Misestimation of water saving in agricultural virtual water trade by not considering the role of irrigation

A R T I C L E I N F O

Keywords: Virtual water Water saving Blue-green water Irrigation

A B S T R A C T

Water saving by agricultural virtual water trade (VWT) is regarded as a new way to address water shortage, and many studies have considered it at local and global scales. However, the existing calculation methods do not consider how agricultural products should be produced in export and import areas without crop trade. We believe that three facts related to irrigation should be considered in water saving in agricultural VWT evaluation:

1) arable land is highly restricted, 2) irrigation increases crop yield significantly, and 3) green water does not require cost. The role of irrigation, which is important for both the export and import region, is very important for determining how to cultivate crops without virtual water trade. In the case of grain VWT between Heilongjiang and Guangdong, China, the national blue water saving in 2010 with this consideration was

−2562.1 Mm³ (water loss), whereas the figure was 975 Mm³ under the existing calculation framework. Therefore, there is a possibility that VWT can be used in agricultural development and water management decision-making while considering the role of irrigation.

1. Introduction

The systematic risks of water resource and arable land shortages facing our planet threaten the global food supply and social stability. In addition to improving the efficiency of water resource utilization (Cao et al., 2020), the importation of water-intensive agricultural products, in the form of virtual water, is regarded as an effective measure to cope with regional water scarcity (Allan, 1998). Virtual water is used to describe the amount of water consumed in the production of products or services, and it refers to the water consumed in the production process of products (Akoto-Danso et al., 2019; Bazrafshan et al., 2019). Virtual water trade (VWT) is the flow of water resources with product trade. VWT makes water resources global resources. A virtual water strategy, which refers to a measure by which water resource-intensive products are imported to replace local production in areas of water shortage, conserves local water resources, and relieves the pressure on water resources, is a specific strategy to allocate water resources from a global perspective (Fu et al., 2018). VWT is considered an important strategy for coping with the global water shortage, and it is based

premise of a virtual water strategy and has attracted the attention of scholars in the field of water and environmental management (Han et al., 2018; Bazrafshan et al., 2020). For instance, Liu et al. (2018) provided a critical review of current research into water savings gen- erated from food trade at the global and regional levels. However, it is difficult to use virtual water evaluation results in regional agricultural water management decision-making (Boelens and Vos, 2012) because the method of quantifying water savings in agricultural VWT still re- quires further study.

2. Existing water saving estimation framework

Water saving of crop VWT-related indicators includes regional water loss (RWL) in a virtual water export area, regional water saving (RWS) in a virtual water inflow area, and global water saving (GWS). Taking the simplest case, one-to-one virtual flow between regions, as an example, the calculation of these parameters in the existing framework is described briefly as follows:

VWT Tprod VWCexp (1)

gions and 2) virtual water flow can save water resources at regional and

RWS = T

× VWC

(3)

⁎ Corresponding author at: College of Agricultural Science and Engineering, Hohai University, Nanjing, Jiangsu, 210098, China.

E-mail address: wumengyang0108@hhu.edu.cn (M. Wu).

https://doi.org/10.1016/j.agwat.2020.106355

Received 4 May 2020; Received in revised form 25 June 2020; Accepted 26 June 2020

0378-3774/©2020PublishedbyElsevierB.V.

Fig. 1. Water trade and saving associated with grain exportation from Heilongjiang to Guangdong, China, in 2010, ignoring the role of irrigation.

GWS = Tprod × (VWCimp − VWCexp)

(4)

this way (Lamastra et al., 2017; Liu et al., 2019; Wang et al., 2019a).

In the aforementioned formulae, Tprod is the trade volume of agricultural product (tons); VWCexp and VWCimp are the virtual water contents of agricultural product in the export and import regions, respectively, (m3/ton); RWLexp is the RWL of the export region (m3), which is the amount of water conservation assuming no trade in agricultural pro- ducts; and RWSimp is the RWS of the import region (m3), which is amount of increase of water investment if the Tprod were produced by the region. GWS should be defined as the national water saving (NWS) if the water saving in agricultural virtual water trade within a country is evaluated. If GWS > 0, virtual flow saves water resources for the whole world (country), and the virtual flow pattern is worth encoura- ging form the perspective of improving global (national) water use ef- ficiency. Otherwise, virtual water flow wastes water resources, and the water use efficiency in the virtual water export region should be im- proved. Taking the VWT in 2010 between China's largest grain exporter (Heilongjiang) and grain importer (Guangdong) as an example, the water saving effect was calculated and is shown in Fig. 1.

VWCH and VWCG are the virtual water contents for producing grain in Heilongjiang and Guangdong, respectively; T is the trade volume of grain from Heilongjiang to Guangdong estimated by Sun et al. (2016); NWS is the national water savings due to grain trade from Heilongjiang to Guangdong; and RWL and RWS are the regional water loss/saving of Heilongjiang and Guangdong, respectively. The data of virtual water content of grain in the total cropland are derived from Cao et al. (2015). Based on this water saving assessment framework, the VWT of grain between Heilongjiang and Guangdong in 2010 was estimated to be 5465.6 Mm³, including 1797.6 Mm³ of blue and 3668.0 Mm³ of green water. This is the volume RWL of Heilongjiang (Fig. 1). The blue and green virtual water contents of grain in Guangdong were 420 and 857 m³/ton, respectively. As a result, the RWS of this province was 6441.4 Mm³, including 1814.7 Mm³ of blue and 3668.0 Mm³ of green water resources. If there was virtual water flow, for China, the virtual water content of 4.28 Mton of grain would be reduced from 1505 to 1227 m³/ ton and the NWS would be 975.8 Mm³. Because both the grain blue and green VWCs of Heilongjiang were lower than those of Guangdong, the virtual water flow saved both blue and green water resources in China. Without considering the endowment of water resources, a virtual water strategy should be implemented between Heilongjiang and Guangdong by the Chinese government to increase the grain transferred from Heilongjiang with its higher water use efficiency to Guangdong with its lower water use efficiency. Almost all of the existing assessments of virtual water trade and its water saving effect have been conducted in

3. Facts pertaining to the role of irrigation

Presently, water savings are evaluated by comparing the assumed no virtual water trade status with the actual situation. Specifically, agricultural product and virtual water import regions expand the scale of crop production according to the existing sowing pattern, including irrigated and rainfed crop areas. For agricultural product and virtual water export regions, irrigated and rainfed crop areas are reduced based on the existing planting structure. In other words, the blue-green water composition and water use efficiency of the observed crops in both the export and import regions are not changed. However, these strategies for achieving the goal of water saving in virtual water trade assessment are not consistent with the actual situation of regional crop sowing, irrigation development, and agricultural water management (Sauer et al., 2010). The following three facts related to irrigation, which plays a vital role in regional crop production and agricultural water man- agement, have been ignored.

Arable land is highly restricted. With the overwhelming trend of urbanization and the booming population, arable land suitable for growing crops has been greatly threatened (Wu et al., 2018). The global cropland area has remained at about 20 Mha through the last decade (FAO, 2019). However, in many areas, a lack of arable land is the main reason for crop product and virtual water importation. Restriction of land resources mandates that the crop planting area cannot be expanded at will.

Irrigation increases crop yield significantly. Water stress is a common problem in crop cultivation, especially in arid areas. Effective precipitation lower than the crop demand inhibits the normal physiological growth of a crop and leads to a reduction of yield. Irrigation solves this problem (Wang et al., 2019b). In de- veloping countries, the yield of irrigated paddy rice is about twice that of rain-fed rice (Cai and Rosegrant, 2003). The grain yield of rainfed cropland in China is approximately 2.91 tons/ha, but the yield for land equipped with irrigation is 6.74 tons/ha (Cao et al., 2015). Expanding the irrigated area under limited arable land is an effective way to mitigate food shortages.

Green water does not require cost. Green water is the water re- source derived from precipitation that is stored in soil and subse- quently returned to the atmosphere (Aldaya et al., 2010) through crop transpiration. There is no need to construct farmland water

conservancy facilities or purchase water conveyance equipment in a green water utilization process. Different from the case of irrigation water withdrawal, it requires low investment of labor in facilities and water resource management. Mostly using green water helps optimize the allocation of social resources.

A rational agricultural and water management department ad- dresses the priority issues pertaining to decision-making on crop pro- duction increase/decrease based on the aforementioned facts related to the role of irrigation. Water saving in agricultural virtual water trade is not determined by water use efficiency (crop VWC). Rather, it is de- termined by the regional crop water footprint (total blue and green water consumptions for crop production [m³]), the scale of agricultural production, and the level of irrigation development. In other words, if crop and virtual water trade does not occur, the effect may be con- sidered to reduce the use of irrigation water in the export region and increase it in the import region.

4. Water saving estimation considering the role of irrigation

Considering the role of irrigation, the situation of water saving ef- fect evaluation in agricultural virtual water trade may be different from that shown in Fig. 1. With consideration of the role of irrigation, it becomes a case of virtual water flow water-saving effect evaluation. The calculation of VWT is the same as in formula (1). However, its impacts on the water resource utilization of the export region, import region, and whole country may be different. In the case of no agricultural product trade, the export region is inclined to reduce irrigation water and hence reduce crop output in the case of water shortage (Dalin et al., 2015; Gao et al., 2020), and the export region is inclined to increase crop output by expanding the irrigation area in the case of limitation of arable land. Therefore, the calculation process of water saving in the case of Heilongjiang and Guangdong should be reexamined. The irri- gation area needed to produce the amount of grain trade (T) in Hei- longjiang is 1.09 Mha (T/(YH,I-YH,R)), which is lower than the total ir- rigated grain cropland (AH,I) of 3.75 Mha (Fig. 2). Therefore, if it did not need to provide grain to Guangdong, Heilongjiang should reduce crop output by reducing the use of irrigation water resources. In this way, it could make full use of the free green water resources and also transfer the saved blue water resources to industrial, commercial, and residential uses, thus improving social welfare. If this were the case, the green and blue water losses in Heilongjiang would be 0 and 6142.7 Mm³, respectively (Fig. 2).

YH,I (YH,R) and YG,I (YG,R) are the grain yield in irrigated (rainfed) crop land for Heilongjiang and Guangdong, respectively; AH,I (AH,R) and AG,I (AG,R) are the irrigated (rainfed) area for grain production in Heilongjiang and Guangdong, respectively; WFH (WFH,b) and WFG (WFG,b) are the blue water footprint of grain in Heilongjiang and Guangdong, respectively; WFH,g,I (WFH,g,R) and WFG,g,I (WFG,g,R) are the green water footprint of grain in irrigated (rainfed) cropland in Heilongjiang and Guangdong, respectively. Other acronyms are ex- plained in Fig. 1.

If food could not be imported from outside of Guangdong, priority should be given to expansion of the irrigation area and then to the area of arable land. An increase of grain of about 2.95 Mton (AG,R×(YG,I- YG,R)) could be attained if all of the rainfed cropland in Guangdong (0.86 Mha) were equipped with irrigation. However, this result still does not meet the grain demand, and an extra 0.21 Mha ((T-2.95)/YG,I) of irrigation area is required. Hence, the amounts of irrigation (blue) and green water to be added are 3580.6 ((0.86 + 0.21)×WFG,b/AG,I) and 1181.3 (0.21×WFG,g,I/AG,I) Mm³ (Fig. 2). The RWS of Guangdong is 7461.9 Mm³ (Fig. 2). The benefit from the grain and virtual water import from Heilongjiang to Guangdong includes 75.2 % blue and 24.8

% green water. Different from the NWS calculated by the existing fra- mework (Fig. 1), the grain virtual water trade between the two pro- vinces lost 1380.8 Mm³ of water resources for the country, even though it saved 1181.3 Mm³ of green water. The waste of adjustable and pre- cious blue water in China reached 2562.1 Mm³ (Fig. 2) in the virtual water trade. Grain virtual water trade between Heilongjiang and Guangdong is unworthily encouraged taking into account the role of irrigation, because it cannot contribute to efficient utilization of water resources in China. This example represents the general situation of a global agricultural virtual water trade pattern, in which flows occur from areas with high water use efficiency to areas with low water use efficiency (Liu et al., 2019).

5. Concluding remarks

It is impossible to consider how the virtual water export area can maintain its position using the existing calculation framework. In other words, how water saving of virtual water trade can be realized is rarely discussed because there are obvious problems that are difficult to solve. We believe that this cannot be practically combined with regional agricultural production water management, which relegates research of virtual water to remain theoretical. At present, it is difficult to propose feasible suggestions for regional agricultural layout, irrigation

Fig. 2. Water trade and saving associated with grain exportation from Heilongjiang to Guangdong, China, in 2010, with consideration of the role of irrigation.

development, and water resource management. However, the result may have more profound and realistic policy implications if the role of irrigation is considered in virtual water assessments. It is suggested that an evaluation framework and empirical research of virtual water trade and its impacts among regions that consider agricultural water use characteristics and the role of irrigation should be implemented. This is expected to promote the virtual water research strategy in response to the global food and water crises.

Declaration of Competing Interest

The authors declared that they have no conflicts of interest to this work.

We declare that we do not have any commercial or associative in- terest that represents a conflict of interest in connection with the work submitted.

Acknowledgments

This work is jointly funded by National Natural Science Foundation of China (51979074; 51609065), the Fundamental Research Funds for the Central Universities (B200202095), and the Social Science Fund of Jiangsu Province (17GLC013).

Appendix A. Supplementary data

Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.agwat.2020.106355.

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