Abstract
Groundwater is the world’s largest accessible source of freshwater to satisfy human water needs. Moreover, groundwater buffers variable precipitation rates over time, thereby effectively sustaining river flows in times of droughts and evaporation in areas with shallow water tables. In this study, building on previous work, we simulate groundwater head fluctuations and groundwater storage changes in both confined and unconfined aquifer systems using a global-scale high-resolution (5′) groundwater model by deriving new estimates of the distribution and thickness of confining layers. Inclusion of confined aquifer systems (estimated 6–20% of the total aquifer area) improves estimates of timing and amplitude of groundwater head fluctuations and changes groundwater flow paths and groundwater-surface water interaction rates. Groundwater flow paths within confining layers are shorter than paths in the underlying aquifer, while flows within the confined aquifer can get disconnected from the local drainage system due to the low conductivity of the confining layer. Lateral groundwater flows between basins are significant in the model, especially for areas with (partially) confined aquifers were long flow paths crossing catchment boundaries are simulated, thereby supporting water budgets of neighboring catchments or aquifer systems. The developed two-layer transient groundwater model is used to identify hot-spots of groundwater depletion. Global groundwater depletion is estimated as 7013 km3 (137 km3y−1) over 1960–2010, which is consistent with estimates of previous studies.
Highlights
As the world’s largest accessible source of freshwater, groundwater plays a vital role in satisfying the basic needs of human society (Gleeson et al, 2016; UNESCO, 2009)
In this paper we present the results of a two-layer transient groundwater flow model that has been built to simulate groundwater head dynamics affected by changes in climate and human water use
We focus the discussion on part of the US (Fig. 9) as this region contains shallow and deep groundwater tables and confined and unconfined aquifer systems, and has been the focus of related previous studies (Gleeson et al, 2011; Schaller and Fan, 2009)
Summary
As the world’s largest accessible source of freshwater, groundwater plays a vital role in satisfying the basic needs of human society (Gleeson et al, 2016; UNESCO, 2009). It serves as a primary source of drinking water and supplies water for agricultural and industrial activities. During periods of low or no rainfall, groundwater storage provides a natural buffer against water shortage, preserves evaporation in areas with shallow water tables, and sustains baseflows to rivers and wetlands, thereby supporting ecosystem habitats and biodiversity (e.g. Bierkens and van den Hurk, 2007; de Graaf et al, 2013; Wada et al, 2014). As a consequence, pumping costs increase, and baseflow to rivers and wetlands is reduced, negatively affecting ecosystems, and compaction of the emptied pore space may lead to land subsidence
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