33.9
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.
1.6.2 Global groundwater depletion and sea level rise
Using a global hydrological model, Wada et al. (2010) assessed the amount of groundwater depletion, defined as the excess of abstraction over recharge replenishment, and estimated that for sub‐humid and arid areas the rate of total global groundwater depletion has increased from 126 ± 32 km3 a−1 in 1960 to 283 ± 40 km3 a−1 in 2000 (Fig. 1.11). Groundwater depletion in 2000 equalled about 40% of the global annual groundwater abstraction, about 2% of the global annual groundwater recharge and about 1% of the global annual continental runoff, contributing a considerable amount (about 25%) of 0.8 ± 0.1 mm a−1 to current sea level rise.
Using a similar approach in which groundwater depletion was directly calculated using calibrated groundwater models, analytical approaches or volumetric budget analyses for multiple aquifer systems, Konikow (2011) estimated an average global groundwater depletion rate of 145 km3 a−1 during the period 2000–2008, equivalent to 0.4 mm a−1 of sea‐level rise, or 13% of the reported rise of 3.1 mm a−1 during this period.
Using an integrated water resources assessment model to simulate global terrestrial water stocks and flows, Pokhrel et al. (2012) estimated that the sum of unsustainable groundwater use, artificial reservoir water impoundment, climate‐driven changes in terrestrial water storage and the loss of water in closed basins, principally the Aral Sea, has contributed a sea‐level rise of about 0.77 mm a−1 between 1961 and 2003, or about 42% of the observed sea‐level rise. Considering a simulated mean annual unsustainable groundwater use during 1951–2000 of about 359 km3 a−1, Pokhrel et al. (2012) estimated, using the assumption of Wada et al. (2010) that 97% of unsustainable groundwater use ends up in the oceans, a cumulative sea‐level rise due
