The Spatiotemporal Relationship Between Water Purification Capacity and Land Use Structure in Fuyang

Abstract

With the rapid development of urbanization and the economy in recent years, increased human activities along the Yinghe River in Fuyang City and industrial expansion have degraded the water quality. Various sewage discharges have elevated nitrogen and phosphorus levels in the water body, disrupting its original ecological balance and exacerbating environmental issues. Therefore, studying the water purification capacity of the Fuyang region is particularly important. Using the InVEST model, this paper analyzes temporal changes and spatial differences in water purification capacity by quantifying nitrogen and phosphorus retention. The results show the following: The water purification capacity of Fuyang exhibits a spatial pattern of higher effectiveness in the north and lower effectiveness in the southwest. This study represents the strength of water purification capacity as the sum of regional output of nitrogen and phosphorus nutrients, based on which different types of areas are divided into water purification capacity deficit areas and water purification capacity control areas, and then combined with the different impacts of different land use types on the regional water purification capacity, corresponding countermeasures are proposed to optimize the water purification capacity of Fuyang City.

1. Introduction

With the rapid economic development, the environment in China is facing serious challenges in many aspects, including air, water, soil and biology. According to relevant policy documents issued in recent years, advanced environmental monitoring and assessment technologies should be adopted to scientifically assess and continuously improve the quality of the ecological environment, thereby promoting positive interaction between the environment and society [1]. This suggests that the enormous pressures on the natural environment and the problems associated with the process of urbanization are receiving sustained attention [2].

Ecosystem services are the processes by which ecosystem habitats and organisms receive ecological benefits or the environment maintains its ecosystem properties [3]. In recent years, attention to ecosystem services has been increasing both at home and abroad, and a great deal of research has been conducted. Water resources are crucial not only for supporting life itself but also in influencing the integrity and stability of the Earth’s ecosystem service functions. However, with the continuous development of urban construction sites and the rapid growth of the population, various sewage discharges have led to increased nitrogen and phosphorus elements in water bodies, which has disrupted the original ecological balance [4]. The nutrient absorption function of ecosystems, particularly for nitrogen and phosphorus, is limited. This restricts the capacity of ecosystem services to purify water, preventing these nutrients from being effectively utilized. While flowing downstream, this will result in a lack of regional ecosystem services [4]. As early as 2013, Mansoor D.K. Leh estimated the indicators of water purification in ecosystem services with the help of InVEST modeling [5]. Liu Yina and other scholars have studied the relationship and influence of water quality purification capacity function and landscape pattern in the Yangtze River basin [6]. Liu Canjun and other scholars explored the relationship between water purification and soil conservation in the Luan River Basin from the perspective of trade-offs and synergies [7]. Mei Yun and other scholars analyzed the spatial and temporal changes of water purification capacity in the Guanting reservoir watershed from the watershed scale and found that the water purification capacity within the study area showed a trend of gradual enhancement [8]. The study of Li Wei and other scholars concluded that each land type has different effects on the changes in nitrogen and phosphorus nutrients and that there is a linear relationship between the total output of nitrogen and phosphorus nutrients for each area occupied by a specific land type [9]. In summary, the water quality purification capacity in the academic research has a deeper foundation, is a more perfect academic concept, and has a degree of concern. Given this observed linear relationship, we believe that land use type distribution can be considered starting at the county level (rather than the watershed level). By doing so, we can specifically link data inputs and outputs at a scale that provides results to inform and guide county government objectives.

Water purification capacity specifically refers to the efficiency of ecosystems in reducing the migration of nitrogen- and phosphorus-rich nutrients or sediments from surface runoff to water bodies through processes such as physical interception, biological uptake, and chemical transformation, and it is essentially an ecosystem’s service of intercepting pollution from opposite sources [10]. This capability is widely found in natural ecosystems such as woodland and grassland. Plant roots and litter layers can slow runoff and adsorb particulate matter; microorganisms remove dissolved nitrogen and phosphorus through assimilation and denitrification; and soil colloids fix pollutants through ion exchange. This service effectively slows the process of eutrophication in lakes and rivers and is a key mechanism for maintaining the health of the watershed’s water environment. Researching the water purification capacity of a region is an extension of concern for the local water environment and ecosystem services. As an important region in central China, Fuyang City has strong universality and representativeness. Researching the water purification capacity of the Fuyang region can provide reference value for most regions in China.

As the largest tributary of the Huaihe River and the main river in Fuyang, the water quality of the Yinghe River has been severely affected by the rapid increase in population and industrial activities along its banks. The increase in pollution sources has had an impact on the quality of its water environment. In addition, other water systems throughout Fuyang have also had an impact on the overall water quality [11]. At the end of 2020, Fuyang City Environmental Protection Inspection Station, after analyzing 19 monitoring sections, found that there are seven sections with water quality of class I~III, accounting for 36.84%, and 12 with class IV~V, accounting for 63.16%. According to the water environment quality report of Fuyang City, as of the end of 2023, the overall water quality condition of surface water in Fuyang City was still mildly polluted. The Yinghe River runs through almost the entire Fuyang city area, and the mildly polluted water quality creates a water shortage in Fuyang. As a typical representative of water-scarce cities, Fuyang has a large amount of agricultural land, and the quality of the urban water environment is crucial to the development of Fuyang. In recent years, Fuyang city has experienced significant urban expansion and excellent economic development. Therefore, the Fuyang region was selected as the research object in this study. The research findings provide a scientific basis for coordinating water ecology protection and economic development in Fuyang City, offering both practical guidance and planning reference value. This can serve as an important reference for future urban land use and development planning.

Water quality purification capacity primarily refers to the natural purification capacity of natural ecosystems. Urban development is often accompanied by changes in land use, so changes in water quality purification capacity are somewhat synchronized with changes in urban land use. Preliminary research suggests that changes in land use structure have a degree of impact on water quality purification capacity in Fuyang. This study tries to explore how the land use structure of Fuyang city affects the water purification capacity function from 2013 to 2023, as well as to conclude how to optimize the capacity of urban water purification services in Fuyang by changing the land use structure. Traditional watershed studies often cross administrative boundaries, making it difficult for local governments to directly apply the research findings [6]. In contrast, this study adopts a county-level administrative scale, aiming to provide more practical guidance and assistance to county governments within the study area.

2. Materials and Methods

2.1. Study Area

Fuyang City currently administers three urban districts, four counties, and one county-level city under its jurisdiction. Specifically, these include Yingzhou District, Yingdong District, and Yingquan District as the three urban districts; Linquan County, Taihe County, Funan County, and Yingshang County as the four counties; and the county-level city of Jieshou. For the scope of this study, districts are urban districts of Fuyang City and form the core part of the city; counties and county-level cities are at the same administrative level, but county-level cities typically have greater economic autonomy and more important regional status. Although the districts, counties, and county-level cities differ in their functional roles and stages of development, they are all classified as county-level administrative divisions in terms of administrative hierarchy and are uniformly under the leadership and management of the Fuyang Municipal Government. Therefore, in terms of urban planning, population aggregation, and public resource allocation, all districts and counties adhere to the municipal government’s overall planning and collectively serve the overall development objectives of the Fuyang region.

The study area of Fuyang is mostly plain and open, and the main river in Fuyang City is the Yinghe River, which is also known as the Shaying River because its main tributary is the Shahe River, and it is the largest tributary of the Huaihe River, a Class IV river that flows through the southern part of Henan Province and the northwestern part of Anhui Province, and it is located at longitude 111°57′–116°43′ E and latitude 32°31′–34°52′ N. The hydrological conditions of the Yinghe River in Fuyang are influenced by a variety of factors, including climate, topography, vegetation cover, and other factors [12]. Among them, different land types (such as woodland, grasslands, and arable land) directly reflect differences in vegetation coverage (Figure 1).

2.2. Data Sources

The data required for this study consist mainly of meteorological data, digital elevation model (DEM) data, land use data, and other regional data. Meteorological data are primarily annual precipitation data and are derived from the National Earth System Science Data Center (https://www.geodata.cn/) (accessed on 1 March 2025). Data with an accuracy of 30 m were selected for a total of five years of 2013, 2015, 2018, 2020 and 2023. DEM data were obtained from the Geospatial Data Cloud (https://www.gscloud.cn/) (accessed on 1 March 2025), and the accuracy is 30 m. Land use data are derived from the Resource and Environmental Science Data Platform (https://www.resdc.cn/) (accessed on 1 March,2025). Data with an accuracy of 1 km were selected for a total of five years, 2013, 2015, 2018, 2020 and 2023. The other regional data were mainly obtained from the website of the Resource and Environmental Sciences Data Platform (https://www.resdc.cn/) (accessed on 1 March 2025) and were cropped using the vector boundaries of the study area, all of which were uniformly converted to the WGS 1984 UTM Zone 48 N projected coordinate system before spatial analysis was undertaken.

2.3. Related Tools

Currently, the main mechanistic modeling tools used to investigate water purification are the InVEST model, the SWAT model [13], and the SWMM model [14]. These tools simulate water quality in river stretches but do not typically provide spatially detailed maps showing where the landscape delivers purification services. Compared with other models, the InVEST model is more suitable for a longer time span because it is often evaluated on an annual scale, whereas SWAT and others use seasons and months as time scales. At the same time, the InVEST model is simpler to operate, requires less data precision, and is able to reflect the output intensity of nitrogen and phosphorus spatially. Most importantly, the water quality purification module of the InVEST model explicitly requires data on urban land use types, and the different types of urban land use in the module have different impacts on the output of nitrogen and phosphorus nutrients, which is more conducive to the direction of this study.

The InVEST model, known as Integrated Valuation of Ecosystem Services and Trade-offs, is a modeling system jointly developed by Stanford University, The Nature Conservancy (TNC), and the World-Wide Fund for Nature (WWF). The model aims to provide a scientific basis for decision makers to weigh the benefits and impacts of human activities by simulating changes in the quality and value of ecosystem services under different land cover scenarios. It was originally designed for effective natural resource management decision-making, by quantifying ecosystem services and expressing them in graphical form. Ecosystem services are more focused on the benefits that humans themselves receive from ecosystems, and the InVEST model is able to identify how to enhance the well-being of both humans and nature, thus supporting the integrated analysis of multiple services and objectives [15]. Therefore, the InVEST model is selected as the relevant tool for this study.

2.4. Research Methods

2.4.1. Nitrogen and Phosphorus Purification

The nitrogen and phosphorus purification amounts are calculated using the Nutrient Delivery Ratio (NDR) module in the InVEST model. The specific formula is shown below [12]:

$${\mathrm{X}}_{\mathrm{e}\mathrm{p}\mathrm{x},\mathrm{j}}={\mathrm{l}\mathrm{o}\mathrm{a}\mathrm{d}}_{\mathrm{s}\mathrm{u}\mathrm{r}\mathrm{f},\mathrm{j}}{\mathrm{N}\mathrm{D}\mathrm{R}}_{\mathrm{s}\mathrm{u}\mathrm{r}\mathrm{f},\mathrm{j}}+{\mathrm{l}\mathrm{o}\mathrm{a}\mathrm{d}}_{\mathrm{s}\mathrm{u}\mathrm{b}\mathrm{s},\mathrm{j}}{\mathrm{N}\mathrm{D}\mathrm{R}}_{\mathrm{s}\mathrm{u}\mathrm{b}\mathrm{s},\mathrm{j}}$$

(1)

where ${\mathrm{X}}_{\mathrm{e}\mathrm{p}\mathrm{x},\mathrm{j}}$ represents the purified nutrient output from each raster cell in the study area; ${\mathrm{l}\mathrm{o}\mathrm{a}\mathrm{d}}_{\mathrm{s}\mathrm{u}\mathrm{r}\mathrm{f},\mathrm{j}}$ represents the load of a nutrient on the surface; ${\mathrm{N}\mathrm{D}\mathrm{R}}_{\mathrm{s}\mathrm{u}\mathrm{r}\mathrm{f},\mathrm{j}}$ represents the rate of transport of a nutrient on the surface; ${\mathrm{l}\mathrm{o}\mathrm{a}\mathrm{d}}_{\mathrm{s}\mathrm{u}\mathrm{b}\mathrm{s},\mathrm{j}}$ represents the load of a nutrient in the subsurface; and ${\mathrm{N}\mathrm{D}\mathrm{R}}_{\mathrm{s}\mathrm{u}\mathrm{b}\mathrm{s},\mathrm{j}}$ represents the rate of transport of a nutrient in the subsurface. Among them, the values of nitrogen and phosphorus outputs represent the values of nitrogen and phosphorus elemental nutrients finally output in each unit in Fuyang area through the water flow of Yinghe River after natural purification of water quality purification capacity function in ecosystem service function. Under the premise that the natural input of nitrogen and phosphorus nutrients remains relatively constant, the higher the value of the final nitrogen and phosphorus output, the weaker the water quality purification capacity function in the ecosystem service function in the study area; on the contrary, the lower the value of the final nitrogen and phosphorus output, the stronger the water quality purification capacity function in the ecosystem service function in the study area.

The relevant parameters were determined based on specific previous studies [12]. It should be emphasized that, in the context of ecosystem services, the core lies in the various benefits that natural ecosystems provide to human society and the natural environment. Therefore, the term “grassland” here refers more to native grasslands as natural ecosystems rather than artificial pastures used for agricultural purposes. The relevant values are shown in

Table 1. Nutrient load and retention efficiency across major land use types.

Type of Land Use	TN Load Value (kg∙hm2∙a−1)	TP Load Value (kg∙hm2∙a−1)	TN, TP Retention Efficiency
Arable land	24.2	5.75	0.25
Woodland	3.68	0.28	0.7
Grassland	8.5	0.55	0.4
Water body	0.01	0.01	0.05
Construction land	14.5	3.85	0.05

:

2.4.2. Watershed Analysis

Watersheds are an essential input for the InVEST model’s Water Purification module to calculate nitrogen and phosphorus nutrient outputs. The delineation of watershed and sub-watershed boundaries within the study area was performed primarily through ArcGIS hydrological analysis tools. This process involved filling sinks in the digital elevation model (DEM), calculating flow direction and flow accumulation, delineating watershed boundaries, extracting the stream network, and identifying outlet points. Sub-watersheds were delineated to minimize computational errors and enhance model accuracy [4].

2.4.3. Information Entropy

In the InVEST model, different sites have different impacts on the output of nitrogen and phosphorus nutrients. In order to explore the comparison of the impacts of various types of land changes on the water purification capacity in the study, this paper adopts the information entropy weighting method to quantitatively compare the impacts of various types of land. Entropy describes the uncertainty of the occurrence of each possible event of the information source, which can also be understood as the degree of confusion or disorder of the information. In 1948, American mathematician Shannon proposed “information entropy,” providing its mathematical formula [16]. The specific formula is shown below [17]. The concept of information entropy solves the problem of quantitative measurement of uncertain information. In this study, the function of information entropy to quantify uncertain information is utilized to measure the degree of change in water purification capacity of various types of land use [18].

$$k={\displaystyle \frac{1}{ln\left(n\right)}}$$

(2)

$$H_n=-k{\Sigma }_i^n{p}_n\cdot ln{p}_n$$

(3)

In this study, $H_n$ represents the final value of the information entropy, which is the extent to which the nth type of land use affects the change in water purification capacity; $p_n$ denotes the share of the nth variable of the study in the sample, that is, the share of the nth land use type in the area of land use in each year in relation to the total study sample area of the nth land use type; $k$ is a constant and is only related to the number of samples in the study. $k$ = 1/ln(5) = 0.62, since only five special years of 2013, 2015, 2018, 2020, and 2023 were selected in this study.

3. Results and Discussion

Using the water purification module of the InVEST model, and incorporating multi-year land use data of the Fuyang region, DEM data adjusted for depressions, annual precipitation data, watershed data, and biophysical tables [19,20], the model automatically simulated the spatial distribution of nitrogen and phosphorus outputs in the Fuyang region for each year. Spatial analysis was then conducted at both the grid and county levels. The final results are shown in Figure 2, Figure 3, Figure 4 and Figure 5.

3.1. Spatial Differences in the Output of Nitrogen and Phosphorus

The spatial differences in the output of nitrogen and phosphorus nutrients at the grid scale are mainly expressed in the degree of color depth of different spatial grids, and the whole is represented in the form of a hotspot map. Among them, the spatial differences in the output values of nitrogen and phosphorus from different sites are caused by the different loading and retention ratios of different site structures for regional nitrogen and phosphorus nutrients.

In terms of the spatial distribution of nitrogen and phosphorus outputs, the highest values of nitrogen and phosphorus outputs at the grid scale were 1.36 kg/km² and 2.01 kg/km², respectively. At the county scale, the highest values were 0.19 kg/km² and 0.57 kg/km², while the lowest were 0.13 kg/km² and 0.42 kg/km². Based on a single year’s data, the maximum and minimum nitrogen and phosphorus outputs varied by approximately 20% among counties, suggesting relatively significant differences in water purification capacity across the study area. Among them, from the point of view of nitrogen output, the water purification capacity of Funan County and Yingzhou District is poor, while Taihe County water purification capacity is better; from the point of view of phosphorus output, the water purification capacity of Linquan County and Funan County is poor, while Yingzhou District water purification capacity is better. In summary, the high values of nitrogen and phosphorus outputs are mainly concentrated in the southwestern part of Fuyang, and the low values are mainly concentrated in the northern part of Fuyang. The water purification capacity of the Fuyang region shows the spatial pattern of strong in the north and weak in the southwest.

3.2. Temporal Changes in the Output of Nitrogen and Phosphorus

In terms of temporal changes in nitrogen and phosphorus output, the highest values of nitrogen and phosphorus output at the grid scale decreased from 1.25 kg/km² to 1.17 kg/km² and then increased to 1.25 kg/km², and decreased from 1.9 kg/km² to 1.87 kg/km² and then increased to 2 kg/km², respectively, between 2013 and 2023. At the county scale, the total county phosphorus or nitrogen output was analyzed for temporal changes using the product of county area and nitrogen or phosphorus output per unit grid to represent the total county phosphorus or nitrogen output. Changes in county nitrogen and phosphorus nutrients over time can, to some extent, reflect changes in the water purification capacity of the county, and the total county nitrogen and phosphorus outputs are shown in

Table 2. Nutrient output at the county level (t).

Regionalization	Funan County	Jieshou City	Linquan County	Taihe County	Yingdong District	Yingquan District	Yingshang County	Yingzhou District
2013	285.53	95.82	291.22	264.44	105.64	95.36	304.81	107.62
2015	287.01	98.71	293.11	264.58	105.19	101.95	302.26	109.44
2018	288.36	105.99	296.39	275.04	111.81	108.29	313.29	124.19
2020	280.86	104.53	290.25	264.09	107.29	102.21	308.45	112.96
2023	281.07	103.65	290.68	267.91	110.19	108.07	305.20	120.59

and

Table 3. Phosphorus output at the county level (t).

Regionalization	Funan County	Jieshou City	Linquan County	Taihe County	Yingdong District	Yingquan District	Yingshang County	Yingzhou District
2013	945.41	331.68	1008.66	882.31	322.93	317.28	991.48	294.93
2015	947.37	335.38	1012.45	887.59	324.13	322.59	994.66	296.73
2018	923.63	324.13	969.59	907.89	324.91	304.25	982.06	278.41
2020	976.72	318.11	961.20	881.89	321.55	307.76	963.79	279.31
2023	962.17	325.89	1011.94	882.71	314.78	301.63	952.87	282.00

.

From 2013 to 2023, the total output of nitrogen and phosphorus in Fuyang decreased from 6645.12 t to 6638.23 t and then to 6621.35 t. The total output of nitrogen and phosphorus was reduced by 23,770 kg during the ten-year period, resulting in an improvement in water purification capacity.

3.3. Changes in Spatial and Temporal Patterns of Water Purification Capacity for Nitrogen and Phosphorus

This study plotted the changes in the spatial and temporal patterns of nitrogen and phosphorus water purification capacities for the four different time periods from 2013 to 2015, 2015 to 2018, 2018 to 2020, and 2020 to 2023, as shown in Figure 6 and Figure 7 below. This study shows that the spatiotemporal pattern of changes in the water purification capacity of nitrogen and phosphorus in the Fuyang region as well as the reduction in the capacity mainly occur in the urban construction land and its surrounding areas. Although, as a whole, the output of nitrogen and phosphorus in all counties and districts showed an overall increasing trend during the decade of 2013–2023, the time period 2018–2023 was particularly special, with changes in the water purification capacity of a large area occurring and the degree of change being more pronounced. This may be due to the fact that in 2018, Fuyang introduced a relevant program to implement the “innovative, coordinated, green, open and shared” development concept, which greatly improved the ecological environment of Fuyang, thus improving the water purification capacity of Fuyang [21]. From the figure, it can be analyzed that the water purification capacity of nitrogen nutrient elements showed a change in the spatial and temporal pattern of large purification capacity improvement in the study time period of 2018–2020. Water purification capacity of phosphorus nutrient element presents a more homogeneous spatiotemporal pattern change from the southwest region to the northeast region, with the middle band of water quality capacity of phosphorus elements unchanged or the degree of change not being the dividing line. There has been a substantial reduction in the phosphorus nutrient water purification capacity in the southwestern region, whereas the northeastern region has shown a marked improvement in its spatiotemporal patterns of phosphorus purification capacity.

However, due to certain inaccuracies and uncertainties in the precision and specific value entropy of the land use maps, elevation DEM data, annual precipitation data, and watershed data, as well as the simplified and static approach adopted by the InVEST model for estimating various pollution sources, discrepancies inevitably arise between the model outputs and actual values. Consequently, the current results of this study are influenced by these limitations and are applicable primarily to the specific conditions of this region. Nevertheless, the impact of land use structure on nitrogen and phosphorus nutrient outputs exhibits greater general applicability.

4. Water Quality Impact Mechanisms and Land Use Structure Optimization Strategies

4.1. Characteristics of Spatial and Temporal Changes in Land Use Structure and Optimization

In recent years, intensified human activities and industrial development in Fuyang City have concentrated predominantly around the Yinghe River. Various urban construction and industrial lands now generate significant pollution affecting the river’s ecology, with primary pollutants including nitrogen, phosphorus, and other organic nutrients. This has elevated the risk of eutrophication in the Yinghe River [22,23]. Water purification capacity is mainly achieved through physical interception, biological absorption, and chemical conversion processes in ecosystems. Biological absorption refers to the absorption and assimilation of pollutants such as nitrogen and phosphorus in water bodies by plants and microorganisms. Since the spatial distribution of plant communities and microbial communities is essentially controlled by land use patterns, this capacity exhibits significant spatial heterogeneity at the regional scale [10]. Alterations in land-use patterns at regional scales disrupt the ecosystem balance, thereby weakening nutrient retention services such as nitrogen and phosphorus purification. To systematically examine the impact of land-use structure changes on water purification capacity in Fuyang, this study first synthesizes recent land-use transitions in the city, then correlates these changes with shifts in purification capacity, and subsequently analyzes the mechanistic influence of land structure on water purification capacity.

In order to visualize the changes in land use in Fuyang City over time, this study, with the help of the ArcGIS10.8 tool and relevant land use data, measured the land area and proportion of land use in Fuyang City in 2013, 2018, and 2023, as shown in the following Table 3. From the viewpoint of land use structure of Fuyang City, the land use of Fuyang City is mainly dominated by arable land, and the total proportion of land use in Fuyang City stays above the 80% level. This means that most of the land in this area is artificially managed farmland used for intensive agricultural production, rather than natural or semi-natural forest and grassland ecosystems.

In terms of changes in land use structure, the changes in land use structure in Fuyang City are mainly reflected in the changes in water body land, construction land, and arable land, of which the more obvious land use changes are the conversion of arable land into water body land and construction land. About 100 km² and 50 km² of arable land were converted to construction land and water body land, respectively. As part of land use restructuring, these newly incorporated water bodies consist primarily of human-made features such as excavated ponds, reservoirs, and ornamental lakes—as opposed to rivers or lakes shaped by natural hydrological processes. Spatially, such water features are often located in close proximity to or interwoven with built-up areas like roads and structures, reflecting a distinct pattern of coordinated development.

In terms of temporal changes, construction land in Fuyang City shows a gradual growth trend, and its proportion in the land use structure increases steadily. In the ten years from 2013 to 2023, the total growth area is about 103.32 km², with a growth rate of about 6.93%. The total growth area is obvious, and the growth rate is relatively considerable. Meanwhile, the total area of water bodies in Fuyang showed a trend of initial increase followed by a subsequent decline during the study period. Starting in 2013, supported by relevant policies, Fuyang achieved progress in ecological protection and restoration, particularly in its water resources and environment. However, in recent years, as Fuyang City has strategically absorbed and integrated industrial capacities relocated from coastal regions, its water environment has experienced significant ecological degradation, including water quality deterioration and functional regression. This industrial restructuring process is likely the primary driver behind the marked decline in Fuyang’s aquatic ecosystem self-purification capacity observed over the past decade [24].

4.2. Differences in Water Purification Capacity Among Districts and Counties in Fuyang City

Since water purification capacity specifically refers to the ecosystem’s ability to reduce the transfer efficiency of nutrient substances or sediments—particularly those rich in nitrogen and phosphorus—in surface runoff through processes such as physical interception, biological absorption, and chemical transformation, it is most appropriate to compare the total nitrogen and phosphorus nutrient outputs of each county when analyzing and comparing the water purification capacity curves.

To delineate zones based on the current water purification capacity in Fuyang City, this study sums the average nitrogen and phosphorus nutrient outputs per unit grid within each district or county. The total output values are then used to determine the relative strength of water purification capacity across different regions. Based on a comprehensive analysis of data from the years 2013, 2015, 2018, 2020, and 2023, this study identifies three natural breakpoints: 0.62 kg/km² to 0.64 kg/km², 0.64 kg/km² to 0.69 kg/km², and 0.69 kg/km² and above, corresponding to low-value, medium-value, and high-value zones, respectively. The thresholds of 0.62 kg/km² and 0.64 kg/km² serve as the dividing points between these categories. In this classification, high-value zones represent relatively weak water purification capacity, while low-value zones indicate stronger purification capacity. Based on this zoning, this study further analyzes the land use structure of each district and county in Fuyang [25], as shown in Figure 8 and

Table 4. Land use changes in Fuyang City from 2013 to 2023.

Type of Land Use		Arable Land	Woodland	Grassland	Water Body	Construction Land
2013	Area (km2)	8500.88	7.32	1	110.96	1491.01
	Proportion (%)	84.07	0.07	0.01	1.10	14.75
2023	Area (km2)	8363.30	8.04	3	140.43	1594.33
	Proportion (%)	82.73	0.08	0.03	1.39	15.77

.

According to Table 4, the land use structure across all districts and counties of Fuyang is predominantly characterized by arable land, followed by construction land. A general trend is observed wherein arable land is gradually decreasing, while construction land is steadily increasing. As shown in

Table 5. Summary of spatial patterns of water purification capacity.

Region	Water Purification Capacity Class	Dominant Land Types	Dynamic Trends
Funan county	high value zone	arable land + low-cover grassland	long-term vulnerability
Jieshou city	lower value zone	mixed arable land + arable land + water	continual improvement
Linquan county	high value zone	arable land dominance	continued deterioration
Taihe county	lower value zone	mixed arable land + arable land + water	stable and good
Yingdong district	lower value zone	water + arable land mosaic	volatility maintenance
Yingquan district	lower value zone	arable land + urban balance	continual improvement
Yingshang county	lower value zone	water continues to expand	continual improvement
Yingzhou district	lower value zone	water + arable land mosaic	continual improvement

, Taihe County, Yingquan District, Yingzhou District, and Yindong District have consistently demonstrated strong water purification capacity, largely due to their relatively diverse and rational land use structures. In contrast, the area of water bodies in Yingshang County has continuously expanded over the study period, contributing to a steady improvement in its water purification capacity. However, Linquan County and Funan County have long experienced poor water purification performance, mainly due to imbalanced land use structures dominated by agricultural land, with some areas even showing signs of continuous degradation.

In this paper, the entropy value method is used to analyze the water purification capacity of various land types in Fuyang City. The coefficients reflecting the influence of each land type on water purification capacity are summarized in

Table 6. Water purification capacity coefficient of various land structures in Fuyang City.

Type of Land Use	Arable Land	Woodland	Grassland	Water Body	Construction Land
$p_1\cdot \mathit{ln}{p}_1$	−0.37	−0.34	−0.32	−0.35	−0.36
$p_2\cdot \mathit{ln}{p}_2$	−0.36	−0.34	−0.32	−0.37	−0.37
$p_3\cdot \mathit{ln}{p}_3$	−0.34	−0.38	−0.31	−0.37	−0.37
${\Sigma }_i^n{p}_n\cdot \mathit{ln}{p}_n$	−1.07	−1.04	−0.95	−1.08	−1.10
$H$ (%)	66.34	64.48	58.90	66.96	68.20

, which shows that construction land, water bodies, and arable land have the greatest impact on changes in water purification capacity, at 68.20%, 66.96%, and 66.34%, respectively, which is consistent with research by Lie Wei et al. [9]. This indicates that arable land significantly impairs water purification capacity, while woodland and grassland enhance it, and construction land weakens surrounding water purification capacity. It can be concluded that construction land and arable land in Fuyang City negatively impact water purification, whereas woodland, grassland, and water bodies positively influence water purification capacity.

In urban areas, the primary causes of reduced purification capacity include insufficient permeable land surface area and reduced vegetation cover, which limit the soil’s ability to absorb nutrients. Additionally, surface runoff directly carries pollutants into water bodies, further exacerbating pollution. In agricultural areas, crop production (especially crops dependent on fertilizers) leads to nutrient excess, exceeding the soil’s own retention and purification capacity. Without effective protective measures, large amounts of nutrients such as nitrogen and phosphorus will ultimately flow into surface water bodies, triggering environmental issues such as eutrophication.

4.3. Optimization Strategy of Water Purification Capacity in Fuyang City

After the above land use analysis, it can be seen that the changes in the area of construction land and watershed land are the key to the changes in the regional water purification capacity of Fuyang City in recent years. By 2035, the amount of water space in Fuyang City should be no less than 859 km², which is an important reference target for how to expand the water land at the level of land use structure, and it is also a hard target to improve the water purification capacity of Fuyang City through the area of water. On the basis of adhering to the principles of ecological priority and green development, it should be ensured that the expansion of waterside land use is coordinated with ecological environmental protection, so that the expansion of waterside land use and ecological environmental protection can be developed in parallel. In addition, given the large area of arable land in Fuyang City, and considering that irrigation of arable land requires a large amount of water resources, which puts pressure on surface water and groundwater resources, it is necessary to optimize the distribution pattern of watersheds in combination with the extent of arable land according to local conditions [26].

For Fuyang region in this study, land use types of water body, woodland, and grassland have positive impacts on urban water purification capacity, among which water bodies have the greatest impact on the improvement of urban water purification capacity. Meanwhile, arable land and construction land have negative impacts on urban water purification capacity, and urban construction land is the most destructive to urban water purification capacity. This indicates that, from the perspective of land use structure, in order to enhance the urban water purification ability, the construction of water bodies, woodland, and grassland should be strengthened, and the focus should be on water bodies [27]. Similarly, in order to prevent the continuous deterioration of urban water purification capacity, the expansion of urban construction land and arable land should be strictly controlled, and focus should be given to the expansion of urban construction land. Based on this framework, and taking land use structure as the starting point, the Fuyang region’s areas requiring water purification capacity adjustment are categorized into water purification capacity deficit areas and water purification capacity regulation areas.

4.3.1. Water Purification Capacity Deficit Area

The water purification capacity deficit area mainly includes Linquan County and Funan County, and the land use types in these areas are more favorable than those in other counties, with arable land and construction land occupying a large proportion of the land use space, which in turn squeezes the land use space for other types of land use. In terms of land use structure, there is a lack of water bodies, woodland, and grassland.

The purification of nutrients such as nitrogen and phosphorus, which can lead to eutrophication, is primarily achieved through the synergistic interaction of physical, chemical, and biological processes. Physical processes include mechanisms such as sedimentation and adsorption of suspended solids; chemical processes involve reaction pathways such as oxidation–reduction reactions; and biological processes rely on microbial degradation and synthesis, as well as the absorption and transformation of nitrogen and phosphorus by plant roots, thereby effectively reducing pollutant loads. Increasing water body land use is one of the most effective ways to enhance purification efficiency in areas with insufficient water quality purification capacity. Such regions should prioritize increasing the proportion of water body land use in urban land use structures and can adopt diverse approaches such as constructing artificial wetland clusters and advancing urban ecological river restoration to systematically optimize water body spatial layout. Research by L. E. Bertassello and colleagues further highlights the critical role wetlands play in nitrogen removal within watersheds [28].

At the same time, it is also necessary to appropriately increase the land area allocated to woodland and grasslands, as these are the cornerstones for maintaining regional ecological security and establishing ecological barriers. The term grassland here specifically refers to natural grasslands as part of the natural ecosystem, rather than artificial pastures established for agricultural production. Such grasslands play a unique and critical role in providing ecosystem services [27]. Forest land and native grassland communities possess strong water retention capacity and permeability characteristics, enabling them to absorb and store large amounts of water during rainfall and release it slowly during dry seasons, thereby continuously replenishing river runoff and enhancing the water system’s purification potential. Additionally, plants in forested areas and native grasslands intercept and purify nitrogen and phosphorus nutrients from pesticide and fertilizer runoff through their roots, stems, and leaves. This not only reduces the natural purification pressure on water bodies but also enhances the ability to maintain and improve water quality at the regional scale.

Unsustainable agricultural practices can also have adverse effects on water quality. Scholars such as Huma Zia have summarized the specific impacts of agricultural activities on water quality and identified four negative practices: excessive fertilizer use, poor water resource management, improper livestock and aquaculture management, and excessive use of pesticides and herbicides [29].

To address the impact of agricultural activities on water quality, the following optimization strategies are recommended: Precision fertilization and nutrient management: use controlled-release fertilizers to improve utilization rates, and install smart sensors to monitor nutrient loss. Improve irrigation and drainage management: upgrade drip irrigation systems, install soil moisture sensors for on-demand irrigation, and construct ecological ditches to purify farm drainage. Standardize livestock farming practices: control livestock density and centrally process animal waste. Promote organic alternatives and green pest control: introduce biological pesticides and natural enemy-based pest control technologies.

4.3.2. Water Purification Capacity Regulation Area

The water purification capacity regulation areas primarily include Taihe County, Yingquan District, Yingzhou District, Yindong District, Yingshang County, and Jieshou County. Compared with the deficit areas, these areas demonstrate relatively strong performance in water purification capacity. Analysis of their land use structure indicates a more balanced and rational allocation of land resources. Therefore, in these regions, the expansion of urban construction land and cropland should be properly regulated.

Among them, urban construction land is the most important factor affecting the weakening of urban water purification capacity, and water purification capacity regulation areas should reasonably control urban development and construction to avoid the phenomenon of weakening of water purification capacity due to the uncontrolled expansion of urban construction land.

It is worth noting that the decline in water purification capacity often occurs in construction land and its surrounding areas, suggesting the possible encroachment of construction land on other land types. To address this issue, it is essential to enforce urban development boundaries and village construction boundaries, as well as to strictly control the scale of urban and rural construction land. The occupation of ecological land—including water bodies—by construction land must be strictly prohibited. At the same time, planners should optimize various types of ecological land within urban areas, such as the “blue-green network,” to guide the rational spatial layout of ecological spaces within towns and cities. This approach aims to establish an integrated urban ecological network that combines blue (water) and green (vegetation) elements, thereby enhancing the water purification capacity both around and within construction land areas [30].

4.3.3. Related Policy Optimization

Water purification is an important part of ecosystem services, and its purification capacity reflects the quality of local water resources to a certain extent [6]. The quality of water resources is critical to human well-being and to every aspect of the ecosystem. Therefore, the water purification capacity of ecosystem services should be strongly supported by relevant policies to protect the health and stability of ecosystems. For example, we should increase the investigation and punishment of illegal behaviors in water body land use and ensure that all laws, regulations, and policy measures are effectively enforced, so as to strengthen the supervision of water body land use and correct problems in a timely manner. Chaohu has established a comprehensive water pollution prevention and control system across the entire watershed, with a particular focus on agricultural non-point source pollution. By constructing a ring of wetlands around the lake, the city has implemented ecological barriers and water quality purification measures. By 2024, the water quality of the entire lake is expected to stabilize at Class IV standards, and its ecological restoration projects have been selected as outstanding cases by the United Nations. Huai’an City has prioritized rural drinking water safety as a key public service, systematically conducting pollution source inspections and issue rectifications in agricultural planting areas and rural residential zones to effectively ensure water source quality. The practices of these cities are similar to Fuyang’s current development stage, and their experiences in wetland construction, non-point source governance, and rural wastewater management provide practical and feasible reference pathways for Fuyang to enhance its water quality purification capabilities.

Zhi Fei et al., in their study, showed that nitrogen and phosphorus nutrients from arable land fertilizer and pesticides enter through rainfall into the water system to exacerbate the phenomenon of water pollution [30]. Studies by Zhi Fei et al. have shown that arable land contributes to increased water pollution due to the runoff of nitrogen and phosphorus nutrients from the use of chemical fertilizers and pesticides entering water bodies through rainfall [2]. Given that the study area contains a large amount of arable land, relevant government authorities should actively promote the adoption of integrated water and fertilizer management projects, as well as related pond and sewage treatment facilities. Implementing these technological measures to intercept pollution sources aims to reduce the burden on the natural water purification capacity of the water systems.

5. Conclusions

This study utilizes the NDR module of the InVEST model for data processing to simulate the water purification capacity of the study area under reasonable environmental conditions. Unlike previous research focusing broadly on water purification capacity, this study specifically examines the impact of land use structure changes on urban water purification capacity. Based on the current analysis of nitrogen and phosphorus nutrient outputs in the Fuyang region, the total combined output of nitrogen and phosphorus is used as the criterion for assessing water purification capacity. This study evaluates the influence of different land use types by analyzing the magnitude of land use changes and their correlation coefficients, ultimately proposing targeted strategies. Given that Fuyang shares common characteristics with other cities in central China, the findings of this research provide valuable insights for assessing the impact of land use structure on water purification capacity in other urban areas within central China.

Potential discrepancies between the results and actual conditions may arise due to the resolution limitations of the mapping scales employed for land use, elevation, and precipitation data, as well as the idealized model assumptions that exclude social factors. The main conclusions of the study are shown below.

(1) County-level division of water purification capacity: There are significant spatial differences in water purification capacity across the Fuyang region, primarily manifested by higher nitrogen and phosphorus output values concentrated in the southwestern area of Fuyang, while lower values are mainly found in the northern part. At the county scale, the maximum outputs of nitrogen and phosphorus nutrients are 0.191 kg/km² and 0.53 kg/km², respectively; the minimum values are 0.137 kg/km² and 0.42 kg/km², respectively. The water purification capacity within the Fuyang watershed exhibits a spatial pattern characterized by stronger capacity in the north and weaker capacity in the southwest. Using the natural breaks classification method, the Fuyang region is divided into three zones based on the total output of nitrogen and phosphorus nutrients, the low-value zone (0.62 kg/km² to 0.64 kg/km²), medium-value zone (0.64 kg/km² to 0.69 kg/km²), and high-value zone (above 0.69 kg/km²), which represent the relative strength of water purification capacity at the county level in Fuyang.

(2) Comparison of the impacts of different land use types: Based on the county-level division of water purification capacity described above, this study compares the relative impacts of various land use types by combining the magnitude of changes in land use structure with correlation coefficient analysis. Incorporating previous research on the positive and negative influences of land use types on water purification capacity, this study concludes that urban construction land and arable land exert negative effects, with urban construction land having a more pronounced detrimental impact. Conversely, water body land, woodland, and grassland demonstrate positive effects, among which water body land has the strongest positive influence in mitigating the negative effects of urban construction land.

(3) Countermeasure recommendations for different types of regions: Based on the division of water purification capacity of the above counties, this study divides Funan County and Linquan County into water purification capacity deficit areas and Taihe County, Yingquan District, Yingzhou District, Yingdong District, Yingshang County, and Jieshou City into regional water purification capacity regulation areas. And from the perspective of land use structure, combined with the strong differences and similarities between the positive and negative impacts of various types of land on water purification capacity, different strategy proposals are put forward for the water purification capacity deficit area and regulation area, respectively, in order to optimize the water purification capacity of the Fuyang region, which is of certain reference value for other areas of the same type.