ABSTRACT
Various data, although relatively unpublicized, exist in support of the quantitative importance of groundwater relative to other components of the hydrologic cycle on the earth. With reference to Table 24.1, excluding the large amount of the earth’s water resting in the oceans and seas at high levels of salinity, groundwater accounts for about one-third of the freshwater resources of the world. If we limit our consideration to the liquid-phase freshwater, groundwater will account for almost the total volume available for use. If we consider only the most active groundwater regimes, for example, in subsurface reservoirs of good hydraulic permeability, water quantity will be reduced just to about two-thirds of the total amount. Considering this, the freshwater breakdown comes to 95% of subsurface groundwater; 3.5% of surface water such as lakes, swamps, reservoirs, and river channels; and 1.5% of soil moisture. From the usability point of view, however, this volumetric superiority of subsurface groundwater should be somewhat moderated by the average residence times listed in the right column of the table. River water has a turnover time on
24.1 Introduction .......................................................................................................................... 841 24.2 Groundwater Velocity and Darcy’s Law ...............................................................................844
