Hydrogeology. Kevin M. Hiscock
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 to groundwater over‐abstraction during this period of 48 mm or about 1 mm a−1.
1.7 Groundwater resources in developed countries
Approaches to the management and protection of groundwater resources have developed in parallel with our understanding of the economic and environmental implications of groundwater exploitation. In developed countries, with the availability of a dense network of field monitoring data and remotely sensed information for the quantification, modelling and reporting of water resources within existing regulatory frameworks, there is a good understanding of the distribution, size and utilization of groundwater resources. In this section, patterns of groundwater abstraction, as well as regulations for the management of groundwater resources, are described for several industrialized regions of the world (the United Kingdom, Europe, Canada and the United States), as well as the rapidly developing country of China.
1.7.1 Groundwater abstraction in the United Kingdom
The distribution of groundwater abstraction volumes in the United Kingdom is largely determined by the relationship between population and geology, with major producing aquifers in the densely populated south and east of the country and minor producing aquifers in the less‐densely populated north and west. Table 1.5 provides a breakdown of water use by purpose and type (surface water and groundwater) for regions of England. Surface water abstraction for electricity generation is the largest category, but most of the freshwater abstracted for cooling purposes is returned to rivers and can be used again downstream. For England, groundwater accounts for 20% of total abstractions (surface water and groundwater) for all water use purposes and 31% of total abstractions for domestic use. In terms of abstractions for domestic water supply, groundwater is especially significant in the Southern (75% dependence on groundwater), Anglian (36%), Thames (34%) and Midlands (32%) regions. In these densely populated regions of south‐east England and the English Midlands, good quality groundwater is obtained from the high‐yielding Cretaceous Chalk and Triassic sandstone aquifers. Groundwater is also important for spray irrigation. The Anglian region in the east of England, an area of intensive arable farming and horticulture, has the highest demand, with 45% of abstractions for spray irrigation obtained from groundwater (Plate 1.5).
Scotland, Wales and Northern Ireland have ample surface water resources that are important for public supplies. Aquifers are typically less productive and/or more localized in these countries, although groundwater is significant for private supplies where properties are not connected to the public supply network. In Scotland, groundwater contributes about 5% to public supplies, with approximately 100 boreholes and springs used to supply some major rural towns (Dochartaigh et al. 2015). The total volume of public supply from groundwater in Scotland is estimated as 235 × 103 m3 day−1 in 2004. In addition, groundwater is important for about 70% of private supplies in Scotland, serving at least 330 000 people. More than 4000 boreholes, as well as some large springs, are used for large private, industrial or agricultural supplies, and approximately 20 000 boreholes, small springs and wells provide private water supplies for at least 80 000 people (Dochartaigh et al. 2015).
Groundwater supplies about 3% of public supplies in Wales, which equates to approximately 40 × 103 m3 day−1, with most groundwater sources operated conjunctively with surface water sources. However, some groundwater sources are critical in supplying local areas that cannot be supplied by other means (Environment Agency 2015; Welsh Water 2019). In Northern Ireland, groundwater is a negligible component of the public water supply, contributing only 0.6% (Northern Ireland Water 2013).
1.7.1.1 Management and protection of groundwater resources in the United Kingdom
In the United Kingdom, it is interesting to follow the introduction of relevant legislation, and how this has increased hydrogeological knowledge, with overviews provided by Downing (1993) and Streetly and Heathcote (2018). Hydrogeological experience prior to 1945 rested on a general awareness of sites likely to provide favourable yields, changes in chemistry down‐gradient from the point of recharge and hazards such as ground subsidence from groundwater over‐exploitation. The Water Act 1945 provided legal control on water abstractions and this prompted an era of water resources assessment that included surveys of groundwater resources, the development of methods to assess recharge amounts (Section 6.5), and the initiation of groundwater