An assessment of the carrying capacity of groundwater resources in North China Plain region-Analysis of potential for development

LIU Min, NIE Zhen-long, WANG Jin-zhe, WANG Li-fang, TIAN Yan-liang

Institute of Hydrogeology and Environmental Geology, Chinese Academy of Geological Sciences, Shijiazhuang 050061,China.

Abstract: Over-exploitation of groundwater in North China Plain (NCP) has resulted in a series of eco-environment problems. Sustainable use of groundwater resources in NCP, in particular management of groundwater resource carrying capacity (GRCC), faces an unprecedented challenge. Here we define GRCC, and a new assessment method is tentatively proposed and applied to evaluate GRCC based on the whole NCP, city administrative units and county administrative units. Our study divided the NCP into three zones, i.e.non-overexploited non-overloaded zone (NNZ), overexploited but non-overloaded zone (ONZ),and overexploited overloaded zone (OOZ). Results confirmed 27.6% of counties belonged to NNZ. However, 58.9% of counties and NCP as a whole belonged to ONZ, and 13.5% of counties belonged to OOZ. Spatially, NNZs were mainly distributed in Beijing, parts of eastern coastal cities and Henan Province. OOZs were mostly distributed in middle-eastern part of Cangzhou, parts of Dezhou, Tianjin and Binzhou, and the remaining areas belonged to ONZs.We suggest two approaches for enhancing GRCC, i) increasing the amount of available groundwater and ii) improving the water use efficiency. An increase of 11.0 billion cubic meters to the available groundwater levels combined with water use efficiency improvements up to 479 CNY per cubic meter of the world mean, the gross domestic product (GDP) sustained by groundwater in the NCP could reach 11.1 trillion CNY and maintain a 20 years of GDP development assuming the current rate of growth.

Keywords: North China Plain; Groundwater resources carrying capacity; Groundwater exploitation ratio; Socio-economic development ratio

Introduction

Water is one of the most valuable natural resources upon which human health, economic development and ecosystem diversity depend.Rapid development of the economy and dramatic growth in China’s population in recent decades has led to a dramatic increase in water demand for agricultural, industrial, and domestic purposes,which has compromised natural ecosystems and exacerbated the conflict between water supply and demand. This conflict is prominent in semi-arid areas and arid areas where lack of sufficient precipitation and surface water resources leads to groundwater being the main (or even the only)water supply. The NPC has a semi-humid/semi-arid climate, with a tendency to warmer and dryer (LIU Min et al. 2010). As one of the primary grain production areas in China, the NCP’s high agricultural output depends predominantly on groundwater irrigation (WENG S Q et al. 2010).Dramatic increases in water consumption due to irrigation, domestic and industrial demands over the past decade has led to over-exploitation of groundwater and a declining regional groundwater depth. Consequently, the NCP has become one of the top three major groundwater depletion areas of the world (Siebert S et al. 2010). Not surprisingly,this situation is responsible for a series of ecological and environmental problems, such as aquifer drainage, land subsidence, surface collapse,and ground fissures, among others. These issues have seriously hampered the harmony and healthy development of the regional society and economy,thus attracting the attention of the Chinese government, the public and the international community. Questions posed by concerned stakeholders are around what the limitation of the groundwater carrying capacity in the North China Plain is, and whether there is any means to improve its carrying capacity. Key to answering these questions, a robust assessment of the NCP groundwater carrying capacity is required, a vital component in research of water cycle and groundwater sustainability in the NCP but also of extreme importance for the preservation of the sustainable development of the socio-economy,water resources, ecosystems and food safety.

The concept of carrying capacity is rooted in demography, biology, and applied ecology (Clarke A L, 2002). Park P E et al. (1969) proposed the concept of carrying capacity for the first time in 1921, in the context of the human population issue.During the 1980s, the United Nations Educational,Scientific, and Cultural Organization (UNESCO)proposed the concept of resource carrying capacity that remains widely used. The broad notion of resource carrying capacity is centered around a correlation between population and resources.More specific research on water resource carrying capacity (WRCC) began relatively late and to date the concept of WRCC has not yet been clearly defined and described. Internationally, few breakthroughs have been achieved in WRCC research, and the topic has only been discussed briefly in theories of sustainable development(Filho W L, 2012; SONG Xiao-meng et al. 2011;Thomas B F and Famiglietti J S, 2015; Tiwari P C and Joshi B, 2012; Varis O and Vakkilainen P,2001; Villarroya F and Aldwell C R, 1998).

In China, the concept of WRCC was first applied to the Urumqi River Basin in 1989 (FENG Shao-yuan et al. 2006; SHI Ya-feng and QU Yao,1992). Since then, studies on WCRR have dramatically increased in number. A wide discussion on the concept of WRCC based on the theories of resource carrying capacity, environmental carrying capacity (ZHU Yong-hua et al.2010) and other similar notions have been made by many researchers. For our purposes, to perform an analysis of the carrying capacity of water resources in the NCP, we divided the definition of WRCC into two accepted categories; one that relies on the maximum water resources exploitation scale, i.e.,the natural system limit of water resources (GAO Yan-chun and LIU Chang-ming, 1997) and the second being the maximum economic or population scale that can be sustained by water resources which can be defined according to sustainable development theory as the maximum social-economical scale sustained by the available regional water resources under the foreseeable level of technology and eco-society in a specific development phase (LI Ling-yue and GAN Hong,2000). This second point of view has been accepted by many researchers (LONG Teng-rui et al. 2004; XI Yang-he et al. 2001), among whom the most notable scholars are SHI Ya-feng and QU Yao (1992) and RUAN Ben-qing and SHEN Jin(1998).

Numerous studies have investigated water resource carrying capacity using different methods.Our analysis divides the type of methods used into three categories; the empirical estimation method,the index system evaluation method and the complicated system method (YUAN Ying et al.2006). The first type of method includes background analysis, empirical formula analysis(WANG Zhong-jing, 1998), and ratio trend analysis(QU Yao-guang and FAN Sheng-yue, 2000)-all methods are generally easy to understand and operate, but with a lower estimation accuracy. The second type of method includes principal components analysis (DOU Ming et al. 2010), fuzzy evaluation methods (GONG Li and JIN Chun-ling 2009), and recurrence calculation of dynamic simulation. These methods are usually easy to perform and have a higher accuracy, but the results of these methods are often dimensionless values.The third type of method includes the system dynamics method (YANG Jun-feng et al. 2015),information diffusion technology (Feng L H and Huang C F, 2008), and multi-objective optimization analysis. These methods are very complicated, with large amounts of data and parameters that are difficult to collect and determine.

Investigations around the carrying capacity of groundwater (GRCC) were relatively scarce until the year 2000, after which GRCC studies gradually became a more popular research field within the WRCC sector (MEN Bao-hui et al. 2003; QU Ji-hong et al. 2008; WAN Xing et al. 2006;WANG Rong-jing et al. 2009; WANG Shun-jiu et al. 2004; ZUO Qi-ting and ZHANG Xiu-yu, 2015).Given the definitions and the assessment methods in most of the research on GRCC continued to use those from WRCC research, the definition of GRCC remained unclear without a systematic and uniform description. Additionally, there was no mature assessment method with sufficient consideration to the utilization of qualitative and quantitative indices. Investigating GRCC assessment is more complicated than the study of surface water resources, thus an urgency surrounds the search for a simple solution via a systematic consideration of the complicated problem.

With the aim to contribute to a more comprehensive understanding of GRCC in China,the outline of our paper is as follows: (1) A definition for groundwater resource carrying capacity that considers the quantity of available groundwater resources and the socio-economic development scale that can be sustained by groundwater resources; (2) combined with water use efficiency, a tentative theory and method of GRCC assessment proposed and applied in the NPC under the current state based on the whole NCP, city administrative unit and county administrative unit;and (3) methods for improving GRCC in the NCP and a quantitative analysis of GRCC potentials.

1 Study area and data

1.1 Study area

The NPC is located in the eastern part of China,surrounded by the Bohai Sea to the east, Taihang Mountains to the west, Yanshan to the North and the Yellow River to the south with a total area of 139 000 km². With a total population of 120 million in 2011, including 21 large and mediumsized cities, the NPC is the political, economic, and cultural center of China. The NCP is both an important national grain production base and an industry base. The gross domestic product (GDP)of the NCP was 5 484.7 billion CNY in 2011,accounting for 11.7% of the national total. Grain output alone is 57.5 million tons, comprising 10.1% of China’s overall grain output. However,the NCP is also one of the most water-deficient areas in China, with a mean annual precipitation of 558.6 mm (ZHANG Zhao-ji et al. 2009), with approximately 80% of rainfall occurring from June through September. Annual water resources amount to 37.2 billion cubic meters (ZHANG Guang-hui et al. 2011), in which groundwater accounts for 60% (LIU Shao-yu et al. 2012). In 2011, 62% of the water consumption was supplied by groundwater resources, and the groundwater supply accounted for greater than 50% of the total water supply in 79% of the counties in the NCP(Fig. 1). Over the past several decades, groundwater pumping has experienced a continuous increase due to rapid agricultural and urban development. As a result, water tables are dropping dramatically at a rate of 1 m/yr in the piedmont plain (LI Qiao-ping and DING Yi-hui, 2012;SHEN Yan-jun et al. 2005). The sustainability of groundwater carrying capacity in the NCP faces an unprecedented challenge.

Fig. 1 The groundwater supply ratio in regions of the North China Plain

1.2 Data

In this study, available groundwater resource data were derived from the National Basic Research Program of China (973 Program), the resource Evolving Mechanisms and Controls of Groundwater in North China Plain. Water supply volumes, water consumption levels (groundwater and total consumption), and groundwater exploitation levels associated with different industries in each city (and county) were primarily sourced from the Yearbook of Water Conservancy Statistics 2011 for Beijing, Yearbook of Water Conservancy Statistics 2011 for Tianjin, Yearbook of Water Conservancy Statistics 2011 for Hebei Province,Yearbook of Water Conservancy Statistics 2011 for Henan Province and Yearbook of Water Conservancy Statistics 2011 for Shandong Province. The economic and population data of each city (and county) were drawn entirely from the statistical yearbooks (2011) for each city in the NCP. The water productivity data cited for other nations are sourced from World Bank Cross Country Data (https://www.quandl.com/data/WORLDBANK).

2 Conceptual approach and methodology

2.1 Definition of groundwater carrying capacity

Groundwater embodies both natural attributes and social attributes. Natural attributes of groundwater are comprised of two aspects. The first is one of valuable natural resources.Groundwater quantity depends on the supply capability of the groundwater system, inevitably influenced by topography, weather, hydrology,geology and hydrogeology. Control by these natural conditions leads to a stable annual average groundwater resource level i.e. the level of natural groundwater resource. The second natural aspect is around the continual interaction of groundwater with the formation and movement processes of the surrounding environment. It is therefore critically important to maintain the natural environment and its evolutionary processes. Humans must give full consideration to the impacts on the natural environment when extracting groundwater, to prevent the gestation of serious environmental issues. Thus, an upper limit to the available groundwater resource exists. However, it is not sufficient to assess GRCC by merely considering its natural attributes in the context of rational exploitation and utilization of groundwater. Rather,the social attributes of groundwater resources should also be considered. Groundwater has become associated with human society and economic development, with different industries varying water use efficiencies in different areas.There are many factors affecting water use efficiency, including those at the economic and technical level, industrial structure, management policy, water saving technology and the level of public awareness. Given equal amounts of the available groundwater resources, improving water use efficiency could dramatic enhance the groundwater resource carrying capacity and its role in economy development. In the meantime,restricted by economic structure and the commitment to high quality of human life, an upper limit of the capability of water use efficiency persists. With limited water resources and limited capacity for improving water use efficiency, the social and economic development scale sustained by groundwater is also limited-i.e. the groundwater carrying capacity.

Clearly, GRCC is related to both natural groundwater attributes and social attributes. GRCC depends on the maximum water supply capacity of the groundwater system under natural environment factors, and is also influenced by the socialeconomic development state of the region and groundwater extraction mechanisms.

Based on the discussion above, the groundwater carrying capacity in this study is defined as the maximum socio-economic development scale sustained by the maximum available groundwater resources amount under the foreseeable level of technology and eco-society development phase based on the sustainable development concept. For our purposes, the GRCC can be expressed by a social and economic development scale, such as GDP and population.

2.2 Methodology

2.2.1 The conceptual approach

To provide a quantitative assessment of the groundwater carrying capacity in the context of continuing socio-economic development, we express the relationship between groundwater availability and the socio-economic development scale sustained by groundwater Equation 1:

In Equation 1, G is the socio-economic development scale, i.e. the gross domestic product(CNY), Q is the amount of water consumption(m3), and α is water use efficiency (CNY/m3).Equation 1 effectively represents the GDP produced by one cubic meter of water. Equation 1 could be expressed as a straight line through the origin of the coordinate system with the Q as the x-axis and G as the y-axis (Fig. 2), where the gradient is the water use efficiency. From Equation 1, we can know that regional groundwater carrying capacity is the function of both available groundwater amount and water use efficiency.Under the same water use efficiency, the more the groundwater available amount, the stronger the groundwater carrying capacity; and for a certain available groundwater amount, the higher the water use efficiency, the stronger the groundwater carrying capacity. For a specified local groundwater system, if the maximum available groundwater amount (Qmax) was obtained by water resources assessment, the socio-economic development scale (Gmax) sustained by groundwater in different water use efficiency could be calculated(Fig. 2).

Fig. 2 The relationship between the groundwater consumption rate and the socio-economic development scale sustained by groundwater in regions throughout the North China Plain

2.2.2 Relative present situation assessment method

Available groundwater resources and the level of water use efficiency vary between regions due to differences in natural conditions, socioeconomic structure, and level of technological advancement. Therefore, to analyze regional groundwater carrying capacity, a uniform criterion is required. With this in mind, we set the highest water use efficiency among the administrative units (e.g. cities) in the study area as a criterion,from which the maximum theoretical groundwater required to sustain GDP in each city could be obtained based on the maximum available groundwater. Finally, the socio-economic development ratio (RG) was derived from the theoretical and realized groundwater levels required to sustain GDP, and the groundwater exploitation ratio was derived from the level of groundwater exploitation and the maximum available groundwater. These reasonings are expressed as Equations 2, 3 and 4 and illustrated in Fig. 3.

In these equations, Rwis the groundwater exploitation ratio, RGis the socio-economic development ratio, We, Waand Gaare the level of groundwater exploitation, total available groundwater and the realized GDP, respectively.

Gais obtained via Equation 4. Gtis the theoretical GDP that could be sustained by groundwater in each administrative unit and can be obtained by Equation 5.

In Equation 4 and 5,maxαis the regional maximum water use efficiency, Q is the amount of groundwater exploitation amount, i represents the ithadministrative unit, and j represents the j industry.

Based on the calculation above, each coordinate point (Rwi, RGi) can be projected into the coordinate system illustrated in Fig. 3(a). The figure can be divided into 3 zones; the NNZ(non-over-exploited non-overloaded zone), the ONZ (over-exploited but non-overloaded zone),and the OOZ (over-exploited and overloaded zone).All these zones are situated below the water use efficiency in Fig. 3(a) because the highest regional water use efficiency valuemaxα was adopted.The NNZ represents cities with a relatively low groundwater resource utilization and exploitation.For the NNZ, water is not a limiting factor in socio-economic development, but this zone also has relatively low water use efficiency levels, and the economic development scale is not overloaded.This indicates that water saving measures should be advocated in this zone and there is potential for further economy development. The ONZ defines regions where groundwater resources are overexploited and relatively low water use efficiency levels are found, where the socio-economic development framework is not over-loaded and where water resources may be a constraint under current water use efficiency levels. Enhancing water use efficiency in the ONZ, could return groundwater to a non-over-exploited state to maintain the current scale of socio-economic development. In effect, for point P1(Rw1, RG1) in Fig. 3(b), enhancing water use efficiency, the RWcoordinate value of point P1(Rw1, RG1) changed from Rw1to R'w1. The OOZ is indicate of regions where not only is groundwater over-exploited but socio-economic development is over-loaded. In the OOZ, although water use efficiency could be improved, the volume of available water resources are an important constraint to future socioeconomic development. No increase in available groundwater means the system will be difficult to return to a balanced state. For example, for point P2(Rw2, RG2) in Fig. 3(b), to maintain a nondecreasing socio-economic development scale by improving water use efficiency, the RWcoordinate value of point P2(Rw2, RG2) changed from Rw2to R'w2, but R'w2was still greater than 100%. In effect,groundwater remained over-exploited even with improved water efficiency.quantitative potential analysis was made.

Fig. 3 Groundwater carrying capacity assessment zones based on the ratio of groundwater exploitation to the socio-economic development ratio for regions of the North China Plain (NNZ - the non-overexploited non-overloaded zone, ONZ - the overexploited but non-overloaded zone,OOZ - the overexploited overloaded zone)

3 NCP case study

Based on the conceptual approach in section 2,we first provide an analysis of the elements employed to assess GRCC. These elements are the available volume of groundwater, groundwater exploitation volume, water use efficiency levels and the scale of socio-economic development sustained by groundwater. In this study, GDP was chosen to represent socio-economic development.Second, assessment method for the relative present situation was applied to evaluate GRCC based on the whole NCP, city administrative unit and county administrative unit. In the end, methods for improving the GRCC were discussed, and a

3.1 GRCC assessment elements analysis

3.1.1 Available groundwater amounts

The volume of available groundwater in this study refers to the amount of groundwater whose total dissolved solid is less than or equal to 1 g/L.According to the latest results of National Basic Research Program of China (973 Program),Evolving Mechanism and Control of Groundwater in the North China Plain (personal correspondence), the available groundwater resource across the entire NCP is 12.3 billion cubic meters. In respect to city units, the volumes of available groundwater sources in descending order were Baoding, Beijing, Shijiazhuang, Xinxiang,Tangshan, Anyang, Handan, Xingtai, Puyang,Langfang, Liaocheng, Cangzhou, Qinhuangdao,Hengshui, Jiaozuo, Tianjin, Dezhou, Hebi, Jinan,Binzhou and Dongying (Fig. 4). Only the top three cities were found to have the available groundwater resources volumes greater than 1 000 million cubic meters. Available groundwater in Jinan, Binzhou and Dongying were less than 100 million. In fact, Dongying currently has no available fresh groundwater resources, although it does have approximately 185 million cubic meters of brackish water. Spatially and on a county basis,the volume of available groundwater generally reduces on a gradient from west to east (Fig. 5(a)).Areas along the west piedmont and the southern part of the NCP were generally found to have available groundwater levels greater than 100 million cubic meters. Around coastal areas such as parts of Cangzhou, Dezhou, Binzhou and Dongying, no available fresh groundwater exists,only brackish and salt water.

Fig. 4 Groundwater carrying capacity assessment elements for the cities in the NCP region

3.1.2 Exploitation levels

The level of groundwater exploitation across the NCP came to 17.5 billion cubic meters. With respect to city analysis, the exploitation levels were greatest in Baoding, Shijiazhuang, Xingtai,Hengshui and Beijing, and smallest in Anyang,Hebi, Dongying and Puyang. Over-exploitation in Baoding and Shijiazhuang was greater than 2 billion cubic meters, while for smallest Dongying and Puyang the figure was zero (Fig. 5(b)).Spatially and on a county basis, exploitation amounts were generally large in the west and north,and decreased in the south and eastern coastal areas (Fig. 5(b)). Statistically, 71.4% of cities were over-exploited, and 95.1% of counties were overexploited. The overall level of over-exploitation in the NCP was 5.3 billion cubic meters.

3.1.3 Water use efficiency

GRCC is based on available groundwater resources, and is also related to water use efficiency in different sectors of water use. In this study, water use efficiency refers to the GDP produced per cubic water otherwise known as water productivity. The average water use efficiency in the NCP was 159.4 CNY/cubic meter, which was more than twice the national Chinese average or 77 CNY/cubic meter, but far behind the world mean value of 479 CNY/cubic meter. Among the cities, Tianjin,Beijing, and Tangshan topped the list for the highest water use efficiency; the values were 511.6, 407.9,and 256.9 CNY/cubic meter, respectively. However,most of these cities had relatively lower water use efficiency (Fig. 4). Statistically, approximately 76.2% of cities had lower water efficiency than the NCP mean value. Hengshui was found to have the lowest water use efficiency levels, merely 54.5 CNY/cubic meter. Based on the county administrative unit analysis, counties with high water efficiency were mostly distributed in Beijing,Tianjin, Tangshan, coastal areas of the Bohai Sea and the urban districts of the cities (Fig. 6(c)). Water use efficiency in the urban districts of Beijing,Tianjin, Tangshan and Shijiazhuang were greater than 500 CNY/cubic meter. While 81% of the county’s water use efficiencies were lower than the NCP mean, 43% were lower than the national Chinese mean and 94% of them were lower than the global mean.

Fig. 5 Spatial distribution of the GRCC assessment elements among the counties of North China Plain where (a) is the available groundwater in volume; (b) is the groundwater exploitation volume; (c) is the water use efficiency level; and (d) is the level of GDP sustained by groundwater

3.1.4 GDP sustained by groundwater resources

GDP sustained by groundwater was calculated by Equation 4 in section 2.2. The groundwatersustained GDP in the NCP as a whole was 2 793.7 billion CNY, accounting for 50.3% of the total GDP. In city administrative units, Beijing,Shijiazhuang, Tangshan took the first three places among all of the cities (Fig. 4); the values were 564.6, 336.3 and 316.2 billion CNY, respectively,which accounted for 39%, 86% and 78%,respectively of the corresponding total GDPs.Puyang and Dongying were locations without groundwater-sustained GDP because their groundwater supply ratios were zero. Based on county administrative unit analysis, high groundwater sustained GDP were mostly distributed in the Beijing-tianjin-Tangshan area, and in most cities in Hebei Province, especially in the areas around their urban districts, low groundwater- sustained GDP were mostly distributed in the cities of Henan Province and the coastal area of Shandong Province (Fig. 5(d)).

Fig. 6 Groundwater carrying capacity assessment zones based on city administrative units in the North China Plain

3.2 Assessment of groundwater resource carrying capacity

Based on the GRCC elements analysis and the method introduced in section 2.2.2, we calculated the theoretical GDP sustained by groundwater in each city (county) according to the maximum water use efficiency of 511.6 CNY/cubic meter in Tianjin among the cities (990.6 CNY/cubic meter in Fengnan County in Tangshan among the counties), and projected each groundwater resource exploitation ratio and GDP development ratio into the (Rw, RG) coordinate system. Results are illustrated in Fig. 6 (cities) and Fig. 7(counties).

Fig. 7 Spatial distribution of groundwater carrying capacity assessment zones based on county administrative units

With respect to the city scale, Dongying,Puyang, Anyang, Hebi, Xinxiang, Jiaozuo and Beijing were positions in the NNZ, Binzhou,Canghzou, Tianjin were in the OOZ, and other cities were in the ONZ (Fig. 6). The analysis comparing county administrative units found similar results, but they were more specifically illustrated (Fig. 7). Almost all of counties in the seven cities of Dongying, Puyang, Anyang, Hebi,Xinxiang, Jiaozuo and Beijing were in the NNZ,but the NNZ were also distributed in urban districts of Tianjin, Baoding and Handan, Fengnan and Tanghai counties of Tangshan, and Wudi and Zhanhua counties of Binzhou. This result suggested that levels of exploitation in groundwater in these counties is relatively small, and currently water does not limit socio-economic development. Although groundwater resources were not over-exploited in Beijing itself, several counties’ of Beijing demonstrated an exploitation ratio close to 100%. Clearly, in these countries water-saving awareness should be advocated. The OOZ was mostly found in the eastern region of Cangzhou, and parts of Dezhou, Tianjin and Binzhou. In these counties, although water use efficiency could still be improved, groundwater levels are incapable of returning to a balanced state unless an increase in available groundwater volume occurs. Given the unlikeliness of increased natural groundwater recharge, future economic development could be severely restricted by water resources. Fortunately, these areas have high volumes of brackish and salt water that are expected to be transformed and utilized in the future. While most of counties belong to the ONZ(Fig. 7) where groundwater is overexploited,current water use efficiencies were found to be relatively low. This indicates that socio-economic development is not actually over-loaded, but constrained by water resources under the current development phase of science and technology. By enhancing water use efficiency, groundwater could be returned to a non-over-exploited state and further economic development is possible. Among the cities, 33.3% of the cities belonged to NNZ,52.4% of the cities belonged to ONZ and 14.3% of the cities to OOZ. Among the counties, NNZ accounted for 27.6% of the counties, ONZ accounted for 58.9%, and OOZ for 13.5%. In summary, the NCP as a whole aligns to the ONZ,whereby groundwater resources are over exploited with an exploitation ratio of approximately 43%.Our findings are in agreement with the results from LIU Ming et al. (2014) and QIAN Yong et al.(2014). LIU Ming et al. (2014) used the fuzzy comprehensive evaluation method to assess the GRCC in NCP, and returned the result that groundwater exploitation in most cities was already close to limits, and parts of them were seriously over-exploited. QIAN Yong et al. (2014)also concluded little potential for groundwater exploitation and utilization remains in the NCP.However, our results also have some differences to them. The authors identified more room for socio-economic development through water use efficiency improvements and increasing the available groundwater volumes via brackish water utilization. These were consistent with the water shortage mitigation plan of exploring new water resources, saving water and improving water use efficiency (WANG Xing et al. 2006). These were also in agreement with the results that there is more brackish water and salt water to be utilized,and groundwater is mainly used in agricultural irrigation (WENG S Q et al. 2010) where water use efficiency is very low and is expected to be urgently improved (LI Zhen-sheng, 2006; YAO Zhi-jun et al. 2000).

3.3 Discussion-Improving GRCC

According to the definition, we know that groundwater carrying capacity is the function of the available groundwater volume and water use efficiency. Thus, the higher the amount of the available groundwater, the higher its carrying capacity and the higher the water use efficiency,the higher its carrying capacity. A corresponding approach may therefore be considered from two aspects. Firstly, an increase in the volume of available groundwater resource is required. By definition, the available groundwater resources is a function referring to hydrogeological conditions,economic and technological conditions, and environment conditions. Therefore, available groundwater resources could be increased on the basis of inherited predominance in different areas,by artificial regulation and storage of rain and floods through regulation and storage capacity improvements of the piedmont alluvial-pluvial fan,by developing technology around transformation and utilization of brackish water, and the mining of low permeability aquifers in the middle-east plain area, reclaimed water use, and groundwater recharge by surplus water of the South-North Water Transfer Project. Secondly, an increase in water use efficiency is required. The heart of this issue lies in the water-saving technology level, and in industrial structure, management policy and public awareness. Increasing water use efficiency could be achieved in different sectors and industries by developing water-saving technology in agriculture, improving water recycling rates in industry, readjusting industrial structure and changing economic growth pattern, promoting the development of high water productivity industries to increase their contribution to the GDP,formulating management policies, and raising public awareness of water conservation by strengthening publicity and education. All these measures may eventually lead to the improvement of water use efficiency across China and the rest of the world.

3.4 Analysis of potential in GRCC

Based on the preceding discussion around possible approaches to improve GRCC, we provide a quantitative analysis of the effects these different approaches may have on improving the GRCC.

The first approach was to increase the volume of the available groundwater resource. There are two ways to increase the available groundwater volume according to the location-specific hydrogeological conditions in different areas of the NCP.First, in the west piedmont areas, artificial rain,flood regulation and storage could be implemented by using the storage capacity of the alluvial and flood fan. According to early results, a storage space of 32.6 to 148 billion cubic meters exists in the west piedmont alluvial-flood fan, with a total storage acreage of approximately 51.8 thousand square kilometers (ZHANG Zhao-ji et al. 2009).Given one quarter of the annual precipitation of 558.6 mm could be converted into runoff in this area (ZHAN Dao-jiang and YE Shou-ze, 2000),there would be approximately 7.1 billion cubic meters of storage water. Further, supposing the groundwater recharge rate was 60% (ZHANG Zhao-ji et al. 2009; LI Sheng-tao et al. 2011; LIU Peng-fei, 2013), there would be 4.3 billion cubic meters of available groundwater annually. The second way involves developing technology to transform and utilize brackish water in the middle-east plain areas. A total of 6.7 billion cubic meters of brackish (total dissolved solid 1 g/L-3 g/L) could be utilized to increase available groundwater. In summary, a total increase of 11.0 billion cubic meters, multiplied by the current rate of water use efficiency in the NCP of 159.4 CNY/cubic meter, results in a total GDP increase of 1 755.6 billion CNY. This represents an increase of 62.8% the GDP sustained by groundwater. In addition, the current overexploitation status could be ameliorated as well.According to the current annual economic development rate of 7%, the increased available groundwater volume could maintain approximately 7.2 years of sustainable economy development.

The second approach was increasing water use efficiency. Currently, the global mean water use efficiency rate is 479 CNY per cubic meter. The top ten countries have an average water use efficiency of 3 978 CNY per cubic meter. We calculated the water use efficiency growth rate in the NCP during the past decade, and the results showed an annual increase rate of approximately 14.3%. Assuming this growth rate, the NCP will achieve the global mean water use efficiency rate in approximately 8 years, and in 24 years will match that of the top ten countries. Thus, by improving water use efficiency to 479 CNY per cubic meter and 3 978 CNY per cubic meter, GDP sustained by groundwater under the current available groundwater volumes could reach between 5.9 trillion CNY and 48.8 trillion CNY,which could maintain approximately 11 and 42 years of sustainable economic development respectively. Overall our findings suggest that improving water use efficiency is an effective measure to increase the GDP sustained by groundwater in the current development phase.

Our general analysis showed that the GDP supported by groundwater could reach 11.1 trillion CNY and 92.5 trillion CNY by both increasing the available groundwater volume and increasing water productivity levels to the global mean and the mean of the top ten countries, respectively,over 20-52 years of sustainable economic development in the NCP under the current GDP growth rate of 7%.

4 Conclusions and discussions

The main outcomes and findings of our work are summarized as follows:

(1) A definition of groundwater resource carrying capacity was tentatively proposed in this study. The groundwater resource carrying capacity can be defined as the maximum socio-economic development scale sustained by the maximum available groundwater resource volume under the foreseeable level of technology and the eco-society development phase based on the sustainable development concept.

(2) A new method of groundwater carrying capacity assessment was attempted, by which the GRCC could briefly be expressed as a function of the available groundwater volume and water use efficiency. A diagram of the groundwater resource exploitation ratio and its sustained GDP development ratio was built and it was divided into three zones, the non-overexploited non-overloaded zone (NNZ), the overexploited but non-overloaded zone (ONZ), and the overexploited overloaded zone (OOZ).

(3) Our results demonstrated that the NCP as a whole is aligned to the ONZ, with an available groundwater volume of 12.3 billion cubic meters,an exploitation volume of 17.5 billion cubic meters,GDP of 2 793.7 billion CNY sustained by groundwater, and a theoretical GDP of 6 271.3 billion CNY sustained by groundwater. Among the cities,33.3% of the cities are aligned to the NNZ, 52.4%of cities to the ONZ, and 14.3% to the OOZ.Among the counties, the NNZ accounted for 27.6% of the counties, the ONZ accounted for 58.9%, and the OOZ made up 13.5%. Spatially, the NNZs are distributed in Beijing, almost all of the cities in Henan Province, as well as Dongying,parts of Tangshan, Binzhou and Cangzhou, urban districts of Tianjin, Baoding and Handan. The OOZs are mostly distributed in eastern part of Cangzhou, and parts of Dezhou, Tianjin and Binzhou. Other areas within counties belong to the ONZ.

(4) Primarily two approaches exist to increase the GDP sustained by groundwater: Increasing the volume of available groundwater resources and improving the water use efficiency. The regulation and storage of rain water and flood water in the alluvial-proluvial fan combined with processing of brackish waters of the middle-east plain, the volume of available groundwater could be increased to 11.0 billion cubic meters. Furthermore,a water use efficiency improvement to the global mean of 479 CNY per cubic meter and 3 978 CNY per cubic meter ( in line with the mean value of the top ten countries in the world), the GDP sustained by groundwater in the NCP could achieve 11.1 trillion CNY and 92.5 trillion CNY, respectively,which could maintain a 20 and 52 years’ GDP development, respectively, under the current growth rate.

However, because the groundwater system is intricate and complex, its carrying capacity is influenced by various natural and anthropogenic factors. We ignored water transfer among the cities(counties) in this study because the data were difficult to collect. Furthermore, the method cannot be used to assess the population growth scale sustained by groundwater due to different factors assuring human quality of life in domestic water use. Thus, it is not the case that a higher water productivity is better. Finally, we lacked specific data on the impact of the South-North Water Transfer Project on water availability. We proposed future investigations are necessary to assess the Project’s impact on surface water volume.

Acknowledgements

We acknowledge the support of the National Basic Research Program of China (973 Program(2010CB428805)), the Fundamental Research Fund (SK201306) of the Central Scientific &Research Institutes, Chinese Academy of Geological Sciences, the Institute of Hydrogeology and Environmental Geology, and the National Natural Science Foundation of China (41502253).