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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.)