Nature-Based Solutions for Landscape Performance Evaluation—Handan Garden Expo Park’s “Clear as a Drain” Artificial Wetland as an Example

Abstract

As a technology for water landscape performance that considers landscape, ecological, and social effects, nature-based solutions play a crucial role in enhancing the functionality of integrated ecosystem services on the micro-scale. This study conducted a systematic investigation into the landscape performance of the “Clear as a Drain” composite sponge facility at Handan Garden Expo Park. The following conclusions were drawn: (1) In terms of ecological restoration support services, the “Clear as a Drain” artificial wetland exhibited diverse habitat types, a rich variety of plant species specific to the site’s region, and high plant diversity indices for shrubs (1.776) and herbaceous aquatic plants (3.352). Reclaimed water reused in the artificial terraced wetland promoted plant growth and diversity while contributing to site self-rehabilitation; plants also significantly contributed to carbon fixation, oxygen release, and carbon emission reduction. (2) Regarding ecological restoration regulation services, the artificial wetland effectively purified reclaimed water with substantial improvements observed in incoming water quality during spring, summer, and autumn—particularly notable purification effects were observed during the summer months. Pollutant reduction rates for COD, BOD₅ ammonia nitrogen, TP, and TN reached 75.8%, 72.1%, 93.8%, 96.7%, and 90.3%, respectively; different independent subsystems within the wetland demonstrated distinct advantages in pollutant removal; park plants displayed strong air purification capabilities; annual energy savings from park plants could fully cover daily energy consumption for nearby residents. This case could serve as guidance for scientific management and design parameter optimization of other composite sponge facilities.

1. Introduction

Nature-based solutions are actions aimed at protecting, sustainably managing, restoring, or improving nature and ecosystems to efficiently solve societal challenges, such as climate change, food security, water security, human health, natural disasters, and social and economic development, while also aiming to provide benefits for human well-being and biodiversity. Among these, the use of nature-based techniques is widely applied in water ecosystem restoration [1].

With the promotion of the concept of nature-based solutions, composite sponge facilities such as sunken green spaces, ecological ponds, surface-flow artificial wetlands, and submerged artificial wetlands are widely used in urban stormwater management [2]. Through the continuous tracking and monitoring of various parameters of completed projects, a comprehensive evaluation of various ecosystem services provided by sponge facilities is carried out, and then the coupling law of various factors is grasped, and thus the optimal and sustainable operation parameters of sponge facilities are proposed. This has a positive effect on broadening the theoretical approach to water landscape performance.

Currently, LEED, SITES, and LPS are the three most influential systems for evaluating landscape performance. These systems are associated with the assessment of objects and factors related to ecological factors such as land, water, habitat, energy utilization, and raw materials. The social benefit index primarily focuses on leisure tourism and similar aspects, while economic research is relatively limited [3,4]. A series of experts and scholars at home and abroad have carried out relevant monitoring studies for different types (ecological pond type, submerged artificial wetland type, composite type, etc.) of completed sponge facilities [5,6]. Specifically, they have included research on the reduction rate of water inlet and outlet of sponge facilities, research on seasonal purification effects, and research on purification effects along the way. The United States government has carried out monitoring and evaluation of ecological restoration in Chesapeake Bay for 40 years, accumulating a large amount of research data, which provide data support for the evaluation of restoration effects, the selection of restoration programs, the selection of restoration technologies, and decisions regarding government-designed impervious ground areas and water purification efficiency to measure the effectiveness of performance-based payment contracts. At the same time, related economic benefits such as increasing employment have begun to be included in the scope of collaborative benefits of ecological restoration work [7]; Seattle’s Thornton Creek Water Quality Channel (TCWQC) was monitored for three years after the project was completed in accordance with the project’s quality assurance plan, including water quality, flow, sediment, vegetation, etc., to demonstrate that it can provide multiple ecological benefits in highly urbanized environments [8]. Chang [9] conducted monitoring experiments on the terraced submerged wetland built in Fengxiang Park in Haikou City and used a multivariate statistical regression model to study the impact of different elements on the pollutant removal rate, and then obtained the influence of each design parameters on the water quality purification effects. Gong proposed reconnaissance verification methods, monitoring methods, and joint monitoring and simulation assessment methods [10], but post-use evaluation and the evaluation of sustainability characteristics have been relatively lacking in terms of actual observation and study of the service quality of these built artificial ecosystems to establish the relationship between design elements, system function and post occupancy evaluation. This is also an organic part of design ecology, which has theoretical and practical significance for the promotion and continuous optimization of design.

At this stage, there are deficiencies in evaluation based on natural technology in the following aspects: insufficient continuous monitoring of the operational effects and water quality evaluation; lack of monitoring of long-term scale and seasonal changes in water quality of built projects; emphasis on a single evaluation of water quality purification effects; and lack of a comprehensive evaluation of the operational status of the composite sponge facilities. It is urgent to establish a comprehensive and systematic evaluation system framework to comprehensively evaluate the comprehensive and sustainable services of the sponge facilities [11,12,13,14,15]. This study took the “Clear as a Drain” artificial wetland in Handan Expo Park as the research object, which is a composite sponge facility designed for water purification and has been standing and operating for a certain period of time, with stable landscape effects. Based on water quality monitoring, plant diversity surveys, model calculation, etc., the ecological restoration performance of the “Clear as a Drain” composite sponge facilities was systematically studied, including landscape support services, landscape regulation services, and influencing factors affecting ecological restoration performance. The results were used to guide the scientific management and optimization of other nature-based composite sponge facilities.

2. Overview of the Garden Expo Park and the Artificial Wetland

Handan Garden Expo Park is located in the Fuxing District of Handan City. It is the source of the Qin River, the mother river of the Handan, which flows into Handan City. Due to urban expansion and industrial development, it became an industrial wasteland. The industrial brownfield area in the park reached 3 square kilometres and caused serious damage to the local air, soil, rivers, habitats, etc. The original “Clear as a Drain” artificial wetland site had a large amount of industrial waste. The surface was mostly hard concrete and bare soil. The plant condition was poor, and the flood storage capacity was weak. The terrain within the site was undulating, with large vertical, with a maximum height difference was about 30 m. The site is surrounded by three natural villages, each housing approximately 700 households within a 500-m radius.

The landscape design of the Garden Expo Park made full use of the existing terrain and water systems to sort and connect the original Qin River waterways, forming a water network that effectively connected the urban ecology and achieving the systematic restoration of the Qin River. The ecological design concept of minimal intervention was adopted for the existing water bodies. The concept was to retain the original wetland texture and replant vegetation appropriately in the wetland edge areas to provide habitat for birds and other creatures; for the steeply sloped areas with large height differences, the terrace texture was formed by cascading fill, and for the surface–flow artificial wetland, submerged artificial wetland and ecological ponds were set up on the terrace. For the problem of soil pollution, the methods of depression filling, soil replacement, and water purification were chosen. Existing slags were excavated, part of the soil was replaced, or wetland water systems were introduced and natural water systems were ecologically purified. Through the above landscape performance strategies, the industrial wasteland was restored to vitality, a complete ecosystem was formed, and natural self-regulation and purification capabilities were utilized and ultimately enabled the Garden Expo Park site to achieve the reproduction and regeneration of the natural landscape (Figure 1).

3. Materials and Methods

3.1. Monitoring of Water Quality Purification Effect

Based on the research purpose, representative wetland single ponds were selected for sampling. Three time points were selected for monitoring: November 2020 (autumn), April 2021 (spring), and July 2021 (summer). Sampling points (water inlet, T1–T5) were set up at the water inlet and 5 independent subsystem water outlets of the “Clear as a Drain” composite sponge facility area to characterize the water quality removal performance of each subsystem.

The principle of selecting continuous series of single-pond wetlands was to start monitoring from the single-pond wetland at the uppermost source of the wetland, to maintain non-crossing series of wetlands that were as long as possible, and to try to encompass each wetland type into the monitoring sampling points: 14 sampling points were set up in the T2 independent subsystem of the terraced flow with better operating conditions and landscape effects (T2-water inlet, T2-1, T2-4, T2-7, T2-9, T2-12, T2-13, T2-16, T2-19, T2-20, T2-21, T2-25, T2-27, T2-water outlet); 11 sampling points were set up in the T3 independent subsystems of the terraced flow with better operating conditions and landscape effects (T3-water inlet, T3-5, T3-13, T3-19, T3-21, T3-22, T3-25, T3-27 T3-30, T3-35, T3-37). The locations of wetland sampling points and observation points at all levels of the “Clear as a Drain” wetland area are shown in Figure 2.

The collected water samples were tested for total phosphorus (TP), total nitrogen (TN), ammonia nitrogen (NH₃-N), chemical oxygen demand (COD), biochemical oxygen demand (BOD₅), and suspended solids (SSs) in the laboratory.

3.2. Biodiversity Maintenance Investigation

Through field investigation, measurement data such as habitat types and plant species were obtained, followed by data analyses and comparative studies, which were compared with the original plant design parameters of the park. The investigation was conducted in the area of “Clear as a Drain” wetland area. The habitat types were investigated via visits, photography, and classification of the plant habitats in the park according to land use types and community type classification. On this basis, the “dominance analysis” method [16] was used to analyze the structures, dominant species, and understory plants of each habitat type, in which the dominant species were determined by calculating the basal area. For plant species, the plant quadrat method was adopted for the investigation and compared with the original plants design parameters. The location distribution of each quadrat is shown in Figure 2. The total area of quadrats for trees and shrubs accounted for 1% of the built-up area of the “Clear as a Drain” wetland area, using a unit area of 10 × 10 m; the total area of herbaceous aquatic plants quadrats accounted for 0.2% of the built-up area of the “Clear as a Drain” wetland area, using a unit area of 2 × 2 m. The site area of the “Clear as a Drain” wetland area was about 103,834 m², the calculated number of tree and shrub quadrats was 10, and the number of herbaceous and aquatic plants quadrats was 50. Within the tree and shrub quadrats, we recorded the species name, quantity, diameter at breast height, crown width, tree height, height under branches, height under leaves, and base diameter (the shrubs were recorded in the same way as the herbaceous aquatic plants); within the herbaceous aquatic plant quadrats, we recorded the species names, height, and covered area.

The diversity index of trees, shrubs, and herbaceous aquatic plants at the site was calculated using the following formula:

$$H=-{\displaystyle \sum _i^S\left(P_i\times \ln P_i\right)}$$

(1)

In the formula: S represents the total number of species within the site, i represents the species, and P_i is the relative importance value of species i.

3.3. Modelling Calculations

The study took the original plant design parameters of Handan Expo Park as the basic data source and used the tree benefit calculator NTBC to calculate the ecological benefits of trees in the park in terms of carbon sequestration and oxygen release; when calculating, the corresponding climate zone should be selected, and the variables to input consist of the species, diameter at breast height, and land types of trees in the park to obtain the amount of carbon sequestration, oxygen release, and carbon reduction per plant of the park’s various species and multiplied by the number of individuals of each tree species, and finally the relevant reference data are combined to further calculate the economic value of the trees (the calculation process was the same as for the benefits evaluation of ecological restoration regulation services of air purification and energy conservation).

The ecological restoration regulation services benefits of air purification and energy conservation refer to the official landscape performance evaluation website [17] and existing research results. The ecological restoration regulation service benefits of air purification and energy saving was evaluated in terms of the reduction of four air pollutants, namely, O₃, NO₂, SO₂ and PM10, and the amount of energy savings in electricity, petroleum, and natural gas, as well as their corresponding economic values. In addition to the combination of construction drawings and field surveys, data were recorded regarding the types, quantities, and diameters breast height of plants in the park. NTBC was used to conduct simulate simulations for each tree to obtain the corresponding functional quantities of each tree, and then the corresponding economic values were converted based on the conversion standards.

4. Evaluation of Landscape Performance in Handan Garden Expo Park

4.1. Assessment of Ecological Restoration Support Services

4.1.1. Habitat Types

Through investigation, it was found that the plant habitats in the “Clear as a Drain” wetland area could be divided into four types, namely deciduous broad-leaved forests, shrubs, grasses, and shallow swamps (Figure 2). At the same time, it was found that the habitat structures, dominant species, and lower-level plants of different habitats in the site were different, with significant differences (

Table 1. Analysis of habitat types in “Clear as a Drain” wetland area.

Habitat Types	Habitat Structures	Dominant Species	Lower Level Plants
Deciduous broad-leaved forest	Mainly composed of deciduous broad-leaved forests, with a combination of tree and shrub layers and herbs	Melia azedarach L. Euonymus maackii Rupr.	Acer negundo L., Pinus, Ligustrum compactum (Wall. ex G. Don) Hook. f., etc.
Shrubs	Combination of shrubs and herbs	Amorpha fruticosa Linn.	Rosa xanthina f.
Grass	No tree layer	Setaria viridis, Axonopus compressus Iris tectorum Maxim. chickweed chaparral Conyza canadensis (L.) Cronq., Salvia japonica Thunb., Echinochloa crusgalli (L.) Beauv	Sonchus asper (L.) Hill, Ophiopogon bodinieri Levl., Humulus scandens (Lour.) Merr. (Humulus scandens), Physostegia virginiana Benth., Beckmannia syzigachne (Steud.) Fern., Artemisia carvifolia Buch.-Ham. ex Roxb. Hort. Beng.
Shallow water swamp	A large number of emergent plants planted in the shallow water area of the wetland	Acorus calamus L., Phragmites australis (Cav.) Trin. ex Steud.	Liatris spicata (L.) Willd., Pontederia cordata L., Canna indica L., et al.

). The habitat structures within the site belonged to the “tree–shrub–grass” three-layer plant structure, but the shrub layer was relatively uniform, mainly composed of Amorpha fruticosa. The dominant species at the site, such as Amorpha fruticosa and gladiolus, showed strong water adaptability, tolerance to barrenness, and salt/alkali conditions, thus achieving the design goals of soil improvement, water quality purification, and ecological restoration; 17 naturally growing wild native plants of 17 genera and 9 families, including Echinochloa crusgali, Setaria viridis, and Kochia scoparia, had appeared in the grass and shallow swamp habitats (

Table 2. Wild Native Plants in “Clear as a Drain”.

The Name of the Plant		Family	Genus
Echinochloa crusgalli (L.) Beauv.		Poaceae	Echinacea
Kochia scoparia (L.) Schrad.		Chenopodiaceae	Kochia
Impatiens balsamina L.		Impatiensaceae	Impatiens
Setaria viridis (L.) Beauv.		Poaceae	Setaria
Cirsium japonicum Fisch. ex DC.		Asteraceae	Thistle
Pogonatherum crinitum (Thunb.) Kunth		Poaceae	Goldengrass
Cichorium endivia L.		Asteraceae	Chicory
Sonchus oleraceus L.		Asteraceae	Gesneria
Chenopodium album L.		Chenopodiaceae	Chenopodium
Solanum nigrum L.		Solanaceae	Solanum
Humulus scandens (Lour.) Merr.		Sanko	Humulus
Portulaca oleracea L.		Portulaceae	Portulaca
Digitaria sanguinalis (L.) Scop.		Poaceae	Crabgrass
Artemisia carvifolia Buch.-Ham. ex Roxb.		Asteraceae	Artemisia
Rumex acetosa L.		Polygonaceae	Sorrel
Beckmannia syzigachne (Steud.) Fern.		Poaceae	Brassica
Cyperus rotundus L.		Poaceae	Cyperus
Total:	17	9 family	17 Genus

).

4.1.2. Plant Diversity

(1)

Analysis of the basic situation and changes of plant species

There were a total of 29 families, 49 genera, and 54 species of plant in the artificial wetlands. Compared with the original design parameters (Figure 3), there were 11 families, 26 genera, and 28 species added. The main community structure was the tree layer and herbaceous layer, and the regional characteristics of the plant community were more obvious. In terms of dominant species, the originally planted vegetation such as Acer miyabei Ligustrum lucidum and other vegetation still accounted for a relatively high proportion; Melia azedarach, Euonymus maackii, Amorpha fruticosa, Setaria viridis, Iris tectorum, Axonopus compressus, and other plants had become new dominant species.

(2)

Analysis of plant diversity index and its changes

The overall diversity index of trees and shrubs was 1.776, and the overall diversity index of herbaceous aquatic plants was 3.352, which is much higher than the diversity index of trees and shrubs, and the diversity index of each quadrat is shown in

Table 3. Comparison of Shannon–Weiner index of quadrat plants after the constructed artificial wetland and at the beginning of the design.

Plant Types	Shannon–Weiner Index
Arbor and bush	Quadrat 1 0.937 (0.637)	Quadrat 2 0 (zero)	Quadrat 3 0.849 (0)	Quadrat 4 0.673 (0.673)	Quadrat 5 0 (0.673)
	Quadrat 6 0 (0.673)	Quadrat 7 0 (0.683)	Quadrat 8 0 (0)	Quadrat 9 0.562 (0)	Quadrat 10 1.011 (0)
Grass	Quadrat 1 0 (0)	Quadrat 2 0.562 (0)	Quadrat 3 0 (0)	Quadrat 4 0.410 (0)	Quadrat 5 0 (0)
	Quadrat 6 0 (0)	Quadrat7 1.674 (0)	Quadrat 8 0.760 (0)	Quadrat 9 0.500 (0)	Quadrat 10 0.325 (0)
	Quadrat 11 1.474 (0)	Quadrat 12 0.693 (0)	Quadrat 13 0 (0)	Quadrat 14 0.637 (0)	Quadrat 15 0 (0)
	Quadrat 16 0.693 (0)	Quadrat 17 0.673 (0)	Quadrat 18 1.560 (0)	Quadrat 19 1.010 (0)	Quadrat 20 0 (0)
	Quadrat 21 0.325 (0)	Quadrat 22 0 (0)	Quadrat 23 0 (0)	Quadrat 24 0 (0)	Quadrat 25 0.758 (0)
	Quadrat 26 0.802 (0)	Quadrat 27 0.858 (0)	Quadrat 28 0.802 (0)	Quadrat 29 0 (0)	Quadrat 30 0 (0)
	Quadrat 31 0.950 (0)	Quadrat 32 0.673 (0)	Quadrat 33 1.089 (0)	Quadrat 34 0.950 (0)	Quadrat 35 0.693 (0)
	Quadrat 36 1.540 (0)	Quadrat 37 0.218 (0)	Quadrat 38 0 (0)	Quadrat 39 0 (0)	Quadrat 40 0.191 (0)
	Quadrat 41 0.245 (0)	Quadrat 42 0 (0)	Quadrat 43 0 (0)	Quadrat 44 0 (0)	Quadrat 45 0.898 (0)
	Quadrat 46 0 (0)	Quadrat 47 0 (0)	Quadrat 48 0.509 (0)	Quadrat 49 0 (0)	Quadrat 50 0 (0)

Note: The values in parentheses represent the Shannon–Weiner index at the beginning of the design.

. The current situation is compared with the original plant design parameters of the exhibition garden. From a longitudinal perspective, 50% of the quadrats exhibited a Shannon index of 0 at the inception of the garden design; from a horizontal comparison, the Shannon index of trees and shrubs were all in the same range, and plant diversity had almost no change. In terms of herbaceous aquatic plants, from a longitudinal perspective, at the beginning of the garden design, the plant quadrats were all of a single species, accounting for 100%, and the plant types were uniform. While the current plant quadrats with Shannon index > 0 accounted for 58%, and the plant diversity was relatively high. In terms of horizontal comparison, the Shannon index of all the quadrats at the beginning of the design was 0, but the Shannon index scores in the current area increased significantly at each interval. After the completion of the park, the habitat in this area was better, and plant diversity was improved. Among these plants, the diversity of the surrounding areas of the terraced wetlands had increased more significantly. Through the establishment of optimal habitats, there will be a sustained enhancement in plant diversity and ecosystem services for the foreseeable future.

From a spatial perspective, in the “Clear as a Drain” wetland area, there was relatively low diversity in the terraced wetlands mainly planted with artificial large-area water purification or soil improvement plants, while the quadrats with higher Shannon–Weiner index were mostly located around the terraced wetlands and in the northern forest land, and at the same time, the plant diversity in the dry terraced wetlands was also higher. The above-mentioned areas with higher diversity are mostly naturally regenerated herbaceous plants, showing a good growth trend, such as Setaria viridis, Setaria viridis, Conyza canadensis, and many other new local herbaceous plant species. Meanwhile, it could be found that plant diversity index increased mainly in the above three areas, namely, around the terraced wetlands, the northern forest land, and the dry terraced wetlands.

4.1.3. Carbon Sequestration and Oxygen Release

Plants have significant functions in sequestering carbon and releasing oxygen, as well as reducing carbon emissions. In recent years, their functions of sequestering carbon and releasing oxygen to improve the environment in cities has gradually received more attention. Plants mainly reduce atmospheric carbon in two ways: firstly, they absorb CO₂ from the air through photosynthesis during growth, convert it into carbohydrate storage bodies, and release O₂, directly reducing the CO₂ content in the atmosphere; secondly, trees can regulate urban climate environments and reduce energy consumption, thereby indirectly reducing carbon emissions from the use of fossil fuels.

After calculation, Handan Garden Expo Park could sequester 748.76 t of carbon, release 545.79 t of oxygen, and reduce carbon emissions by 965.90 t in a year. According to the data provided by NTBC, an ordinary medium-sized car traveling about 20,000 kilometres per year will produce about 5 t of carbon dioxide. The fixed annual CO₂ emissions of Handan Garden Expo Park will be about 150 medium-sized cars, and this value will be even more significant in the future with the growth of the park’s trees. Meanwhile, referring to the unit price of 251.40 yuan/t for carbon sequestration and carbon sequestration functions in China’s afforestation cost method, the CO₂ fixation amount and CO₂ emission reduction in the park will be converted into the corresponding economic value of CNY 431,066.27 (

Table 4. Oxygen sequestration by vegetation in Handan Garden Expo Park.

Carbon Sequestration (t/Year)	Oxygen Release (t/Year)	Reduction of CO2 Emissions (t/Year)	Total Atmospheric CO2 Reduction (t/Year)	Economic Value ($/Year)
748.76	545.79	965.90	1714.66	431,066.27

).

4.2. Evaluation of Ecological Restoration Regulation Services

4.2.1. Water Purification

At the same time, the design parameters of each single pond wetland along the route were recorded to study the effects of different types of wetlands and design parameters on water quality purification. The locations of wetland sampling points and observation points at all levels of the “Clear as a Drain” wetland area are shown in Figure 2.

The detection results of different subsystems were sorted and combined according to the serial order of the wetlands, and the change diagram of the concentration of each pollutant was obtained. In Figure 2, the abscissa represents the number of the wetland group; for example, T2-1 refers to the first level wetland through which the inlet water flows in the T2 subsystem, and the vertical axis represents the concentration of pollutants. Thus, research on the seasonal purification effects of terraced wetlands was carried out.

(1)

Pollutant Removal Effects of “Clear as a Drain” Artificial Wetland in Spring

The average values of COD, BOD₅, ammonia nitrogen, TP, and TN at the sampling points in spring were 17.9 mg/L, 4.2 mg/L, 0.07 mg/L, 0.01 mg/L, and 0.32 mg/L, respectively; the pollutant removal rates were 45.9%, 46%, 40.9%, 85%, and 66.7%, respectively (Figure 4). The purification effect was more obvious during the closure period of the Garden Expo Park in spring, when the plant maintenance in “Clear as a Drain” was poor, and the number of blue-green algae in the water body increased, resulting in significant fluctuations in dissolved oxygen within a day. At the same time, some plants withered and died without timely harvesting, causing nutrients such as nitrogen and phosphorus to re-enter the water body.

(2)

The Removal Effects of Pollutants in “Clear as a Drain” Artificial Wetland in Summer

The average value of COD, BOD₅, ammonia nitrogen, TP, and TN at summer sampling points were 22.1 mg/L, 6.77 mg/L, 0.61 mg/L, 0.08 mg/L, and 0.95 mg/L, respectively; the pollutant removal rates were 57.2%, 33.9%, 95.8%, 96.3%, and 73%, respectively (Figure 5). The water quality changed from Class V incoming water to Class II water, and the water quality standard reached the design expectations. The research results show that the dissolved oxygen concentration in the “Clear as a Drain” artificial wetland increased, and COD and BOD₅ concentrations showed the opposite trend, with significant negative correlations. With the use of a large number of aquatic plants to sequester carbon and release oxygen, and ecological ponds and other falling water aeration facilities, the dissolved oxygen concentration gradually increased, and the pollutant purification effect gradually became apparent. Both water quality indicators and reduction rates performed best in summer [18]. This shows that the temperature and plant growth in summer have a significant impact on water purification.

(3)

The Removal Effects of Pollutants in “Clear as a Drain” Artificial Wetland in Autumn

The average values of COD, BOD₅, ammonia nitrogen, TP, and TN at the autumn sampling points were 18.01 mg/L, 7.15 mg/L, 0.27 mg/L, 0.03 mg/L, and 5.46 mg/L, respectively, and the removal rates were 51.8%, 35%, 42.8%, 72%, and 4.8%, respectively (Figure 6). Due to the obvious aerobic response of composite sponge facilities such as surface flow constructed wetlands and ecological ponds, the ammonia and nitrogen removal rate of sponge facilities was relatively high, reaching 43%, and the changes along the process were obvious. However, the removal effect of the composite sponge facility on TN was not obvious, and no obvious changes were seen in levels of TN (the outlet water quality did not meet the standard). It is possible that the system was not stable in the early plant growth stage and required long-term monitoring. In addition, the C/N ratio of the incoming water was relatively low, and denitrification would consume a large amount of carbon, affecting the denitrification of TN [19].

From the above research results, it can be seen that the reduction rates of water pollutants in the inlet and outlet water in the three seasons were relatively significant. The purification efficiency of wetland water in summer was significantly higher than that in spring and autumn, and the water quality changed from Class V inflow to Class II. This shows that this composite constructed wetland had good water purification effects, and the water quality standard met the design expectations. It is worth noting that due to many uncontrollable influencing factors during actual site operation, the concentration of pollutants increased instead of decreasing. There are various reasons for this; it may be due to the poor vegetation growth of the single-pond wetland, resulting in an increase in pollutants, or due to local blockage of the wetland, leading to the accumulation of pollutants, or possibly due to the instability of the pollutants themselves, which were not fully decomposed in reversible reactions [20,21].

4.2.2. Air Purification

Using NTBC to analyze and calculate the plants in the park, the annual absorption of ozone, nitrogen dioxide, sulfur dioxide, and particulate matter by the vegetation in the park was obtained [22]. Then, based on the amount of air purification, the economic value brought by the vegetation’s absorption of various pollutants could be converted. It could be concluded that the total amount of air purified by the vegetation in Handan Garden Expo Park throughout the whole year is about 13,836.56 kg, including 2181.73 kg of purified ozone, 4481.32 kg of nitrogen dioxide, 3422.85 kg of sulfur dioxide, and 3750.66 kg of particulate matter. It was found that the vegetation in Garden Expo Park had the strongest purification ability for nitrogen dioxide, followed by the purification ability for sulfur dioxide and particulate matter, and the purification ability for ozone was the weakest.

Due to the lack of a unified and clear economic conversion standard for ecological benefits in China, this study converted the economic benefits of plant purification of air pollutants based on the standards of the National Public Service Commission in the United States. The conversion criteria and economic benefits are as follows (

Table 5. Price conversion criteria for the economic value of vegetation purification of atmospheric pollutants.

Atmospheric Pollutants	Relevant Economic Value (CNY/kg)	Amount of Air Purified (kg/year)	Economic Value (USD/Year)
O3	54.23	2181.73	118,318.35
NO2	54.18	4481.32	243,067.55
SO2	23.22	3422.85	79,478.68
PM10	36.13	3750.66	135,511.75
Overall amount		13,836.56	576,376.33

).

4.2.3. Energy Saving

Buildings such as venues, exhibition halls, toilets, and shops in the park would generate energy consumption [23]. Reasonable plant configurations could generate certain economic benefits in terms of energy consumption. For example, in hot summer weather, the shading effect of plants lowered the indoor temperature of the building, thereby reducing electricity consumption; in cold winter weather, the design of plant groups blocked the cold wind and played a role in keeping warm, reducing the consumption of natural gas [24]. In addition, reasonable vegetation design was conducive to creating a good environment, improving local microclimate, and saving related electricity and air purification costs for nearby residents [25].

This study calculated and evaluated the energy saving benefits of plants in the park with the help of NTBC, and found that the total amount of electricity saved by the trees in Handan Garden Expo Park over the year was 812,665 kw·h, and the amount of oil or natural gas reduced was 851,402.67 m³. Based on a monthly electricity consumption of 300 kw·h per household, the annual energy saved by plants in the park was enough for 100 households to use in 2.3 years; based on a monthly use of 30 m³ of natural gas by each household, the annual amount of natural gas saved by the park’s vegetation was enough for 100 households for 23.7 years.

Since Handan residents have implemented time-of-use electricity prices, 8 a.m. and 10 p.m. were used as the dividing point; the peak period was 8:00–22:00 (electricity price of CNY 0.55), and the valley period was 22:00–8:00 (electricity price of CNY 0.30). This study used the standard of 0.55 to convert the economic value of electricity. Oil and natural gas were according to the latest sales pricing of 2.68 yuan/cubic meter adjusted by the Handan Municipal Development and Reform Commission. It was calculated that the total economic value of energy saving by plants in the park in 2021 reached 2,646,556 yuan (

Table 6. Energy saving status of vegetation in Handan Garden Expo Park.

	Power Cooling	Oil/Gas Reduction	Total Value
Energy savings	812,665 (kw·h/year)	300,849 (therm/year)
Energy savings	812,665 (kw·h/year)	851,402.67 (m3/year)
Economic value ($/year)	364,796.85	2,281,759.15	2,646,556.00

).

5. Conclusions

This study was guided by concepts of water control in the new era and uses nature-based technology to solve China’s present water problems. It aimed to transition from simple water conservancy project construction and water quality standards to comprehensively improving the service functions of water ecosystems and restoring the natural virtuous cycle of water bodies. This study focused on the actual landscape performance and influencing factors of the green technology that comprehensively enhance the ecosystem service function of composite sponge facilities. Empirical research was conducted using the example of the “Clear as a Drain” artificial wetland in Handan Garden Expo Park. The results showed that constructed wetlands play an important role in ecosystem support services and ecosystem regulation services, especially in water purification and biodiversity enhancement in summer. Relevant conclusions are as follows:

(1) The “Clear as a Drain” artificial wetland in Handan Garden Expo Park had a variety of habitat types, maintaining material circulation and the normal operation of the ecosystem. The dominant species presented adaptive landscape characteristics of purifying water quality and improving soil, and had strong ecological restoration functions. The rich plant species in the site has accelerated the natural succession process of native plants, with an increasing trend in plant species, especially the addition of many new local plants, which grew well and provided a good habitat for other organisms, demonstrating the good sustainability of the site. Meanwhile, plant diversity was relatively high, especially in the area around the terraced wetland, the northern forest land, and the dry terraced wetlands, where plant diversity had significantly increased compared with before. In addition, the ecological benefits of the site plants in carbon sequestration, oxygen release, and reducing carbon emissions were also significant. In short, the scientific and reasonable plant design of Handan Garden not only met the aesthetic functional requirements, but also provided good conditions for the self-restoration and regeneration of the site’s habitats, thereby providing sustainable natural services for the city and its residents.

(2) Handan Expo Park had played a positive role in ecosystem regulation. Among them, the “Clear as a Drain” artificial wetlands had a more significant effect on purifying recycled water. The inflow water quality significantly improved in spring, summer, and autumn, with the highest purification effect in summer. Therefore, during the operation of Garden Expo Park, the treatment volume in summer should be increased and the proportion of deep treatment of recycled water in summer should be increased throughout the year. The water quality gradually became better as the steps increased, and the reduction rate was obvious, which effectively conformed to the basic principle of first-order kinetic reaction. There was an obvious negative correlation between pollutant concentration and dissolved oxygen, and the effect of oxygen explosion facilities was obvious. The wetland purification effects of different independent subsystems had their own advantages in the removal of pollutants, and the length of the water flow path was positively correlated with the pollutant removal rate; the environment and air quality of the park had been greatly improved, and the plants in the park had shown strong air purification functions; the vegetation in the park was like a natural sponge, absorbing and digesting rainwater as a precious resource; the annual energy savings of plants in the park can fully cover the daily energy consumption of nearby residents in Qi Village.

(3) At the beginning of the planning of the constructed wetland, the sustainability of the ecosystem was fully considered. In the construction process, local renewable and recyclable materials, including steel slag, plant materials, wood, etc. were given priority to reduce the “ecological footprint” and “life cycle cost”. In terms of plant selection, a large number of native plants are selected, which can effectively adapt to local climate and soil conditions while reducing maintenance costs and water consumption. The establishment of good habitats helps to support the sustainable provision of services in local ecosystems. In the future, with the continuous investment in operation and maintenance, the artificial wetland will provide more systematic and integrated ecosystem services, such as stormwater management (reducing regional water security risks), aesthetic enlightenment (promoting regional culture), and so on.

Through empirical research on the “Clear as a Drain” artificial wetland in Handan Garden Expo Park, a quantitative study on the multiple comprehensive values of ecological restoration was carried out based on a comprehensive evaluation framework. This research results can provide support for the operation and maintenance of the “Clear as a Drain” artificial wetland in the Garden Expo Park and the subsequent design of composite sponge facilities. In the future, the project should be continuously tracked and studied, and the time dimension should be integrated into the open system for comprehensive analysis, in order to reflect the sustainability of the ecological restoration performance of composite sponge facilities.