3 Hydrological Regime of the Mega Aquifer Systems
The hydrological regime of an aquifer system characterizes its dynamics and is defined by the system’s water inflows (recharge), outflows (discharge) and the resulting changes over time in the volume of stored groundwater. In principle, all these terms are subject to spatial and temporal variation. For practical reasons, however, this section pays hardly any attention to the variations inside each of the mega aquifer systems. The horizontal spatial variations are ignored by adopting a lumped approach to the individual mega aquifer systems. Occasionally some differentiation is made between the shallow and deeper domains of the aquifer systems, but otherwise vertical variations are ignored. Regarding the variations over time, only the changes in groundwater storage are considered in some detail; storage variations have been monitored by the GRACE satellite (over the period 2002–2016) for all mega aquifer systems, and for a few, area-wide terrestrial-monitoring records cover rather long periods. To quantify groundwater recharge and discharge, more or less synchronous estimates are presented, that are reasonably representative of average conditions during the first two decades of the 21st century. Despite these simplifications, it has not been possible to find information on most of the relevant variables for some of the mega aquifer systems, while the available information for some others does not always appear to be reliable. Nevertheless, the contents of this section will give a first impression of the mega aquifer system hydrological regimes, which is a first step towards understanding their dynamics and their relevance for humans, the biosphere and the environment.