Hydrogeology. Kevin M. Hiscock

In regions of the world where the rate of groundwater abstraction exceeds the rate of natural groundwater recharge over extensive areas and for long time periods, over‐exploitation or persistent groundwater depletion is the consequence. As shown by the global overview of Wada et al. (2010), in the year 2000, the rate of total global groundwater depletion is estimated to have increased to 283 km³ a⁻¹ (Fig. 1.11). Groundwater depletion rates were found to be highest in some of the world's major agricultural regions including north‐east Pakistan and north‐west India, north‐east China, the Ogallala aquifer in the central United States (see Section 10.2.1), the San Joaquin aquifer in the Central Valley of California (see Box 2.7), Iran, Yemen and south‐east Spain.

Table 1.3 Countries with estimated groundwater abstraction greater than 10 km³ a⁻¹ (Margat and van der Gun 2013).

      (Source: Adapted from Margat, J. and van der Gun, J. (2013) Groundwater Around the World: A Geographic Synopsis. CRC Press/Balkema, EH Leiden.)

Box 1.4 Groundwater depletion from global irrigated crop production and trade

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 (%)
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