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The over‐abstraction of groundwater in major food producing areas of the world such as north‐west India, North China, the central United States and California is rapidly depleting groundwater storage in large aquifer systems. The global scale of groundwater depletion due to irrigation is difficult to assess given the lack of research integrating crop water use, groundwater depletion and the embedding of groundwater in international food trade. In the study by Dalin et al. (2017), groundwater depletion linked to irrigation (GWD) (in which GWD is defined as the volume of groundwater abstracted for irrigation use in excess of the natural recharge rate and irrigation return flow, and allowing for environmental flow requirements) is estimated based on 26 crop classes and bilateral trade flow for 360 commodities. The results shown in Table 1.4 for 2010, show the volume of GWD embedded in food production and trade for the top 10 countries with the most GWD.

Table 1.4 Groundwater depletion (GWD) for irrigation embedded in national food production, imported and exported GWD, and corresponding fractions of GWD in global food production, national food consumption and national food production for the year 2010 (Dalin et al. 2017).

(Source: Adapted from Dalin, C., Wada, Y., Kastner, T. and Puma, M.J. (2017) Groundwater depletion embedded in international food trade. Nature 543: 700–704.)

Country	GWD in production (km3 a−1)	Fraction of global GWD (%)	GWD in imports (km3 a−1)	Fraction of GWD in national consumption (%)	GWD in exports (km3 a−1)	Fraction of GWD in national production (%)
India	73.5	33.9	0.2	0.3	3.0	4.0
Iran	33.3	15.4	1.4	4.2	1.2	3.5
Pakistan	27.5	12.7	0.2	1.2	7.3	26.4
China	24.0	11.1	2.2	8.5	0.3	1.1
USA	16.2	7.5	1.7	15.3	6.9	42.4
Saudi Arabia	12.5	5.7	0.8	6.0	0.4	3.5
Mexico	11.1	5.1	1.0	10.6	2.5	22.6
Libya	2.5	1.1	0.1	2.4	0	0.1
Turkey	2.0	0.9	0.5	22.6	0.4	18.0
Italy	2.0	0.9	0.5	27.9	0.8	39.2
Total top ten	204.6	84.8	8.6	4.5	22.8	11.1
Total world	241.4	100	25.6	NA	25.6	NA

Note: Also shown are totals for these ten countries and for the world. NA, not applicable.

Global GWD increased to 292 km³ in 2010, mostly due to increases in India, China and the United States. The crops accounting for most depletion globally, both in terms of their large production and GWD intensity, are wheat (22% of global GWD, or 65 km³), rice (17%), sugar crops (7%), cotton (7%) and maize (5%) (Dalin et al. 2017). The countries irrigating crops from over‐exploited aquifers export these crops in various proportions: India retains most of its large GWD‐based crop production for domestic use (only 4% of GWD exported), while the United States, Pakistan and Mexico export significant portions of their GWD‐based crop production (Table 1.4). Globally, about 11% of GWD is embedded in international food trade, of which exports from Pakistan, the United States and India alone account for more than two‐thirds of all embedded GWD. Pakistan is the largest exporter, with 29% of the global GWD trade volume, followed by the United States (27%) and India (12%) (Dalin et al. 2017).

Five of the 10 countries shown in Table 1.4 with the most GWD (the United States, Mexico, Iran, Saudi Arabia and China) are also the top importers of GWD via food trade. Critically, these countries import or export crops irrigated from the world's most stressed aquifer systems. As demonstrated by Gleeson et al. (2012), food production relying on these aquifers is particularly unsustainable with extraction rates 20–50 times higher than required for sustainable groundwater use (see further Table 6.1 and Section 10.2). For example, the United States imports about 1.5 times as much GWD from Mexico (mainly via citrus and sugar crops) as it exports there (mainly via cotton and maize) (Dalin et al. 2017).

Therefore, it is of concern that exhaustion of aquifers in areas that are hotspots of water and food security related to GWD threaten the food supply both domestically and in their water‐stressed trade partners. Clearly, solutions are required to improve the sustainability of water use and food production for those regions, crops and trade relationships that are most reliant on over‐exploited aquifers. In the food producing countries, solutions could include water‐saving strategies such as improving irrigation efficiency and growing more drought‐resistant crops, together with targeted measures such as metering and regulation of groundwater pumping, while accounting for local socio‐economic, cultural and environmental requirements (Dalin et al. 2017). In addition, food importing countries can assist these solutions by promoting and supporting sustainable irrigation practices with their trade partners.