The VWC value of each commodity was calculated by using provincial crop yield estimates from CHINAGRO (15) (for rainfed and irrigated lands) and ET simulated with the H08 global hydrological model (16, 17). We combine these trade flows with province-level estimates of commodities’ virtual water content to build China’s domestic and foreign virtual water trade network. In particular, the water savings associated with trade of soy products (41 km3) largely dominate water savings from Chinese domestic and international VWT (Fig. 2A).

The VWC (kilograms of water per kilograms of product) of raw crops is defined as the evapotranspiration (ET) during a cropping period (kilograms of water per square meter) divided by the crop yield (kilograms of crop per square meter).

China’s domestic food trade is efficient in terms of green water resources but inefficient regarding blue water resources. The international harbors in Tianjin, Liaoning, and Shanghai participate in 95% of the foreign trade analyzed here and export domestically more than 60% of the foreign commodities they import (SI Appendix, Fig. S10). Reducing provincial VWC requires improving both water-use efficiency and crop yields. VWC ranges from about 700 kgwater/kgcrop for crops produced in Tibet to more than 2,300 kgwater/kgcrop in Hainan province, and from about 2,000 kgwater/kgmeat for meat produced in Guangdong to nearly 5,500 kgwater/kgmeat in Inner Mongolia (Fig. Today China is the world's biggest water user, accounting for 13 percent of the world's freshwater consumption. Specific provinces also might be targets for improvements of water-use efficiency: provinces that export large quantities of crops while relying significantly on irrigation, such as Inner Mongolia—which is the largest corn exporter and uses a great deal of irrigation—and the northeastern provinces (e.g., Heilongjiang for rice and Hebei for pork). China’s role in the global virtual water trade (VWT) network has been increasingly important, with its food imports contributing to 36% of the global water savings associated with international food trade (7). Copyright © 2020 National Academy of Sciences. Ministry of Water Resources of the People's Republic of China China Water Resources Bulletin 2011 (China Water & Power Press, 2012). These findings are essential to inform sound policies aimed at improving agricultural sustainability in China. In this paper, we construct and analyze China’s interprovincial and foreign VWT network to address the following questions: (i) What is the connectivity and flow structure of China’s interprovincial VWT network? These provinces might be targets for agricultural policies that would improve water-use efficiency.

China’s interprovincial VWT may be described as a weighted and directed network, in which link direction is given by the orientation of trade (i.e., from exporting to importing province), and link weights are the volumes of virtual water traded between provinces.

Importantly, when incorporating foreign imports, China’s soy trade switches from an inefficient system to a particularly efficient one for saving water resources (20 km3/y irrigation water savings, 41 km3/y total).

The funding support of the School of Engineering and Applied Sciences, the Global Collaborative Networks Fund, and the Council for International Teaching and Research of Princeton University is gratefully acknowledged. 18; Materials and Methods and SI Appendix).

Water Resources in China This is the sum of renewable groundwater and renewable surface water resources (both internal and external) This reflects a climate with very little rainfall during the growing season. This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1404749111/-/DCSupplemental.

Moreover, domestic and international food trade may help save water at the national scale by encouraging exchanges of virtual water from highly productive countries or provinces to less productive locales, resulting in a smaller water use per unit crop grown (6). Total water savings due to (A) domestic trade of local food only (2.8 km3) and (B) all domestic and foreign food trade (47 km3) in 2005, by product and water source. Chinese water savings from foreign imports (at the national scale) actually are even greater than the global water savings (at the world scale), because foreign imported soy does not require any use of China’s water resources. A significant share of water flows is from international imports (20%), which are dominated by soy (93%). They constitute an essential input for designing policies (e.g., targeted investment in agricultural research and development) and provide a framework for analyzing how these policies might change China’s VWT network and irrigation use in the near future. Water Resources of China # #China is situated in the southeastern part of the Eurasian Continent.The distribution of precipitation and water resources in this country varies largely in both time and space due to the influence &the varied topographic features and monsoon climate. We found that foreign trade plays an important role in the Chinese VWT network. Consumption is depicted at the regional level, separately for the urban and rural populations, and domestic trade is interregional (SI Appendix). Most of these studies have been carried out at the large regional scale (i.e., eight administrative divisions). Top exporting provinces are colored with the same scheme as in Fig. The need for China to include virtual water in its national policy has been pointed out (8, 9)—especially as the country’s virtual water imports will likely increase further because of projected population and economic growth (10)—and a fine-scale domestic analysis of the country’s virtual water trade is key to guiding such policy planning.

Interregional commodity trade [from the CHINAGRO economic model (15)] is downscaled to interprovincial trade flows through a linear programming optimization aimed at minimizing trade costs (Materials and Methods and SI Appendix). • World Bank:Stepping up - Improving the performance of China's urban water utilities, by Greg Browder et al., 2007 4B). Because arable land is available mainly in the water-scarce north, irrigation has become widespread, covering 45% of the country’s agricultural land and accounting for 65% of national water withdrawal [Food and Agriculture Organization of the United Nations (FAO), http://fao.org/nr/water/aquastat/countries_regions/china/index.stm]. When combining international and domestic trade flows, we observe the major role of Chinese imports from abroad: VWT associated with direct international imports account for 20% of Chinese total VWT (i.e., domestic trade of local goods and direct international trade) (Fig. Hydroelectricity is one of China's most important energy sources since it is the country's largest and most valuable renewable energy source. China’s crop yields have improved significantly in the past decades [e.g., corn yield per unit area nearly doubled since 1983 (FAO, http://faostat3.fao.org)], but some “yield gaps” remain to be filled. 3. Click on the map to get infos about China’s river basins. China’s uneven distribution of water sources presents unique difficulties to China as demand for water is increasing rapidly. Indeed, the values of undirected node strength—VWT imports and exports of a given province—span a range of less than two orders of magnitude: (7.4 × 108; 5.6 × 1010 m3/y) vs. (1011; 1015 m3/y) for the global network (7). It still remains as an arduous task to ensure the modernization process in China It still remains as an arduous task to ensure the modernization process in China against the risks of floods and the shortage of clean water sources. The tendency seems to allow miniaturized organisms to pack enough cells into a tiny form to build a complex physiology. Recently, this threat to water availability and quality triggered grand water projects, such as very large dams and canals [e.g., South–North Water Transfer, which has the opposite direction as most VWT flows, but does not compensate them (12)]. Thank you for your interest in spreading the word on PNAS. Water withdrawal from other sources is on the increase in China, both the descriptive and empirical results show that water withdrawal from other sources is contributing immensely to total water supply in China. The authors declare no conflict of interest. 80% of China’s water supply lies in southern China. We use a general equilibrium welfare model and linear programming optimization to model interprovincial food trade in China. The quality of groundwater or surface water is a major problem in China, be it because of man-made water pollution or natural contamination. China’s water supply problems are well-known globally. acknowledges the support of the National Natural Science Foundation of China (71222302, 71073154) and the support of the Newton Fellowship, United Kingdom.

In particular, soy always requires much more water than the other three main crops: rice, corn, and wheat (SI Appendix and Fig. The map at the lower right provides a key to the color scheme. Ti,j,xloc and Ti,j,xfor are the volumes traded from i to j of commodity x locally or internationally produced, respectively. Focusing on China’s domestic food trade alone, we find that the network is efficient in saving green water (5.9 km3), but contributes to a significant loss of blue water resources (3.1 km3). Officially, Beijing tap water has been declared safe to drink under China’s new national drinking water standards for 106 contaminants, but complaints in parts of the city indicate that local sources of contamination still affect quality, particularly the old dis-

acknowledges the support of the Science, Technology and Environmental Policy (STEP) Fellowship and the Hack Graduate Award from Princeton Environmental Institute, and the support of the Global Systemic Risk Fellowship from Princeton Institute for International and Regional Studies. Note the spatial variability and the general dominance of livestock VWC (which accounts for feed provenience) vs. crop VWC.

In their global study, Foley et al. Note that the largest water-saving links are foreign imports by Tianjin and Liaoning, and exports from Shandong to Hainan. Although Inner Mongolia is both a top corn exporter and a province that relies largely on irrigation to produce this crop (58% of VWC from blue water; Fig. China’s water resources are under increasing pressure from socioeconomic development, diet shifts, and climate change. A balance of both measures thus would be required to reduce the national water use in agriculture while avoiding an excessively geographically centralized agricultural system.

Blue and green portions of the bars indicate irrigation water and rainfall, respectively. Comparison across Chinese provinces of commodities’ VWC (kilograms of water per kilograms of product) averaged over corn, rice, soy, and wheat for crops (C; lower bars) and pork, poultry, and ruminant for livestock (L; upper bars) in 2005. Note in Eq. Because we focus on China’s domestic and international trade, all global water savings correspond to national savings for China, except direct exports to abroad, which are negligible (i.e., 3% of Chinese total VWT), and direct international imports, which lead to national savings for China that are even greater than global savings. 4A). However, Foley et al.

These savings represent about 13% of Chinese irrigation water use in 2005 (FAO, http://fao.org/nr/water/aquastat/countries_regions/china/index.stm). However, in the case of wheat, top exporting provinces (Henan, Shaanxi, and Anhui) are relatively more water efficient than others (SI Appendix, Fig.



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