The virtual Water flow of crops between intraregional and interregional in mainland China

Agricultural Water Management 208 (2018) 204–213

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The virtual Water flow of crops between intraregional and interregional in mainland China

T

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YiCheng Fu , Jinyong Zhao, Chengli Wang, Wenqi Peng, Qi Wang, Chunling Zhang State Key Laboratory of Simulation and Regulation of River Basin Water Cycle, China Institute of Water Resources and Hydropower Research, Beijing, 100038, China

A R T I C LE I N FO

A B S T R A C T

Keywords: Virtual water Virtual water flow Virtual water trade China

Agriculture is the main consumer of freshwater in the world. The paper described the process of virtual water content and flow in crops. Differences in climatic conditions of different regions in China result in large differences in water consumption during crop growth. The virtual water trade in crops connects water flows within and between regions, linking the actual water consumption with the invisible water trade as a whole. The objective of the paper was supplying the agricultural products virtual water trade evaluation system, determining quantitatively the virtual water flow within the region & regional. Using meteorological and agricultural data from 2003 to 2010, a comprehensive analysis of China's domestic & international virtual water trade of agricultural products has been undertaken. The virtual water for the three primary crops and virtual water trade are discussed. The virtual water content of grain crops in northern & southern China was 1293 m3/t & 942 m3/t, respectively; the national average value was 1117 m3/t; and the regional differences in virtual water content for each crop were significant. China's inter-regional agricultural products virtual water trade was not consistent with water resource endowment expectations. The transfer of crops from northern to southern regions would have a significant impact on the sustainable utilization of water resources and would exacerbate water resources shortages in northern regions. China had a trade surplus in global virtual water trade of agricultural products. The exported agricultural products virtual water amounted to 31.5 billion m3/yr., and the imported amount was 145 billion m3/yr. The net import of virtual water embedded in agricultural products increased from 44 billion m3/yr. in 2003 to 178 billion m3/yr. in 2010. It is further concluded that the trend for agricultural products total virtual water, green water, and blue water is that China is increasing its imports year on year. A large increase in imports of agricultural products has led to a decline in the rate of self-sufficiency in domestic agricultural production. The paper provided the basis for the comprehensive evaluation of crop planting structure adjustment, grain import & export, and the potential of regional water resources development and utilization.

1. Introduction The populations of most nations consume products of both domestic and foreign origin, importing together with the products the water which is expended abroad for their production (termed ‘virtual water’) (Allan, 1994). The scholar Tony Allan (1997) first proposed the concept of virtual water, which was further extended by Hoekstra (2003a,b,c) to the currently recognized concept of virtual water, the quantity of water needed to produce goods and services. The related theory of virtual water has provided new ideas for solving the problem of water resources in China. Agricultural virtual water refers to the water consumed during the production of agricultural products (including crops and livestock products). The virtual water, as non-real water yield congealed in products and services, is internalized in product in the

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Corresponding author. E-mail address: [email protected] (Y. Fu).

https://doi.org/10.1016/j.agwat.2018.06.023 Received 3 April 2018; Received in revised form 18 June 2018; Accepted 19 June 2018 0378-3774/ © 2018 Elsevier B.V. All rights reserved.

form of "virtual", i.e. "embedded water" or "exogenous water". The virtual water is derived with the development of merchandise trade based on resource flow, resource substitution, and comparative advantage theory. Since the Israeli economists stated the diseconomy to the development of high-water-consumption agriculture in watershortage countries/regions in the 1980s, the decision-making guidance role of "virtual water" as countries/regions to alleviate water resource shortages has been highlighted. With the increasing globalization of commodity exchange trading, Hoekstra further expanded virtual water as the amount of water resources needed to produce commodities and services (Hoekstra and Hung, 2002). Since 2002, the theories of virtual water have been internationally expanded, while the quantitative calculation system has been gradually elaborated. Therefore, for a region, the smaller the total amount of virtual water for agricultural products

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concept of water footprint when investigating the consumption of water resources by production and consumption activities in the region. The concept of virtual water was often used when studying the consumption of water resources in cross-regional consumption and trade (Lenzen, 2009; Mekonnen and Hoekstra, 2012). When product is exported, its water footprint becomes virtual water trade or transfer. To realize the water balance among different countries, Aldaya et al. (2012) suggested water footprint should instead of virtual water for a product. The concept of water footprint is defined for a product “volume of freshwater appropriated to produce the product, considering the volumes of water consumed and polluted in the different steps of the supply chain”. Therefore, we can determine the amount of water necessary to produce a series of products (Table 1). Several authors began calculating the “water foot prints” of goods and services a few years after the notion of virtual water was introduced, with the goal of describing the direct and indirect water use by consumers and producers (Hoekstra et al., 2011; Ercin and Hoekstra, 2014; Brindha, 2017). The water footprint of virtual water consumption and trades are interrelated relationship, which is illustrated in Fig. 2 (Hoekstra and Mekonnen, 2012). The water footprint relates virtual water with the actual water consumption of human beings, truly reflecting the demand for and occupancy of water resources by the population and the degree of human pressures on water resources systems. In the field of water footprint assessment, with most studies carried out at the watershed, national and global level, application at the urban level is at its infancy. Yu et al. (2010) developed a regional input-output model to assess domestic water footprint for various consumption categories for the southeast and the northeast of England. Paterson et al. (2015) provide the first review of research on the water footprint of cities, and a range of methods for estimating the water footprint of consumption is being used. Rushforth and Ruddell (2015) quantify the water footprint of the Phoenix Metropolitan Area in Arizona, contributing methodologically by calculating intra-metropolitan areavirtual water flows using commodity and labor flows. De Miguel et al. (2015a) considers water footprint at the river basin level by considering the sustainability of the water footprint of crop production in the Duero River Basin in Spain. The various countries in the world are not entirely equal in virtual water trading. The virtual water strategy refers to the purchase of water-intensive products from the water-enriched countries (regions) by the water-shortage countries (regions) by way of trade, which is to guarantee water and food security. Water withdrawals for irrigation are limited by water scarcity in major river basins from Mexico to North Africa, West Asia, Central Asia and North China (Brown et al., 2009). It is widely agreed that virtual water trade as a strategy is likely to play a more prominent role in the future in global food production. Measuring

means that the smaller the amount of agricultural water consumed, the more water resources that can be used in other industries will be conducive to the increase of regional economic output value. To understand the development trend of the total amount of virtual water for agricultural products in a region, it is particularly important to find out the drivers of the change in the total amount of virtual water in agricultural products in mainland China. Research on virtual water and related theories at home and abroad is mainly focused on the issue of agricultural products (especially food). Scholars from various countries have conducted in-depth discussions on the driving factors of virtual water trade (flow) from the actual conditions of their own countries (Ma et al., 2006; Wang et al., 2015; Kumar and Singh, 2005; Hoekstra, 2003a,b,c). The proposition of the concept of virtual water causes the diversion of the study on water resource issues from "water entity" to "generalized water", which promotes the system innovation of water resources and makes a second configuration of water resources come true. And in the meantime, it broadens the research field of water resources and lays a solid foundation for solving of regional water resource issues by employing the concept and method of "macro-water resources development" system and has remarkably practical significance on the studies of rational allocation, carrying capacity and efficient use of water resources in China. Virtual water flow represents the volume of water that is embedded in traded commodities (Chouchane et al., 2018). Water traded in this way is referred to as ‘virtual water’ and constitutes a significant portion of global water consumption (Hoekstra and Mekonnen, 2012). The “flows of virtual water” should be considered when designing national strategies regarding water and agriculture, and when evaluating guidelines pertaining to international trade (Duarte et al., 2014; Shi et al., 2014; Zoumides et al., 2014). Virtual water trade in mainly agricultural products can be considered as a substitute for trade in freshwater, or non-virtual water. The theory of comparative advantage suggests that if two countries have relative differences in production processes, expressed as differences in opportunity costs, there will be scope for trade that will benefit both countries (Wichelns, 2010). The virtual water trade process may prove significant for improving global water use efficiency and alleviating pressure on local water resources (Goswami and Nishad, 2015). The development process of global virtual water trade is shown in Fig. 1, while the globalization of food trading and the digitization of metering system as the signs of the development of new water resources have greatly propelled the process of virtual water trade. The concept of water footprint is similar to the concept of virtual water. In many cases, the water footprint of product and the virtual water contained in the product can be exchanged. People often used the

Fig. 1. The development process of global virtual water trade. In the figure, the IHE, WWC and FAO is the abbreviation of International Health Evaluation Association, World Water Council, and Food and Agriculture Organization. 205

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Table 1 Water footprints of a series of products for calculating virtual water. Source: Water Footprint Network website. Plant products

The flow of virtual water to produce (L)

Animal products/food

The flow of virtual water to produce (L)

Banana (200 g) Cabbage (1 kg) Cotton (1 kg fabric) Cucumber (1 kg) Dates (1 kg) Lettuce (1 kg) Maize (1 kg) Olives (1 kg) Orange (1 of, 150 g) Peach (1 of, 140 g) Apple (1 of, 150 g) Potato (1 kg) Rice (1 kg) Sugar (1 kg, from cane) Tea (3 g in a cup) Tomato (1 of, 250 g) Biodiesel-soya bean (1 L) Bioethanol-maize (1 L) Bioethanol-sugar bee

160 280 10,000 350 2280 240 1220 3020 80 140 125 290 1670 210 30 50 11,400 2854 1188

Beef (1 kg) Butter (1 kg) Cheese (1 kg) Chicken (1 kg) Egg (1 of, 60 g) Leather (bovine, 1 kg) Milk (1 L) Pork (1 kg Sheep (1 kg) Bread (1 kg) Chocolate (1 kg) Coffee (1 cup) Pasta (1 kg, dry weight) Pizza (1 of, 750 g, margherita) Wine (1 glass, 250 ml) Beer (1 L)

15400 5550 5060 4330 200 17000 1020 5990 10400 1608 17,000 130 1850 1260 110 298

2. Methodology and data 2.1. Study area The study region contains 31 provinces, autonomous regions and municipalities in mainland China, but doesn’t include Hong Kong, Macao and Taiwan. The study region is divided into two primary regions and eight sub-regions in accordance with geographical location, climatic conditions and agricultural production conditions. The eight sub-regions are North China, Northeast China, Huang-huai-hai Region, Northwest China, Southeast China, the Middle–Lower Reaches of the Yangtze River, South China and Southwest China (Ma et al., 2006). (1) The North China contains Beijing city, Tianjin city, and Shanxi province; (2) The Northeast China contains Heilongjiang, Jilin, Liaoning province, and Inner Mongolia Autonomous Region; (3) The HuangHuai-Hai region contains Hebei, Henan, Shandong, and Anhui province; (4) The Northwest China contains Shaanxi, Gansu, Qinghai province, the Ningxia Hui Autonomous Region and the Xinjiang Uygur Autonomous region; (5) The Southeast China includes Shanghai city, Zhejiang and Fujian province; (6) The Middle and Lower Yangtze river region includes Jiangsu, Hubei, Hunan, and Jiangxi province; (7) The South China includes Guangdong province, Hainan province, and Guangxi zhuang autonomous region; (8) The southwest China includes Chongqing city, Sichuan, Guizhou, Yunnan province, and the Tibet Autonomous region (Fig. 3). The northern region includes the region from (1) to (4), the southern region includes the region from (5) to (8). The figure has been supplied by www.geodata.cn, which is a national science and technology basic conditions platform and an earth system science data sharing platform. The figure information is public.

Fig. 2. Calculation framework of the water footprint was based on different relationships. In the figure, + represents adding together, and the grey portion represents a summation value. In the right column, each data is the sum of the two columns on the left, which is expressed in “summation value”, therefore, the data is increased from left to right. The data from top to bottom also have the similar characteristics.

virtual water is a useful concept in assessing water management as it permits the comparison of crops and livestock from the perspective of embedded. To evaluate trade-offs in water allocation in countries like Canada with large regional variability in climate, virtual water should be calculated on a watershed scale (European Environment Agency, 2012). China has one of the most intensive agricultural systems in the world and is a major importer of virtual water in the form of meat (Schmidt and Benítez-sanz, 2012). The global virtual water trade evaluation lists the USA, Australia and Canada as the largest virtual water exporters in the world, while Japan, the Netherlands, South Korea and China are the largest virtual water importers (Lamastra et al., 2017). Countries that have additional capacity to produce food (USA, Canada, Brazil and Argentina) will likely increase their virtual water exports particularly in the form of meat which is more water demanding than grains. This paper is structured as illustrated below. We first provide an overview and comprehensive analysis of the virtual water trade, water footprint, and virtual water strategy following the recent literature. Section 2 details our accounting approach and the virtual water data that we use. Section 3 present a case study and discuss the results. Finally, we present our conclusions and comments on the issue.

2.2. Methods The consumption of goods and services often creates stress on the water resources of production sites. Empirical studies suggest that a range of economic, hydro-climatic, and political factors is important for the occurrence of conflicts over water (Ye et al., 2017; Zhao et al., 2017). The potential for conflict is decreased by virtual water trade if the water-abundant country uses a sufficiently small share of the resource (Ansink, 2010). The virtual water trade (import/export) was calculated as the food trade volume (t/yr) times its virtual water content (m3/t). The virtual water content of an agricultural product (m3/t) is estimated from the volume of water used during the crop’s growth period (m3/hm2), called the crop water requirement (Hoekstra et al., 206

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consumed by the given region or population, then the virtual water is obtained by the summation of all the multiplication of the consumed goods and services by their respective virtual water intensities (Hoekstra, 2009). The virtual water content of crop products is the key to measure virtual water. The calculation method for the virtual water content of primary crops is limited by inferior territorial, ratione materiae, reproducibility and comparability. The virtual water content of non-water-intensive products was calculated in line with the balanced nutrition rule in general. The steps to calculate the virtual water trade (import/export) for the major grain crops are shown in Fig. 4. Grey water footprint is a method of quantitatively describing the state of water environment pollution. Hoekstra (2003a,b,c) first proposed its concept in 2002, which is defined as the amount of fresh water required to dilute pollutants loading to the maximum concentration allowed by the standard based on a certain water quality standard. The grey water footprint considers that the same water body can dilute different pollutants. The grey water footprint quantitatively shows the impact of water pollution on the quantity of water resources and provides a new method for the sustainable study of water resources. Research on the gray water footprint is still at the primary stage. At present, research abroad mainly focuses on the calculation and analysis of gray water footprints of industrial and agricultural products, calculation and analysis of gray water footprints in national regions or watershed, and evaluation of water resources sustainability based on grey water footprint measurement, et al (Mekonnen and Hoekstra, 2014; Vanham and Bidoglio, 2014; Chapagain and Hoekstra, 2011; Ercin and Hoekstra, 2014). The study of greywater footprint domestic is mainly about the study and application of abroad research methods. Based on the gray water footprint theory, the study of effective assessment of regional real water pollution remains to be further discussed in the future. Therefore, the paper only calculated the blue and green water footprint.

Fig. 3. Basic information of China.

2011; Hoekstra and Chapagain, 2008). The volume of water used by a crop has two components: green water and blue water. The use of green water in agriculture is related to more sustainable practices than the use of blue water (Aldaya et al., 2008). There are two widely accepted categories of methodology for virtual water accounting, the down-top approach works well when providing detailed and separate virtual water information of specific goods, such as rice, meat or even T-shirt (Vanham and Bidoglio, 2013). As item-by-item approach, the first step is to collect the data of all the individual goods or services that are

2.2.1. Virtual water flow The basic estimate of water embodied in external trade is to multiply the import and export trade figures and the corresponding water intensity coefficients. The imported virtual water (IW) is the sum of virtual water embodied in imported goods (IWg) and imported services (IWs) (Chen and Li, 2015). The IW can be calculated as,

Fig. 4. The steps to calculate the virtual water trade (import/export) for the major grain crops. The abbreviations were given in the paper. The schematic of estimating the blue, green and evaporative water use and the respective components of the virtual water content of primary crops. The bold arrow line means the calculation process of virtual water flow, and the thin arrow line means lateral influencing factors, and the bidirectional arrow line means the two-way relationships among different factors.

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IW = IWg + IWs =

∑ ∑ (Mi j × Nij) i

Where, VWCg and VWCb are the green and blue components of virtual water content for a crop (m3/t), respectively; WRCg and WRCb are green and blue water consumption over the crop-growing period (m3/ha); ETg and ETb are green and blue water evapotranspiration over the crop growing period (mm). ETg and ETb during the crop-growing period can be estimated using the FAO’s equation (Hoekstra et al., 2011),

(1)

j

Where, Mij is the imported item i from j country(region), Nij is the corresponding virtual water intensity of import i from j country(region). The exported virtual water (EW) can be calculated by virtual water embodied in exported goods (EWg) and exported services (EWs) (Lamastra et al., 2017). The EW can be calculated as,

EW = EWg + EWs =

∑ (Mk × Nk )

⎧ ETg = min (ETc , Pe ) ⎨ ETb = max (0, ETc − Pe ) ⎩

(2)

k

Where, Mk represent the export k, and Nk is the corresponding virtual water content of k. The virtual water is computed using Eq. (3),

Where, Pe is the effective rainfall over the crop-growing period (mm). Understanding the pattern of China's virtual water trade and exploring virtual water policy is one of the important ways to achieve optimal allocation of water resources. It can provide a new perspective for China to ease water conflicts and solve the water crisis. We estimated the international trade volume of virtual water: (1) Calculating the overall international trade volume of virtual water. According to the data of China Statistical Yearbook 2004–2011, we obtained grain import and export volume in the international trade of grain in the past 8 years in China. We calculated the virtual water content of the three crops (Rice, Wheat, and Maize) and their import and export volume, and obtained the Import and export trade volume of virtual water. (2) Virtual water trade of unit crops (Rice, Wheat, and Maize) in international trade. According to the data of China Statistical Yearbook 2004–2011, we obtained the number of imports and exports of a certain crop in the international trade of grain in China in the past 8 years. The unit virtual water content of a certain crop was multiplied by its import and export volume to obtain the corresponding virtual Water imports, exports and net imports.

(3)

VW = IW + EW

2.2.2. Volume of virtual water This paragraph provided a more detailed calculation process to evaluate the virtual water volume embedded in food trade between different regions. Under a rain-fed scenario, green water is equal to the total virtual water of the crop (Sun et al., 2013). The per unit water value virtual water content, which refers to the value per unit water value, was calculated for the analysis of virtual water strategy (Shao et al., 2017). The virtual water content is used as the reference value for the tracking of water movement across international boundaries in the agricultural and industry product trades. The virtual water content of primary crops can be calculated according to the methodology as follows (Hoekstra and Hung, 2002), (4)

VWC = WRC / YC

(9)

3

Where, VWC is the virtual water content of the primary crop (m /t), WRC is the crop water requirement (m3/ha), YC is the crop yield per unit area (t/ha). The WRC is the evapotranspiration (ETc, mm/d) over the cropgrowing period (Hoekstra and Hung, 2002):

2.3. Data Climate data were taken from the CLIMWAT database, which were available from www.fao.org/nr/water/infores_databases_climwat.html. The data include monthly average maximum temperature, monthly average minimum temperature, relative humidity, wind speed, sunshine hours and precipitation, from 2003-2010. We obtained the data from China Meteorological Administration, as well as the provinces, autonomous regions and municipalities bureau of Meteorology. Agricultural data includes farmland area, agricultural inputs (fertilizers, pesticides, agricultural machinery, etc.), irrigation data, and crop yield, which were taken from the China Statistical Yearbook and Chinese agricultural statistics data, et.al. We got agricultural inputs data from the “China Statistical Yearbook 2004-2011″ and “China Rural Statistics Yearbook 2004-2011″. The irrigation data were obtained from “China Water Resources Bulletin 2003-2011″ and “Yearbook of China Water Resources 2004-2011″. The crop yield and farmland area come from “China Rural Statistics Yearbook 2004-2011″. Virtual water trade data were obtained from the FAOSTAT database (http://faostat.fao.org/site/406/default.aspx). We reconstructed 8 years (2003–2010) of international global trade data for 32 crops and animal products. The food trade data for each commodity were converted into virtual water trade values (Hoekstra et al., 2011).

lp

WRC = 10 ×

∑ ETC

(5)

d=1

Where, factor 10 is intended to convert water depth into water volume per land surface (1mm = m3/ha); lp is the number of days throughout the growing season (d). The evapotranspiration ETC over the crop-growing period (mm) is calculated as follows, (6)

ETC = ET0 × kC

Where, Kc is the crop coefficient, ET0 is the reference crop evapotranspiration (mm). The ET0 is calculated according to the FAO Penman–Monteith equation as follows,

(e − ea ) ⎤ ⎡ r / Δ + γ ⎛1 + s ⎞ ⎤ ET0 = ⎡Δ (Rn − G ) + ρa × cp × s ⎥ ⎢ ⎥ ⎢ ra r a ⎠⎦ ⎣ ⎦ ⎝ ⎣ ⎜

⎟

(7)

Where, Rn is the net radiation, G is the soil heat flux, (es-ea) represents the vapor pressure deficit of the air, ρa is the mean air density at constant pressure, cp is the specific heat of the air, Δ represents the slope of the saturation vapor pressure temperature relationship, γ is the psychrometric constant, and rs and ra are the (bulk) surface and aerodynamic resistances. The values for green and blue water in virtual water are calculated by computing the accumulation of daily evapotranspiration over the crop growing period (Ercin et al., 2013). The virtual water flow related to crop water requirement WRCg, WRCb can be calculated as follows,

⎧VWCg = WRCg / YC = 10 × (ETg / YC ) ⎨ ⎩VWCb = WRCb / YC = 10 × (ETb/ YC )

3. Results and discussion Although there are many quantification studies on virtual water, there are still some problems in its calculation methods. As for the virtual water content of agricultural products, many researches lacked relevant meteorological data and water data, and the experimental equipment and technical conditions for measuring crop water requirements were relatively backward, therefore, in practice, many years average values or common reference values were often used (Yang et al., 2015). However, due to the spatial and temporal differences in climate, the degree of dryness and humidity in different regions and

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different years was different. Even if the water demand of the same type of crop was different, the virtual water content of the calculated agricultural product may have large errors. From the perspective of the virtual water change mechanism analysis method, the current method is not strict enough to identify virtual water change factors. In most studies, water use efficiency, industrial structure, and production scale are selected as the main factors for analysis. Too few factors cause the decomposition analysis to be too simple and general, which has limited significance for the practical work of guiding the virtual water strategy. Due to the lack of trade statistics, a large number of product trades between regions were estimated through historical data or economic models rather than actual surveys, which directly leaded to the deviations in the calculation of total virtual water.

Table 2 The water resources content of different crops in China. Crops

Main growing region

Range value (m3/ha)

Average value (m3/ha)

Rice

The Middle and Lower Yangtze river area, The Huang-Huai-Hai region The Huang-Huai-Hai region, The Middle and Lower Yangtze river area The Huang-Huai-Hai region, The Northeast China The Northeast China, The Huang-Huai-Hai region The North China The Northwest China, The North China The North China The North China The Northeast China The South China, The southwest China – The Huang-Huai-Hai region The South China The Northwest China The North China The North China The North China The North China The North China The North China The North China The North China The North China The Huang-Huai-Hai region, The Middle and Lower Yangtze river region The North China, The Northwest China The Northeast China, The southwest China The Northeast China – – – – The Southeast China, The South China

2300–6670

4440

3000–5500

4208

2800–6000

4356

3200–5500

4307

4000–4650 2800–4900

4282 3812

3000 4000 3300–5200 7200–10000

3000 4000 4208 8514

3500–6000 3200 11000–15000 5800 3700–5600 3500–4500 5500 3900–5700 3000 5400 4100–4600 1300–3400 3000–4500 4500–6000

4703 3200 12870 5800 4604 3960 5500 4752 3000 5400 4307 2327 3713 5198

3000–6000

4455

1200–4800

2970

3600–4500 2600–4100 4500–5600 7500–11000 4500–5500 6000–13000

4010 3317 5000 9158 4950 9405

Wheat

Maize Soybean Millet Sorghum

3.1. Virtual water of different crops

Potato Barley Apple Citus

The virtual water content doesn’t depend on the actual condition of crop growth except the climate condition. There is less related to the state of irrigation, seed, agriculture technology and so on. The virtual water content of crop crops is in correlation with the crop quality, water consumption quota, yield, geographical conditions of growth, conversion efficiency of products in different regions. Differences of water and cultivated land resources led to differences in grain production and water use between regions (Sun et al., 2016). The matching of the geographical distribution of the virtual farmland of agricultural products in China and the economic and economic elements of each resource is not high. This paper analyzed the virtual water of China's primary agricultural product trade. The main goal is to find out the volume of virtual water trade in different regions in China and calculate the value of the virtual water import and export in agricultural trade with other countries in the future to effectively use the international market to solve the problem of water resources shortage. We use grain production and water use for grain production in mainland China to calculate the water requirement water resources content of different crops. The range and average value of water resources content of major crop is given in Table 2. In China, rice, wheat, and maize are the three major crops, and the rice has the largest cultivated area. Based on our calculation, water resources content for rice is about 4440 m3/ha, and the percolation for rice ranges from 2300 m3/ha to 6670 m3/ha (Zhang et al., 2016). Therefore, the national average of 8000 m3/ha was used as the water resources content of rice in China. Owing to the differences in climatic condition, crop yields and crop management between regions, the regional differences of virtual water content for the same crop were significant. The average virtual water content (2003–2010) was calculated by dividing the crop water resources content with the average crop yield during this period (WRCg, WRCb). Annual virtual water content was used for calculating virtual water trade in the corresponding year. Table 3 shows the green and blue virtual water content of major three crops in each region. The highest value and lowest value of wheat virtual water content was 2303, 1438 m3/t, respectively. The national average virtual water content of wheat, rice, and maize were as follows: wheat (1307 m3/t), rice (1604 m3/t), and maize (1061 m3/t). These results are approximately consistent with the results of former studies (Sun et al., 2013; Wang et al., 2015), which were wheat (1071 m3/t), rice (1294 m3/t), and maize (830 m3/t). The virtual water content of grain crops in the northern and southern China was 1293 m3/t and 942 m3/t, respectively. The national average value was 1117 m3/t and shows that the regional differences of virtual water content for crop were significant. Taking wheat as an example, the virtual water content of wheat was relatively low in Huang-huai-hai region, parts of the South China, which were less than 1000 m3/t; the high values of virtual water content for wheat were mainly distributed in Northeast China, the North China, and part of Northwest China, and were more than 2000 m3/t. The virtual water content of maize was between 850 and 1342 m3/t,

Pear Water melon Banana Grape Tomato Cabbage Carrot Cucumber Lettuce Onion Pea Spinach Pepper Cotton

Groundnut Rapeseed Sunflower Sesame Sugar beet Sugarcane Tobacco Tea

“–” denotes the crop planting has no definite regional distribution.

and the spatial distribution was slightly different from that of wheat. As rice production needs more water and heat, the high values of virtual water content for rice located in the South China and North China (in the west of Inner Mongolia), which were more than 1400 m3/t. The VWCg proportion for wheat, maize and rice was increased gradually from northern to southern regions. 3.2. Virtual water trade 3.2.1. Transfer between regions in China Water scarcity has become an increasingly pressing constraint to agricultural production in China. There is a mismatch between the distribution of water and the Cultivated Land resources in China. Water resources in southern China account for more than 80% of China's total water resources, however, while cultivated land is mainly in the Northern regions (Wang et al., 2014). We Analyzed according to the information provided by the China National Grain and Oil Information Center, the export regions of wheat were mainly located in Huanghuaihai region, Northwest and Southwest China; the North China, Northeast China, Huang-huai-hai region, Northwest China and Southwest China were the main output regions of maize; and the surplus 209

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Table 3 The VWCg and VWCb of each crop in different regions of China (m3/t). Regions

Wheat

North China Northeast China Huang-Huai-Hai region Northwest China The Northern China Southeast China M & L Yangtze river South China Southwest China The Southern China The average

Rice

Maize

Total

VWCg

VWCb

VWC

VWCg

VWCb

VWC

VWCg

VWCb

VWC

VWC

526 532 813 355 562 984 864 1096 633 912 809

878 766 511 1032 805 366 442 391 509 436 498

1404 1298 1324 1387 1367 1349 1306 1487 1142 1348 1307

509 795 767 490 647 1587 1257 1450 1239 1411 818

1531 1508 671 1619 1345 64 279 24 933 331 786

2040 2303 1438 2109 1992 1652 1535 1474 2172 1742 1604

765 773 838 570 744 1262 842 1150 969 1077 809

281 355 132 717 375 79 8 24 5 30 252

1046 1128 970 1287 1119 1342 850 1174 974 1106 1061

1264 1384 1074 1449 1293 876 832 1027 1031 942 1117

M & L Yangtze river is the abbreviation of the Middle and Lower Yangtze river region. The shaded parts are summation data. The VWCg, VWCb, and VWC denotes virtual water content, green water content, and blue water content of primary crop. Table 4 The virtual water flows transfer for the major crops among different regions (108m3). Regions

North China Northeast China Huang-Huai-Hai region Northwest China The Northern China Southeast China M & L Yangtze river South China Southwest China The Southern China

Rice

Wheat

Maize

Total

VWCg

VWCb

VWC

VWCg

VWCb

VWC

VWCg

VWCb

VWC

VWCg

VWCb

VWC

2.93 −4.86 0.78 1.93 0.78 7.39 −11.44 3.26 / −0.78

4.21 −7.00 0.40 2.80 0.40 3.78 −5.86 1.67 / −0.40

7.13 −11.86 1.18 4.73 1.18 11.17 −17.30 4.93 / −1.18

2.13 0.47 −7.85 −5.25 −5.25 2.25 / 3.00 / 5.25

1.86 0.41 −6.86 −4.58 −4.58 1.96 / 2.62 / 4.58

3.99 0.89 −14.71 −9.83 −9.83 4.21 / 5.62 / 9.83

/ −8.98 −1.66 −2.64 −13.28 3.16 7.49 5.23 −2.60 13.28

/ −4.12 −0.26 −3.32 −7.70 0.95 3.43 3.34 −0.02 7.70

/ −13.10 −1.92 −5.96 −20.98 4.11 10.91 8.57 −2.62 20.98

5.05 −13.37 −8.74 −5.96 −17.75 12.80 −3.95 11.49 −2.60 17.75

6.07 −10.71 −6.71 −5.10 −11.88 6.69 −2.43 7.63 −0.02 11.88

11.12 −24.07 −15.44 −11.06 −29.63 19.48 −6.38 19.13 −2.62 29.63

“/” denotes there is no virtual water flows transfer; M & L Yangtze river is the abbreviation of the Middle and Lower Yangtze river region; the shaded parts are summation data; the VWC, VWCg and VWCb, and denotes virtual water content, green water content, and blue water content of primary crop; the positive value in the table indicates a virtual water inflow (virtual water import); the negative value in the table indicates the outflow of virtual water (virtual water export).

regions in China (Antonelli et al., 2017). Uneven economic development across regions is an important factor. As east coastal regions are the most developed areas in China, they depend heavily on raw materials for production and food and export less water-intensive and high value-added products and services to other regions. The spatial distribution of water-intensive production activities is determined by multiple production factors. Therefore, the uneven distribution of land and water resources, and irrational planting pattern, structure of regional crops would consume large quantities of water and put pressure on water resources of northern regions. Thus, the transfer of crops from northern to southern regions would have a significant impact on the water resources sustainable utilization and exacerbate the situation of water resources shortage in northern regions.

regions of rice were distributed in Northeast China, the Middle and Lower Yangtze river region, and Southwest China. Based on the calculation results of virtual water content for wheat, maize and rice, combining with the grain trade data among different regions in China, the transfer of virtual water flows was given in Table 4. The calculation results are like the literature (Sun et al., 2013, 2016). Northeast China, Huang-Huai-Hai region, and Northwest China are the major grain production regions as well as the major virtual water export regions. The virtual water content output from the three regions accounts for nearly 85% of the total virtual water content output from the grain export regions. The virtual water outflow with grain export is 24.07 × 108m3 in the Northeast and 15.44 × 108m3 in the HuangHuai-Hai regions. The Southeast China, South China, and North China regions are the major grain virtual water content import regions, which were 19.48 × 108, 19.13 × 108, and 11.12 × 108 m3, respectively. The virtual water content reaches 49.73 × 108 m3 above three regions. The virtual water content import of Southeast China, South China, and North China were 19.78 × 108, 19.42 × 108, and 11.29 × 108 m3, respectively (Sun et al., 2013). The growth of maize mainly relies on green water, therefore, the average green water proportion of virtual water content for maize was the highest, about 68.78%, followed by rice and wheat. Precipitation is concentrated mostly in summer and autumn, and maize need a large quantity of water to grow during the period. Therefore, the green water proportion in maize virtual water content was higher. China's inter-regional virtual water trade is not consistent with water resource endowments expectations. The relatively abundant water resources in south China do not become a comparative advantage in terms of producing water intensive products to trade with other

3.2.2. Transfer between nations in the world The research object of virtual water accounting could be either the virtual water contained in an intermediate product or final product in the production chain, or the virtual water contained in all products consumed or produced by consumers or producers. Different virtual water concepts have corresponding virtual water accounting methods, therefore, the virtual water content in different spatial scale (such as industrial parks, watersheds, regions, administrative regions, countries, or the world) and time scales is different. The virtual water content of each crop imports is not as same as in China. However, in order to compare the calculated results, we used the same value for the virtual water content of the same crop. The virtual water calculation method adopted in this paper mainly considered direct water use in agricultural production and ignored the virtual water of intermediate products (products supplied to other producers) in the industrial chain (Hoekstra 210

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production process should be calculated separately. The green and blue water footprint of crop products was mainly calculated by formulas (8)(9) based on crop water requirement, growth period precipitation, and irrigation water volume, using Cropwat model estimation or field test data. Under drought conditions, the evapotranspiration of blue water within the crop growth period is 0 (mm), and the actual water consumption (etc) of the crop is the green water evapotranspiration (ETg, mm). The water consumption in the facility is the blue water evapotranspiration. The amount of blue water evapotranspiration can be calculated based on the principle of soil water balance in the facility. Different countries or regions have great differences in terms of geography, climate, production technology, and production. Therefore, the estimation of the average global water footprint of agricultural products is difficult to truly reflect the actual situation in different countries or regions. For example, the water footprint of Spanish pork production and processing is 19.5 billion m3/yr. The green water, blue water, and gray water footprints account for 82%, 8%, and 10%, respectively (de Miguel et al., 2015b). The calculation method and calculation results for the global average water footprint is obviously different. In the lack of detailed spatial geography and other information, as well as the results of the global scale water footprint literature study cannot accurately reflect the changes in the global water footprint, to reduce the uncertainty of the calculation results, we used the same virtual water content between China's grain and the imported grain. At present, China has played an important role in the field of global water resources. The agricultural products virtual water that China exported to the world was 50 billion m3 and 24 billion m3 in 2003 and 2013. The agricultural products virtual water total amount was 252 billion m3; the average was 31.5 billion m3/yr., approximately 73.4% green water, 26.6% blue water. The total virtual water, green water, blue water China exported per year were on the decline. The main exported virtual water was from crop products, approximately 89% of the total, and livestock products accounted for 11% (Zhang et al., 2016). virtual water flows of China mainly exported to Northeast Asia and Southeast Asia, accounting for 30% and 20% of total exported virtual water. China plays a more important role in the world trade market and imports more virtual water every year. The agricultural products virtual water that China imported from the world was 90 billion m3 and 210 billion m3 in 2001 and 2013. The agricultural products virtual water total amount was 1160 billion m3 from 2003 to 2010; the average was 145 billion m3/yr; approximately 91% green water, 9% blue water. Green water was the most important water resource in agricultural production. The tendency of total virtual water, green water, blue water, and gray water that China imported per year were on the rising. The massive imports of agricultural products led to the decline of selfsufficiency rate of domestic agricultural products. The agricultural products virtual water flows of China mainly imported from America and Southeast Asia, accounting for 60% and 20% of total exported virtual water. Imported virtual water flows of China was relative concentration.

et al., 2011). Since the amount of water used by agriculture itself was much greater than the amount of water used by these intermediate products, research on agricultural virtual water could virtually ignore the virtual water of intermediate products. Most of the international virtual water trade relates to trade in food. The virtual water balance can lead to the export and import data of water by country. There are many developed countries that import more virtual water than they export even if they are typically in water balance surplus, or exporting, countries in conventional hydrological terms. There are also countries conventionally viewed as water scarce where there is net virtual water export (Hoekstra and Mekonnen, 2012). Agriculture accounts for 70% of freshwater withdrawals and 85% of consumptions worldwide. Chapagain and Hoekstra estimated that 895 billion m3/yr are transferred inter-regionally (Ercin et al., 2013). This compares with world annual water withdrawals and consumption of 4430 billion m3 and 2304 billion m3, respectively, and annual desalination of 0.017 billion m3 (International desalination association, 2011). The increasing focus on interactions between water, land, food, and climate leads to a call to consider water, land, carbon and other footprints more coherently and put the different footprints in the context of local, regional or global planetary boundaries (Hoekstra and Wiedmann, 2014). Fang et al. (2015) have taken up the challenge to connect the environmental footprint literature to the planetary boundaries literature in a comprehensive study putting the carbon, water and land footprints of 28 selected nations in the context of allocated planetary boundaries. By analyzing the virtual water balance among different countries, total virtual water import and export at the scale of the globe balanced to zero. In the world, water exporters are the USA, South America, India and Australia; the importers are Western Europe, North Africa, Japan, Mexico and the Middle East. The virtual water trade in the world has increased considerably over time, and high-value exports is beneficial for low-income countries from a regional economic water efficiency perspective (Clark et al., 2015; Jackson et al., 2015). Many studies have attempted to quantify the link between China’s WTO accession and agricultural outcomes, comparative advantage in agriculture will lead China to increase the export of labor-intensive products and the import of land-intensive products (Liao et al., 2008). The community structure of virtual water trade also reflects some of the trade partnerships resulting from political alliances and that some of the changes associated with political events. Considering both water resource constraints and political considerations, a more plausible scenario is that China will increase imports of wheat and maize while exporting vegetables. On the base of analyzing China's international virtual water flows of agricultural products trade from 2003 to 2010, China was trade surplus in global virtual water trade of agricultural products. The exported virtual water amounted to 31.5 billion m3/yr., and imported amount was 145 billion m3/yr. The exported/imported virtual water flows of China from 2003 to 2010 was shown in Table 5. Due to the different opportunity costs and impacts of the green and blue water footprints, the water footprint in the global virtual water trade and product

Table 5 The exported/imported virtual water flows of China from 2003 to 2010 (billion m3/yr.).

The shaded parts are summation data. 211

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exporting more virtual water than they withdraw or consume domestically (Mubako and Lant, 2013).

The net import of virtual water embedded in agricultural products increased from 44 billion m3/yr. in 2003 to 178 billion m3/yr. in 2010. Compared to water consumption in 2010 (318.22 billion m3) (The Ministry of Water Resources of the People's Republic of China, 2011), the net import in 2010 accounted for 56%. Net import of virtual water was in a rising trend. For the virtual water trade with other countries, China is basically in a state of surplus. Therefore, virtual water & water footprint are two closely inter-connected activities, it becomes an opportunity to realize the liberalization of international commerce. From 2003 to 2010, the agricultural products virtual water trade in China's grain international trade has been maintaining a large deficit. In 8 years, the cumulative net imported virtual water through grain international trade is 573.63 billion m3, which is equivalent to saving the same amount of grain production water consumption, which effectively relieved the contradiction of water shortage in China. At the practical application level, the calculation of the virtual water content of a specific product is an important content of virtual water research in the world in recent years. At present, the virtual water accounting of the global agriculture, forestry, animal products and its derivatives has been relatively perfect, and the virtual water content for some industrial products and tertiary industry services have also been studied. From the point of virtual water calculation method for industrial and tertiary industries, the calculation of the virtual water content involved in the production process is a complicated process. The differences in production technology, production scale and production environment all caused the inaccurate of virtual water calculation. Therefore, even if the distribution of virtual water in the products and services of various industries and tertiary industries was avoided or omitted or repeated, the result of the calculation may also have large errors. Future research should continue to improve the accuracy of calculation methods and enrich the types of factors considered in order to obtain more accurate virtual water accounting results. China, US, South Asia and India are the most important countries in the international virtual water trade. Hoekstra's study found that North America, South America and Australia are the main agricultural production virtual water resources outflow regions, while Africa, East Asia and Europe are the main virtual water inflow regions. Fracasso (2014) used the gravitational model to study the virtual water trade in 145 countries and considered that the flow of virtual water was affected by the country’s trade level, the endowment of water resources, water pressure and other factors. The virtual water import from agricultural production is increasing in China from 2003. China's agricultural virtual water trade shows China is the net importer of agricultural production virtual water. China did not use the whole import virtual water for local consumption and used a part of to supply the manufacturing industry as an intermediate input. China's manufacturing industry is in a net export state, and in 2003–2007 net exports of virtual water accelerated year by year. However, since 2008, the net exports of virtual water from the manufacturing industry have fallen, while agriculture was still accelerating its net imports. As a result, the net imports of virtual water from agricultural production continued to rise. Although agriculture is a major consumer of water in the national economy, the importance of agricultural production is self-evident because of the needs of the national life and the raw materials for industrial products. Therefore, the virtual water net import status of agriculture can not only satisfy China's huge demand for agricultural products, but also save a large amount of domestic water resources. The virtual water export of United States is increasing from 2003. Despite its large water footprint per capita (1688 m3/yr.), the U.S. is the leading net virtual water exporting country (5 billion m3), but this mask interstate flows of 191 billion m3 per year of virtual water, larger than withdrawals in many states. Virtual water flows are large compared to total water withdrawals, evapotranspiration from rain-fed crops, or total water footprint in nearly every state, with several Eastern states importing roughly half their water footprint and some Midwestern states

4. Conclusions As a new water source conception for the 21st Century, agricultural virtual water has become the strategic decision to solve the global water resources and crop security issues. On the base of analyzing the virtual water trade, water footprint, and virtual water strategy following the recent literature, the paper details our accounting approach and the virtual water data that we use. The virtual water calculation includes virtual water flow and virtual water volume. The virtual water of different crops and virtual water trade were discussion. The virtual water content of grain crops in the northern and southern China was 1293 m3/t and 942 m3/t from 2003 to 2010; the national average value was 1117 m3/t; the regional differences of virtual water content for crop were significant. China's inter-regional virtual water trade is not consistent with water resource endowments expectations. The transfer of crops from northern to southern regions would have a significant impact on the water resources sustainable utilization and exacerbate the situation of water resources shortage in northern regions. China was trade surplus in global virtual water trade of agricultural products from 2003 to 2010. The exported agricultural products virtual water amounted to 31.5 billion m3/yr., and imported amount was 145 billion m3/yr. The net import of agricultural products virtual water embedded in agricultural products increased from 44 billion m3/yr. in 2003 to 178 billion m3/yr. in 2010. Net import of agricultural products virtual water was in a rising trend. The paper supplied a comprehensive analysis of China's domestic and international agricultural products virtual water trade of agricultural products. The structure of calculating interregional and international agricultural products virtual water flows and changes was feasible, however, the subjects were highly concentrated on several trade partners and agricultural products. While the virtual water trade of agricultural products in China has received more attention, the studies on the international trade need to be enriched in the future. The paper used virtual water strategy analysis to optimize the agricultural water use structure, analyzed the current direction of virtual water changes in various agricultural production based on driving factors, and reduced the consumption of local water resources. Based on the results, we obtained policy implications and provided new thinking for China's water resources management, regional development, and ecological security: the introduction of water management methods that integrating physical water and virtual water; and the establishment of a virtual water compensation mechanism to coordinate regional development; Maintain the ecological security of the balance between green water and blue water. South Asia and India mainly focuses on the qualitative research on the role of virtual water and virtual farmland. Few scholars quantitatively analyzed the value of international virtual resource flows combining the virtual water and virtual cultivated land research. The analysis of the economic benefits generated by the flow of virtual resources between regions is the key direction for future research. Conflict of interest statement We would like to submit the enclosed manuscript entitled “Agricultural Virtual Water-A Green Insight to Water Scarcity”, which we wish to be considered for publication in “Agricultural water management”. No conflict of interest exits in the submission of this manuscript, and manuscript is approved by all authors for publication. We would like to declare on behalf of my co-authors that the work described was original research that has not been published previously, and not under consideration for publication elsewhere, in whole or in part. All the authors listed have approved the manuscript that is enclosed. 212

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In this work, we evaluated the manuscript is a part of our present research achievement, and which is a good paper. I hope this paper is suitable for “Agricultural water management”. We have tried our best to revise the manuscript to hope to meet with approval. The manuscript has been thoroughly checked again and revised as suggested with the help of an English teacher. It is believed that the revised paper will be readable and could meet the standard generally for publication.

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