The return of groundwater quantity: a mega-scale and interdisciplinary “future of hydrogeology”?

Abstract

A series of recent papers suggest groundwater quantity may be returning to prominence in hydrogeology research. The unsustainable depletion of groundwater has been documented on both regional (Rodell et al. 2009; Tiwari et al. 2009; Famiglietti et al. 2011) and global scales (Wada et al. 2010; Konikow 2011; Wada et al. 2012) using data synthesis and the GRACE satellite data. Additionally, how groundwater resources will be impacted by global change remains important but uncertain and difficult to predict (Green et al. 2011; Taylor et al. 2013). Recent discussions on groundwater sustainability have suggested applying cutting-edge sustainability concepts such as multigenerational goal setting and adaptive management to groundwater quantity problems (Gleeson et al. 2010, 2012). At its core, groundwater quantity is a water budget question of fluxes and stores. The critical applied questions of groundwater quantity are “how much groundwater is available for sustainable use, and what is the impact of the various uses on interconnected social, economic and environmental systems” From the perspective of the authors, as young and possibly naive early-career hydrogeologists, it is suggested that this overall question is one “future of hydrogeology”, like the other futures of hydrogeology described in the Hydrogeology Journal special edition of 2005 (Voss 2005). In the following, this vast question is tentatively divided into a series of smaller questions. Undoubtedly, other researchers will raise other questions or suggest other angles or avenues of research. The purpose of this essay is to encourage this discussion. The term “return” is used to reflect the historical trends in hydrogeologic research. In Fig. 1, the evolution of hydrogeologic research has been divided into three overlapping phases, based on trends in citations and benchmark papers (Schwartz et al. 2005; Anderson 2008). Prominent research in early quantitative hydrogeology focused on questions of “capacity” or “safe yield” when studying aquifers (Meinzer 1923; Theis 1935, 1940). Over the past ∼30–40 years, the community has been largely focused on issues of groundwater contamination and quality as well as more recently on groundwater/surface-water interactions. This research remains important and can be integrated into a holistic view of groundwater and sustainability. The trajectory of hydrogeology research has been increasing in scope, interdisciplinarity and complexity (Fig. 1). It is suggested that a return to groundwater quantity research at the mega-scale, which addresses long-term issues of sustainability, equity, ecology and economics, may have a high return-oninvestment for science and society. Here, the focus is on groundwater systems at regional (≈10-km length scale) to continental (>1000 km) scales, herein called “mega-scale”. Since the 1970s, hydrogeology has often, but not exclusively, focused on site-scale (<1 km) research to examine important water resource and contamination problems and how groundwater interacts with surface water. Regional-scale groundwater systems were first modeled in the 1960s (Toth 1963; Freeze and Witherspoon 1967; Garven 1995; Person et al. 1996) but the numerical simulation of groundwater systems over entire continents has only been recently possible (Fan et al. 2007; Lemieux et al. 2008). Additionally, continental-scale remote sensing from the GRACE satellites have only recently documented realtime groundwater depletion at the mega-scale. Numerous fundamental questions of the spatio-temporal variability of groundwater fluxes and stores remain, especially at the mega-scale. These questions resemble recurring issues in groundwater science and engineering but with a new large-scale twist: