Globally, groundwater represents a critical natural resource that is affected by changes in natural supply and renewal, as well as by increasing human demand and consumption. However, despite its critical role, groundwater is difficult to accurately quantify as it is beneath the Earth surface. Here, we review several state-of-the-art remote sensing techniques useful for local- to global-scale groundwater monitoring and assessment, including proxies for groundwater extraction. These include inferring changes in subsurface water from mass changes using gravitational measurements, and analyzing changes in the Earth surface height using Interferometric Synthetic Aperture Radar, Light Detection and Ranging, Airborne Electromagnetic Systems, and satellite altimetry. Remote sensing information is often used in tandem with ground-based observations such as hydraulic head in wells, Global Navigational Satellite System monitoring, and numerical modeling to complement the space-based approaches. In the future, fusing different remote sensing techniques capable of operating in various environments will yield additional insight on the state and rate of use for groundwater across the globe. 1. Scientific Necessity and Overview Groundwater is Earth's largest reservoir of fresh, liquid water. Accounting for more than 20% of water usage worldwide and 43% of irrigation water (Earman & Dettinger, 2011; Fetter, 2001; Zektser & Everett, 2004), groundwater serves as the primary source of freshwater for over 2 billion people across the globe (Alley et al., 2002; Famiglietti et al., 2011; Gleeson et al., 2012; WWAP, 2015). Its contributions are expected to increase with rising global population and changing climate, as surface water becomes a less reliable resource (FAO, 2005; OECD, 2011; WWAP, 2015). It is estimated that by 2050, 2 billion additional people will need to be fed, increasing demand on agricultural land use for improved rates of food production (OECD, 2011; WWAP, 2015). As climate change continues to alter patterns of drought and regional recharge dynamics, groundwater will continue to establish itself as an increasingly critical component of the water cycle, as groundwater variability directly impacts surface water (Döll, 2009; Earman & Dettinger, 2011; Maxwell & Kollet, 2008; Scibek & Allen, 2006). Groundwater also has important implications for the energy cycle, as it can act as a thermal energy storage (Arola et al., 2016; Dickinson et al., 2009) or an energy consumer during its abstraction (Kumar, 2005; Scott & Sharma, 2009; Wang et al., 2012). Groundwater supply, therefore, is directly linked to global food safety, climate change, and energy security (Famiglietti, 2014; Giordano, 2009; McCallum et al., 2020; OECD, 2011; Sharma, 2009; WWAP, 2015). However, groundwater depletion is a significant issue globally, and it is estimated that over 20% of the world's aquifers are overexploited (Gleeson et al., 2012; Richey et al., 2015; Wada et al., 2010). Use of groundwater and its eventual depletion is not an isolated problem, and entails various side effects, including land subsidence (Erban et al., 2014; Farr & Liu, 2015; Galloway & Burbey, 2011), coastal saltwater intrusion (Ferguson & Gleeson, 2012; Konikow, 2011; Michael et al., 2017; Werner & Simmons, 2009), decreased baseflow and consequent basin salinization (Farber et al., 2004; Pauloo et al., 2020; Warner et al., 2013), desertification (Sheridan, 1981; Van Dijck et al., 2006; Yang et al., 2015), and increased political conflict across transboundary aquifers (Giordano, 2009; Jarvis et al., 2005; Wolf, 2007; Zeitoun & Mirumachi, 2008). The Intelligence Community Assessment (NIC, 2012) has identified water stress to be a potential driver of regional instability. Therefore, understanding the availability of groundwater in the world's aquifers as it is exploited by humans over time is a key component for decision-makers interested in populations at risk of drought and consequent conflict.
NASA GRACE/GRACE FO Science Team
