The service function value assessment analysis of urban wetland ecosystem--A case study of Xi’an Chan-Ba wetland

To analyze the service function value of urban wetland ecosystems, a functional value assessment system was done for Xi’an Chan-Ba wetland. Using market value method, carbon tax law and afforestation cost method, alternative cost method, travelling cost method and so on, those values were converted into economic value and a summary of the same was made for comparison. The total service function value of Chan-Ba wetland ecosystem is 3.87 × 108 ￥. Of this, the value of wetland leisure tourism is the highest (accounting for 55.76%), followed by the value of carbon fixation and oxygen release from the wetland (accounting for 25.56%), other functional values accounting for smaller proportions. The value of ecosystem functions in Xi’an Chan-Ba wetland plays an important role in development of the urban economy. In that context, it should be reasonable to develop and protect the urban wetland ecosystem, and promote the virtuous circle of the urban wetland ecosystem.


Introduction
As a special ecosystem in the city, an urban wetland has significant economic, social and ecological values in terms of material production, ecological balance, flood regulation, leisure tourism and scientific research [1,2]. In order to fully understand the ecological function and service value of an urban wetland, it is necessary to analyze and evaluate the ecological function of the wetland.
Costanza et al. estimated the global ecosystem services value is $33.3×10 12 , which has 1.8-2 times of the global GDP. Among them, the wetlands (only 6% of the land area) have an ecological service value of $4.9×10 12 , accounting for 14.7% of the total ecosystem [3][4][5][6]. Chen estimated the total economic value of China's ecological services is￥7.8×10 12 , which is 1.73 times of the Gross National Product [7]. Han estimated the Yellow River delta wetland ecosystem service value is￥1.7608×10 10 , and per unit area value 52,809￥/hm 2 [8]. Zhuang assessed the ecological function value of Dongting Lake wetland in the northeast of Hubei Province; the results showed that the value is about two times of average annual revenue of the Lake District [9]. Cui assessed the ecosystem service function value of Pan-Yang Lake wetland as ￥3.627×10 10 [10]. Zhi evaluated the ecosystem service functions value of Baotou Nanhai wetland according to Gaia hypothesis (the earth self-regulation theory) and market value method, and found that the ecological function of wetland core area changed significantly with the seasons [11]. These results highlight the great economic value of wetland ecosystem service function. However, due to long-term unreasonable exploitation and neglect of the ecological environment protection, the ecological functions of urban wetlands are affected, resulting in the 2 degradation of urban ecosystems in China. This paper takes Xi'an Chan-Ba wetland (in arid and semi humid areas) as the research area, and studies its service function value. This research will help people to be more intuitive to realize the importance of urban wetland ecosystem, and provide scientific theoretical basis for the protection and management of such wetland ecosystem.

The study area
Chan-Ba wetland is located in the east of Xi'an city center in Shaanxi province. Its north is the south bank of Weihe River. The wetlands are mainly distributed in Chanhe River, Bahe River, Chan-Ba delta, the floodplain zone of Ba-Wei intersection and the Guangyuntan wetland scenic spot (see figure 1). The area of Chan-Ba wetland is about 2533 hm 2 , of which river wetland is 304 hm 2 , floodplain wetland is 1456 hm 2 , lake is 773 hm 2 (including natural lakes 233 hm 2 and the rest artificial).

Study method
The total economic value of wetland ecosystem service functions of Chan-Ba wetland was divided into three categories, namely material production function, ecological regulation function, social service function. Following the principles of feasibility and operability, this paper uses market value method, travel cost method, shadow engineering method, ecological method, carbon tax method, afforestation cost method and alternative cost method to estimate the values of wetland ecosystem functions (see table 2). It is a method of evaluating the value of ecosystem products and functions that has market price [13]. It is mainly used for evaluation of material products produced by ecosystems, and reflected through the market. The formula adopted for estimation is as follows: In the formula, V is the value of material products, P i is the market price of the i-th wetland products, Y i is the per unit area yield of the i-th wetland products, S i is the distributing area of the i-th wetland products.

Travel cost method.
Travel cost method is commonly used to evaluate the value of natural scenic spots and environmental resources for which a direct market price is difficult to estimate. The economic value of Landscape Tourism function is the sum of travel expenses (transportation, accommodation, etc.), consumer surplus and travel time value.

Shadow engineering method.
This method uses the cost of artificial construction project to replace the service value of wetland ecosystem function. The formula used is as follows: In the formula, Q is the value of wetland flood storage, V t is the amount of wetland water transfer, t is the average cost of construction unit volume capacity investment.

Carbon tax method and afforestation cost method.
Carbon tax is estimated based on the equation of photosynthesis (as in equation (3)) that relates the amount of the CO 2 absorption and O 2 release for unit material production. The economic value of fixed CO 2 and released O 2 is estimated based on international and Chinese standards of CO 2 emission by which the ecological indicators are converted into economic indicators [14].
Afforestation cost method is the product of the plant carbon net growth per unit area, the cost of 4 afforestation, and the total area under wetland plants [15].

Alternative cost method.
When a specific functional service value cannot be estimated in terms of market price, the market price of a substitute is used as the basis for determining the value of the service.

Evaluation of material production function value
The Chan-Ba wetland is rich in biological diversity. Many kinds of species, which can provide a variety of physical materials, including making paper, construction, living and medical materials, have high economic value. In terms of the raw materials the wetland has reed, cattail, weeping willow, grassland, hornworts (Ceratophyllaceae) and carp (in animals). Taking into account all these, it is estimated that the material production function value of Chan-Ba wetland is 1.41252×10 7 ￥/a that include 1.03752×10 7 ￥/a of crops and 3.7500×10 6 ￥/a of aquatic products (see table 3).  [16]. Then according to the equation of photosynthesis (equation (3)) we know that one gram dry matter produced by plant fix 1.63 g CO 2 and release 1.2g O 2 . Based on the current international standard of carbon tax of 150 dollars per ton (considering the exchange rate as 1:6.66 between RMB and dollar) and the afforestation cost in China for 260.90￥/t, it is concluded that the average 625.45 ￥/t as the carbon tax standards. The value of the release O 2 was calculated with the cost of industrial oxygen for 0.4 ￥/kg. In this study, reed and grass as representative of green vegetation were used to calculate the value and reed and grassland area were taken as 98 hm 2 , 1258 hm 2 , and paddy wetland area (estimated by planting area of aquatic plants) were 27 hm 2 . It is concluded that the total value of carbon fixation and oxygen release in Chan-Ba wetland is 9.90076×10 7 ￥, in which the value of carbon fixation is 3.63149×10 7 ￥ and the value of oxygen release 6.29627×10 7 ￥ (see table 4).  [17]. The total area of Chan-Ba wetland being 2533 hm 2 , the function value of pollution treatment is 4.07473×10 7 ￥/ a.

The function value of habitats.
Chan-Ba wetland is rich in diversity of birds including rare and endangered species. There are 63 kinds of protected animals (14 species of national protected animals and 49 species of protected animals-the endemic soft-shell turtle Pelodiscus sinensis and so on) in Shaanxi province. The function value of habitats in Chan-Ba wetland were estimated based on the values given by Costanza (1989) that the per unit area function value of habitats is 304 $/ hm 2 (converted into RMB it is 2024.64￥/hm 2 ) [18], it is concluded that the function value of habitats in this study area is 3.56337 ×10 6 ￥/ a.

The function value of flood storage.
Wetlands with flood storage function include river wetland, floodplain wetland, swamp wetland, paddy wetland and lakes. The swamp wetland and paddy wetland are important in the flood storage function of Chan-Ba wetland ecological system. Hence, these two types were used to calculate the value of homogenizing flood, using shadow engineering method to calculate. Based on the results of Meng, the per hectare water storage of swamp wetland and paddy wetland is 8100 m 3 [19]. Thus the total water storage of Chan-Ba wetland is 1.0125 ×10 6 m 3 . The capacity cost is 6.1 ￥ /m 3 according to the provisions of Assessment criteria for forest ecosystem services (LY/T1721-2008) [20]. It is concluded that the function value of homogenizing flood (Homogenization of the flood distribution) in Chan-Ba wetland is 6.1763 ×10 6 ￥/ a (see table 5). 6 wages is the 1/3 times of cost wages, the foreign tourists to take 300￥/d, the domestic tourist to take 50￥/d, so the total value of travel time is 6.66667 ×10 7 ￥. In summary, the function value of leisure tourism in Chan-Ba wetland is 2.159907×10 8 ￥/ a.

The value of scientific research and cultural.
The landscape of river wetland, floodplain wetland that lies in the intersection of Ba River and Wei River, Guangyuntan artificial lake wetland are the main tourist zone in Chan-Ba wetland ecosystem. Rich and varied water body and plants, landscape, and animals provide objects, materials and experimental base for scientific research and teaching. It also provides place for abundant visitors to know the wetland plants and animals, especially the wetland birds. It makes people appreciate the natural beauty, understand the significance of wetland function and the need for protecting wetland. Based on Chen and Costanza that the scientific research and cultural value of wetland ecosystem is 382￥/hm 2 [7] in China and 861$/hm 2 in the whole world, it is concluded that the average value of scientific research and cultural value is 3058.13￥/hm 2 .The total area of Chan-Ba wetland is 2533 hm 2 , thus the total value is 7.7462 ×10 6 ￥/a. From table 6, it can be seen that the ecological function value of Chan-Ba wetland is about 3.87×10 8 ￥/a, in which the contributions of material production, ecological, and social service function are 3.65%, 38.59% and 57.76% respectively. The leisure tourism value is the highest in Chan-Ba wetland ecosystem service function values and that can help promoting the economic development of Chan-Ba ecological area. The value of carbon fixation and oxygen release also play leading roles and would be reflected in regulation of climate, slowing greenhouse gas effect, and in general improving the environment. Other ecosystem functions accounted for relatively smaller proportions. The order of eight ecological service function values from big to small is leisure tourism, carbon fixation and oxygen release, pollution purification, plant resources, scientific research and cultural, flood amelioration, aquatic products and biological habitat. It can be seen that the value of Chan-Ba wetland reflects a reasonable combination of environmental protection, education and entertainment, and the value is huge.

Compared with the global and China's ecosystem service function value
The global wetland area is 5.1625×10 8 km 2 , the total value of its ecosystem services is 2.86105×10 14 ￥ and the unit value of ecological services is 5542￥/hm 2 [8]. The wetland area in China is 1.433×10 7 km 2 , the total value of ecosystem services 7.76657×10 12 ￥, and per unit value of ecological  [9]. The Chan-Ba wetland area is 2533 hm 2 , the value of ecosystem services is 3.87×10 8 ￥, and per unit value of ecological services 152,924￥/hm 2 [8]. Comparing the value with that of the country or global, it can be seen that the unit area ecosystem service value of Chan-Ba wetland is considerably higher. There are three points to be considered while explaining the research result. Firstly, due to the limitation of the data and estimation methods, there is a certain error between the estimated results and the actual value. Secondly, the research is based on the latest data, and these latest data are larger than earlier data because of strengthening of regional ecological governance. Thirdly, the Chan-Ba National Wetland Park is an important part of the Chan-Ba wetland system, it is a 4A level scenic spot, the number of tourists is nearly two million each year, and with the third phase construction there is a growth trend, and hence the value of leisure tourism function occupies a large proportion in the total value of ecosystem services. To end with, Chan-Ba wetland species diversity is rich, the water resources is adequate, the ecological system is stable, and provides good ecological services for promoting harmonious development of social economy in Xi'an.

Conclusions
In this paper, taking Xi'an Chan-Ba wetland as the research object, the service function value evaluation index system is established to assess the three types of eight kinds of ecological service function values. The results show that the service function value of Chan-Ba wetland is about 3.87×10 8 ￥/a. Among them，the social service function value is 2.237369×10 8 ￥ and it is the largest, accounting for 57.76% of the total value. Ecological regulation function value ranking second is 1.494946×10 8 ￥, accounting for 38.59% of the total value. The material production function value is 1.41252×10 7 ￥, accounting for 3.65% of the total value. From table 6, the function value of leisure tourism, carbon fixation and oxygen release, pollution purification are primary in Chan-Ba wetland ecosystem service functions. Leisure tourism is the most important functions and has huge economic benefits. Through this study, it can be seen that the service function value of urban wetland play an important role in the urban economic and social development, and ecological setup. The evaluation of urban wetland value can not only provide scientific basis for further management of ecological system, but also make people aware of the importance of wetland. We should rationally develop and protect the urban wetland ecosystem, realize the sustainable utilization of resources, and ensure the sustainable development of urban economy.