amount of freshwater (<0.3% of total water) is readily available to humans and other biota (Maurits la Rivière 1989).
(Source: Maurits la Rivière, J.W. (1989) Threats to the world’s water. Scientific American 261, 48–55.)
The interfaces between hydrological compartments in the water cycle have important implications for water quantity and quality. Processes at the interfaces between the hydrological compartments (for example, soil‐atmosphere or soil‐groundwater) determine the age distribution of the water fluxes between these compartments and can, thus, greatly influence water travel and residence times (Sprenger et al. 2019). The age distribution of water spans over a wide range of temporal scales. In the ‘critical zone’, the Earth's boundary layer ranging from the top of the vegetation layer to the bottom of the groundwater storage, water ages range from hours to millennia (Fig. 1.10).
By taking the constant volume of water in a given reservoir and dividing by the rate of addition (or loss) of water to (from) it enables the calculation of a residence time for that reservoir. For the oceans, the volume of water present (1370 × 106 km3; see Fig. 1.8) divided by the rate of river runoff to the oceans (0.037 × 106 km3 a−1) gives an average time that a water molecule spends in the ocean of about 37 000 years. Lakes, rivers, glaciers and shallow groundwater have residence times ranging between days and thousands of years. Because of extreme variability in volumes and precipitation and evaporation rates, no simple average residence time can be given for each of these reservoirs. As a rough calculation, and with reference to Fig. 1.8 and Table 1.1, if about 6% (2220 km3 a−1) of runoff from land is taken as active groundwater circulation, then the time taken to replenish the volume (4.2 × 106 km3) of shallow groundwater stored below the Earth's surface is of the order of 2000 years. In reality, groundwater residence times vary from about 2 weeks to 10 000 years (Nace 1971), and longer (Edmunds 2001). A similar estimation for rivers provides a value of about 20 days. These estimates, although a gross simplification of the natural variability, do serve to emphasize the potential longevity of groundwater pollution compared to more rapid flushing of contaminants from river systems.
1.5.1 Groundwater occurrence in the upper continental crust
Focusing on the upper 2 km of the continental crust in which most hydrogeological observations are made, Gleeson et al. (2015) combined multiple approaches using geospatial datasets, tritium age dating of groundwater and numerical modelling to show that less than 6% of the groundwater in the uppermost portion of the Earth's land mass is less than 50 years old, representing modern groundwater that is the most recently recharged. Gleeson et al. (2015) found that the total groundwater volume in the upper 2 km of continental crust is approximately 22.6 × 106 km3, of which 0.1–5.0 × 106 km3 is less than 50 years old. The distribution of this modern groundwater is spatially heterogeneous, with very little in arid regions. Although modern groundwater represents a small percentage of the total groundwater storage on Earth, the volume of this component is still very significant, equivalent to a water depth of about 3 m spread over the world's continents.
1.5.2 Groundwater‐related tipping points
Several potential groundwater‐related tipping points are associated with the storage function of groundwater (Gleeson et al. 2020). Most critical for aquatic ecosystems is the role of groundwater as a stable supply of baseflow, and therefore a key tipping point is when a stream transitions from perennial to intermittent due to groundwater depletion (see Section 6.8). Groundwater‐related tipping points are also present for terrestrial groundwater‐dependent ecosystems. Groundwater within or near the root zone provides a stable supply of water, particularly during drought, for many natural and agricultural crops via capillary rise and direct groundwater uptake (see Section 6.4.1). Since groundwater is estimated to influence terrestrial ecosystems over 7–17% of global land area (Fan et al. 2013) and can contribute substantially to evapotranspiration, it is likely that groundwater constitutes an important component of terrestrial evapotranspiration (Gleeson et al. 2020). For instance, groundwater is an essential contributor to evapotranspiration in the Amazon Basin (Fang et al. 2017).
Fig. 1.10 Conceptual diagram showing hypothetical age distributions in the Earth's critical zone. The envelopes shown indicate the mixing of water with different ages at the interfaces between hydrological compartments (Sprenger et al. 2019).
(Source: Adapted from Sprenger, M., Stumpp, C., Weiler, Met al. (2019) The demographics of water: a review of water ages in the critical zone. Reviews of Geophysics 57, DOI: 10.1029/2018RG000633.)
1.5.3 Groundwater discharge to the oceans
The approximate breakdown of direct groundwater discharge from continents to adjacent oceans and seas was estimated by Zektser and Loaiciga (1993) as follows: Australia 24 km3 a−1; Europe 153 km3 a−1; Africa 236 km3 a−1; Asia 328 km3 a−1; the Americas 729 km3 a−1; and major islands 914 km3 a−1. The low contribution from the Australian continent of direct groundwater discharge, despite its relatively large territory, is attributed to the widespread occurrence of low‐permeability surface rocks that cover the continent. At the other extreme, the overall proximity of recharge areas to discharge areas is the reason why major islands of the world contribute over one‐third of the world’s direct groundwater discharge to the oceans. The largest direct groundwater flows to oceans are found in mountainous areas of tropical and humid zones and can reach 10–15 × 10−3 m3 s−1 km−2. The smallest direct groundwater discharge values of 0.2–0.5 × 10−3 m3 s−1 km−2 occur in arid and arctic regions that have unfavourable recharge and permeability conditions (Zektser and Loaiciga 1993).
In a later study presented by Luijendijk et al. (2020), the application of a spatially resolved, density‐driven global model of coastal groundwater discharge showed that the contribution of fresh groundwater to the world’s oceans is equal to 224 (range 1.4–500) km3 a−1, and accounts for approximately 0.6% (range 0.004–1.3%) of the total freshwater input and approximately 2% (range 0.003–7.7%) of the solute input of carbon, nitrogen, silica and strontium. The uncertainty ranges reported are mostly caused by the high uncertainty of the values of permeability that were used, which is on average two orders of magnitude. Additional sources of uncertainty are the representative topographic gradient of coastal watersheds, groundwater recharge, and the size of the area that contributes to coastal groundwater discharge.
The coastal discharge of freshwater showed a high spatial variability. For an estimated 26% (0.4–39%) of the world's estuaries, 17% (0.3–31%) of salt marshes and 14% (0.1–26%) of coral reefs, the flux of terrestrial groundwater exceeds 25% of the river flux and poses a risk for pollution and eutrophication. Catchments with hotspots of coastal groundwater discharge, where coastal groundwater discharge exceeds 100 m2 a−1 and 25% of the river discharge, were located predominantly in areas with a steep coastal topography due to glacio‐isostatic rebound, active tectonics
