-{\displaystyle \frac{AE{T}_{xj}}{P_x}})\times P_x$$

(1)

where “Y” denotes the water yield of raster x in category j land use; “AET” denotes the actual annual evapotranspiration; and “P” is the annual precipitation in cell i. The description of the formulae is detailed in the literature [14,15].

$$Retention=\mathit{min}(1,{\displaystyle \frac{249}{Velocity}})\times \mathit{min}(1,{\displaystyle \frac{0.9\times TI}{3}})\times \mathit{min}(1,{\displaystyle \frac{Ksat}{300}})\times Yield$$

(2)

$$TI=({\displaystyle \frac{Drainage\text{\_}Area}{Soil\text{\_}Depth\times Percent\text{\_}Slope}})$$

(3)

where “Retention” is the water retention; “Velocity” is the flow rate coefficient; “Ksat” is the saturated hydraulic conductivity of the soil, which was calculated based on the floor surface, which was composed of clay, powder, and the proportion of sand present in the soil; “TI” is the topographic index; “Drainage_Area” is the number of rasters in the catchment area; “Soil_Depth” is the level of soil depth; and “Percent_Slope” is the slope percentage.

2.3.2. Soil and Water Conservation

The calculation of soil retention was selected from the “Universal Soil Loss Equation” (USLE), which was devised by American scholars Smith and Wischmeier in the 1950s, in order to ascertain the precise level of soil erosion in the specified area, taking into account six key factors. The formula is as follows:

$$A_r=R\times K\times L\times S\times C\times P$$

(4)

$$A_p=R\times K\times L\times S$$

(5)

$$A_c=A_p-A_r$$

(6)

where “A_r” is the actual soil erosion per unit area; “A_c” is the soil retention per unit area; and “A_p” is the potential soil erosion per unit area.

$$R={\displaystyle \sum _{i=1}^{12}1.735\times {10}^{\left[1.5\times \lg \left(P_i^2/P\right)-0.8188\right]}}$$

(7)

$$\begin{array}{l}K=\left\{0.2\right.+0.3\exp \left[ -\right.0.0256SAN1-{\displaystyle \frac{SIL}{100}}\left.\right]\left.\right\}\\ {\left[\right.{\displaystyle \frac{SIL}{CLA+SIL}}\left.\right]}^{0.3}\left[\right.1-{\displaystyle \frac{0.25C}{C+\exp 3.72-2.95C}}\left.\right]\\ \times \left[\right.1-{\displaystyle \frac{0.7SNI}{SNI+\exp (-5.51+22.9SNI)}}\left.\right]\end{array}$$

(8)

$$S=\left\{\right.\begin{array}{cc}10.8\sin \theta +0.03& (\theta <5°)\\ 16.8\sin \theta -0.5& (5°\le \theta \le 14°)\\ 21.9\sin \theta -0.96& (\theta \ge 14°)\end{array}$$

(9)

$$L={\left(\lambda /22.13\right)}^{{\displaystyle \frac{\left(\right.\sin \theta /0.0896\left.\right)/\left[\right.3{\left(\right.\sin \theta \left.\right)}^8+0.56\left.\right]}{\left(\right.\sin \theta /0.0896\left.\right)/\left[\right.3{\left(\right.\sin \theta \left.\right)}^8+0.56\left.\right]+1}}}$$

(10)

where “R” is the rainfall erosivity factor; “P_i” is the average monthly rainfall; “K” is the soil erosivity factor; “SAN”, “SIL”, and “CLA” are the percentages of sand, silt, and clay content, respectively; “C” is the percentage of organic matter content; “L” and “S” are slope length and slope factor, respectively; and “θ” is slope.

2.3.3. Wind and Sand Control

The role of ecosystems in reducing soil erosion caused by wind is to stabilize the soil through their structures and processes [16]. The calculation uses the wind erosion process in the national ecological environment standard of the People’s Republic of China [17]. The formula is as follows:

$$S{L}_r={\displaystyle \frac{2z}{S_r^2}}{Q}_{rmax}\times e^{{-(z/S_r)}^2}$$

(11)

$$Q_{rmax}=109.8\times (WF\times EF\times SCF\times K^{\prime })$$

(12)

$$S_r=150.71\times {(WF\times EF\times SCF\times K^{\prime })}^{-0.3711}$$

(13)

where “SL_r” denotes potential wind erosion; “Q_rmax” denotes the maximum sand transport capacity of potential wind; “S_r” denotes the length of a potential key plot; “z” denotes the calculated downwind distance, and 50 m was taken for this calculation; “WF” is the climate factor; “EF” is the percentage of soil erodibility; “SCF” is the soil crust factor (dimensionless); “K’” denotes the soil roughness factor (dimensionless); and “C” denotes the vegetation factor (dimensionless).

2.3.4. Habitat Quality

The quality of a habitat is contingent upon its accessibility to anthropogenic activities and the extent of those activities [18]. The formula is as follows:

$$Q_{xj}=H_j\left(\right.1-{\displaystyle \frac{D_{xj}^z}{D_{xj}^z+k^z}}\left.\right)$$

(14)

where “Q_xj” is the habitat quality of raster cell “x” in land-use type “j”; “H_j” is the habitat suitability of land-use type j; “D_xj” denotes the degree of habitat degradation; and “K” is the half-saturation covariate.

This study considered the application of the InVEST approach and integrated the available data and fieldwork results. Two distinct categories of land use were identified as potential threats: arable land and construction land. The impact weights and maximum impact ranges of the aforementioned threat factors were assigned with reference to the values recommended by the model and the field situation (

Table 1. Threat factor attribute assignment.

Threat Factor	Weight	Maximum Impact Distance	Attenuation
Farmland	2	0.6	Linear
Construction land	5	0.8	Linear

). The suitability of the habitat types and their vulnerability to potential threats were established through a process of integration, whereby the recommended values of the reference model were combined with pertinent studies and field observations (

Table 2. Habitat suitability of different land-use types and their sensitivity to various threat factors.

Land-Use Type	Habitat Suitability	Farmland	Construction Land
Farmland	0.3	0	0.4
Woodland	1	0.3	0.5
Grassland	0.8	0.5	0.2
Water	1	0.1	0.4
Construction land	0	0.1	0
Unused land	0.1	0.1	0.1

).

2.3.5. AHP

The analytic hierarchy process (AHP) is a system analysis method initially proposed by A. L. Saaty, a professor at the University of Pittsburgh, in the 1970s. The method employs a systematic approach to data analysis, combining qualitative and quantitative techniques to synthesize subjective judgments [19]. This enables the integration of qualitative analysis with quantitative decision making, facilitating the generation of objective and evidence-based insights. This study employs Yaaph (V12.11.8293) software to construct hierarchical structure models for water and soil conservation, as well as wind and sand control [20]. The judgement matrix is developed based on the varying weights of terrain, soil, temperature, rainfall, wind speed, and other pertinent influencing factors. Subsequently, a consistency test is conducted to calculate the weights of the ecological quality composite index (Figure 2).

Therefore, the hierarchical analysis method is used to determine the degree of importance and obtain the ecological service function importance weights with the following formula:

$$ES=C_{1}Retention+C_{2}R+C_{3}SLr\mathrm{R}$$

(15)

where C₁ is the weighting factor for water retention, 0.3400; C₂ is the weighting factor for rainfall erosivity, 0.4459; and C₃ is the weighting factor for potential wind erosion, 0.2141.

3. Characteristics of Spatial and Temporal Evolution of Impact Factors

3.1. Characterisation of LULC

The most prevalent land-use category in the Hexi region is that of unused land, which constitutes a considerable proportion of the total area [21]. Subsequently, the land is divided into categories of farmland and woodland, with a relatively limited amount of construction land. The diversity of land types is contingent upon a number of factors, including the regional soil type, geomorphological features, and the distribution of water resources (Figure 3).

The distribution of land use exhibits a clear spatial heterogeneity, with forest and grassland predominantly situated in regions characterized by a gentle topography and abundant water sources. Conversely, deserts and barrens are primarily concentrated in arid and semi-arid zones. The transformation of land use is influenced by the combined impact of natural factors, such as climate and topography, and human-driven factors, including economic development and policy orientation. The Sankey diagram of land-use change shows that between 2000 and 2010 a significant proportion of previously unused land became cropland and grassland (Figure 4). This trend reflects a relatively favorable ecological environment during that period. From 2010 to 2020, there was a notable increase in the proportion of grassland converted into farmland, residential construction land, and unused land.

3.2. Water Availability

The temporal pattern of water per unit area in the Hexi Corridor from 2000 to 2020 evinces a relatively consistent distribution, with higher precipitation levels observed in the southeast and lower levels in the northwest (Figure 5). The majority of the region displays a restricted capacity for water preservation, with only a few areas in the south-east exhibiting a noteworthy capacity for water conservation. On average, approximately 7.80 mm of water is conserved per unit area in the Hexi Corridor over a five-year period. The maximum and minimum values are observed in 2015 and 2020, at 8.53 mm and 6.64 mm, respectively. The period between 2000 and 2005 exhibited an upward trajectory, with a 0.93 mm increase in water conservation per unit area; however, from 2005 to 2010 there was a notable decline, with an average reduction of 0.18 mm. Subsequently, from 2010 to 2015, following a period of growth, there was a rebound in water yield per unit area, with an average increase of 0.4 mm; however, from 2015 to 2020, there was a clear downward trend in water yield per unit area, with an average decrease of 1.89 mm, representing the largest change observed during this period.

Overall, the water source availability of the Hexi Corridor demonstrated a gradual decline from 2000 to 2020, exhibiting an average reduction of 0.75 mm per unit area. The most significant expansion in water source capacity was observed between 2000 and 2005, while the most rapid contraction occurred from 2015 to 2020. The dynamics of the water source nutrient quantity distribution and fluctuation were influenced by a multitude of factors. In recent years, the northwestern region has shown a trend of warming and humidification, with increased precipitation and surface runoff, creating good conditions for water conservation. Concurrently, the execution of soil and water conservation initiatives, the establishment of artificial forests and an array of ecological remediation projects, and a notable decrease in the land area necessary for industrial and mining operations, through the ongoing rectification and administration of the mining region, enhance the surface vegetation [22].

3.3. Soil and Water Conservation

The geographic variation in soil and water preservation across the Hexi Corridor exhibited a consistent upward trajectory from the southern to the northern regions from 2000 to 2020. This was accompanied by a relatively weak soil and water conservation capacity (Figure 6). There were notable variations in soil conservation trends across different time periods [23]. Significant differences were observed in soil conservation changes. From 2000 to 2005, the region exhibited an increase in soil conservation capacity, with an average increase of 0.11 t/hm² in the quantity of soil and water per unit area. Between 2005 and 2010, there was a decline in the capacity for soil and water preservation in the region, as evidenced by an average reduction of 0.25 t/hm² in soil and water per unit area; however, the trend reversed in 2015, with an average increase in soil and water conservation compared to 2010.

The overall trend in soil and water volume in the Hexi Corridor from 2000 to 2020 was characterized by a dynamic change, initially increasing and subsequently decreasing. The quantity of soil and water conservation in the Hexi Corridor demonstrated a slight upward trajectory, with an estimated increase of approximately 0.19 t/hm². During the period 2000–2010, the discrepancy in soil and water preservation across regions was relatively minimal, indicating a general consistency in regional soil and water conservation functions throughout this period. In contrast, the change over the period 2010–2020 in the amount of soil and water preservation in the Hexi Corridor per unit area was subject to considerable fluctuations, an indication that the ecological environment of the Hexi corridor is susceptible to external disturbances.

3.4. Wind and Sand Defences

The concentration of sand fixity exhibited by the Hexi Corridor displays a pronounced peak in the regions situated towards the center and west of the area, whereas the remaining regions display a comparatively lower concentration (Figure 7). The mean value of sand fixation per unit area in the Hexi Corridor over the period under consideration is approximately 7.51 t/hm² per 5a. The maximum value of sand fixation per unit area is observed in 2010, at 22.426 t/hm², while the minimum value is recorded in 2005, at 0.329 t/hm². The sand fixation capacity of the Hexi Corridor exhibited a notable surge between 2005 and 2010, accompanied by a decline in the average amount of sand fixity per unit area, amounting to a reduction of 4.32 t/hm² between 2010 and 2015; however, between 2015 and 2020, the sand fixation capacity of the Hexi Corridor exhibited a notable increase, with an average growth of 3.06 t/hm².

The amount of windbreaks and sand fixity in the Hexi Corridor from 2000 to 2020 reflected a weak upward trend, and the average increase in windbreaks and sand fixation per unit area in the 20a was about 2.43 t/hm², which reflected a gradual increase in the windbreaks and sand fixation capacity of the Hexi Corridor. Two factors can be considered for the enhancement of the wind and sand conservation capacity: on the one hand [24], the growth of vegetation can be improved through the solution and management of ecological problems as well as active ecological protection initiatives; on the other hand, reducing the area of industrial and mining land, cleaning up the waste residue of the mining area, managing the mining roads, stopping the mining and prospecting activities, and promoting the expansion of the area of grasslands, forests, and thickets to increase the coverage of vegetation, thus improving the vegetation cover and vegetation productivity in addition to enhancing the wind and sand conservation function [25]. This will increase vegetation cover as well as productivity and enhance the function of wind and sand control.

3.5. Habitat Quality

The temporal pattern of habitat quality in the Hexi Corridor from 2000 to 2020 is characterized by a gradient from high suitability in the south to low suitability towards the north (Figure 8). The geometric intersection of the habitat quality in different periods of every five years was 0.0059. The geometric intersection of the habitat quality in different periods was 0.2917, 0.2920, 0.2924, 0.2926, and 0.2964. The most pronounced increase was seen from 2015 to 2020, when habitat quality rose by 0.2924, 0.2926, and 0.2964. The remaining years, 2000–2005, 2005–2010, and 2010–2015, exhibited minimal fluctuations in habitat quality; however, an upward trajectory was observed in all cases, with average increases of 0.0003, 0.0004, and 0.0002, respectively.

The quality of habitats in the Hexi Corridor as a whole demonstrates a gradual but consistent increase, which reflects the observable warming and humidifying trend in the climate in northwest China since 2000 [22]. The quality of the habitat is subject to change as a result of different land-use types. The central part is subject to greater influence from anthropogenic factors, largely due to the distribution of cultivated land and construction land. In the west, there are extensive areas of bare land with low surface vegetation cover, poor natural conditions, and a fragile environment [26]. These areas exhibit significant environmental heterogeneity.

4. Assessing the Significance of Ecological Service Functions and Influencing Factors

4.1. The Significance of Water Conservation

The significance of the water-holding function of the Hexi Corridor from 2000 to 2020 was assessed over a five-year period. The results demonstrate that the region exhibited a general tendency towards better distribution in the central and southern regions, with a comparatively worse distribution in the northwestern region. This distribution was observed to be diffuse and decreasing, particularly within the Heihe River Basin. With regard to the timescale, the West Corridor exhibited the greatest extent of favorable water quality in 2010, and the total area of favorable water quality in 2020 was markedly larger than that observed in 2000 (Figure 9).

In the year 2000, the proportion of water quality adaptation areas in the Hexi Corridor in relation to the total regional water quality was 7.06%. This increased to 10.11% in 2010, representing the maximum value observed during the study period. Subsequently, there was a slight decline in this proportion until 2020. The proportion of general adaptation areas for water availability exhibits a similar trend to that observed for good adaptation areas, reaching a peak of 8.64% in 2010. Additionally, the proportion of areas with minimal water-holding functionality was found to be the highest, averaging at 84.29% over several years. When viewed in conjunction with the region’s topography, this suggests that low-lying areas are predominantly concentrated in unused lands, such as deserts, wastelands, the Gobi Desert, and other forms of land use.

4.2. The Significance of Soil and Water Conservation Functions

The functional importance of soil and water conservation in the Hexi Corridor from 2000 to 2020 was evaluated with 5a as one period. The results show that the proportion of area with good functional importance of soil and water conservation first decreased from 0.12% to 0.06%, then rebounded in 2010, rapidly increased to 0.17% in 2015 to reach the maximum value, and then showed a slight decline. The proportion of areas of medium functional importance for soil and water conservation peaked at 3.02% in 2010, and the proportion of areas of poor functional importance for soil and water conservation, in particular, declined significantly in 2020, from 8.66% to 5.24% (Figure 10).

4.3. The Significance of Wind and Sand Control Functions

The functional significance of windbreaks and sand fixation in the Hexi Corridor from 2000 to 2020 was assessed using 5a as the unit of analysis. The results demonstrate that the area of significant functional importance for wind and sand prevention and fixation is primarily concentrated in the central region, exhibiting a fluctuating increasing trend from 9.04% to 9.09%. The areas with the highest functional importance for sand and wind protection were 11.16% in 2015, while the areas with the lowest functional importance for sand and wind protection were 5.82% in 2020. Overall, there was an improvement in this phenomenon (Figure 11).

4.4. The Significance of Ecosystem Service Functions

In order to gain a comprehensive understanding of the changes in ecosystem service functions in the Hexi Corridor, the results of each ecosystem service function were used to evaluate them (Figure 12). These functions mainly included changes in hydrology, soil, and the ecological environment. In the calculation, the weights assigned to wind and sand control, soil and water conservation, and water conservation were 0.2141, 0.4459, and 0.3400, respectively. The physical dimensions of the ecosystem services of wind and sand control, soil and water preservation, and water preservation differed, resulting in a disparate order of magnitude values; therefore, it was necessary to deformalize the various indicators and then superimpose them to obtain the following results.

This study demonstrated that, between 2000 and 2020, ecosystem service function was of general importance, particularly in the northern and western parcels of the Hexi Corridor, which collectively accounted for 76.96% of the total area. Additionally, the southwestern region was identified as an important area for ecosystem service function, with a small portion of it being classified as an extremely important area. The Heihe River Basin, located within the central and southern regions of the Hexi Corridor, was identified as an area of significant ecological importance, contributing 4.43% of the total area. The areas with the most significant ecosystem service functions were of paramount importance for the provision of ecological resources and products across the entire Hexi Corridor.

5. Conclusions

This study employs a comprehensive analysis of the change trends in different ecological service functions in the Hexi Corridor from 2000 to 2020. The objective was to account for changes in hydrology, soil, and the ecological environment, and to combine the actual situation of the study area with the degree of importance of the various service functions to the study area. The main conclusions are as follows:

(1) The water conservation volume of the Hexi Corridor also exhibited a weak trend; there was a discernible decline in this measure from 2000 to 2020, with an average reduction of 0.75 mm per unit area. This suggests that the evapotranspiration of the Hexi Corridor is considerable and requires intervention through artificial means.

(2) The quantity of soil and water conservation in the Hexi Corridor demonstrated a fluctuating pattern, initially showing growth and subsequent decline. During the period between 2000 and 2010, the disparity in the quantity of soil and water conservation per unit area was relatively minimal, suggesting a general stability in the regional soil and water conservation function during this time. In contrast, between 2010 and 2020, the difference was larger, indicating that the region’s ecological environment is particularly vulnerable to external disturbances.

(3) From 2000 to 2020, there was a slight but consistent increase in the amount of windbreaks and sand fixation in the Hexi Corridor, with an average annual growth of approximately 2.43 t/hm². This indicates that the Hexi Corridor’s capacity to withstand wind and fix sand has been gradually enhanced.

(4) From 2000 to 2020, the ecosystem service function of the Hexi Corridor will be of general importance, particularly in the northern and western parcels, which collectively account for 76.96%, whereas the Heihe River Basin in the central and southern regions will be of significant importance for ecosystem service function, representing 4.43% of the total area. The Hexi Corridor continues to experience challenges in balancing human activities with the need for environmental protection.