1.5.5 Human influence on the water cycle
Although rarely depicted in diagrams of the water cycle, human activity alters the water cycle in three distinct, but inter‐related ways (Abbott et al. 2019). First, humans appropriate water through: (1) livestock, crop and forestry use of soil moisture that eventually flows back to the atmosphere as evapotranspiration (so‐called ‘green water’ use with an estimated flux of 15–22 × 103 km3 a−1); (2) as surface water and groundwater abstraction (‘blue water’ use with an estimated flux of 3.8–6.0 × 103 km3 a−1); and (3) as water required to assimilate pollution (‘grey water’ use with an estimated flux of 1.0–2.0 × 103 km3 a−1) (Abbott et al. 2019; Schyns et al. 2019). Second, humans have disturbed approximately three‐quarters of the Earth's ice‐free land surface through activities that include agriculture, deforestation and wetland destruction (Ellis et al. 2010). These disturbances alter evapotranspiration, groundwater recharge, river discharge and precipitation at continental scales (Falkenmark et al. 2019). Third, climate change is disrupting patterns of water flow and storage at local to global scales (Haddeland et al. 2014). These human interferences with the water cycle have created problems for billions of individuals and many ecosystems worldwide. The estimates of human green, blue and grey water use (about 24 × 103 km3 a−1) indicates that human freshwater appropriation redistributes the equivalent of half of global river discharge or double global groundwater recharge each year (Abbott et al. 2019).
1.6 Global groundwater resources
Groundwater is an important natural resource. Worldwide, more than two billion people depend on groundwater for their daily supply (Kemper 2004). Total global fresh water use is estimated at about 4000 km3 a−1 (Margat and Andréassian 2008) with 99% of the irrigation, domestic, industrial and energy use met by abstractions from renewable sources, either surface water or groundwater. Less than 1% (currently estimated at 30 km3 a−1) is obtained from non‐renewable (fossil groundwater) sources mainly in three countries: Algeria, Libya and Saudi Arabia.
The increase in global groundwater exploitation has been stimulated by the development of low‐cost, power‐driven pumps and by individual investment for irrigation (Plate 1.5) and urban uses. Currently, aquifers supply approximately 20% of total water used globally, with this share rising rapidly, particularly in dry areas (IWMI 2007). Globally, 65% of groundwater utilization is devoted to irrigation, 25% to the supply of drinking water and 10% to industry.
Groundwater resources are often the only source of supply in arid and semi‐arid zones (for example, 100% in Saudi Arabia and Malta, 95% in Tunisia and 75% in Morocco). As demonstrated for the case of the North‐west Sahara Aquifer System (Box 1.3), irrigated agriculture is the principal user of groundwater from the major sedimentary aquifers of the Middle East, North Africa, North America and the Asian alluvial plains of the Punjab and Terai (WWAP 2009). Groundwater has been most intensively developed in South Asia and North America, where it provides 57 and 54%, respectively, of all irrigation water. The rapid growth in the construction of irrigation water‐wells now supplies 3.9 × 106 km2 of irrigated land in India, 1.9 × 106 km2 in China, 1.7 × 106 km2 in the United States and also across large areas of Pakistan and Bangladesh (Konikow et al. 2015).
Whether groundwater or surface water is exploited for water supply is largely dependent on the location of aquifers relative to the point of demand. A large urban population with a high demand for water would only be able to exploit groundwater if the aquifer, typically a sedimentary rock, has favourable storage and transmission properties, whereas in a sparsely populated rural district more limited but essential water supplies might be found in poor aquifers, such as weathered basement rock.
Box 1.3 The North‐west Sahara Aquifer System and the Ouargla Oasis, Algeria
The Sahara Basin covers an area of about 780 000 km2 and includes two sub‐basins separated by the M’zab High. The western sub‐basin occupies about 280 000 km2 and is covered by sand dunes of the Grand Erg Occidental and the eastern sub‐basin extends over about 500 000 km2 and is covered by the desert of the Grand Erg Oriental. The Sahara Basin is underlain by two major aquifers that comprise the Northwest Sahara Aquifer System (NWSAS) of Cretaceous age that extends below Algeria, Tunisia and Libya in North Africa (see Plate 2.7). The Lower Aquifer (the Continental Intercalaire) is composed of continental sandstone alternating with argillaceous layers and the Upper Aquifer (the Complex Terminal) is a multi‐layered aquifer consisting of sandstones and limestones. The thickness of the thicker, more extensive Lower Aquifer ranges between 200 and 1000 m, decreasing north‐eastwards to 125 m. The lower confining unit consists of argillaceous and marly formations of Devonian‐Triassic age while the upper confining units consist of evaporites and clays of Upper Cretaceous age (Zektser and Everett 2004).
In western Algeria, the Lower and Upper Aquifers are almost independent but towards the Mediterranean coast the aquifers become interconnected or merge to form one aquifer system. Groundwater movement in the NWSAS is towards the south and south‐west in the western sub‐basin. In the eastern sub‐basin, where the Complex Terminal aquifer is heavily exploited in Algeria and Tunisia, groundwater flows towards discharge areas, mainly desert depressions or oases known as ‘chotts’. The chotts supply irrigation water through traditional qanat systems (foggaras), with some 570 foggaras discharging about 90 × 106 m3 a−1 (Zektser and Everett 2004).
In the western sub‐basin, the total dissolved solids content of groundwater in the Lower Aquifer ranges from 0.5 to 1 g L−1. In the eastern sub‐basin, salinity increases to 5 g L−1. The concentration of total dissolved solids in the Upper Aquifer is about 2 g L−1 in southern areas of the Grand Erg Oriental, increasing in concentration north‐eastwards from 2 to 5 g L−1 at Tozeur in Tunisia. At Ouargla in Algeria, concentrations reach 8 g L−1 in discharge areas (Zektser and Everett 2004).
Groundwater reserves in the NWSAS are estimated to be 60 000 km3 although, given the low rainfall amount, the aquifer system is generally considered a non‐renewable aquifer system. Use of the superficial water table of the NWSAS extends back to ancient times and, from the middle of the nineteenth century, boreholes were drilled to access deeper parts of the aquifer. In Algeria, exploitation of groundwater from the aquifer system was about 150 × 106 m3 until