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<br />18 <br /> <br />The overland flow of water occurs "hen the rate of rainfall or snow <br />meltwater exceeds the absorptive capacity of the land surface. Overland <br />flow is most cOlDlllon in semiarid and arid regions. but is common in all <br />areas, including humid areas, where the original vegetstion or soil <br />structure have been disturbed. Heavily forested areas, although highly <br />porous, cannot absorb water and facilita,te overlan,d flow. <br /> <br />Ground Water <br /> <br />Intercepted water not undergoing evaporation ,nay per co lat.! through the <br />surface and subsoil to replenish the ground water ,;upply. Ground water is <br />that water which lies in the saturated region of the ground. l:he upper <br />surface of this saturated region is the water tabh (Figure 10). Most of <br />the ground water is so deep in the ground that for "all practical purposes <br />it does not participate in the hydrological cycle ILDd is not aVailable for <br />use. However, it is that groundwater in the upper region of the earth's <br />crust which does participate in active exchange, 8IIld into which rain-water <br />percolates and runs off into streams, la,ltes, and the, ocean. <br /> <br /> <br />(ltYCW"""''''.11<11011 <br /> <br />A <br /> <br />. <br /> <br />Flgu!e 10. <br />::Or~tion ollJll'Mlnd lur1;act to .'altt Ilblc .'\ llt~~m clw"nd 0'- rill h;u '1 <br />......11( ~_lf~, . thr dl&nnd bollo'" II at I lawn e-In<:,uon lhiln [.._... (t <br />W"'ler I.lh...",_ (USG.S.) "" <br /> <br />(Modified frolll: Water A Prilller by L. 8. Leopold. W. R. <br />1neaaD .and Coapany. San Funcll1co. n 7.) <br /> <br />Deep water in the oceans may have a t'esidence time of 1,000 years. <br />The residence ti_ of continental water r,anges from ,a few weeks to many <br />lB111enniums,' depending on whether it is located in the ground, a lake, <br />a glacier, wetlands, or a stream. The ground water :reservoir shows the <br />greatest variation in residence time from a few weeks in karst (limestone) <br />aquifers, to a few weeks or months in gravel streamb<!ds, a few mc>nths or <br />'years in alluvial depOSits, a few yeat's or decades ill other unconfined <br />aquifers snd a few decades, centuries, orlllillenniumEI in confined aquifers. <br />