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<br /> <br /> <br />-~-- <br /> <br />GENERAL FACTS AND CONCEPTS <br />ABOUT GROUND WATER <br /> <br />The following review of some basic facts and concepts about ground water serves asbackgi'ound <br />for the discussion of ground-water sustainability, <br /> <br />. <br /> <br />Ground water occurs almost everywhere <br />beneath the land surface, The widespread <br />occurrence of potable ground water is the <br />reason that it is used as a source of water <br />supply by about one-half the population of <br />the, United States, including almost all of the <br />population that is served by domestic water- <br />supply systems. <br /> <br />Natural sources of freshwater that become <br />ground water are (1) areal recharge from <br />precipitation that percolates through <br />the unsaturated zone to the water table <br />(Figure 4) and (2) losses of water from <br />streams and other bodies of surface water <br />such as lakes and wetlands. Areal recharge <br />ranges from a tiny fraction to about one-half <br />of average annual precipitation. Because <br />areal recharge occurs over broad areas, <br />even small average rates of recharge (for <br />example, a few inches per year) represent <br />significant volumes of inflow to ground <br />water. Streams and other surface-water <br />bodies may either gain water from ground <br />water or lose (recharge) water to ground <br />water, Streams commonly are a significant <br />source of recharge to ground water down- <br />stream from mountain fronts and steep <br />hillslopes in arid and semiarid areas and <br />in karst terrains (areas underlain by lime- <br />stone and other soluble rocks). <br /> <br />,1 <br />! <br />p <br />H <br />~ <br />i <br /> <br />. <br /> <br />I <br />I <br />, <br /> <br />il!AIiiIii~1IM0i' <br /> <br />""'-""'j.Iil,....,,,_cI'I\.. <br /> <br />~ <br /> <br />~ <br /> <br />.ar!>l>l~[1i>lI.n; <br /> <br />~ EvapOlransplra'ton <br /> <br /> <br />I' . <br />I :t 2 <br />- - - - - - -,- - - - -,.--I':-ll <br />I "I ~ <br />RaCh~rg9JQI' I Water table: ! <br />water table .;:5 <br />Capillary fringe'" v <br /> <br />Saturated zone <br />below the water table <br /> <br />(Ground water) <br /> <br />. <br /> <br />The top of the subsurface ground,water body, <br />the water table, is a surface, generally below <br />the land surface, that fluctuafes s~asonaliy <br />and from year to year in reSi'onse to changes <br />in recharge from precipitation andsurfa<;e- <br />water bodies, On a regional scale, the config- <br />uration of the water tablecomm6n1y'is'a <br />subdued replica of the land-sui face topog"' <br />raphy. The depth to the water table varies. <br />In some settings, it can be at or' near the land <br />surface; for example, near boClies of surface <br />water in humid climates. In other settings, the <br />depth to the water table can be hundreds of <br />feet below land surface. <br /> <br />m'"",,,,,,,,m,ell:;Iii"":"''''''''''''' <br /> <br />6 <br />