Abstract

With rapid economic development and expansion of urban boundaries, increasingly damaged wetland resources have seriously threatened the ecosystem. The study of eco-environmental requirements of wetlands is not only the basis of water resources allocation in development and utilization, but also for creating a sustainable system to maintain and improve the overall ecosystem. In this study, we used the Shuangtaizi Estuary Wetland as our study area. The breakdown of wetland cover types was extracted based on multi-source remote sensing data, providing the graphic database for ecological water requirement calculation. According to the characteristics of the Shuangtaizi Estuary Wetland ecosystem, the methods of quantifying the components of ecological water requirements were determined. The results showed that the optimum ecological water requirement of the total wetland was 239 million m3. The minimum, 75th percentile frequency, and 95th percentile frequency water requirements were 670 million m3, 921 million m3, and 1,078 million m3, respectively.

INTRODUCTION

Wetlands are referred to as the Earth's kidneys. They are crucial in maintaining ecological balance, and play a crucial role in the conservation of biodiversity on the planet (Wu & Li 2003). Wetlands not only provide abundant animal and plant resources (Gleick 1998, 2000), but also regulate and store flood water, regulate water supply, purify water, and provide other ecosystem services (Cui & Yang 2002). Recently, population and industrial growth combined with the expansion of urban boundaries have caused wetlands to be severely damaged. These human intrusions have resulted in weakened ecological functions of wetlands, and have seriously threatened habitat for red-crowned cranes (Wang et al. 2004). As people become more aware of the importance of wetlands and understand the relationship between wetlands and water, an increasing number of experts and scholars are beginning to seek new ways to help maintain and improve wetland health. To do so, researchers are trying to balance ecological water requirements with the rational allocation of water resources. Achieving this balance can ensure normal wetlands water circulation in order to improve the ecological function of the overall system, with the goal of restoring function and reconstructing wetland ecosystems (Robertson et al. 1998; Chen 2012). Based on multi-source remote sensing data and related meteorological data, this research on the eco-environment water requirement in the Shuangtaizi Estuary Wetland quantifies the suitable and minimal ecological water requirements, and provides the basis for the optimal allocation of water resources in wetlands.

MATERIALS AND METHODS

Description of the study area

Shuangtaizi Estuary Wetland is a national wetland nature reserve, located in the mouth of the Shuangtaizi River in Panjin City in the northern part of Liaodong Bay in Liaoning Province. The geographical coordinates range from 121° 30′ to 122° 00E, and from 40° 45′ to 41° 10′N. Its drainage area is 1,281.03 km2. The total number of species in the wetland has been estimated to be 1,096. There are a variety of plant species and rare wild animals (Luo et al. 2011). According to the statistics, there are 5 species of Country grade I protected animals and 30 species of Country grade II protected animals. The average annual evaporation in the wetland is 942.1 mm with an average annual rainfall of 640.9 mm (mainly concentrated in the summer). A significant characteristic of this area is that the annual evaporation is greater than the rainfall. The location of Shuangtaizi Estuary Wetland National Nature Reserve is shown in Figure 1.

Figure 1

Location of Shuangtaizi Estuary Wetland Nature Reserve.

Figure 1

Location of Shuangtaizi Estuary Wetland Nature Reserve.

Data processing

Since the establishment of the Shuangtaizi Estuary Wetland Nature Reserve in 1985, the government has implemented three phases of the project. The first phase of infrastructure construction started in 1991. The second phase of infrastructure construction was started in early 2004 and completed in 2005. The wetland protection and construction project began in 2006 and was completed in 2008. Therefore, the three phases of wetland cover information were extracted based on 3S technology for the purpose of providing calculated data.

The basic data in the Shuangtaizi Estuary Wetland Nature Reserve include image data, topographic map data, field collection data, and related auxiliary data (Gillibrand & Balls 1998). The remote sensing images are multi-band scan images for 1996, 2006 acquired by thematic mapper of US Landsat 4–5 and Landsat 8 OLI_TIRS image for 2016 (Goodrich et al. 2000). The topographic data include a CAD line of the protected areas in 2010. The supporting materials include the water system (2015), administrative boundary map (2010), land use status map (2015), vegetation map (2010), functional area map (2010), relevant texts, and the graphic data.

The remote sensing image processing software, ErdasImagine, corrected the images for platform radiation and geometry. Classification and interpretation of remote sensing images were carried out in the ErdasImagine platform. The graphic database was established with the field detection of GPS (global positioning system) and the topographical features on the images. The land cover classification attribute database was built using the standard of the Liaoning Province classification system. According to the characteristics of the nature reserve (Wang & Liu 2009), the land cover types were divided into reed, paddy fields, dry land, beach, lake, residential, and river. Referring to characteristics and ecological environment, wetlands were divided into core area, buffer area and transition area.

The land cover distribution maps in 1996, 2006, and 2016 are shown in Figure 2. The various areas of land cover types in Shuangtaizi Estuary Wetland Nature Reserve are shown in Table 1.

Table 1

Areas of land cover types in Shuangtaizi Estuary Wetland (km2)

Types 1996
 
2006
 
2016
 
Core area Whole wetland Core area Whole wetland Core area Whole wetland 
Reed 133.07 489.77 130.11 470.28 128.64 479.77 
River 264.73 452.59 281.51 470.17 287.50 469.23 
Lake 1.15 19.53 2.11 25.29 2.45 30.17 
Beach 65.35 94.38 63.25 87.32 60.55 86.88 
Residences 46.27 99.68 30.79 100.56 31.19 102.93 
Paddy fields 2.11 106.85 4.92 107.82 2.36 94.71 
Dry land 0.01 18.23 19.59 17.34 
Sum 512.69 1,281.03 512.69 1,281.03 512.69 1,281.03 
Types 1996
 
2006
 
2016
 
Core area Whole wetland Core area Whole wetland Core area Whole wetland 
Reed 133.07 489.77 130.11 470.28 128.64 479.77 
River 264.73 452.59 281.51 470.17 287.50 469.23 
Lake 1.15 19.53 2.11 25.29 2.45 30.17 
Beach 65.35 94.38 63.25 87.32 60.55 86.88 
Residences 46.27 99.68 30.79 100.56 31.19 102.93 
Paddy fields 2.11 106.85 4.92 107.82 2.36 94.71 
Dry land 0.01 18.23 19.59 17.34 
Sum 512.69 1,281.03 512.69 1,281.03 512.69 1,281.03 
Figure 2

The land cover distribution in 1996, 2006 and 2016.

Figure 2

The land cover distribution in 1996, 2006 and 2016.

Methodology

Calculation type

Wetland eco-environment water requirements is a comprehensive concept, including both ecological and environmental water requirements. The ecological water requirement of wetlands refers to the amount of water needed by wetlands to maintain growth and protect biodiversity. This includes wetland vegetation, soil, and biological habitat water requirements. Wetland environmental water requirement refers to the amount of water needed to support and protect natural ecosystems and ecological processes including the support and protection of human and biological resources, groundwater recharge (Groendveld et al. 2007), lake ecological water requirements, purification of water pollutants, and basic ecological requirements of rivers (Ni et al. 2002).

The dry land and residents in the study area are mainly distributed in the transitional area, so they are not accounted for in the calculation. Due to evaporation being greater than the rainfall in the study area, a certain amount of water must be added to the consumption of evaporation in order to maintain the water balance equalizing the groundwater level (Lopez & Quintana 2003). Lake water consumption was included in the calculation as an important part of the wetland ecosystem. The most common plants in the Shuangtaizi Estuary Wetland were also included in the calculation. In addition, paddy fields and wetland soils play an important role in maintaining wetland health in protected areas (Sklar & Browder 1998). The ecological functions of wetlands can be maintained by ensuring the above types of basic ecological water requirements, while the wetlands can provide the necessary resources for an abundance of species to achieve the goal of conserving biodiversity.

Due to the different goals, the ecological water requirement of wetlands varies. In this research, the wetland eco-environment water requirement is divided into the minimum and optimum requirements (Sun & Yang 2005). The minimum ecological water requirement refers to the amount of water required to maintain the basic functions of the wetland. The optimum ecological water requirement is the amount of water required to restore the ideal wetland (Yang et al. 2006).

Lake surface evaporation water requirement

When the water requirements for the lake reach a steady state, the water storage in the lake will not change. However, in this area, wetland evaporation is greater than the rainfall (Cui & Zhao 2005). The amount of evapotranspiration from the lake's surface must be calculated so as to maintain dynamic equilibrium with the groundwater level. The lake surface evaporation water requirement was computed as:  
formula
(1)
where WE is the water requirement of evaporation from the lake's surface, m3; A is the surface area of the lake, m2; E is evaporation from the lake's surface, mm; and P is rainfall over the lake, mm.

Wetland plant evapotranspiration water requirement

Plant water requirements include: water consumption from a plant's surface evaporation, water consumption from soil surface evaporation, the amount of water the plant contains, and water consumption for the plants (Liu et al. 2006). The amount of water the plant contains and water consumption from the plant assimilation process, which is only 1% of the total plant water consumption, may not be calculated. The water consumption of the plant from surface and soil surface evaporation was computed as:  
formula
(2)
where Wp is the water requirement of wetland plant evapotranspiration, m3; ETm is evapotranspiration, mm; and t is the time in months.

Wetland soils water requirement

Water in the soil helps to maintain a healthy ecosystem, and to keep the soil water in a certain range. Requirements for water in wetland soils was computed as:  
formula
(3)

where S is the water requirement of wetland soils, m3; α is the percentage of the field's water holding capacity; h is the depth of the soil, m; and A is the soil area of the wetland, m2.

In the above three types of water demand, the lake surface water requirement and wetland plant evapotranspiration water requirement are consumable, however the wetland soil water requirement is non-consumable. This means that the three types of water demand are non-duplicative. Therefore, we can integrate the water requirement of these three parts as:  
formula
(4)

where WE is the sum of the water requirements calculated above.

RESULTS AND DISCUSSION

Meteorological data

Meteorological data were provided by the Northeast Regional Meteorological Center, China Meteorological Administration (King & Louw 1998). Rainfall data used in this study were monthly observation data from 1978–2015 from the Antun and Jinzhou hydrological stations. Water surface evaporation data were monthly observed data from 1972 through 2015 in the Jinzhou hydrological station. According to data from April through September, the annual evapotranspiration of the paddy field was 650 mm. The measurements from April through September were 55.99, 86.30, 116.43, 164.79, 141.37, 85.12 mm. According to data from April through September, the annual evapotranspiration of reed was 700 mm. The monthly measurements were 60.30, 92.94, 125.39, 177.46, 152.24 and 91.67 mm.

Water requirement calculation

Lake surface evaporation water requirement

The average monthly rainfall, monthly surface water evaporation, and lake area were used to calculate the total water requirement. The results are shown in Table 2.

Table 2

Evaporation water requirement of the lake in Shuangtaizi Estuary Wetland (104 m3)

Year Jan Feb Mar Apr May Jun July Aug Sept Oct Nov Dec Whole year 
1996 183 403 828 1,612 1,925 1,002 – – 495 889 490 196 8,023 
2006 238 525 1,077 2,098 2,506 1,305 – – 644 1,156 647 257 10,453 
2016 283 623 1,279 2,491 2,976 1,550 – – 765 1,374 768 305 12,414 
Year Jan Feb Mar Apr May Jun July Aug Sept Oct Nov Dec Whole year 
1996 183 403 828 1,612 1,925 1,002 – – 495 889 490 196 8,023 
2006 238 525 1,077 2,098 2,506 1,305 – – 644 1,156 647 257 10,453 
2016 283 623 1,279 2,491 2,976 1,550 – – 765 1,374 768 305 12,414 

Wetland plant evapotranspiration water requirement

Because there were a wide range of wetland plants, only the most common plants were chosen for calculating the plant evapotranspiration water requirement. In this research, reeds and paddy fields were chosen for analysis (Tang et al. 2006). According to the monthly transpiration of reeds and paddy fields from April to September, the evapotranspiration of major plants can be obtained. The results are shown in Table 3.

Table 3

Evapotranspiration water requirement of reed and paddy fields in Shuangtaizi Estuary Wetland (104m3)

Plant Year Apr May Jun July Aug Sept Whole year 
Reed 1996 2,953 4,551 6,141 8,690 7,455 4,489 34,279 
2006 2,836 4,371 5,898 8,348 7,161 4,312 32,926 
2016 2,894 4,459 6,017 8,516 7,305 4,398 33,589 
Paddy fields 1996 598 922 1,244 1,761 1,510 909 6,944 
2006 604 931 1,255 1,778 1,525 918 7,011 
2016 531 818 1,103 1,561 1,339 807 6,159 
Plant Year Apr May Jun July Aug Sept Whole year 
Reed 1996 2,953 4,551 6,141 8,690 7,455 4,489 34,279 
2006 2,836 4,371 5,898 8,348 7,161 4,312 32,926 
2016 2,894 4,459 6,017 8,516 7,305 4,398 33,589 
Paddy fields 1996 598 922 1,244 1,761 1,510 909 6,944 
2006 604 931 1,255 1,778 1,525 918 7,011 
2016 531 818 1,103 1,561 1,339 807 6,159 

Wetland soil water requirement

Swamp and paddy soil under the reeds are major soil types in the Shuangtaizi Estuary Wetland. The volume percentage of swamp soil was between 45 and 55%, and the volume percentage of paddy soil was between 60 and 70%. In this study, 50% and 65% soil percentages were used, respectively. We used a plant root soil thickness of 0.8 m for the calculation. The results are shown in Table 4.

Table 4

Water requirement of swamp soil and paddy soil in Shuangtaizi Estuary Wetland (104m3)

Soil type Year Apr May Jun July Aug Sept Oct Whole year 
Swamp soil 1996 27.9 27.9 27.9 27.9 27.9 27.9 27.9 195.3 
2006 26.8 26.8 26.8 26.8 26.8 26.8 26.8 187.6 
2016 27.5 27.5 27.5 27.5 27.5 27.5 27.5 192.5 
Paddy soil 1996 11.9 11.9 11.9 11.9 11.9 11.9 11.9 83.3 
2006 12.1 12.1 12.1 12.1 12.1 12.1 12.1 84.7 
2016 10.6 10.6 10.6 10.6 10.6 10.6 10.6 74.2 
Soil type Year Apr May Jun July Aug Sept Oct Whole year 
Swamp soil 1996 27.9 27.9 27.9 27.9 27.9 27.9 27.9 195.3 
2006 26.8 26.8 26.8 26.8 26.8 26.8 26.8 187.6 
2016 27.5 27.5 27.5 27.5 27.5 27.5 27.5 192.5 
Paddy soil 1996 11.9 11.9 11.9 11.9 11.9 11.9 11.9 83.3 
2006 12.1 12.1 12.1 12.1 12.1 12.1 12.1 84.7 
2016 10.6 10.6 10.6 10.6 10.6 10.6 10.6 74.2 

Total wetland water requirement

The total wetland water requirement can be calculated by adding the lake surface evaporation, wetland plant evapotranspiration, and wetland soils water requirements. Table 5 shows that the water requirement of the entire wetland ecosystem in 1996, 2006 and 2016, was 4.95, 5.07 and 5.24 million m3, respectively. The water requirement of the core area of the Shuangtaizi Estuary Wetland can be calculated using the same method. The results are shown in Table 5.

Table 5

The total and the core area water requirement of Shuangtaizi Estuary Wetland (104m3)

Region Year Jan Feb Mar Apr May Jun July Aug Sept Oct Nov Dec Whole year 
Total wetland 1996 183 403 828 5,203 7,438 8,427 10,491 9,005 5,933 929 490 196 49,526 
2006 238 525 1,077 5,577 7,847 8,497 10,165 8,725 5,913 1,195 647 257 50,663 
2016 283 623 1,279 5,954 8,291 8,708 10,115 8,682 6,008 1,412 768 305 52,428 
Core area 1996 0.1 0.3 0.6 1,620.8 2,010.2 2,500.5 3,208.2 2,867.8 2,045.8 800.69 0.3 0.1 15,098.9 
2006 1.6 3.5 10.1 1,620.8 1,725.7 2,315.2 3,118.1 2,787.5 1,919.8 591.5 4.3 1.7 14,099.8 
2016 2.1 4.6 9.3 1,466.5 1,888.6 2,334.6 3,065.2 2,730.2 1,913.4 669.2 5.6 2.2 14,091.5 
Region Year Jan Feb Mar Apr May Jun July Aug Sept Oct Nov Dec Whole year 
Total wetland 1996 183 403 828 5,203 7,438 8,427 10,491 9,005 5,933 929 490 196 49,526 
2006 238 525 1,077 5,577 7,847 8,497 10,165 8,725 5,913 1,195 647 257 50,663 
2016 283 623 1,279 5,954 8,291 8,708 10,115 8,682 6,008 1,412 768 305 52,428 
Core area 1996 0.1 0.3 0.6 1,620.8 2,010.2 2,500.5 3,208.2 2,867.8 2,045.8 800.69 0.3 0.1 15,098.9 
2006 1.6 3.5 10.1 1,620.8 1,725.7 2,315.2 3,118.1 2,787.5 1,919.8 591.5 4.3 1.7 14,099.8 
2016 2.1 4.6 9.3 1,466.5 1,888.6 2,334.6 3,065.2 2,730.2 1,913.4 669.2 5.6 2.2 14,091.5 

The optimum and minimum ecological water requirement

The optimum ecological water requirement

Differences in the amount of wetland water will lead to different ecological characteristics, with particularly significant changes in wetland boundaries. The Shuangtaizi Estuary Wetland in 2006 was at a hydrologically ‘good’ level according to a field survey (Luo et al. 2011). The water requirements in 2006 can be considered the appropriate ecological water requirement. The calculation of appropriate ecological water requirements for the lake considered rainfall, while those for reed and paddy fields do not. Therefore, the rainfall factor was deducted to avoid errors in the resulting data. The average rainfall in the study area from April to September was 571.24 mm and the reed area was 47,028 hm2, so the total rainfall was deducted from 268 million m3. Similarly, the core area of reed is 13,011 km2; the rainfall was deducted from 74 million m3. Therefore, the optimum ecological water requirement of the total wetland and the core was 239 million m3 and 67 million m3. The results are shown in Table 6.

Table 6

The optimum ecological water requirement of Shuangtaizi Estuary Wetland (104m3)

Type Jan Feb Mar Apr May Jun July Aug Sept Oct Nov Dec Whole year 
The wetland 238 525 1,077 4,363 5,285 5,140 2,661 336 2,139 1,195 647 257 23,863 
Core area 1.6 3.5 10.1 1,171.6 1,059 1,233 1,157 552 914 592 4.3 1.7 6,699.8 
Type Jan Feb Mar Apr May Jun July Aug Sept Oct Nov Dec Whole year 
The wetland 238 525 1,077 4,363 5,285 5,140 2,661 336 2,139 1,195 647 257 23,863 
Core area 1.6 3.5 10.1 1,171.6 1,059 1,233 1,157 552 914 592 4.3 1.7 6,699.8 

The minimum ecological water requirement

The core zone was the region delimited to protect identified rare and endangered plants and animals. The whole wetland will lose value if this region is lost. To maintain the stable function and ecological water requirement of the core area, the minimum ecological water requirement of Shuangtaizi Estuary Wetland must be met, which also equals the optimum ecological water requirement of the core zone, of 67 million m3. As the minimum ecological water requirement, a year with less rainfall cannot meet these requirements will threaten the health of the wetland ecosystem. Therefore, we took precipitation into consideration (Figure 2), on the basis of the average precipitation rate, to calculate the minimum ecological water requirement. If the precipitation was less than the annual average precipitation with a probability of 75%, then the shortage of ecological water requirement caused by the other 25% of precipitation probability was added to make a precise calculation of minimum ecological water requirement. As seen in Figure 3, 75% of the corresponding frequency of precipitation compared with the average precipitation was needed to increase the precipitation by 113.46 mm, so the amount of water required to be calculated is 15 million m3/a, and the minimum ecological water requirement should be 92 million m3/a. The precipitation corresponding to 95% of the frequency compared with the annual average precipitation was necessary to increase the precipitation by 225.82 mm, increasing the water volume to 31 million m3/a and the corresponding minimum ecological water requirement to 107 million m3/a.

Figure 3

The results of the frequency curve of average rainfall in the Shuangtaizi Estuary Wetland.

Figure 3

The results of the frequency curve of average rainfall in the Shuangtaizi Estuary Wetland.

The optimum ecological water requirement and the minimum ecological water requirement under average rainfall and the monthly minimum ecological water requirement under different frequencies are shown in Table 7.

Table 7

Ecological water requirements in the Shuangtaizi Estuary Wetland (104m3)

Water requirement Jan Feb Mar Apr May Jun July Aug Sept Oct Nov Dec Whole year 
Optimum 238 525 1,077 4,363 5,285 5,140 2,661 336 2,139 1,195 647 257 23,863 
Minimum 1.6 3.5 10.1 1,171.6 1,059 1,233 1,157 552 914 592 4.3 1.7 6,700 
75% 1.6 3.5 11.1 1,451 1,673 1,648 1,528 872 1,244 1,083 5.3 9,215 
95% 2.6 4.5 12.1 1,678 1,834 1,811 1,754 1,012 1,481 1,187 5.3 10,784 
Water requirement Jan Feb Mar Apr May Jun July Aug Sept Oct Nov Dec Whole year 
Optimum 238 525 1,077 4,363 5,285 5,140 2,661 336 2,139 1,195 647 257 23,863 
Minimum 1.6 3.5 10.1 1,171.6 1,059 1,233 1,157 552 914 592 4.3 1.7 6,700 
75% 1.6 3.5 11.1 1,451 1,673 1,648 1,528 872 1,244 1,083 5.3 9,215 
95% 2.6 4.5 12.1 1,678 1,834 1,811 1,754 1,012 1,481 1,187 5.3 10,784 

CONCLUSIONS

In this study, topographical features were extracted from the images based on 3S technology establishing the wetland cover information database. Based on the wetland cover information data, the ecological environment water requirements of the Shuangtaizi Estuary Wetland were calculated. Results showed that: the ecological environment water requirements of the whole wetland in 1996, 2006 and 2016, were 495, 506 and 524 million m3; the requirements of the core zone in 1996, 2006 and 2016, were 151, 141 and 141 million m3; the optimum requirement of the total wetland was 239 million m3 which reduced the rainfall; under average rainfall, the minimum, 75th percentile frequency and 95th percentile frequency requirements were 670 million m3, 921 million m3 and 1,078 million m3.

Through calculating the ecological water requirement for a wetland ecosystem, the optimum and minimum ecological water requirements can be provided for the optimal allocation of water resources to achieve scientifically sound and sustainable management practices.

ACKNOWLEDGEMENTS

This study is supported by the Specialized Research Fund for Cultivation Plan for Youth Agricultural Science and Technology Innovative Talents in Liaoning Province, China (No. 2014038) and by the Provincial Department of Education research project in Liaoning, China (No. L2014250).

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