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<br />
<br />NATURAL PROCESSES OF GROUND-WATER
<br />AND SURFACE-WATER INTERACTION
<br />
<br />The Hydrologic Cycle and Interactions
<br />of Ground Water and Surface Water
<br />
<br />The hydrologic cycle describes the contin-
<br />uous movement of water above, on, and below the
<br />surface of the Earth, The water on the Earth's
<br />surface--surface water-occurs as streams, lakes,
<br />and wetlands, as well as bays and oceans, Surface
<br />water also includes the solid forms of water-
<br />snow and ice, The water below the surface of the
<br />Earth primarily is ground water, but it also
<br />includes soil water,
<br />The hydrologic cycle commonly is portrayed
<br />by a very simplified diagram that shows only
<br />major transfers of water between continents and
<br />oceans, as in Figure 1. However, for under-
<br />standing hydrologic processes and managing
<br />water resources, the hydrologic cycle needs to be
<br />viewed at a wide range of scales and as having a
<br />
<br />Figure 1, Groulld water is the secolld
<br />smallest of the four maill pools of
<br />water 011 Earth, alld river flow to the
<br />oceallS is olle of the smallest fluxes,
<br />yet groulld water alld surface water
<br />are the compollellts of the hydrologic
<br />system that Imma/ls use most,
<br />(Modified from Schelesillger, W,H.,
<br />1991, Biogeochemistry-All a 11 a lys is
<br />of global challge: Academic Press,
<br />Sail Diego, Califomia,) (Used with
<br />permissioll.)
<br />
<br />Precipitation
<br />on land
<br />27,000
<br />
<br />Ice
<br />6,600,000
<br />
<br />Ground water
<br />2,000,000
<br />
<br />great deal of variability in time and space, Precipi-
<br />tation, which is the source of virtually all fresh-
<br />water in the hydrologic cycle, falls nearly
<br />everywhere, but its distribution is highly variable,
<br />Similarly, evaporation and transpiration return
<br />water to the atmosphere nearly everywhere, but
<br />evaporation and transpiration rates vary consider-
<br />ably according to climatic conditions, As a result,
<br />much of the precipitation never reaches the oceans
<br />as surface and subsurface runoff before the water
<br />is returned to the atmosphere, The relative magni-
<br />tudes of the individual components of the hydro-
<br />logic cycle, such as evapotranspiration, may differ
<br />significantly even at small scales, as between an
<br />agricultural field and a nearby woodland,
<br />
<br />Atmosphere
<br />3,000
<br />
<br />Net transport
<br />to land
<br />10,000
<br />
<br />\
<br />
<br />Evapatranspi ration
<br />1"~~6~~d
<br />
<br />.$ R;ve' flow to ocean,
<br />.. 10,000
<br />
<br />p,ec;p;taH/"
<br />on oceans
<br />( 92,000
<br />
<br />Evaporation
<br />from oceans
<br />102,000
<br />
<br />~
<br />
<br />
<br />Oceans
<br />322.600,000
<br />
<br />Pools are in cubic miles
<br />Fluxes are in cubic miles per year
<br />
<br />2
<br />
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