Evaluation of the Effectiveness of Water Ecological Restoration Based on the Relationship between the Supply and Demand of Ecological Products—A Case Study of the Yellow River Delta

Abstract

The imbalance between the supply and demand of ecological products between society and ecosystems is an important cause of a series of water ecological problems, and water ecological restoration projects aim to improve the above supply–demand relationship by means of inputs from the social side. For this reason, this paper takes the Yellow River Delta region as an example to launch a study on the assessment of the effectiveness of water ecological restoration projects from the perspective of the supply and demand of ecological products. Specifically, the level of the supply and demand of ecological products, as well as the relationship between the supply and the demand in the studied area, were measured using the equivalent factor approach and the water footprint approach, and the effects of the Yellow River Delta hydro-ecological restoration project were assessed by integrating the following four metrics: land use, supply of ecological products (ecosystem services), demand for ecological products, and the relationship between the supply of and demand for ecological products. The results of this study show that although the hydro-ecological restoration project continues to replenish water resources in the Yellow River Delta region, and promotes the level of ecological product supply in the study area through the restoration of wetlands and water, the growing and excessive demand for ecological products in the study area still puts the local ecosystems at risk of degradation. In the future, the Yellow River Delta region should continue to control the scale of land for production and living on the supply side of ecological products and increase its investment in water ecological restoration, while establishing a highly efficient mode of ecological product development and utilization and a reasonable mechanism for the payment of ecological products on the demand side. In addition, the Yellow River Delta region needs to pay attention to the impacts of rising sea levels and other climatic problems on ecological restoration.

1. Introduction

Water is a fundamental element that supports the normal functioning of ecological and social systems, and fragile water ecology has become a key constraint to high-quality socio-economic development. Water ecosystem is destroyed via a combination of human and natural factors. The “Tragedy of the Commons” is a good metaphor for the core social reason for ecosystem destruction [1]. The simple fact is that everyone tends to overutilize public resources, which results in the depletion of resources and the fragility of ecosystems. Meanwhile, climate change also affects water ecology, such as rising sea levels, droughts, floods, and other negative impacts caused by climate change. Water ecological restoration projects are an important way to restore or rebuild damaged water ecosystems. However, the complexity of society and water ecosystems often makes it difficult for water ecological restoration projects to achieve their theoretical effects. Therefore, an evaluation of the effectiveness of the implementation of water ecological restoration projects can provide support for the planning and implementation of future water ecological restoration projects, and can point out the optimal direction for future water ecological protection practices.

These ecological restoration projects are projects of the Chinese government to manage regional ecosystems at the macro level, aiming for the sustainable development of ecosystems [2]. Over the past two decades, many ecological restoration projects have been carried out in the Yellow River Delta region. For example, ecological water recharge projects, the most influential of which were the “Wuwanmu” project completed in 2003 and the “Shiwanmu” project completed in 2006, have controlled the rapid degradation of various wetlands. Since the beginning of 2019, the Chinese government has further emphasized the importance of ecological restoration projects, and 16 ecological protection and restoration projects, including the Yellow River Delta Wetland of International Importance Protection and Restoration Project, have been implemented. Through water system micro-circulation, micro-habitat modification, water recharging, and other restoration and construction techniques, the wetland ecosystem of the Yellow River Delta region has been significantly improved, and the diversity of its wetlands has been significantly enhanced. At present, ecological restoration projects in the Yellow River Delta region are mainly centered on the ecological restoration of degraded wetlands and the comprehensive management of land and sea environments, which mainly include the construction of ecological coastlines, planning for the connectivity of water systems, freshwater recharge projects, restoration of fallow wetlands and beach lands, restoration of the habitats of rare and endangered wildlife and plants, and restoration of degraded areas of saline and alkaline lands.

In the field of assessing the effects of ecological restoration projects, existing research has progressed from evaluations of a single environmental quality to evaluations of the function of the whole ecosystem, and from assessments of a single ecological benefit to comprehensive benefit assessments. Earlier studies focused on whether ecological restoration projects played a role in specific environmental assessment indicators, such as total phosphorus in water, soil pH, biodiversity, etc. [2,3,4,5,6]. As the complexity of ecosystems is better understood, scholars are paying more attention to whether ecological restoration projects can promote the normal functioning of the whole ecosystem, including the assessment of ecosystem sensitivity and resilience, before and after the implementation of the project [7,8,9,10]. In addition to the above assessment of the ecological benefits of ecological restoration projects, many scholars have included economic and social benefits in their assessments of the effectiveness of ecological restoration projects, with the aim of comprehensively reflecting the impact of ecological restoration projects on the entire socio-ecological system [7,8,11,12].

With the rise in research on ecological products and their supply–demand relationship in recent years, scholars have gradually realized that the essence of the water ecology problem lies in the imbalance between the supply and demand of ecological products within the social-ecological system. Supply and demand reflect the flow of ecosystem services through ecosystems and human social systems. The relationship between the supply of and demand for ecosystem services can effectively reflect the state of regional economic development and the state of the ecological environment [13]. On the one hand, the amount of ecological products and services that water ecosystems can provide is no longer able to meet the increasing demand needed for human society to develop, and on the other hand, human society’s investment in water ecology is not enough to support the restoration of water ecosystems’ ability to supply goods [14,15,16]. This imbalance between the supply and demand of ecological products has continuously weakened the supply capacity of water ecosystem products, thus triggering a series of water ecological problems such as soil salinization and wetland degradation.

Therefore, the purpose of implementing water ecological restoration projects is to enhance the supply level of water ecosystem products by means of inputs from human society, so as to promote the balance of the supply and demand of ecological products between society and water ecology. For this reason, scholars have begun to pay attention to the effectiveness of ecological restoration projects in restoring the supply capacity of ecosystem products. In particular, the ecosystem services value can be used to reflect the supply capacity of ecosystem products as well as to measure the demand for ecological products in human social systems; thus, more and more studies have used the changes in the ecosystem services value before and after the implementation of the project to assess the effectiveness of ecological restoration projects [17,18,19,20]. Existing studies mainly evaluate the effect of ecological restoration projects from the perspective of ecological product supply, for example, analyzing the change in the ecosystem service value supply after the implementation of ecological restoration projects. However, few scholars have paid attention to the impact of ecological restoration projects on the relationship of the supply and demand of ecological products between society and ecosystems, and it is difficult for existing studies to accurately reflect the effects of ecological restoration projects on improving the balance between the supply and demand of ecological products. In the meantime, water ecological services are essentially generated through a combination of natural, built, and human capital [21]. Natural capital is therefore an important factor to consider when studying water ecology. For example, other factors such as the precipitation, slope, soil type, location, and distance between two pressure points can have a significant impact on water ecosystem services [22]. Since gross primary productivity, transpiration, and other factors have different sensitivities to multi-year precipitation fluctuations, the efficiency of water ecosystem use will increase in wet years and decrease in dry years [23].

For this reason, this paper will take the Yellow River Delta as an example to begin evaluating the effect of ecological restoration projects under the perspective of ecological product supply and demand, while taking into account the impact of natural factors. The Yellow River Delta is the most complete and youngest wetland ecosystem in China’s warm–temperate zone, and is strategically important in maintaining the ecological balance of the Bohai Bay and the lower Yellow River Basin. It is abundant in land, oil, and gas resources, biological and marine life, and warm temperate climates [24]. Meanwhile, the Yellow River Delta region has rich wetland resources, making it the supply of lots of ecosystem services for human beings [25]. Since 1900, global wetland areas have degraded by 50 percent [26]. In the context of global wetland degradation, wetland hydro-ecology has been severely damaged and further destabilized. Although some countries have taken measures to curb their wetland degradation, the results have not been significant [27,28]. The first 10 pioneering initiatives to restore the natural world, selected by the United Nations, reveal the best examples of large-scale and long-term ecosystem restoration. Examples include the Atlantic Forest Tripartite Agreement, Abu Dhabi’s Ocean Restoration, and China’s Shan-Shui Initiative. The water ecology restoration projects are a key component of China’s Shan-Shui Initiative. Affected by natural and socio-economic development, the Yellow River Delta region has long been facing the problem of freshwater shortage, and the wetlands in the Yellow River Delta region are under threats of ecological degradation, such as the shrinking of the natural wetland area. Evaluating the effectiveness of the existing water ecological restoration projects in the Yellow River Delta region based on the perspective of the supply and demand of its ecological products can provide theoretical guidance for the ecological restoration of wetlands in the Yellow River Delta region and other regions of the world.

This paper intends to use the ecosystem services value to measure the relationship between the supply of and demand for ecological products. Specifically, this paper uses remote sensing data to obtain the land use status of the study area from 2000 to 2020, and applies the equivalent factor method to calculate the supply level of ecological products in the study area, combined with a water footprint calculation to obtain the demand level of ecological products in the study area, and then obtain the ratio of ecological product supply and demand in the study area. This paper is intended to evaluate the effect of water ecological restoration projects implemented in the study area based on the changes in this area’s ecological product supply level and its supply–demand ratio.

2. Methodology and Data Sources

2.1. Methodology

In this paper, based on the CICES framework from the Routledge Handbook of Ecosystem Services, ecological products are classified into four categories: provision, regulation, support, and cultural services [29] (pp. 31–36) (see

Table 1. Classification of ecological products.

Ecological Products/Ecosystem Services
Provision services	Food production
	Raw materials
Regulation services	Gas regulation
	Climate regulation
	Waste treatment
	Hydrological regulation
Support services	Soil conservation
	Biodiversity
Cultural services	Aesthetics

), and the corresponding ecosystem service metrics are applied to measure the level of ecological product supply and demand. This paper mainly uses the equivalent factor method to calculate the level of ecological product supply in the study area. For the demand of ecological products, considering that water resources are the basic elements that support the functioning of natural ecological and socio-economic systems and that they are key resources for ensuring the ecological restoration of the Yellow River Delta region, referring to the existing literature, this paper uses the ratio of the water footprint to the amount of water resources to obtain the coefficient of water resource consumption in the study area; this characterizes the coefficient of ecological product consumption in the study area, and then obtains the level of ecological product demand. This paper will analyze the impact of the Yellow River Delta ecological restoration project on the supply and demand of local ecological products by using the ratio between the level of ecological product supply and the level of demand.

2.2. Data Sources and Processing

The research data used in this study mainly include Landsat satellite remote sensing data, Google Earth image data, Dongying water resources data, and Dongying socio-economic statistics. In particular, Landsat5, Landsat7, and Landsat8 satellite imagery data from April to May and October to November for the years 2000, 2005, 2010, 2015, and 2020 were obtained from the website of the USGS Global Visualization Viewer (GloVis) (http://glovis.usgs.gov/, accessed on 12 September 2022). All satellite image data were processed for radiometric calibration and atmospheric correction using ENVI5.3. The water resources data were from Shandong Province Water Resources Bulletin (http://www.sd.gov.cn/, accessed on 13 September 2023); the Dongying socio-economic statistics were from the Shandong Provincial Statistical Yearbook and the Dongying Municipal Statistical Yearbook (http://dystjj.dongying.gov.cn/, accessed on 19 March 2023). Data on the profits per unit area for major crops are from the National Compendium of Agricultural Cost and Benefit Information (http://www.stats.gov.cn/, accessed on 19 March 2023).

With reference to existing studies and the land use characteristics of the study area, this paper classifies the types of land use in the modern Yellow River Delta into a total of eight types: farmland, forest, grass, water, wetland, salt field, aquaculture pond, and construction land. Among them, considering its small size within the study area, this paper incorporates the sea area into the water type; thus, the water type includes all rivers, lakes, reservoirs, pits and ponds, and sea areas within the study area. Remote sensing image interpretation flags were established based on the land use classification system. In order to distinguish between different land use types more clearly, this paper synthesized the remote sensing images of April to May and October to November in the same year to obtain the images used for interpretation in five years, and the interpretation mark (ROI) of the synthesized images in each year was greater than 1.8. In this paper, we used the ENVI5.3 support vector machine method to decipher the remote sensing images and manually correct the land use classification results by combining them with Google Earth images. Finally, the overall interpretation accuracies for the five periods of remote sensing images were all greater than 90.89%, and the Kappa coefficients were all greater than 0.89, so the interpretation accuracies met the requirements.

The results for land use classification and land use changes in the Yellow River Delta region from 2000 to 2020 are shown in

Table 2. Yellow River Delta land use classification results from 2000 to 2020 (unit: hectare).

Classification	Farmland	Forest	Grass	Water	Wetland	Salt Field	Aquaculture Pond	Construction Land
2000	81,357.39	2222.1	90,674.55	17,191.44	63,483.93	1313.64	12,112.2	6502.23
2005	106,432.56	3936.78	51,354.81	21,194.55	59,521.5	1375.47	19,920.96	11,112.75
2010	107,157.6	2907.54	46,247.49	25,715.25	38,849.58	4733.37	37,419.39	11,813.22
2015	99,732.15	2613.6	50,681.97	20,431.62	42,222.06	10,174.23	37,014.84	11,962.8
2020	87,498.9	1972.62	46,603.62	28,059.03	41,611.14	16,893.9	34,447.59	17,769.42

and Figure 1. The area of cultivated land increased continuously between 2000 and 2010, and showed a decreasing trend after 2010; but the area of cultivated land in 2015 and 2020 was higher than in 2000. The areas of construction land, salt fields, and aquaculture ponds increased from 2000 to 2020. Among them, construction land increased from 6502.33 hectares in 2000 to 17,769.42 hectares in 2020, salt field area increased from 1313.64 hectares in 2000 to 16,893.9 hectares in 2020, and aquaculture pond area increased from 12,112.2 hectares in 2000 to 34,447.59 hectares in 2020.

Against the background of the rapidly expanding land for cultivation and living mentioned above, the areas of grasslands and wetlands in the Yellow River Delta region shrunk significantly between 2000 and 2020. Across all areas, the expansion of the salt field and aquaculture pond areas is the main cause of this degradation of grasslands and wetlands in the Yellow River Delta. However, the decrease in grassland area slowed down significantly after 2010, and the wetland area increased slightly, indicating that the series of ecological restoration projects of the previous 10 years prevented further degradation of the grassland and wetland ecosystems in the Yellow River Delta region.

In addition, the area of forested land in the Yellow River Delta region has basically remained stable, and the area of water as a whole has shown an increasing trend. The former reflects the remarkable effect of a series of ecological projects, such as the ecological water replenishment project. The latter may stem from rising sea levels due to climate change, and the fact that the growth of shallow waters has to some extent eroded the coastal wetland area.

2.2.1. Methodology for Measuring the Supply Level of Ecological Products

Equivalent factor values per unit area of farmland, forest, grassland, and water in this study were derived from the study by Xie et al. (2008) [30]. Equivalent factor values per unit area of coastal wetlands, salt fields, and aquaculture ponds were derived from the study by Liu et al. (2020) [31]. Salt fields refer to land used for the purpose of producing salt, including salt tanning sites, salt ponds, and land used for ancillary facilities. This study does not consider the supply of ecological products from construction land. Therefore, this equivalent factor value per unit of ecosystem services in the Yellow River Delta region is shown in

Table 3. The equivalent factor values per unit of ecosystem services in the Yellow River Delta region.

Ecological Products	Farmland	Forest	Grass	Water	Wetland	Salt Field	Aquaculture Pond
Food production	1.00	0.33	0.43	0.53	0.36	0.00	18.09
Raw material	0.39	2.98	0.36	0.35	0.24	16.60	0.00
Gas regulation	0.72	4.32	1.50	0.51	2.41	0.50	0.51
Climate regulation	0.97	4.07	1.56	2.06	13.55	0.00	2.06
Waste treatment	1.39	1.72	1.32	14.85	14.40	0.35	0.00
Hydrological regulation	0.77	4.09	1.52	18.77	13.44	4.13	0.00
Soil conservation	1.47	4.02	2.24	0.41	1.99	0.00	0.00
Biodiversity	1.02	4.51	1.87	3.43	3.69	2.30	3.43
Aesthetics	0.17	2.08	0.87	4.44	4.69	2.32	2.32

Xie et al. (2015) defined the ecosystem service value of one standard equivalent factor as the net profit of food production per unit area of farmland ecosystem [32], which was calculated as follows:

$$D=\sum _c{F}_c{S}_c$$

(1)

where: D represents the value of ecosystem services for one standard equivalent factor, $F_c$ is the net profit per unit area of crop c (USD/ hectare), $S_c$ is the proportion of the area sown under crop c to the total area sown. Based on the crop data from the study area, the equivalent factor applicable to the Yellow River Delta region can be obtained. In order to further calibrate the equivalent factors, this paper adjusted the crop prices for each year to the 2020 price based on the inflation rate. Moreover, since the price of the crops is in the Chinese currency, it has been converted to US dollars using the 2020 exchange rate (1 USD to 6.8974 RMB). The average D value for each year from 2000 to 2020 is taken as the final D value.

By multiplying the area of each type of land use in the study area with its ecosystem service value per unit area, the levels of ecological product supply of different land use types in the study area can be obtained.

$$EPS=\sum _i\sum _j{A}_i{E}_{ij}D$$

(2)

where: $EPS$ indicates the level of ecological product supply (which is the ecosystem service values) in the study area, $A_i$ is the area of land use type i, and $E_{ij}$ is the equivalent factor of the j type of ecological product for the i type of land use.

2.2.2. Methodology for Measuring the Level of Demand of Ecological Products

Ecological products have diversity and complexity; examples such as food production, raw material production, and the production of other ecological such as timber, etc. can be freely circulated in the market, with strong market attributes. However, services such as hydrological regulation, climate regulation, biodiversity, etc. are considered public goods, the demand and value of which are difficult to fully reflect in the market. Therefore, this paper divides the demand for ecological products into a market segment (provision-type ecological products) and a non-market segment (regulation-, support-, and cultural-type ecological products), and the level of demand for these ecological products is obtained by calculating the sum of the two, respectively.

(1) Calculation of demand for ecological products in the market segment

The demand of human society for this segment of ecological services is tied to the actual consumption of products, such as food and meat, or to the actual demands of industrial development and people’s lives. Water resources are fundamental to productive activities and are key resources for water ecological restoration. Therefore, this paper refers to the study of Wang et al. (2016) [12]. The consumption of water resources indicates the consumption of supply-based ecological products. Specifically, this paper intends to use the water footprint to represent the water consumption in the study area, and the ratio of the water footprint to the total water resources as the consumption coefficient of provision-type ecological products, which will be applied to the supple of provision-type ecological products in order to obtain the demand of ecological products in the market segment.

Water footprint refers to the amount of water required for all products and services consumed by a country, region, or individual over a period of time, and is an actual water demand or consumption situation. In this paper, we measured the regional water footprint according to three aspects: agricultural, industrial, and domestic use, as follows:

$$WF=W{F}_{agr}+W{F}_{ind}+W{F}_{dom}$$

(3)

where $WF$ is the water footprint of the study area (10⁸ m³), $W{F}_{agr}$ is the agricultural water footprint (10⁸ m³), $W{F}_{ind}$ is the industrial water footprint (10⁸ m³), and $W{F}_{dom}$ is the domestic water footprint (10⁸ m³).

Considering the huge amount of water used in agricultural production, this paper’s proposed method expresses the agricultural water footprint in terms of its virtual water. In industrial production, the virtual water content of industrial products was ignored in this paper due to the variety of products and the small virtual water content compared to agricultural products. Therefore, the agricultural water footprint was multiplied by the virtual water content per unit of product and the product output, while the industrial and domestic water footprints used the corresponding water consumption data from the Water Resources Bulletin.

The agricultural water footprint was calculated as follows:

$$W{F}_{\mathit{agr}}=\sum _{c=1}^c{D}_c\times V{T}_c\times P$$

(4)

where $D_c$ is the per capita demand for agricultural product c in the study area (kg), $V{T}_c$ is the virtual water content per unit of agricultural product c (${\mathrm{m}}^3/\mathrm{kg}$), and $P$ is the number of residents populating the study area.

The virtual water content data per unit of agricultural products and virtual water for animal products were obtained using the study from Mekonnen et al. on China’s water footprint [33,34]. The aquatic products data were obtained from the calculations of Yuan (2016) [35]. The water footprint values per unit mass of product and per capita consumption data used in the paper are shown in

Table 4. The water footprint values per unit mass of product and per capita consumption data.

Main Food	Crop	Fresh Vegetable	Fresh Fruit	Pork	Beef	Lamb	Chicken	Egg	Milk	Aquatic Product
Water footprint values per unit mass of product (m3/kg)	1.467	0.366	1.761	6.103	13.688	5.813	3.971	3.094	1.282	3.110
Per capita consumption (kg)	126.754	88.281	62.590	13.464	1.081	0.986	6.027	16.822	16.774	12.607

.

Based on the results of the water footprint model, the ratio of water demand to total water resources was used as the consumption coefficient of ecological products in the market segment. Water demand was characterized by the water footprint and water supply was characterized by the water availability. Specifically, the level of demand for ecological products in the market segment was calculated as follows:

$$EP{D}_M=EP{S}_S\times \frac{WF}{WS}$$

(5)

where $EP{D}_M$ is the demand level of ecological products in the market segment, $EP{S}_S$ is the supply level of provision-type ecological products, and $WS$ is the total water resources in the study area.

(2) Calculation of demand for ecological products in the non-market segment

Regulation, support, and cultural services provided by nature to human society are generally considered to be in the category of public goods. Due to the non-competitive and non-exclusive nature of public goods, the demand or consumption of such services was taken to be equal for each individual within the study scope and the total demand was determined by the total supply. Here, the total demand for the non-market segment of the ecological products is used to equal the total supply.

2.2.3. Methodology for Evaluating Ecological Products’ Supply–Demand Relationship

In this paper, the supply–demand ratio (SDR) was used to evaluate the supply–demand relationship of ecological products, and the formula for this is as follows:

$$\mathit{SDR}=\frac{EPS}{EPD}$$

(6)

The SDR > 1 indicates that the supply is greater than the demand, indicating that the ecosystem’s ecological product supply capacity is strong and can satisfy the demand for ecological products on the social side; the SDR < 1 indicates that the supply is less than the demand, indicating that the ecosystem’s ecological product supply capacity is weak, and there is an imbalance between the ecological product supply and demand of the social and water ecosystems; the SDR = 1 indicates that the supply is equal to the demand, indicating that the ecosystem’s ecological product supply capacity can precisely satisfy the demand for ecological products on the social side.

In addition to the SDR, this paper standardizes the z-score for the level of supply and demand of ecological products in the study area and plots it in a two-dimensional coordinate system. Based on the quadrant wherein the supply and demand values are located, the supply and demand relationship of ecological products is divided into four regions, namely, high supply–high demand, low supply–high demand, low supply–low demand, and high supply–low demand. This paper analyzes the restoration effect of the water ecological restoration project by combining the SDR and the changes in the supply–demand relationship during the studied period.

2.3. Study Area

The Yellow River Delta is a land surface delta formed by Yellow River sediment and artificial reclamation since the Yellow River was diverted into the Bohai Sea in 1855, and can be divided into the ancient Yellow River Delta and the modern Yellow River Delta according to the age. The modern Yellow River Delta is a fan-shaped area of about 2400 km² with Yuwa as the apex, from the mouth of the Tiao River in the north to the Song Chunrong gully in the south [36]. Considering the scope of influence of the Yellow River water resources, the distribution of natural wetlands in the Yellow River Delta region and the geographic location of major ecological restoration projects in the Yellow River Delta region, the scope of this paper is the modern Yellow River Delta (see Figure 2), which mainly involves the Hekou District, the Kenli District, and the Lijin County in Dongying City, Shandong Province. In addition, considering the time period in which a series of ecological restoration projects such as ecological water replenishment were implemented in the Yellow River Delta National Ecological Reserve, the timeframe of this study was from 2000 to 2020.

3. Results

3.1. Changes in the Level of Supply and Demand for Ecological Products in the Yellow River Delta Region from 2000 to 2020

(1) Analysis of the supply level of ecological products

The main food crops in the Yellow River Delta region are wheat, rice, and maize. Based on the average value of the profit per unit area of these three main food crops from 2000 to 2020 (calculated at the 2020 price level), the economic value of one standardized equivalent factor was calculated to be USD 436.06. Based on the standard equivalent factor economic value and the equivalent factor value per unit, the ecosystem service value per unit area of different land use types in the study area was obtained (see

Table 5. Ecosystem service values per unit area in the Yellow River Delta region (USD/hm²).

	Ecosystem Services Value (USD/hm2)
	Farmland	Forest	Grass	Water	Wetland	Salt Field	Aquaculture Pond
Food production	436.06	143.90	187.50	231.11	156.98	0.00	7890.11
Raw materials	170.06	1299.45	156.98	152.62	104.65	7238.02	0.00
Gas regulation	313.96	1883.76	654.08	222.39	1050.90	217.57	222.39
Climate regulation	422.97	1774.75	680.25	898.27	5908.56	0.00	898.27
Waste treatment	606.12	750.02	575.59	6475.43	6279.21	152.30	0.00
Hydrological regulation	335.76	1783.47	662.81	8184.77	5860.59	1800.87	0.00
Soil conservation	641.00	1752.94	976.76	178.78	867.75	0.00	0.00
Biodiversity	444.78	1966.61	815.42	1495.67	1609.05	1000.82	1495.67
Aesthetics	74.13	907.00	379.37	1936.09	2045.10	1013.34	1013.34

), which was then calculated to obtain the level of ecological product supply in the Yellow River Delta region from 2000 to 2020 (see Figure 3).

Overall, the supply level of ecological products in the Yellow River Delta region shows a trend of first decreasing and then increasing, and the total supply level of ecological products in the Yellow River Delta region in 2020 has basically recovered to the level it was in 2000. The average supply of ecological products from 2000 to 2020 was USD 2679 million; The average supply of provision, regulation, support, and cultural ecological products was USD 376 million, 1666 million, 426 million, and 211 million, respectively. The supply of regulation-type ecological products was always the highest, and the supply of cultural-type ecological products was always the lowest. In 2015, the standard deviation of the supply of various types of ecological products was the smallest, which was 584; the standard deviation of the ecological products in 2000 was the largest, at 801, as it was affected by the high supply of regulation-type ecological products in 2000. Furthermore, because of the limited quantity of data presented in this document, they were not suitable for parameter testing.

In terms of ecological product types, the supply level of support and cultural ecological products has basically remained stable over the past 20 years, while the supply level of provision and regulation ecological products has been the main thing triggering changes in the overall ecological product supply level in the Yellow River Delta region.

Figure 4 illustrates the supply levels of provision-type and regulation-type ecological products for various types of land use in the Yellow River Delta region between 2000 and 2020. As can be seen from this figure, salt fields and aquaculture ponds are the main supply sources of provision-type ecological products in the Yellow River Delta, and the increase in the area of these two types of land use in the last 20 years has greatly driven the increase in provision-type ecological products in the Yellow River Delta region. Wetlands and water areas are the main supply sources of regulation-type ecological products in the Yellow River Delta, and the recovery of both these areas since 2010 has led to an increase in the supply levels of regulation-type ecological products in the Yellow River Delta.

Between 2010 and 2020, provision-type ecological products increased by 53 million and regulation-type ecological products increased by 75 million, indicating that regulation-type ecological products are the main driving force for the growth of ecological product supply level in the Yellow River Delta. The restoration of wetlands and water ecosystems is the direct cause of these increases, indicating that the wetland ecological restoration projects between 2010 and 2020 increased the level of local ecological product supply. However, due to the significant decrease in wetland areas compared with 2000, the supply level of regulation-type ecological products in the Yellow River Delta in 2020 is significantly lower than that in 2000, indicating that the Yellow River Delta region still needs to increase its ecological restoration efforts to further promote the recovery of local wetland ecosystems.

(2) Analysis of the demand level of ecological products

In view of the fact that the study area of this paper is not a complete administrative region, and considering the availability of data, the consumption coefficient calculated from the data of Dongying City was used as the consumption coefficient of provision-type ecological products in the Yellow River Delta region. Due to the lack of data on the per capita consumption of major agricultural products in Dongying, the average level of per capita consumption of major agricultural products in Shandong Province, from 2013 to 2020, was used for these data. Combining the total water resources of Dongying City, based on the aforementioned calculation methods, the water footprint of the Yellow River Delta, the provision-type ecological product consumption coefficient, and the ecological product demand level were obtained (see Figure 5).

Except for the year 2000, the total water resources of Dongying City were generally stable, between 500 and 600 million m³. In terms of the water footprint, Dongying’s water footprint decreased significantly from 2000 to 2005 due to a significant decrease in industrial water use, and then increased with the continuous increase in water use for production and domestic use. Under the condition of less total water resources and a higher water footprint, the demand for provision-type ecological products in the Yellow River Delta region in 2000 was the highest level of any year during the studied period. During the period of 2005 to 2020, the demand for provision-type ecological products in the Yellow River Delta region followed an increasing trend due to an increase in production- and living-related consumption demand.

In general terms, the non-market segment demand was always higher than the market segment demand. The demand for provision-type ecological products was the main driver of changes in the overall level of demand. Influenced by the more exceptional total water resources and water footprint in 2000, the demand for ecological products in the Yellow River Delta region in 2000 amounted to USD 3907 billion, which is significantly higher than the demand levels of 2005 to 2020. In addition, the level of demand for ecological products in the Yellow River Delta region has also shown an increasing trend since 2005.

3.2. Analysis of Ecological Products’ Supply–Demand Relationship in the Yellow River Delta Region from 2000 to 2020

The supply and demand levels of ecological products in the Yellow River Delta region were subjected to z-score processing in this paper, and the supply–demand relationship of ecological products was plotted in Figure 6a,b. From this figure, it can be seen that the ratio of supply to demand of ecological products in the Yellow River Delta region from 2000 to 2020 was always less than 1, i.e., the Yellow River Delta region has been in a constant state of imbalance between its supply and demand for the past 20 years.

The supply and demand relationship of ecological products in the Yellow River Delta region has been through two main stages. The first stage was from 2000 to 2010, during which both the supply and the demand of ecological products in the Yellow River Delta region decreased. Within this, the period from 2000 to 2005 mainly showed a decrease in the demand for ecological products and an increase in the supply–demand ratio of ecological products in the Yellow River Delta region, which means that the decrease in the level of demand for ecological products on the social side in this period relatively improved the supply capacity of ecological products in the Yellow River Delta region. Between 2005 and 2010, the demand for ecological products in the Yellow River Delta region decreased slightly while the supply of ecological products decreased significantly, which led to a decrease in the supply–demand ratio of ecological products, indicating that the supply capacity of ecological products in the Yellow River Delta region became weaker. The above ecological product supply–demand relationship indicates that the wetland restoration project at this stage failed to directly bring about an increase in the ecological product supply capacity of the local water ecology, while the reduction in ecological product demand for water products on the social side of the local community greatly alleviated the local ecological pressure, thus improving the local ecological product supply–demand relationship from the social side.

The second stage was from 2010 to 2020, during which the supply of ecological products in the Yellow River Delta region showed a growing trend. Within this stage, during the period of 2010 to 2015, the Yellow River Delta showed a significant increase in the demand for ecological products; although the supply level of ecological products was restored to a certain extent, this significant increase in the demand for ecological products on the social side led to a further decline in the local ecological product supply–demand ratio. This shows that the investment in local ecological restoration projects improved the supply capacity of ecological products to a certain extent, but was unable to meet the rapidly growing social demand and failed to effectively improve the relationship between the supply and demand of local ecological products. During the period of 2015 to 2020, the supply level of ecological products in the Yellow River Delta region increased significantly, while the demand level of ecological products remained essentially unchanged, thus driving an increase in the ecological product supply–demand ratio. This shows that the ecological restoration project significantly improved the supply capacity of ecological products in the Yellow River Delta region at this stage, and the demand for ecological products on the social side was been further expanded, and the improvement of both the supply and demand sides jointly pushed forward the improvement of the relationship between the supply and demand of local ecological products.

In order to excavate the factors driving these changes in the supply–demand relationship in the Yellow River Delta region, this paper conducted a gray correlation analysis between the supply–demand ratio of ecological products and the agricultural water footprint; the industrial water footprint; the domestic water footprint; the total amount of water resources; and the area of various types of land use, and obtained the gray correlation between each factor and the supply–demand ratio of ecological products, as shown in

Table 6. Result of the gray correlation analysis.

Driving Factor	Gray Correlation
Agricultural water footprint	0.640
Industrial water footprint	0.541
Domestic water footprint	0.553
Total water resources	0.650
Farmland area	0.780
Forest area	0.750
Grassland area	0.542
Water area	0.698
Wetland area	0.625
Salt field area	0.554
Aquaculture pond area	0.581

. The gray correlations of the factors, from largest to smallest, are farmland area > forest area > water area > total water resources > agricultural water footprint > wetland area > aquaculture pond area > salt field area > domestic water footprint > grassland area > industrial water footprint.

3.3. Contribution of Ecological Restoration Projects

The volume of ecological water consumption represents the amount of artificial water ecological recharge, which can be quantified to represent the water ecological restoration project. The volume of ecological water consumption increased year on year, indicating that the ecological restoration project was expanding by focusing on the volume of upstream water inflow. After correlation analysis, the ecological water replenishment and upstream water inflow were significantly correlated at a level of 0.05. It was found that with the progress of ecological restoration projects, the volume of water entering the Yellow River Delta is increasing. Specifically, from 2003 to 2020, the ecological restoration project achieved results, and the amount of water coming in from the upstream in 2020 was 3.3 times that of 2000 (see

Table 7. Water resources in the study area between 2000 and 2020.

Factors (108 m³)	Volume of Annual Rainfall	Volume of Water Consumption	Volume of Ecological Water Replenishment	Volume of Upstream Water Inflow	Volume of Water Entering the Sea
2000	21.3	15.05	0	136.9	48.6
2005	48.4	8.5	1.23	234.4	206.8
2010	44.37	9.2808	3.33	258.3	193
2015	45.13	9.56	4.79	229.18	113.6
2020	51.42	9.83	9.88	459.94	359.6

), which greatly supplemented the water resources of the Yellow River Delta region and played an important role in wetland restoration. Meanwhile, the ecological restoration project also played a phased role. The “Wuwanmu” project in 2003 brought more water to the Yellow River Delta by increasing the amount of water coming from the upstream between 2000 and 2005. The 2006 “Shiwanmu” project continued to increase ecological water replenishment, further increasing the amount of water coming from the upper reaches of the Yellow River Delta. In 2019, the Chinese government once again emphasized the importance of ecological restoration projects; investment in restoration projects aimed at ecological restoration increased. The current upstream water inflow level is significantly higher than any previous year, and the ecological water consumption is the highest in history, which proves that the ecological restoration project has brought about an improvement in the water ecology.

Through the analysis of other water-related indicators, we found that the year 2000 was a partial dry year, which resulted in the annual rainfall and the volume of water entering the sea from the Yellow River being significantly smaller than in other years. Annual rainfall and water consumption increased significantly after the year 2000. Therefore, changes in the supply of ecological products between 2000 and 2005 may be due to changes in rainfall, and substantial changes in the amount of water entering the sea from the Yellow River will mainly affect the speed of the expansion of the Yellow River Delta region [37]. Meanwhile, after statistical analysis, the ecological water replenishment was not found to correlate with the amount of water entering the sea. Therefore, the amount of water entering the sea from the Yellow River is not considered the main factor affecting changes in the ecological product supply. The annual levels of rainfall and water consumption between 2005 and 2020 remained basically stable, while the supply and demand of ecological products changed significantly; so, annual rainfall and water consumption may not be the main factors leading to changes in the supply and demand of ecological products during this period.

4. Discussion and Conclusions

4.1. Discussion

This section discusses four aspects of the supply–demand relationship of ecological products, the contribution of ecological projects, and the related ecological pressures.

(1) In terms of ecological product supply, the supply level of ecological products in the Yellow River Delta region has gradually risen since 2010, influenced by the growth of supply-type and regulation-type ecological products. The increase in regulation-type ecological products driven by natural wetland restoration is the main reason for increases in the supply level of ecological products in the study area, indicating that the ecological restoration project in the Yellow River Delta region wetlands has increased the supply of local ecological products. However, the increase in provision-type ecological products driven by the growth of artificial wetlands is also an important factor leading to the increase in the supply level of ecological products in the study area, implying that future studies on ecological product supply and the assessment of the effects of ecological restoration projects should not only focus on the increase in the overall supply level of ecological products, but should also identify the main driving issues leading to this increase.

On the demand side of ecological products, the overall demand for ecological products in the Yellow River Delta region has been increasing in recent years, indicating that local high-quality ecological products are gradually becoming scarce resources. This also reflects the possibility of using mechanisms such as ecological compensation [38] to guide the payment of consumers of ecological products, which means that the government can use the payment behavior of the consumers of ecological products to increase the level of ecological restoration inputs and supply of ecological products, thus improving the supply and demand relationship of ecological products between the society and the ecosystem.

(2) From the perspective of the supply–demand relationship, although the supply level of ecological products in the Yellow River Delta region has gradually increased since 2010, this growth rate is significantly lower than the growth rate of the demand for ecological products, and the supply–demand ratio of ecological products in the region is still on a downward trend. Therefore, although the Yellow River Delta ecological restoration project has achieved some success in the supply side of ecological products, it has failed to effectively improve the supply–demand relationship of ecological products between the local society and the ecosystem. This indicates, on the one hand, an insufficiency of ecological restoration inputs on the supply side, and on the other hand, reflects the irrationality of ecological consumption on the demand side.

(3) In the past few ecological restoration projects, positive phased and overall results have been achieved, and the amount of water entering the Yellow River from the upstream has been increasing, effectively improving the water ecology of the Yellow River Delta region. However, due to the rising demand for ecological products, the supply–demand ratio in the Yellow River Delta region still declined from 2005 to 2015. From 2015 to 2020, the supply–demand ratio began to increase again. In the future, with increasing investments in ecological restoration projects and the government’s macro-control of the supply and demand, the water ecological quality of the Yellow River Delta region will be further improved.

(4) This article is based on the pressure perspective to analyze the supply and demand of ecological products in the Yellow River Delta. Many scholars have already used the water footprint method to calculate the ecological pressure [39,40]. This pressure is reflected in the demand for ecological products. It is important to note that supply is not the same as demand, and that meeting ecological demand does not necessarily require the consumption of all ecological outputs.

4.2. Conclusions and Suggestions

Based on the land use classification of the Yellow River Delta region using Landsat remote sensing image data, this paper synthesizes the equivalent factor method and the water footprint method to calculate the supply and demand levels and supply–demand ratios of ecological products in the Yellow River Delta region from 2000 to 2020, and evaluates the effects of ecological restoration projects in the Yellow River Delta region. The results show that the ecological restoration project continues to replenish the water resources of the Yellow River Delta region by increasing the amount of water coming from the upper reaches of this region. And, after 2010, the ecological restoration project in the Yellow River Delta region effectively curbed the reduction in grassland areas and realized the restoration of a certain amount of wetland areas, therefore promoting the growth of the local ecological product supply level. However, this growth in the supply of ecological products driven by the ecological restoration project has lagged behind the growth in the level of demand for ecological products, and the supply–demand ratio of ecological products in the Yellow River Delta region has hence been declining. The ecological restoration project has failed to improve the supply–demand relationship of ecological products within the Yellow River Delta region socio-ecological system, and the excessive consumption of ecological products has put the ecosystems of the Yellow River Delta region at risk of further degradation.

In order to further restore the ecosystem of the Yellow River Delta and promote the balance between the supply and demand of local social-ecological system ecological products, this paper puts forward the following recommendations: (1) For the supply of ecological products, the Yellow River Delta region should further control the utilization of land for local production and living, and increase the investment in ecological restoration projects in order to enhance the supply of local ecological products. (2) For ecological production demand, the Yellow River Delta region should explore the establishment of an efficient model for the development and utilization of ecological products and a reasonable payment mechanism for consumers of ecological products, so as to increase the compensation for producers of ecological products while reducing the level of consumption, to incentivize producers’ ecological restoration and protection behaviors, and to promote the growth of the level of supply of ecological products; (3) The Yellow River Delta region needs to increase its investment in coastal ecological restoration projects, such as coastal protection forests, in order to cope with climate change issues such as rising sea levels.

The aim of ecological restoration projects is to improve the relationship between the supply and demand of ecological products within society and ecosystems in the form of social inputs. Therefore, evaluating the effects of ecological restoration projects in terms of the supply and demand of ecological products is of great significance for ecological protection and restoration, and this paper is an attempt in this regard. The findings of this paper show that although ecological restoration projects can effectively increase the supply of ecological products, it is difficult to mitigate the risk of ecosystem degradation triggered by irrational and excessive ecological product consumption behaviors. Therefore, the future practice of ecological restoration should not only pay attention to the supply side of ecological products, but should also pay attention to the management of the demand for ecological products, guide the formation of a reasonable ecological product demand and consumption habits, and establish an appropriate payment mechanism for ecological products, so as to pay back the value of the ecological restoration project.

In addition, research on ecological products and ecological restoration needs to pay attention to identifying the factors that drive the growth of ecological product supply. The results of this paper show that the expansion of productive land can also drive the growth of ecological product supply, but this growth may actually cause degradation of the natural ecosystems. Future research could focus on the cost of converting natural ecological land to productive land and construction land in order to more realistically reflect the ecological product supply capacity and restoration status of natural ecosystems.